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Context.The fast solar wind is known to emanate from polar coronal holes.Aims.This Letter reports the first estimate of the expansion rate of polar coronal flows performed by the Metis coronagraph on board Solar Orbiter.Methods.By exploiting simultaneous measurements in polarized white light and ultraviolet intensity of the neutral hydrogen Lyman-αline, it was possible to extend observations of the outflow velocity of the main component of the solar wind from polar coronal holes out to 5.5 R⊙, the limit of diagnostic applicability and observational capabilities.Results.We complement the results obtained with analogous polar observations performed with the UltraViolet Coronagraph Spectrometer on board the SOlar and Heliospheric Observatory during the previous full solar activity cycle, and find them to be satisfactorily reproduced by a magnetohydrodynamic turbulence model.Conclusions.This suggests that the dissipation of 2D turbulence energy is a viable mechanism for coronal plasma heating and the subsequent acceleration of the fast solar wind.

Funding: The research leading to these results has received funding from the UK Science and Technology Facilities Council (consolidated grant ST/N000609/1), the European Union Horizon 2020 research and innovation programme (grant agreement No. 647214). IDM received funding from the Research Council of Norway through its Centres of Excellence scheme, project number 262622. This work used the DiRAC Data Analytic system at the University of Cambridge, operated by the University of Cambridge High Performance Computing Service on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment was funded by BEIS National E-infrastructure capital grant (ST/K001590/1), STFC capital grants ST/H008861/1 and ST/H00887X/1, and STFC DiRAC Operations 650 grant ST/K00333X/1. DiRAC is part of the National e-Infrastructure. Why the atmosphere of the Sun is orders of magnitudes hotter than its surface is a long-standing question in solar physics. Over the years, many studies have looked at the potential role of magnetohydrodynamic (MHD) waves in sustaining these high temperatures. In this study, we use 3D MHD simulations to investigate (driven) transverse waves in a coronal loop. As the boundary-driven transverse waves propagate along the flux tube, the radial density profile leads to resonant absorption (or mode coupling) and phase mixing in the boundaries of the flux tube and the large velocity shears are subject to the Kelvin Helmholtz instability (KHI). The combination of these effects leads to enhanced energy dissipation and wave heating. Considering both resonant and non-resonant boundary driving as well as different densities for the flux tube, we show that only wave heating associated with a resonant driver in a lower density loop (with a loop core density ~5x10-13 kg/m-3) is able to balance radiative losses in the loop shell. Changing the model parameters to consider a denser loop or a driver with a non-resonant frequency, or both, leads to cooling of the coronal loop as the energy losses are greater than the energy injection and dissipation rates. Publisher PDF

Funding: JR and AWH gratefully acknowledge the financial support of STFC through Consolidated Grants ST/S000402/1 and ST/W001195/1 to the University of St Andrews. JT is grateful for support from the Division of Games Technology and Mathematics at Abertay University. AWH further acknowledges support from ERC Synergy grant ‘The Whole Sun’ (810218). The DiRAC component of CSD3 was funded by BEIS capital funding via STFC capital grants ST/P002307/1 and ST/R002452/1 and STFC operations grant ST/R00689X/1. DiRAC is part of the National e-Infrastructure. Straightened cylindrical models of coronal loops have been standard for decades, and shown to support nanoflare-like heating, but the influence of geometric curvature in models upon the heating produced has not been discussed in depth. Heating, its spatio-temporal distributions, and the associated mechanisms responsible are discussed, and compared with those from straightened models of a coronal loop. Previously, MHD avalanches have been generalized to curved loops, and shown to be viable. From that study, the associated heating is analysed and discussed in depth. Heating is seen to arise from processes originally instigated, yet not dominated, by magnetic reconnection, producing bursty, aperiodic nanoflares, dispersed evenly throughout the corona, but with a modest bias away from footpoints. One novelty arising is the simultaneous and independent occurrence of nanoflare-like events at disjoint sites along individual strands, anticipating some features recently seen in the ‘campfires’ from Solar Orbiter. With a view to future refinements in the model and to the inclusion of additional physical effects, the implications of this analysis are discussed. Publisher PDF

Funding: D.H.M. would like to thank the STFC for support via consolidated grant ST/N000609/1 and, the Leverhulme trust, and the ERC under the Synergy Grant: The Whole Sun (grant agreement no. 810218) for financial support. L.A.U. was supported by the NASA Living With a Star program (NNH16ZDA010N-LWS) and by the National Science Foundation Atmospheric and Geospace Sciences Postdoctoral Research Fellowship Program (Award #1624438). The solar corona exhibits a wide range of phenomena, from highly non-potential objects such as solar filaments to near-potential structures such as the open magnetic flux. For any global model to be useful in space weather applications, the model must on a single day reproduce all of these phenomena in the same simulation, using the same set of coronal physics and parameters. The purpose of the present paper is to evaluate whether the evolving magnetofrictional model can achieve this goal. Twenty-eight separate simulations are analyzed, where each tries to reproduce both highly non-potential and near-potential phenomena observed in the solar corona on the same day. This day is chosen to be 2015 March 20, the date of the solar eclipse. The study evaluates how the cadence of bipole data, ideal or nonideal coronal physics, and the variety of helicity injection mechanisms affect the accuracy of the simulations. Results show that significantly better agreement arises when using Advective Flux Transport (AFT) synoptic maps to drive the simulations, as compared to 27-day Carrington rotation synoptic maps. Using the nonideal effect of hyperdiffusion leads to the worst agreement with all coronal phenomena. Alternatively, when running either ideal or ohmic diffusion simulations with helicity condensation or bipoles with a self-helicity, a good agreement with both on-disk and limb structures can be found. This suggests that future studies aiming to simulate the corona and reproduce multiple phenomena on a given day should use data products such as AFT and avoid using the nonideal physics of hyperdiffusion. Publisher PDF

Funding: JR acknowledges the support of the Carnegie Trust for the Universities of Scotland. JR and AWH acknowledge the financial support of STFC through the Consolidated grant, ST/S000402/1, to the University of St Andrews. AWH acknowledges support from ERC Synergy grant ‘The Whole Sun’ (810218). CDJ acknowledges funding from the European Research Council (ERC) under grant agreement No. 647214. A ‘proof of principle’ is presented, whereby the Ohmic and viscous heating determined by a three-dimensional (3D) MHD model of a coronal avalanche are used as the coronal heating input for a series of field-aligned, one-dimensional (1D) hydrodynamic models. Three-dimensional coronal MHD models require large computational resources. For current numerical parameters, it is difficult to model both the magnetic field evolution and the energy transport along field lines for coronal temperatures much hotter than 1MK⁠, because of severe constraints on the time step from parallel thermal conduction. Using the 3D MHD heating derived from a simulation and evaluated on a single field line, the 1D models give coronal temperatures of 1MK and densities 1014--1015m−3 for a coronal loop length of 80Mm⁠. While the temperatures and densities vary smoothly along the field lines, the heating function leads to strong asymmetries in the plasma flows. The magnitudes of the velocities in the 1D model are comparable with those seen in 3D reconnection jets in our earlier work. Advantages and drawbacks of this approach for coronal modelling are discussed. Publisher PDF

El trabajo de esta tesis se centra en la caracterización observacional de la estructura termodinámica tridimensional (3D) de la corona solar a escala global durante mínimos de actividad solar, mediante reconstrucciones tomográficas. Asimismo, se implementan simulaciones magnetohidrodinámicas (MHD) 3D de la corona y el viento solar, que permiten analizar los resultados tomográficos en diversas estructuras magnéticas coronales. El estudio abarca los mínimos posteriores a los tres últimos ciclos solares (CSs), 22 a 24, durante los cuales los indicadores de actividad solar disminuyeron progresivamente. Las reconstrucciones tomográficas se basan en imágenes coronales en extremo ultravioleta tomadas por tres generaciones de telescopios espaciales, EIT/SoHO,EUVI/STeReO y AIA/SDO, y en luz visible por el coronógrafo espacial LASCO- C2/SoHO. Las simulaciones MHD, implementadas mediante el modelo AWSOM (Alfven Wave Solar atmosphere Model), se basan en magnetogramas fotosfericos sinópticos del instrumento espacial MDI/SoHO y la red terrestre GONG. La estructura coronal de mınimo exhibe caracterısticas globales persistentes independientemente del nivel de actividad del CS precedente. Entre ellas se destacan la estructura 3D de densidad y temperatura, el grado de contraste en densidad y temperatura entre Streamers y CHs, y la presencia de arcos coronales con temperatura decreciente con la altura dentro de Streamers. Se encuentra que durante el mínimo CS 23/24, el más prolongado y con menor nivel de actividad de los tres períodos estudiados, la corona exhibió temperaturas algo menores. Se discute la implicancia de distintos resultados DEMT para el estudio del problema del calentamiento coronal. El modelo MHD reproduce la reconstrucción tomográfica de la distribución 3D de densidad y temperatura en la corona interna (r < 1.25 R), pero sobrestima la densidad reconstruida a alturas mayores (2.5 − 6.0 R). Se discuten las causas de discrepancias entre el modelo y las reconstrucciones. El campo de velocidad terminal del modelo, que reproduce mediciones in situ, se anticorrelaciona con los valores de densidad y temperatura reconstruidos topográficamente en su región fuente en la corona interna y se correlaciona con su latitud terminal. Estos resultados apoyan la identificación de las líneas magnéticamente abiertas que rodean a Streamers como región fuente del viento solar lento. The work of this thesis focuses on the observational characterization of the three- dimensional (3D) thermodynamic structure of the solar corona on a global scale during solar activity minima, by means of tomographic reconstructions. Also, 3D magnetohydrodynamic (MHD) simulations of the corona and solar wind are implemented, which allow the analysis of tomographic results in various coronal magnetic structures. The study covers the minima after the last three solar cycles (CSs), 22 to 24, during which solar activity indicators progressively decreased. The tomographic reconstructions are based on coronal images in extreme ultraviolet taken by three generations of space telescopes, EIT/SoHO, EUVI/STeReO and AIA/SDO, and in visible light by the LASCO-C2/SoHO space coronagraph. The MHD simulations, implemented using the Alfv ́en Wave Solar atmosphere Model (awsom), are based on synoptic photospheric magnetograms from the MDI/SoHO space instrument and the GONG ground-based network. The coronal minimum structure exhibits persistent global characteristics regardless of the level of activity of the preceding CS. These include the 3D structure of density and temperature, the degree of contrast in density and temperature between Streamers and CHs, and the presence of coronal arcs with decreasing temperature with height within Streamers. It is found that during the CS 23/24 minimum, the longest and lowest activity level of the three periods studied, the corona exhibited somewhat lower temperatures. The implications of different DEMT results for the study of the coronal heating problem are discussed. The MHD model reproduces the tomographic reconstruction of the 3D density and temperature distribution in the inner corona (r < 1.25 R ), but overestimates the reconstructed density at higher altitudes (2.5 − 6.0 R). The causes of discrepancies between the model and the reconstructions are discussed. The model’s terminal velocity field, which reproduces in situ measurements, anticorrelates with the tomographically reconstructed density and temperature values in its source region in the inner corona and correlates with its terminal latitude. These results support the identification of the magnetically open lines surrounding Streamers as the source region of the slow solar wind. Fil: Lloveras, Diego Gustavo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

We study a series of relations between physical parameters in coronal loops of the quiet Sun reconstructed by combining tomographic techniques and modeling of the coronal magnetic field. We use differential emission measure tomography (DEMT) to determine the three-dimensional distribution of the electron density and temperature in the corona, and we model the magnetic field with a potential-field source-surface (PFSS) extrapolation of a synoptic magnetogram. By tracing the DEMT products along the extrapolated magnetic field lines, we obtain loop-averaged electron density and temperature. Also, loop-integrated energy-related quantities are computed for each closed magnetic field line. We apply the procedure to Carrington rotation 2082, during the activity minimum between Solar Cycles 23 and 24, using data from the Extreme Ultraviolet Imager on board the Solar Terrestrial Relations Observatory (STEREO) spacecraft. We find a scaling law between the loop-average density N and loop length L,Nm∼L-0.35, but we do not find a significant relation between loop-average temperature and loop length. We confirm though the previously found result that loop-average temperatures at the equatorial latitudes are lower than at higher latitudes. We associate this behavior with the presence at the equatorial latitudes of loops with decreasing temperatures along their length (“down” loops), which are in general colder than loops with increasing temperatures (“up” loops). We also discuss the role of “down” loops in the obtained scaling laws of heating flux versus loop length for different heliographic latitudes. We find that the obtained scalings for quiet-Sun loops do not generally agree with those found in the case of AR loops from previous observational and theoretical studies. We suggest that to better understand the relations found, it is necessary to forward model the reconstructed loops using hydrodynamic codes working under the physical conditions of the quiet-Sun corona. Fil: Mac Cormack, Cecilia. Universidad Nacional de Tres de Febrero; Argentina. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Lopez Fuentes, Marcelo Claudio. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Mandrini, Cristina Hemilse. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Lloveras, Diego Gustavo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Poisson, Mariano. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina. Universidad Nacional de Tres de Febrero; Argentina Fil: Vasquez, Alberto Marcos. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina. Universidad Nacional de Tres de Febrero; Argentina

Since the 1970s it has been empirically known that the area of solar coronal holes affects the properties of high-speed solar wind streams (HSSs) at Earth. We derive a simple analytical model for the propagation of HSSs from the Sun to Earth and thereby show how the area of coronal holes and the size of their boundary regions affect the HSS velocity, temperature, and density near Earth. We assume that velocity, temperature, and density profiles form across the HSS cross section close to the Sun and that these spatial profiles translate into corresponding temporal profiles in a given radial direction due to the solar rotation. These temporal distributions drive the stream interface to the preceding slow solar wind plasma and disperse with distance from the Sun. The HSS properties at 1 AU are then given by all HSS plasma parcels launched from the Sun that did not run into the stream interface at Earth distance. We show that the velocity plateau region of HSSs as seen at 1 AU, if apparent, originates from the center region of the HSS close to the Sun, whereas the velocity tail at 1 AU originates from the trailing boundary region. Small HSSs can be described to entirely consist of boundary region plasma, which intrinsically results in smaller peak velocities. The peak velocity of HSSs at Earth further depends on the longitudinal width of the HSS close to the Sun. The shorter the longitudinal width of an HSS close to the Sun, the more of its “fastest” HSS plasma parcels from the HSS core and trailing boundary region have impinged upon the stream interface with the preceding slow solar wind, and the smaller is the peak velocity of the HSS at Earth. As the longitudinal width is statistically correlated to the area of coronal holes, this also explains the well-known empirical relationship between coronal hole areas and HSS peak velocities. Further, the temperature and density of HSS plasma parcels at Earth depend on their radial expansion from the Sun to Earth. The radial expansion is determined by the velocity gradient across the HSS boundary region close to the Sun and gives the velocity-temperature and density-temperature relationships at Earth their specific shape. When considering a large number of HSSs, the assumed correlation between the HSS velocities and temperatures close to the Sun degrades only slightly up to 1 AU, but the correlation between the velocities and densities is strongly disrupted up to 1 AU due to the radial expansion. Finally, we show how the number of particles of the piled-up slow solar wind in the stream interaction region depends on the velocities and densities of the HSS and preceding slow solar wind plasma.

The present study investigates a mini-filament eruption associated with cancelling magnetic fluxes. The eruption originates from a small-scale loop complex commonly known as a Coronal Bright Point (CBP). The event is uniquely recorded in both the imaging and spectroscopic data taken with IRIS. We analyse IRIS spectroscopic and slit-jaw imaging observations as well as images taken in the extreme-ultraviolet channels of AIA, and line-of-sight magnetic-field data from HMI onboard the SDO. We also employ an NLFFF relaxation approach based on the HMI magnetogram time series. We identify a strong small-scale brightening as a micro-flare in a CBP. The mini-eruption manifests with the ejection of hot (CBP loops) and cool (mini-filament) plasma recorded in both the imaging and spectroscopic data. The micro-flare is preceded by the appearance of an elongated bright feature in the IRIS slit-jaw 1400 A images located above the polarity inversion line. The micro-flare starts with an IRIS pixel size brightening and propagates bi-directionally along the elongated feature. We detect in both the spectral and imaging IRIS data and AIA data, strong flows along and at the edges of the elongated feature which we believe represent reconnection outflows. Both edges of the elongated feature that wrap around the edges of the erupting MF evolve into a J-type shape creating a sigmoid appearance. A quasi-separatrix layer (QSL) is identified in the vicinity of the polarity inversion line by computing the squashing factor Q in different horizontal planes of the NLFFF model. The QSL reconnection site has the same spectral appearance as the so-called explosive events identified by strong blue- and red-shifted emission, thus answering a long outstanding question about the true nature of this spectral phenomenon., accepted for publication in A&A, the abstract is reduced in size, for the movies contact MM

On the Sun,jets in light bridges are frequently observed with high-resolution instruments.The respective roles played by convection and the magnetic field in triggering such jets are not yet clear.We report a small fan-shaped jet along a LB observed by the 1.6m Goode Solar Telescope(GST) with the TiO Broadband Filter Imager(BFI),the Visible Imaging Spectrometer(VIS) in H{\alpha},and the Near-InfraRed Imaging Spectropolarimeter(NIRIS),along with the Stokes parameters.The high spatial and temporal resolution of those instruments allowed us to analyze the features identified during the jet event.By constructing the H{\alpha} Dopplergrams,we found that the plasma is first moving upward,whereas during the second phase of the jet,the plasma is flowing back.Working with time slice diagrams,we investigated the propagation-projected speed of the fan and its bright base.The fan-shaped jet developed within a few minutes,with diverging beams. At its base,a bright point was slipping along the LB and ultimately invaded the umbra of the sunspot.The H{\alpha} profiles of the bright points enhanced the intensity in the wings, similarly to the case of Ellerman bombs.Co-temporally,the extreme ultraviolet brightenings developed at the front of the dark material jet and moved at the same speed as the fan, leading us to propose that the fan-shaped jet material compressed and heated the ambient plasma at its extremities in the corona.Our multi-wavelength analysis indicates that the fan-shaped jet could result from magnetic reconnection across the highly diverging field low in the chromosphere,leading to an apparent slipping motion of the jet material along the LB.However,we did not find any opposite magnetic polarity at the jet base,as would typically be expected in such a configuration.We therefore discuss other plausible physical mechanisms,based on waves and convection, that may have triggered the event., Comment: 10 pages, 7 figures

Full list of authors: Kahil, F.; Hirzberger, J.; Solanki, S. K.; Chitta, L. P.; Peter, H.; Auchère, F.; Sinjan, J.; Orozco Suárez, D.; Albert, K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero, A.; Blanco Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero, L.; Gutiérrez Márquez, P.; Kolleck, M.; del Toro Iniesta, J. C.; Volkmer, R. ; Woch, J.; Fiethe, B.; Gómez Cama, J. M.; Pérez-Grande, I.; Sanchis Kilders, E.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Calchetti, D.; Carmona, M.; Deutsch, W.; Fernández-Rico, G.; Fernández-Medina, A.; García Parejo, P.; Gasent-Blesa, J. L.; Gizon, L.; Grauf, B.; Heerlein, K.; Lagg, A.; Lange, T.; López Jiménez, A.; Maue, T.; Meller, R.; Michalik, H.; Moreno Vacas, A.; Müller, R.; Nakai, E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub, J.; Strecker, H.; Torralbo, I.; Valori, G.; Aznar Cuadrado, R.; Teriaca, L.; Berghmans, D.; Verbeeck, C.; Kraaikamp, E.; Gissot, S.-- This is an Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., Context. The Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter (SO) spacecraft observed small extreme ultraviolet (EUV) bursts, termed campfires, that have been proposed to be brightenings near the apexes of low-lying loops in the quiet-Sun atmosphere. The underlying magnetic processes driving these campfires are not understood. Aims. During the cruise phase of SO and at a distance of 0.523 AU from the Sun, the Polarimetric and Helioseismic Imager on Solar Orbiter (SO/PHI) observed a quiet-Sun region jointly with SO/EUI, offering the possibility to investigate the surface magnetic field dynamics underlying campfires at a spatial resolution of about 380 km. Methods. We used co-spatial and co-temporal data of the quiet-Sun network at disc centre acquired with the High Resolution Imager of SO/EUI at 17.4 nm (HRIEUV, cadence 2 s) and the High Resolution Telescope of SO/PHI at 617.3 nm (HRT, cadence 2.5 min). Campfires that are within the SO/PHI−SO/EUI common field of view were isolated and categorised according to the underlying magnetic activity. Results. In 71% of the 38 isolated events, campfires are confined between bipolar magnetic features, which seem to exhibit signatures of magnetic flux cancellation. The flux cancellation occurs either between the two main footpoints, or between one of the footpoints of the loop housing the campfire and a nearby opposite polarity patch. In one particularly clear-cut case, we detected the emergence of a small-scale magnetic loop in the internetwork followed soon afterwards by a campfire brightening adjacent to the location of the linear polarisation signal in the photosphere, that is to say near where the apex of the emerging loop lays. The rest of the events were observed over small scattered magnetic features, which could not be identified as magnetic footpoints of the campfire hosting loops. Conclusions. The majority of campfires could be driven by magnetic reconnection triggered at the footpoints, similar to the physical processes occurring in the burst-like EUV events discussed in the literature. About a quarter of all analysed campfires, however, are not associated to such magnetic activity in the photosphere, which implies that other heating mechanisms are energising these small-scale EUV brightenings. © F. Kahil et al. 2022., Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. We are grateful to the ESA SOC and MOC teams for their support. The German contribution to SO/PHI is funded by the BMWi through DLR and by MPG central funds. The Spanish contribution is funded by FEDER/AEI/MCIU (RTI2018-096886-C5), a “Center of Excellence Severo Ochoa” award to IAA-CSIC (SEV-2017-0709), and a Ramón y Cajal fellowship awarded to DOS. The French contribution is funded by CNES. The EUI instrument was built by CSL, IAS, MPS, MSSL/UCL, PMOD/WRC, ROB, LCF/IO with funding from the Belgian Federal Science Policy Office (BELSPO/PRODEX PEA 4000112292); the Centre National d’Etudes Spatiales (CNES); the UK Space Agency (UKSA); the Bundesministerium für Wirtschaft und Energie (BMWi) through the Deutsches Zentrum für Luft- und Raumfahrt (DLR); and the Swiss Space Office (SSO).

Funding: The research leading to these results has received funding from the UK Science and Technology Facilities Council (consolidated grant ST/N000609/1), the European Union Horizon 2020 research and innovation programme (grant agreement No. 647214). IDM received funding from the Research Council of Norway through its Centres of Excellence scheme, project number 262622. We consider the behaviour of Alfvén waves propagating in a medium with random density perturbations. The imposed density perturbations have a broadband spectrum and their characteristic spatial scale may be defined according to the peak in the spectrum. The interaction of the boundary driven Alfvén waves with the medium generates reflections most efficiently when their wavelength is comparable to the spatial scale of the density perturbations. For our monotonic driver, this leads to the generation of quasi-periodic oscillations. The periods of oscillation of the propagating Alfvén waves is no longer only associated with the driver. Additional periodicities may be associated with one or more characteristic spatial scales in the density profile, or with beating between other spectral components. Multiple wave reflections cause oscillatory power to be retained at low altitudes, increasing opportunities to contribute to heating at those locations. Publisher PDF

We have carried out a comprehensive study of the temperature structure of plasmoids, which successively occurred in recurrent active region jets. The multithermal plasmoids were seen to be travelling along the multi-threaded spire as well as at the footpoint region in the EUV/UV images recorded by the Atmospheric Imaging Assembly (AIA). The Differential Emission Measure (DEM) analysis was performed using EUV AIA images, and the high-temperature part of the DEM was constrained by combining X-ray images from the X-ray telescope (XRT/Hinode). We observed a systematic rise and fall in brightness, electron number densities and the peak temperatures of the spire plasmoid during its propagation along the jet. The plasmoids at the footpoint (FPs) (1.0-2.5 MK) and plasmoids at the spire (SPs) (1.0-2.24 MK) were found to have similar peak temperatures, whereas the FPs have higher DEM weighted temperatures (2.2-5.7 MK) than the SPs (1.3-3.0 MK). A lower limit to the electron number densities of plasmoids - SPs (FPs) were obtained that ranged between 3.4-6.1$\times$10$^{8}$ (3.3-5.9$\times$10$^{8}$) cm$^{-3}$ whereas for the spire, it ranged from 2.6-3.2$\times$10$^{8}$ cm$^{-3}$. Our analysis shows that the emission of these plasmoids starts close to the base of the jet(s), where we believe that a strong current interface is formed. This suggests that the blobs are plasmoids induced by a tearing-mode instability., Comment: 13 pages, 10 figures, Accepted for publication in Monthly Notices of the Royal Astronomical Society

Context. We apply our recently developed method to reconstruct synoptic maps of the photospheric magnetic field from observations of chromospheric plages and the magnetic polarity of sunspots. Here, we apply the method to an extended time interval from 1915 to 1985. Aims. Systematic magnetographic observations of the solar photospheric magnetic field were initiated as recently as the 1970s and the lack of earlier observations limits our ability to study and understand the long-term evolution of the Solar global field. This study is aimed at creating synoptic maps of magnetic fields for the pre-magnetograph era and using these maps as input for modern simulation models to investigate the long-term (centennial) evolution of the Sun’s global magnetic fields. Methods. We reconstructed active Solar regions by identifying chromospheric plages from Ca II K line synoptic maps and assigning magnetic polarities based on the observed polarity of sunspots. We used a surface flux transport (SFT) model to simulate the evolution of the photospheric magnetic field from the reconstructed active regions. We used the potential field source surface (PFSS) model to determine the amount of open magnetic flux from the reconstruction and from magnetographic observations. We also reconstructed the coronal field during two eclipses and compared the result with eclipse drawings. Results. We successfully reconstructed the photospheric magnetic field from 1915 to 1985. The number and total magnetic flux of the reconstructed active regions shows a realistic cyclic behavior that mostly follows the evolution of the sunspot number, even on relatively short timescales. The polar field strengths of cycles 19 and 20 do not reflect the evolution of the sunspot number very accurately, which may be related to problems related to the calcium data during cycle 19 and the long data gap during cycle 20. The polarity of polar fields and the amount of open field both at high and low latitudes all demonstrate the expected cyclic behavior. The agreement of the modeled coronal structure with eclipse drawings in 1922 and 1923 is fair.

Funding: D.H.M. would like to thank both the UK STFC and the ERC (Synergy grant: WHOLE SUN, grant Agreement No. 810218) for financial support. D.H.M. would like to thank STFC for IAA funding under grant number SMC1-XAS012. ARY thanks STFC for support under grant ST/5000321/1. An important element of 3D data-driven simulations of solar magnetic fields is the determination of the horizontal electric field at the solar photosphere.This electric field is used to drive the 3D simulation and inject energy and helicity into the solar corona. One outstanding problem is the localisation of the horizontal electric field such that it is consistent with Ohm’s law. Yeates (ApJ, 836, 131, 2017) put forward a new “sparse” technique for computing the horizontal electric field from normal component magnetograms that minimises the number of non-zero values. This aims to produce a better representation of Ohm’s law compared to previously used “non-sparse” techniques. To test this new approach we apply it to active region (AR) 10977, along with the previously developed non-sparse technique of Mackay, Green and van Ballegooi-jen (ApJ, 729, 97, 2011). A detailed comparison of the two techniques with coronal observations is used to determine which is the most successful. Results show that the non-sparse technique of Mackay, Green, and van Ballegooijen (2011) produces the best representation for the formation and structure of the sigmoid above AR 10977. In contrast, the Yeates (2017) approach injects strong horizontal fields between spatially separated, evolving magnetic polarities. This injection produces highly twisted unphysical field lines with significantly higher magnetic energy and helicity. It is also demonstrated that the Yeates (2017) approach produces significantly different results that can be inconsistent with the observations depending on whether the horizontal electric field is solved for directly or indirectly through the magnetic vector potential. In contrast, the Mackay, Green, and van Ballegooijen (2011) method produces consistent results using either approach. The sparse technique of Yeates (2017) has significant pitfalls when applied to spatially resolved solar data, where future studies need to investigate why these problems arise. Publisher PDF

Aims. We here demonstrate how the recently developed Lightweaver framework makes non-LTE (NLTE) spectral synthesis feasible on a new 3D ab-initio magnetohydrodynamic (MHD) filament/prominence simulation, in a post-processing step. Methods. We clarify the need to introduce filament/prominent-specific Lightweaver boundary conditions that accurately model incident chromospheric radiation, and include a self-consistent and smoothly varying limb darkening function. Results. Progressing from isothermal/isobaric models to the self-consistently generated stratifications within a fully 3D MHD filament/prominence simulation, we find excellent agreement between our 1.5D non local thermodynamic equilibrium Lightweaver synthesis and a popular Hydrogen Hα proxy. We compute additional lines including Ca~\textsc{ii} 8542 alongside the more optically-thick Ca~\textsc{ii} H&K & Mg~\textsc{ii} h&k lines, for which no comparable proxy exists, and explore their formation properties within filament/prominence atmospheres. Conclusions. The versatility of the Lightweaver framework is demonstrated with this extension to 1.5D filament/prominence models, where each vertical column of the instantaneous 3D MHD state is spectrally analysed separately, without accounting for (important) multi-dimensional radiative effects. The general agreement found in the line core contrast of both observations and the Lightweaver-synthesised simulation further validates the current generation of solar filaments/prominences models constructed numerically with MPI-AMRVAC., 16+2 pages with appendix, 12+1 figures, accepted for publication in Astronomy & Astrophysics

We have demonstrated that the Transition Region Adaptive Conduction (TRAC) method permits fast and accurate numerical solutions of the field-aligned hydrodynamic equations, successfully removing the influence of numerical resolution on the coronal density response to impulsive heating. This is achieved by adjusting the parallel thermal conductivity, radiative loss, and heating rates to broaden the transition region (TR), below a global cutoff temperature, so that the steep gradients are spatially resolved even when using coarse numerical grids. Implementing the original 1D formulation of TRAC in multi-dimensional magnetohydrodynamic (MHD) models would require tracing a large number of magnetic field lines at every time step in order to prescribe a global cutoff temperature to each field line. In this paper, we present a highly efficient formulation of the TRAC method for use in multi-dimensional MHD simulations, which does not rely on field line tracing. In the TR, adaptive local cutoff temperatures are used instead of global cutoff temperatures to broaden any unresolved parts of the atmosphere. These local cutoff temperatures are calculated using only local grid cell quantities, enabling the MHD extension of TRAC to efficiently account for the magnetic field evolution, without tracing field lines. Consistent with analytical predictions, we show that this approach successfully preserves the properties of the original TRAC method. In particular, the total radiative losses and heating remain conserved under the MHD formulation. Results from 2D MHD simulations of impulsive heating in unsheared and sheared arcades of coronal loops are also presented. These simulations benchmark the MHD TRAC method against a series of 1D models and demonstrate the versatility and robustness of the method in multi-dimensional magnetic fields. We show, for the first time, that pressure differences, ..., Comment: 18 pages, 11 figures, accepted for publication in A&A

Aims. Estimates of coronal wave energy remain uncertain as a large fraction of the energy is likely hidden in the non-thermal line widths of emission lines. In order to estimate these wave energies, many previous studies have considered the root mean squared wave amplitudes to be a factor of $\sqrt{2}$ greater than the non-thermal line widths. However, other studies have used different factors. To investigate this problem, we consider the relation between wave amplitudes and the non-thermal line widths within a variety of 3D magnetohydrodynamic (MHD) simulations. Methods. We consider the following 3D numerical models: Alfv\'en waves in a uniform magnetic field, transverse waves in a complex braided magnetic field, and two simulations of coronal heating in an arcade. We applied the forward modelling code FoMo to generate the synthetic emission data required to analyse the non-thermal line widths. Results. Determining a single value for the ratio between the non-thermal line widths and the root mean squared wave amplitudes is not possible across multiple simulations. It was found to depend on a variety of factors, including line-of-sight angles, velocity magnitudes, wave interference, and exposure time. Indeed, some of our models achieved the values claimed in recent articles while other more complex models deviated from these ratios. Conclusions. To estimate wave energies, an appropriate relation between the non-thermal line widths and root mean squared wave amplitudes is required. However, evaluating this ratio to be a singular value, or even providing a lower or upper bound on it, is not realistically possible given its sensitivity to various MHD models and factors. As the ratio between wave amplitudes and non-thermal line widths is not constant across our models, we suggest that this widely used method for estimating wave energy is not robust., Comment: 11 pages and 11 figures

Many prominences are supported by magnetic flux ropes. One important question is how we can determine whether the flux rope is weakly-twisted or strongly-twisted. In this paper, we attempted to check whether prominences supported by weakly-twisted and strongly-twisted flux ropes can manifest different features so that we might distinguish the two types of magnetic structures by their appearance. We performed pseudo three-dimensional simulations of two magnetic flux ropes with different twists. We found that the resulting two prominences differ in many aspects. The prominence supported by a weakly-twisted flux rope is composed mainly of transient threads, forming high-speed flows inside the prominence. Its horns are evident. Conversely, the one supported by a highly-twisted flux rope consists mainly of stable quasi-stationary threads, including longer independently trapped threads and shorter magnetically connected threads. It is also revealed that the prominence spine deviates from the flux rope axis in the vertical direction and from the photospheric polarity inversion line projected on the solar surface, especially for the weakly-twisted magnetic flux rope. The two types of prominences differ significantly in appearance. It is also suggested that a piling-up of short threads in highly-twisted flux ropes might account for the vertical-like threads in some prominences., 10 pages, 7 figures, accepted for publication in A&A

In solar physics, a severe numerical challenge for modern simulations is properly representing a transition region between the million-degree hot corona and a much cooler plasma of about 10000 K (e.g., the upper chromosphere or a prominence). In previous 1D hydrodynamic simulations, the transition region adaptive conduction (TRAC) method has been proven to capture aspects better that are related to mass evaporation and energy exchange. We aim to extend this method to fully multidimensional magnetohydrodynamic (MHD) settings, as required for any realistic application in the solar atmosphere. Because modern MHD simulation tools efficiently exploit parallel supercomputers and can handle automated grid refinement, we design strategies for any-dimensional block grid-adaptive MHD simulations. We propose two different strategies and demonstrate their working with our open-source MPI-AMRVAC code. We benchmark both strategies on 2D prominence formation based on the evaporation--condensation scenario, where chromospheric plasma is evaporated through the transition region and then is collected and ultimately condenses in the corona. A field-line-based TRACL method and a block-based TRACB method are introduced and compared in block grid-adaptive 2D MHD simulations. Both methods yield similar results and are shown to satisfactorily correct the underestimated chromospheric evaporation, which comes from a poor spatial resolution in the transition region. Because fully resolving the transition region in multidimensional MHD settings is virtually impossible, TRACB or TRACL methods will be needed in any 2D or 3D simulations involving transition region physics., Accepted by A&A

Wimmer-Schweingruber, R. F., et al., [Context] Solar Orbiter strives to unveil how the Sun controls and shapes the heliosphere and fills it with energetic particle radiation. To this end, its Energetic Particle Detector (EPD) has now been in operation, providing excellent data, for just over a year. [Aims] EPD measures suprathermal and energetic particles in the energy range from a few keV up to (near-) relativistic energies (few MeV for electrons and about 500 MeV nuc-1 for ions). We present an overview of the initial results from the first year of operations and we provide a first assessment of issues and limitations. In addition, we present areas where EPD excels and provides opportunities for significant scientific progress in understanding how our Sun shapes the heliosphere. [Methods] We used the solar particle events observed by Solar Orbiter on 21 July and between 10 and 11 December 2020 to discuss the capabilities, along with updates and open issues related to EPD on Solar Orbiter. We also give some words of caution and caveats related to the use of EPD-derived data. [Results] During this first year of operations of the Solar Orbiter mission, EPD has recorded several particle events at distances between 0.5 and 1 au from the Sun. We present dynamic and time-averaged energy spectra for ions that were measured with a combination of all four EPD sensors, namely: the SupraThermal Electron and Proton sensor (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) as well as the associated energy spectra for electrons measured with STEP and EPT. We illustrate the capabilities of the EPD suite using the 10 and 11 December 2020 solar particle event. This event showed an enrichment of heavy ions as well as 3He, for which we also present dynamic spectra measured with SIS. The high anisotropy of electrons at the onset of the event and its temporal evolution is also shown using data from these sensors. We discuss the ongoing in-flight calibration and a few open instrumental issues using data from the 21 July and the 10 and 11 December 2020 events and give guidelines and examples for the usage of the EPD data. We explain how spacecraft operations may affect EPD data and we present a list of such time periods in the appendix. A list of the most significant particle enhancements as observed by EPT during this first year is also provided., We acknowledge the Spanish Ministerio de Ciencia, Innovación y Universidades for their invaluable support on the development of the EPD suite and the ICU under grants FEDER/MCIU – Agencia Estatal de Investigación/Projects ESP2015-68266-R, ESP2017-88436-R and PID2019-104863RB-I00/AEI/10.13039/501100011033. We also thank acknowledge the financial support by the Spanish MINECO-FPI-2016 predoctoral grant with FSE. We thank the German Federal Ministry for Economic Affairs and Energy and the German Space Agency (Deutsches Zentrum für Luft- und Raumfahrt, e.V., (DLR)) for their unwavering support of STEP, EPT, and HET under grants numbers 50OT0901, 50OT1202, 50OT1702, and 50OT2002; and ESA for supporting the build of SIS under contract number SOL.ASTR.CON.00004, as well as the University of Kiel and the Land Schleswig-Holstein for their support of SIS. We thank NASA headquarters and the GSFC Solar Orbiter project office for support of SIS at APL under contract NNN06AA01C. A.A. acknowledges the support by the Spanish Ministerio de Ciencia e Innovación (MICINN) under grant PID2019- 105510GB-C31 and through the ‘Center of Excellence María de Maeztu 2020-2023’ award to the ICCUB (CEX2019-000918- M). Solar Orbiter magnetometer operations are funded by the UK Space Agency (grant ST/T001062/1). Tim Horbury is supported by STFC grant ST/S000364/1. We acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 101004159 (SERPENTINE).

Context. The recent discovery of much greater magnetic flux cancellation taking place at the photosphere than previously realised has led us in our previous works to suggest magnetic reconnection driven by flux cancellation as the cause of a wide range of dynamic phenomena, including jets of various kinds and solar atmospheric heating. Aims. Previously, the theory considered energy release at a two-dimensional current sheet. Here we develop the theory further by extending it to an axisymmetric current sheet in three dimensions without resorting to complex variable theory. Methods. We analytically study reconnection and treat the current sheet as a three-dimensional structure. We apply the theory to the cancellation of two fragments of equal but opposite flux that approach each another and are located in an overlying horizontal magnetic field. Results. The energy release occurs in two phases. During Phase 1, a separator is formed and reconnection is driven at it as it rises to a maximum height and then moves back down to the photosphere, heating the plasma and accelerating a plasma jet as it does so. During Phase 2 the fluxes cancel in the photosphere and accelerate a mixture of cool and hot plasma upwards., Comment: 10 pages, 10 figures

The coronal heating problem addresses how temperatures in the solar atmosphere increase by millions of degrees, moving up from the cool surface, into the upper reaches of the atmosphere. This heating, known to be attributable to the magnetic field, has attracted several possible explanations. Attention here focuses on the viability and onset of magnetohydrodynamic avalanches. One means of transferring energy from interior convective motions is through photospheric motions, for example granulation and super-granulation. The power transferred by these photospheric motions stores energy in the global coronal magnetic field, and in coronal loops within active regions. The fields become highly braided, such that an ideal kink-mode instability occurs within one strand of a loop. Then, this instability rapidly expands, destabilizing the surrounding magnetic field, and destroying neighbouring threads. Bursty events occur in an intermittent, impulsive series, Parker's so-called nanoflares, above a largely steady background. This heating is then investigated in order to determine the respective contributions of the physical mechanisms of viscosity and resistivity. Arbitrary distributions of non-uniform heating occur across the domain. The localization and impulsiveness are investigated in respect of field-aligned heating properties. These heating profiles are tested in a hydrodynamic model of single coronal strands. This heating is found capable of maintaining approximately coronal conditions. Interestingly, three-dimensional MHD simulations and field-aligned, one-dimensional models produce similar behaviours, apart from in velocity, and this may have observable consequences. Throughout, magnetic reconnection is vital in contributing to coronal heating. Therefore, the onset and locations of reconnection are identified, and compared with several possible indicators. The widely used squashing factor, Q, little agrees with the sites of reconnection in a braided MHD field. Based on these results, magnetohydrodynamic avalanches can occur and produce sufficient energy to maintain the elevated temperatures of coronal loops.

All authors: Rouillard, A. P.; Pinto, R. F.; Vourlidas, A.; De Groof, A.; Thompson, W. T.; Bemporad, A.; Dolei, S.; Indurain, M.; Buchlin, E.; Sasso, C.; Spadaro, D.; Dalmasse, K.; Hirzberger, J.; Zouganelis, I.; Strugarek, A.; Brun, A. S.; Alexandre, M.; Berghmans, D.; Raouafi, N. E.; Wiegelmann, T.; Pagano, P.; Arge, C. N.; Nieves-Chinchilla, T.; Lavarra, M.; Poirier, N.; Amari, T.; Aran, A.; Andretta, V.; Antonucci, E.; Anastasiadis, A.; Auchère, F.; Bellot Rubio, L.; Nicula, B.; Bonnin, X.; Bouchemit, M.; Budnik, E.; Caminade, S.; Cecconi, B.; Carlyle, J.; Cernuda, I.; Davila, J. M.; Etesi, L.; Espinosa Lara, F.; Fedorov, A.; Fineschi, S.; Fludra, A.; Génot, V.; Georgoulis, M. K.; Gilbert, H. R.; Giunta, A.; Gomez-Herrero, R.; Guest, S.; Haberreiter, M.; Hassler, D.; Henney, C. J.; Howard, R. A.; Horbury, T. S.; Janvier, M.; Jones, S. I.; Kozarev, K.; Kraaikamp, E.; Kouloumvakos, A.; Krucker, S.; Lagg, A.; Linker, J.; Lavraud, B.; Louarn, P.; Maksimovic, M.; Maloney, S.; Mann, G.; Masson, A.; Müller, D.; Önel, H.; Osuna, P.; Orozco Suarez, D.; Owen, C. J.; Papaioannou, A.; Pérez-Suárez, D.; Rodriguez-Pacheco, J.; Parenti, S.; Pariat, E.; Peter, H.; Plunkett, S.; Pomoell, J.; Raines, J. M.; Riethmüller, T. L.; Rich, N.; Rodriguez, L.; Romoli, M.; Sanchez, L.; Solanki, S. K.; St Cyr, O. C.; Straus, T.; Susino, R.; Teriaca, L.; del Toro Iniesta, J. C.; Ventura, R.; Verbeeck, C.; Vilmer, N.; Warmuth, A.; Walsh, A. P.; Watson, C.; Williams, D.; Wu, Y.; Zhukov, A. N.-- Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., Context. The Solar Orbiter spacecraft will be equipped with a wide range of remote-sensing (RS) and in situ (IS) instruments to record novel and unprecedented measurements of the solar atmosphere and the inner heliosphere. To take full advantage of these new datasets, tools and techniques must be developed to ease multi-instrument and multi-spacecraft studies. In particular the currently inaccessible low solar corona below two solar radii can only be observed remotely. Furthermore techniques must be used to retrieve coronal plasma properties in time and in three dimensional (3D) space. Solar Orbiter will run complex observation campaigns that provide interesting opportunities to maximise the likelihood of linking IS data to their source region near the Sun. Several RS instruments can be directed to specific targets situated on the solar disk just days before data acquisition. To compare IS and RS, data we must improve our understanding of how heliospheric probes magnetically connect to the solar disk. Aims. The aim of the present paper is to briefly review how the current modelling of the Sun and its atmosphere can support Solar Orbiter science. We describe the results of a community-led effort by European Space Agency's Modelling and Data Analysis Working Group (MADAWG) to develop different models, tools, and techniques deemed necessary to test different theories for the physical processes that may occur in the solar plasma. The focus here is on the large scales and little is described with regards to kinetic processes. To exploit future IS and RS data fully, many techniques have been adapted to model the evolving 3D solar magneto-plasma from the solar interior to the solar wind. A particular focus in the paper is placed on techniques that can estimate how Solar Orbiter will connect magnetically through the complex coronal magnetic fields to various photospheric and coronal features in support of spacecraft operations and future scientific studies. Methods. Recent missions such as STEREO, provided great opportunities for RS, IS, and multi-spacecraft studies. We summarise the achievements and highlight the challenges faced during these investigations, many of which motivated the Solar Orbiter mission. We present the new tools and techniques developed by the MADAWG to support the science operations and the analysis of the data from the many instruments on Solar Orbiter. Results. This article reviews current modelling and tool developments that ease the comparison of model results with RS and IS data made available by current and upcoming missions. It also describes the modelling strategy to support the science operations and subsequent exploitation of Solar Orbiter data in order to maximise the scientific output of the mission. Conclusions. The on-going community effort presented in this paper has provided new models and tools necessary to support mission operations as well as the science exploitation of the Solar Orbiter data. The tools and techniques will no doubt evolve significantly as we refine our procedure and methodology during the first year of operations of this highly promising mission. © 2020 A. P. Rouillard et al., Solar Orbiter is a joint ESA and NASA mission. A. Vourlidas' Solar Orbiter effort is supported by NRL grant N00173-16-1-G029. P. Pagano would like to thank D. H. Mackay and S. L. Yardley for their valuable contributions, the European Research Council (ERC) under the European Union Horizon 2020 research and innovation program (grant agreement No. 647214), and the DiRAC Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (http://www.dirac.ac.uk).This equipment was funded by a BIS National E-infrastructure capital grant ST/K00042X/1, STFC capital grant ST/K00087X/1, DiRAC Operations grant ST/K003267/1 and Durham University. DiRAC is part of the National E-Infrastructure. A. Rouillard, V. Genot, M. Janvier, Elie Soubrier, F. Auchere, E. Buchlin and E. Pariat acknowledge support from the French space agency (Centre National d'Etudes Spatiales; CNES; https://cnes.fr/fr) that funds activity in plasma physics data center (Centre de Donnees de la Physique des Plasmas; CDPP; http://cdpp.eu/) and the Multi Experiment Data and Operation Center (MEDOC; https://idoc.ias.u-psud.fr/MEDOC), and the space weather team in Toulouse (Solar-Terrestrial Observations and Modelling Service; STORMS; http://storms-service.irap.omp.eu/).This includes funding for Gaia-DEM, the data mining tools AMDA (http://amda.cdpp.eu/), CLWEB (clweb.cesr. fr/) and the propagation tool (http://propagationtool.cdpp.eu).R.Pinto, M. Lavarra, Y. Wu and A. Kouloumvakos acknowledge financial support from the ANR project SLOW_ SOURCE No. ANR-17-CE31-0006-01, ANR project COROSHOCK No. ANR-18-ERC1-0006-01 and FP7 HELCATS project https://www.helcats-fp7.eu/under the FP7 EU contract number 606692. A. Warmuth acknowledges the support by DLR under grant No. 50 QL 0001. The STEREO SECCHI data are produced by a consortium of RAL (UK), NRL (USA), LMSAL (USA), GSFC (USA), MPS (Germany), CSL (Belgium), IOTA (France) and IAS (France). The ACE data were obtained from the ACE science center. The WIND data were obtained from the Space Physics Data Facility. Javier Rodriguez-Pacheco acknowledges Spanish Project: FEDER/MCIU-AEI/Project ESP2017-88436-R.

Context. The relative importance of AC and DC heating in maintaining the temperature of the corona is not well constrained. Aims. Investigate the effects of the characteristic time scales of photospheric driving on the injection and dissipation of energy within a coronal arcade. Methods. We have conducted three dimensional MHD simulations of foot point driving imposed on an arcade. We modified the typical driving time scales to understand the efficiency of heating obtained using AC and DC drivers. We considered the implications for the injected Poynting flux and the nature of the energy release in dissipative regimes. Results. For the same driver amplitude and complexity, long time scale motions are able to inject a much greater Poynting flux into the corona. Consequently, in non-ideal regimes, slow stressing motions result in a greater increase in plasma temperature than for wave-like driving. In dissipative simulations, Ohmic heating is found to be much more significant than viscous heating. For all drivers in our parameter space, energy dissipation is greatest close to the base of the arcade where the magnetic field strength is strongest and at separatrix surfaces, where the field connectivity changes. Across all simulations, the background field is stressed with random foot point motions (in a manner typical of DC heating studies) and even for short time scale driving, the injected Poynting flux is large given the small amplitude flows considered. For long time scale driving, the rate of energy injection was comparable to the expected requirements in active regions. The heating rates were found to scale with the perturbed magnetic field strength and not the total field strength. Conclusions. Alongside recent studies which show power within the corona is dominated by low frequency motions, our results suggest that in the closed corona, DC heating is more significant than AC heating., 14 pages, 16 figures

This paper investigates the changes in spatial properties when magnetohydrodynamic (MHD) waves undergo resonant damping in the Alfvén continuum. The analysis is carried out for a 1D cylindrical pressure-less plasma with a straight magnetic field. The effect of the damping on the spatial wave variables is determined by using complex frequencies that arise as a result of the resonant damping. Compression and vorticity are used to characterise the spatial evolution of the MHD wave. The most striking result is the huge spatial variation in the vorticity component parallel to the magnetic field. Parallel vorticity vanishes in the uniform part of the equilibrium. However, when the MHD wave moves into the non-uniform part, parallel vorticity explodes to values that are orders of magnitude higher than those attained by the transverse components in planes normal to the straight magnetic field. In the non-uniform part of the equilibrium plasma, the MHD wave is controlled by parallel vorticity and resembles an Alfvén wave, with the unfamiliar property that it has pressure variations even in the linear regime.

A single case study of a CBP in an equatorial coronal hole with an exceptionally large size is investigated to extend our understanding of the formation of mini-filaments, their destabilisation and the origin of the eruption triggering the formation of jet-like features recorded in the extreme-ultraviolet (EUV) and X-ray emission. We aim to explore the nature of the so-called micro-flares in CBPs associated with jets in coronal holes and mini coronal mass ejections in the quiet Sun. Co-observations from the Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory, and GONG Halpha images are used together with a Non-Linear Force Free Field (NLFFF) relaxation approach, where the latter is based on a time series of HMI line-of-sight magnetograms. A mini-filament (MF) that formed beneath the CBP arcade around 3-4 h before the eruption is seen in the Halpha and EUV AIA images to lift up and erupt triggering the formation of an X-ray jet. No significant photospheric magnetic flux concentration displacement (convergence) is observed and neither is magnetic flux cancellation between the two main magnetic polarities forming the CBP in the time period leading to the MF liftoff. The CBP micro-flare is associated with three flare kernels that formed shortly after the MF liftoff. No observational signature is found for reconnection beneath the erupting MF. The applied NLFFF modelling successfully reproduces both the CBP loop complex as well as the magnetic flux rope that hosts the MF during the build-up to the eruption., 13 pages, 11 figures, the abstract has been reduced from its original length, accepted by A&A

The large field-of-view of the Sun Watcher using Active Pixel System detector and Image Processing (SWAP) instrument on board the PRoject for Onboard Autonomy 2 (PROBA2) spacecraft provides a unique opportunity to study extended coronal structures observed in EUV in conjunction with global coronal magnetic field simulations. A global non-potential magnetic field model is used to simulate the evolution of the global corona from 1 September 2014 to 31 March 2015, driven by newly emerging bipolar active regions determined from Helioseismic and Magnetic Imager (HMI) magnetograms. We compare the large-scale structure of the simulated magnetic field with structures seen off-limb in SWAP EUV observations. In particular, we investigate how successful the model is in reproducing regions of closed and open structures; the scale of structures; and compare the evolution of a coronal fan observed over several rotations. The model is found to accurately reproduce observed large-scale off-limb structures. When discrepancies do arise they mainly occur off the east solar limb due to active regions emerging on the far side of the Sun, which cannot be incorporated into the model until they are observed on the Earth-facing side. When such ``late'' active region emergences are incorporated into the model, we find that the simulated corona self-corrects within a few days, so that simulated structures off the west limb more closely match what is observed. Where the model is less successful, we consider how this may be addressed, through model developments or additional observational products., Comment: 26 pages, 11 figures

Aims. We investigate the formation of small scales and the dissipation of MHD wave energy through non-linear interactions of counter-propagating, phase-mixed Alfvenic waves in a complex magnetic field. Methods. We conducted fully 3-D, non-ideal MHD simulations of transverse waves in complex magnetic fields. Continuous wave drivers were imposed on the foot points of magnetic field lines and the system was evolved for several Alfven travel times. Phase-mixed waves were allowed to reflect off the upper boundary and the interactions between the resultant counter-streaming wave packets were analysed. Results. The complex nature of the background magnetic field encourages the development of phase mixing, leading to a growth in currents and vorticities. Counter-propagating phase-mixed waves induce a cascade of energy to small scales and result in more efficient energy dissipation. This effect is enhanced in simulations with more complex magnetic fields. High-frequency drivers excite localised field line resonances and produce efficient wave heating. However, this relies on the formation of large amplitude oscillations on resonant field lines. Drivers with smaller frequencies than the fundamental frequencies of field lines are not able to excite resonances and thus do not inject sufficient Poynting flux to power coronal heating. Even in the case of high-frequency oscillations, the rate of dissipation is likely too slow to balance coronal energy losses, even within the quiet Sun. Conclusions. For the generalised phase-mixing presented here, complex background field structures enhance the rate of wave energy dissipation. However, it remains difficult for realistic wave drivers to inject sufficient Poynting flux to heat the corona. Indeed, significant heating only occurs in cases which exhibit amplitudes that are much larger than those currently observed in the solar atmosphere., 14 pages, 19 figures

The Kelvin-Helmholtz instability has been proposed as a mechanism to extract energy from magnetohydrodynamic (MHD) kink waves in flux tubes, and to drive dissipation of this wave energy through turbulence. It is therefore a potentially important process in heating the solar corona. However, it is unclear how the instability is influenced by the oscillatory shear flow associated with an MHD wave. We investigate the linear stability of a discontinuous oscillatory shear flow in the presence of a horizontal magnetic field within a Cartesian framework that captures the essential features of MHD oscillations in flux tubes. We derive a Mathieu equation for the Lagrangian displacement of the interface and analyse its properties, identifying two different instabilities: a Kelvin-Helmholtz instability and a parametric instability involving resonance between the oscillatory shear flow and two surface Alfv\'{e}n waves. The latter occurs when the system is Kelvin-Helmholtz stable, thus favouring modes that vary along the flux tube, and as a consequence provides an important and additional mechanism to extract energy. When applied to flows with the characteristic properties of kink waves in the solar corona, both instabilities can grow, with the parametric instability capable of generating smaller scale disturbances along the magnetic field than possible via the Kelvin-Helmholtz instability. The characteristic time-scale for these instabilities is $\sim 100$ s, for wavelengths of $200$ km. The parametric instability is more likely to occur for smaller density contrasts and larger velocity shears, making its development more likely on coronal loops than on prominence threads., Comment: 12 pages, 4 figures, accepted for publication MNRAS

Modelling the solar Transition Region with the use of an Adaptive Conduction (TRAC) method permits fast and accurate numerical solutions of the field-aligned hydrodynamic equations, capturing the enthalpy exchange between the corona and transition region, when the corona undergoes impulsive heating. The TRAC method eliminates the need for highly resolved numerical grids in the transition region and the commensurate very short time steps that are required for numerical stability. When employed with coarse spatial resolutions, typically achieved in multi-dimensional magnetohydrodynamic codes, the errors at peak density are less than 5% and the computation time is three orders of magnitude faster than fully resolved field-aligned models. This paper presents further examples that demonstrate the versatility and robustness of the method over a range of heating events, including impulsive and quasi-steady footpoint heating. A detailed analytical assessment of the TRAC method is also presented, showing that the approach works through all phases of an impulsive heating event because (i) the total radiative losses and (ii) the total heating when integrated over the transition region are both preserved at all temperatures under the broadening modifications of the method. The results from the numerical simulations complement this conclusion., 20 pages, 9 figures, 1 table, accepted for publication in A&A

Phase mixing of Alfv\'en waves has been studied extensively as a possible coronal heating mechanism but without the full thermodynamic consequences considered self-consistently. It has been argued that in some cases, the thermodynamic feedback of the heating could substantially affect the transverse density gradient and even inhibit the phase mixing process. In this paper, we use MHD simulations with the appropriate thermodynamical terms included to quantify the evaporation following heating by phase mixing of Alfv\'en waves in a coronal loop and the effect of this evaporation on the transverse density profile. The numerical simulations were performed using the Lare2D code. We set up a 2D loop model consisting of a field-aligned thermodynamic equilibrium and a cross-field (background) heating profile. A continuous, sinusoidal, high-frequency Alfv\'en wave driver was implemented. As the Alfv\'en waves propagate along the field, they undergo phase mixing due to the cross-field density gradient in the coronal part of the loop. We investigated the presence of field-aligned flows, heating from the dissipation of the phase-mixed Alfv\'en waves, and the subsequent evaporation from the lower atmosphere. We find that phase mixing of Alfv\'en waves leads to modest heating in the shell regions of the loop and evaporation of chromospheric material into the corona with upflows of the order of only 5-20 m/s. Although the evaporation leads to a mass increase in the shell regions of the loop, the effect on the density gradient and, hence, on the phase mixing process, is insignificant. This paper self-consistently investigates the effect of chromospheric evaporation on the cross-field density gradient and the phase mixing process in a coronal loop. We found that the effects in our particular setup (small amplitude, high frequency waves) are too small to significantly change the density gradient., Comment: 11 pages, 12 figures, accepted for publication in A&A

We analyse the capability of different type of perturbations -associated with usual environment energy fluctuations of the solar corona- to excite slow and sausage modes in solar flaring loops. We perform numerical simulations of the MHD ideal equations considering straight plasma magnetic tubes subject to local and global energy depositions. We find that local loop energy depositions of typical microflares [$\sim$($10^{27}$-$10^{30}$) erg] are prone to drive slow shock waves that induce slow mode patterns. The slow mode features are obtained for every tested local energy deposition inside the loop. Meanwhile, to obtain an observable sausage mode pattern a global perturbation, capable to modify instantaneously the internal loop temperature, is required, i.e. the characteristic conductive heating time must be much smaller than the radiative cooling one. Experiments carried out by varying parameter $\beta$ show us that the excitation of sausage modes does not depend significantly on the value of this parameter but on the global or local character of the energy source., Comment: 9 pages, 14 figures

Coronal plasma in the cores of solar active regions is impulsively heated to more than 5 MK. The nature and location of the magnetic energy source responsible for such impulsive heating is poorly understood. Using observations of seven active regions from the Solar Dynamics Observatory, we found that a majority of coronal loops hosting hot plasma have at least one footpoint rooted in regions of interacting mixed magnetic polarity at the solar surface. In cases when co-temporal observations from the Interface Region Imaging Spectrograph space mission are available, we found spectroscopic evidence for magnetic reconnection at the base of the hot coronal loops. Our analysis suggests that interactions of magnetic patches of opposite polarity at the solar surface and the associated energy release during reconnection are key to impulsive coronal heating., Comment: Published in Astronomy & Astrophysics. Online movies available at https://www.aanda.org/articles/aa/olm/2020/12/aa39099-20/aa39099-20.html

Aim. We investigate synthetic observational signatures generated from numerical models of transverse waves propagating in braided magnetic fields. Methods. We examine two simulations with different levels of magnetic field braiding and impose a periodic, transverse wave driver at the lower boundary. After waves reflect off the top boundary, a complex pattern of wave interference forms. We analyse the synthetic emission produced by the forward modelling code, FoMo. We examine line intensity, Doppler shifts and kinetic energy along several line-of-sight (LOS) angles. Results. The Doppler shifts showed the presence of transverse waves. However, when analysing the intensity, waves are less easily observed for more complex magnetic fields and may be mistaken for background noise. Depending on the LOS angle, the observable signatures of waves reflect some of the magnetic field braiding, particularly when multiple emission lines are available. In the more braided simulation, signatures of phase mixing can be identified. We identify possible ambiguities in the interpretation of wave modes based on the synthetic emission signatures. Conclusions. Most of the observables within this article behave as expected, given knowledge of the evolution of the parameters in 3D space. However, some intriguing observational signatures are present. Detecting regions of magnetic field complexity is somewhat possible when waves are present. However, simultaneous spectroscopic imaging from different lines is important to identify these locations. Care needs to be taken when interpreting intensity and Doppler velocity signatures as torsional motions as, in our setup, such signatures were a consequence of the magnetic field complexity and not torsional waves. Finally, the kinetic energy, estimated with Doppler velocities, is dependent on the polarisation of the wave, complexity of the background field and the LOS., Comment: 15 pages and 22 figures

The study of the evolution of coronal holes (CHs) is especially important in the context of high-speed solar wind streams (HSS) emanating from them. Stream interaction regions may deliver large amount of energy into the Earths system, cause geomagnetic storms, and shape interplanetary space. By statistically analysing 16 long-living CHs observed by the SDO, we focus on coronal, morphological and underlying photospheric magnetic field characteristics as well as investigate the evolution of the associated HSSs. We use CATCH to extract and analyse CHs using observations taken by AIA and HMI. We derive changes in the CH properties and correlate them to the CH evolution. Further we analyse the properties of the HSS signatures near 1au from OMNI data by manually extracting the peak bulk velocity of the solar wind plasma. We find that the area evolution of CHs mostly shows a rough trend of growing to a maximum followed by a decay. No correlation of the area evolution to the evolution of the signed magnetic flux and signed magnetic flux density enclosed in the projected CH area was found. From this we conclude that the magnetic flux within the extracted CH boundaries is not the main cause for its area evolution. We derive CH area change rates (growth and decay) of 14.2 +/- 15.0 * 10^8 km^2/day showing a reasonable anti-correlation (cc =-0.48) to the solar activity, approximated by the sunspot number. The change rates of the signed mean magnetic flux density (27.3 +/- 32.2 mG/day) and the signed magnetic flux (30.3 +/- 31.5 * 10^18 Mx/day) were also found to be dependent on solar activity (cc =0.50 and cc =0.69 respectively) rather than on the individual CH evolutions. Further we find that the CH area-to-HSS peak velocity relation is valid for each CH over its evolution but revealing significant variations in the slopes of the regression lines., Accepted at A&A

Context. Coronal loops are the basic building blocks of the solar corona, which are related to the mass supply and heating of solar plasmas in the corona. However, their fundamental magnetic structures are still not well understood. Most coronal loops do not expand significantly, whereas the diverging magnetic field would have an expansion factor of about 5-10 over one pressure scale height. Aims. In this study, we investigate a unique coronal loop with a roughly constant cross section, it is ultra long and quite thin. A coronal loop model with magnetic helicity is presented to explain the small expansion of the loop width. Methods. This coronal loop was predominantly detectable in the 171 A channel of the Atmospheric Imaging Assembly (AIA). Then, the local magnetic field line was extrapolated by a Potential-Field-Source-Surface model. Finally, the differential emission measure analysis made from six AIA bandpasses was applied to obtain the thermal properties of this loop. Results. This coronal loop has a projected length of roughly 130 Mm, a width of about 1.5 +(-) 0.5 Mm and a lifetime of around 90 minutes. It follows an open magnetic field line. The cross section expanded very little (i.e., 1.5-2.0) along the loop length during its whole lifetime. This loop has a nearly constant temperature at about 0.7 +(-) 0.2 MK, whereas its density exhibits the typical structure of a stratified atmosphere. Conclusions. We use a thin twisted flux tube theory to construct a model for this non-expanding loop, and find that indeed with sufficient twist a coronal loop can attain equilibrium. However, we can not rule out other possibilities such as footpoint heating by small-scale reconnection, elevated scale height by a steady flow along the loop etc., Comment: 8 pages, 5 figures, 1 tables

Funding: Carnegie Trust for the Universities of Scotland; Science and Technology Facilities Council (grants ST/N000609/1 and ST/P000320/1). The plasma heating associated with an avalanche involving three twisted magnetic threads within a coronal loop is investigated using three-dimensional magnetohydrodynamic simulations. The avalanche is triggered by the kink instability of one thread, with the others being engulfed as a consequence. The heating as a function of both time and location along the strands is evaluated. It is shown to be bursty at all times but to have no preferred spatial location. While there appears to be a level of "background" heating, this is shown to be comprised of individual, small heating events. A comparison between viscous and resistive (Ohmic) heating demonstrates that the strongest heating events are largely associated with the Ohmic heating that arises when the current exceeds a critical value. Viscous heating is largely (but not entirely) associated with smaller events. Ohmic heating dominates viscous heating only at the time of the initial kink instability. It is also demonstrated that a variety of viscous models lead to similar heating rates, suggesting that the system adjusts to dissipate the same amount of energy. Postprint

Context. Some of the most dynamic active regions are associated with complex photospheric magnetic configurations such as quadrupolar regions, and especially ones with a $\delta$-spot configuration and a strong Polarity Inversion Line (PIL). Aims. We study the formation and eruption of magnetic flux ropes in quadrupolar regions. Methods. 3D MHD simulations of the partial emergence of a highly twisted flux tube from the solar interior into a non-magnetized stratified atmosphere. Results. We emerge two $\Omega$-shaped loops forming a quadrupolar region. The emerging flux forms two initially separated bipoles, that later come in contact creating a $\delta$-spot central region. Above the two bipoles, two magnetic lobes expand and interact through a series of current sheets at the interface between them. Two recurrent confined eruptions are produced. In both cases, reconnection between sheared low-lying field lines forms a flux rope. Reconnection between the two lobes high in the atmosphere forms retracting field lines that push against the flux rope, creating a current sheet between them. It also forms a third magnetic lobe between the two emerged ones, that later acts as a strapping field. The flux rope eruptions are triggered when the reconnection between the flux ropes and the field above them becomes efficient enough to remove the tension of the overlying field. These reconnection events occur internally in the system. The first erupting flux rope almost fully reconnects with the overlying field. The second eruption is confined by the overlying field. During the confined eruption, the flux rope is enhanced in size, flux and twist, similar to confined-flare-to-flux-rope observations. Proxies of the emission reveal the two erupting filaments channels. A flare arcade is formed only in the second eruption due to the longer-lasting and more efficient reconnection below the flux rope., Comment: 15 pages, 19 figures, to appear in Astronomy and Astrophysics

The rapid damping of slow magnetoacoustic waves in the solar corona has been extensively studied in previous years. Most studies suggest that thermal conduction is a dominant contributor to this damping, albeit with a few exceptions. Employing extreme-ultraviolet (EUV) imaging data from SDO/AIA, we measure the damping lengths of propagating slow magnetoacoustic waves observed in several fan-like loop structures using two independent methods. The dependence of the damping length on temperature has been studied for the first time. The results do not indicate any apparent decrease in damping length with temperature, which is in contrast to the existing viewpoint. Comparing with the corresponding theoretical values calculated from damping due to thermal conduction, it is inferred that thermal conduction is suppressed in hotter loops. An alternative interpretation that suggests thermal conduction is not the dominant damping mechanism, even for short period waves in warm active region loops, is also presented., Accepted for publication in Frontiers in Astronomy and Space Sciences

Some high-resolution observations have revealed that the active-region solar corona is filled with myriads of thin strands even in apparently uniform regions with no resolved loops. This fine structure can host collective oscillations involving a large portion of the corona due to the coupling of the motions of the neighbouring strands. We study these vibrations and the possible observational effects. Here we theoretically investigate the collective oscillations inherent to the fine structure of the corona. We have called them fundamental vibrations because they cannot exist in a uniform medium. We use the T-matrix technique to find the normal modes of random arrangements of parallel strands. We consider an increasing number of tubes to understand the vibrations of a huge number of tubes of a large portion of the corona. We additionally generate synthetic time-distance Doppler and line broadening diagrams of the vibrations of a coronal region to compare with observations. We have found that the fundamental vibrations are in the form of clusters of tubes where not all the tubes participate in the collective mode. The periods are distributed over a wide band of values. The width of the band increases with the number of strands but rapidly reaches an approximately constant value. The frequency band associated with the fine structure of the corona depends on the minimum separation between strands. We have found that the coupling between the strands is of large extent. The synthetic Dopplergrams and line-broadening maps show signatures of collective vibrations, not present in the case of purely random individual kink vibrations. We conclude that the fundamental vibrations of the corona can contribute to the energy budget of the corona and they may have an observational signature., Accepted for publication in the Astronomy and Astrophysics

Context. Reliable estimates of the mass of coronal mass ejections (CMEs) are required to quantify their energy and predict how they affect space weather. When a CME propagates near the observer's line of sight, these tasks involve considerable errors, which motivated us to develop alternative means for estimating the CME mass. Aims. We aim at further developing and testing a method that allows estimating the mass of CMEs that propagate approximately along the observer's line of sight. Methods. We analyzed the temporal evolution of the mass of 32 white-light CMEs propagating across heliocentric heights of 2.5-15 R, in combination with that of the mass evacuated from the associated low coronal dimming regions. The mass of the white-light CMEs was determined through existing methods, while the mass evacuated by each CME in the low corona was estimated using a recently developed technique that analyzes the dimming in extreme-UV (EUV) images. The combined white-light and EUV analyses allow the quantification of an empirical function that describes the evolution of CME mass with height. Results. The analysis of 32 events yielded reliable estimates of the masses of front-side CMEs. We quantified the success of the method by calculating the relative error with respect to the mass of CMEs determined from white-light STEREO data, where the CMEs propagate close to the plane of sky. The median for the relative error in absolute values is ≈30%; 75% of the events in our sample have an absolute relative error smaller than 51%. The sources of uncertainty include the lack of knowledge of piled-up material, subsequent additional mass supply from the dimming region, and limitations in the mass-loss estimation from EUV data. The proposed method does not rely on assumptions of CME size or distance to the observer's plane of sky and is solely based on the determination of the mass that is evacuated in the low corona. It therefore represents a valuable tool for estimating the mass of Earth-directed events. Fil: López, F. M.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; Argentina Fil: Cremades Fernandez, Maria Hebe. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Tecnológica Nacional; Argentina Fil: Balmaceda, Laura Antonia. George Mason University; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; Argentina Fil: Nuevo, Federico Alberto. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Vásquez, A. M.. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina. Universidad Nacional de Tres de Febrero; Argentina

PA acknowledges funding from his STFC Ernest Rutherford Fellowship (No. ST/R004285/1). TVD was funded by GOA-2015-014 (KU Leuven). This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 724326). The inhomogeneous solar corona is continuously disturbed by transverse MHD waves. In the inhomogeneous environment of coronal flux tubes, these waves are subject to resonant absorption, a physical mechanism of mode conversion in which the wave energy is transferred to the transition boundary layers at the edge between these flux tubes and the ambient corona. Recently, transverse MHD waves have also been shown to trigger the Kelvin-Helmholtz instability (KHI) due to the velocity shear flows across the boundary layer. Also, continuous driving of kink modes in loops has been shown to lead to fully turbulent loops. It has been speculated that resonant absorption fuels the instability by amplifying the shear flows. In this work, we show that this is indeed the case by performing simulations of impulsively triggered transverse MHD waves in loops with and without an initially present boundary layer, and with and without enhanced viscosity that prevents the onset of KHI. In the absence of the boundary layer, the first unstable modes have high azimuthal wavenumber. A boundary layer is generated relatively late due to the mixing process of KHI vortices, which allows the late onset of resonant absorption. As the resonance grows, lower azimuthal wavenumbers become unstable, in what appears as an inverse energy cascade. Regardless of the thickness of the initial boundary layer, the velocity shear from the resonance also triggers higher order azimuthal unstable modes radially inwards inside the loop and a self-inducing process of KHI vortices occurs gradually deeper at a steady rate until basically all the loop is covered by small-scale vortices. We can therefore make the generalisation that all loops with transverse MHD waves become fully turbulent and that resonant absorption plays a key role in energising and spreading the transverse wave-induced KHI rolls all over the loop. Publisher PDF

Here we report on the unique observation of flaring coronal loops at the solar limb using high resolution imaging spectropolarimetry from the Swedish 1-meter Solar Telescope. The vantage position, orientation and nature of the chromospheric material that filled the flare loops allowed us to determine their magnetic field with unprecedented accuracy using the weak-field approximation method. Our analysis reveals coronal magnetic field strengths as high as 350 Gauss at heights up to 25 Mm above the solar limb. These measurements are substantially higher than a number of previous estimates and may have considerable implications for our current understanding of the extended solar atmosphere., 12 pages, 14 figures, accepted in ApJ

Thermal non-equilibrium (TNE) is believed to be a potentially important process in understanding some properties of the magnetically closed solar corona. Through one-dimensional hydrodynamic models, this paper addresses the importance of the numerical spatial resolution, footpoint heating timescales and background heating on TNE. Inadequate transition region (TR) resolution can lead to significant discrepancies in TNE cycle behaviour, with TNE being suppressed in under-resolved loops. A convergence on the periodicity and plasma properties associated with TNE required spatial resolutions of less than 2 km for a loop of length 180 Mm. These numerical problems can be resolved using an approximate method that models the TR as a discontinuity using a jump condition, as proposed by Johnston et al. (2017a,b). The resolution requirements (and so computational cost) are greatly reduced while retaining good agreement with fully resolved results. Using this approximate method we (i) identify different regimes for the response of coronal loops to time-dependent footpoint heating including one where TNE does not arise and (ii) demonstrate that TNE in a loop with footpoint heating is suppressed unless the background heating is sufficiently small. The implications for the generality of TNE are discussed., 15 pages, 10 figures, 1 table, accepted for publication in A&A

This paper investigates the mixed properties of MHD waves in a non-uniform plasma. It starts with a short revision of MHD waves in a uniform plasma of infinite extent. In that case the MHD waves do not have mixed properties. They can be separated in Alfv\'{e}n waves and magneto-sonic waves. The Alfv\'{e}n waves propagate parallel vorticity and are incompressible. In addition they have no parallel displacement component. The magneto-sonic waves are compressible and in general do have a parallel component of displacement but do not propagate parallel vorticity. This clear separation has been the reason why there has been a strong inclination in the literature to use this classification in the study of MHD waves in non-uniform plasmas. The main part of this paper is concerned with MHD waves in a non-uniform plasma. It is shown that the MHD waves in that situation in general propagate both vorticity and compression and hence have mixed properties. Finally, the close connection between resonant absorption and MHD waves with mixed properties is discussed.

AH acknowledges funding from the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1. DP and TVD were supported by the GOA-2015-014 (KU Leuven) and the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 724326). The transverse structure of coronal loops plays a key role in the physics but the small transverse scales can be difficult to observe directly. For wider loops the density profile may be estimated by forward modelling of the transverse intensity profile. The transverse density profile may also be estimated seismologically using kink oscillations in coronal loops. The strong damping of kink oscillations is attributed to resonant absorption and the damping profile contains information about the transverse structure of the loop. However, the analytical descriptions for damping by resonant absorption presently only describe the behaviour for thin inhomogeneous layers. Previous numerical studies have demonstrated that this thin boundary approximation produces poor estimates of the damping behaviour in loops with wider inhomogeneous layers. Both the seismological and forward modelling approaches suggest loops have a range of layer widths and so there is a need for a description of the damping behaviour that accurately describes such loops. We perform a parametric study of the damping of standing kink oscillations by resonant absorption for a wide range of inhomogeneous layer widths and density contrast ratios, with a focus on the values most relevant to observational cases. We describe the damping profile produced by our numerical simulations without prior assumption of its shape and compile our results into a lookup table which may be used to produce accurate seismological estimates for kink oscillation observations. Publisher PDF

This research has received funding from the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1 and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 647214). Aims . This paper presents 2.5D numerical experiments of Alfvén wave phase mixing and aims to assess the effects of nonlinearities on the wave behaviour and dissipation. In addition, this paper aims to quantify how effective the model presented here is at providing energy to the coronal volume. Methods . The model is presented and explored through the use of several numerical experiments which were carried outusing the Lare2D code (Arber et al. 2001). The experiments study footpoint driven Alfvén waves in the neighbourhood of a two-dimensional x-type null point, with initially uniform density and plasma pressure. A continuous sinusoidal driver with a constant frequency is used. Each experiment uses different driver amplitudes to compare weakly nonlinear experiments with linear experiments. Results . It was found that the wave trains phase-mix due to variations in the length of each field line as well asvariations in the field strength. The nonlinearities reduce the amount of energy entering the domain, as they reduce the effectiveness of the driver, but they have relatively little effect on the damping rate (for the range of amplitudes studied). The nonlinearities produce density structures which change the natural frequencies of the field lines and hence cause the resonant locations to move. The shifting of the resonant location causes the Poynting flux associated with the driver to decrease. Reducing the magnetic diffusivity increased the energy build-up on the resonant field lines, however, it has little effect on the total amount of energy entering the system. From an order of magnitude estimate, it was shown that the Poynting flux in our experiments was comparable to the energy requirements of the Quiet Sun corona, although a (possibly unphysically) large amount of magnetic diffusion was used, and it remains unclear if the model is able to provide enough energy under actual coronal conditions. Postprint

Forced magnetic reconnection, a reconnection event triggered by external perturbation, should be ubiquitous in the solar corona. Energy released during such cases can be much greater than that which was introduced by the perturbation. It is unclear how the properties of the external perturbation and the initial current sheet affect the reconnection region properties, and thereby the reconnection dynamics and energy release profile. We investigate the effect of the form of the external perturbation and initial current sheet on the evolution of the reconnection region and the energy release process. Chiefly we explore the non-linear interactions between multiple, simultaneous perturbations, which represent more realistic scenarios. Future work will use these results in test particle simulations to investigate particle acceleration over multiple reconnection events. Simulations are performed using Lare2d, a 2.5D Lagrangian-remap solver for the visco-resistive MHD equations. The model of forced reconnection is extended to include superpositions of sinusoidal driving disturbances, including localised Gaussian perturbations. A transient perturbation is applied to the boundaries of a region containing a force-free current sheet. The simulation domain is sufficiently wide to allow multiple magnetic islands to form and coalesce. Island coalescence contributes significantly to energy release and involves rapid reconnection. Long wavelength modes in perturbations dominate the evolution, without the presence of which reconnection is either slow, as in the case of short wavelength modes, or the initial current sheet remains stable, as in the case of noise perturbations. Multiple perturbations combine in a highly non-linear manner: reconnection is typically faster than when either disturbance is applied individually, with multiple low-energy events contributing to the same total energy release., Comment: 13 pages (including appendix)

© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. Two kinds of processes could occur during the flare decay phase: processes of energy release or processes of energy relaxation. Quasi-periodic pulsations (QPPs) of the broad-band emission are a good tool for the verification of mechanisms. We aim to study the processes during the decay phase of the X-class solar flare SOL2014-03-29T17:48. The observations in X-ray, microwave, and extreme ultraviolet bands are exploited to study the fine temporal, spatial, and spectral structures of the flare. The periods, amplitudes, and phases of both the fluxes and physical parameters [emission measure (EM), temperature] are studied using standard methods of correlation, Fourier, and wavelet analyses. It is found that the source of the QPPs is associated with the uniform post-flare loop. The X-ray source is located at the top of the arcade. QPPs with the similar characteristic time scales of P ≈ 74-80 s are found in the X-ray (3-25 keV) and microwave (15.7 GHz) emissions. Besides, QPPs with the same period are found in the time profiles of both the temperature (T e) and EM. The QPPs in temperature and the QPPs in EM demonstrate antiphase behaviour. The analysis reveals the quasi-periodic process of energy relaxation, without any additional source of energy during the decay phase. The periods of the QPPs are in a good agreement with second harmonic of standing slow magneto-acoustic wave in the arcade that could be triggered by a Moreton wave initiated by the flare in the direct vicinity of the arcade. ispartof: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY vol:483 issue:4 pages:5499-5507 status: published