Search

International audience

Both coronal holes and active regions are source regions of the solar wind. The distribution of these coronal structures across both space and time is well known, but it is unclear how much each source contributes to the solar wind. In this study we use photospheric magnetic field maps observed over the past four solar cycles to estimate what fraction of magnetic open solar flux is rooted in active regions, a proxy for the fraction of all solar wind originating in active regions. We find that the fractional contribution of active regions to the solar wind varies between 30% to 80% at any one time during solar maximum and is negligible at solar minimum, showing a strong correlation with sunspot number. While active regions are typically confined to latitudes $\pm$30$^{\circ}$ in the corona, the solar wind they produce can reach latitudes up to $\pm$60$^{\circ}$. Their fractional contribution to the solar wind also correlates with coronal mass ejection rate, and is highly variable, changing by $\pm$20% on monthly timescales within individual solar maxima. We speculate that these variations could be driven by coronal mass ejections causing reconfigurations of the coronal magnetic field on sub-monthly timescales., Comment: 18 pages, 7 figures. Published in Solar Physics

One of the major discoveries of Hinode’s Extreme-ultraviolet Imaging Spectrometer (EIS) is the presence of upflows at the edges of active regions. As active regions are magnetically connected to the large-scale field of the corona, these upflows are a likely contributor to the global mass cycle in the corona. Here we examine the driving mechanism(s) of the very strong upflows with velocities in excess of 70 km s−1, known as blue-wing asymmetries, observed during the eruption of a flux rope in AR 10977 (eruptive flare SOL2007-12-07T04:50). We use Hinode/EIS spectroscopic observations combined with magnetic-field modeling to investigate the possible link between the magnetic topology of the active region and the strong upflows. A Potential Field Source Surface (PFSS) extrapolation of the large-scale field shows a quadrupolar configuration with a separator lying above the flux rope. Field lines formed by induced reconnection along the separator before and during the flux-rope eruption are spatially linked to the strongest blue-wing asymmetries in the upflow regions. The flows are driven by the pressure gradient created when the dense and hot arcade loops of the active region reconnect with the extended and tenuous loops overlying it. In view of the fact that separator reconnection is a specific form of the more general quasi-separatrix (QSL) reconnection, we conclude that the mechanism driving the strongest upflows is, in fact, the same as the one driving the persistent upflows of ≈10 – 20 km s−1 observed in all active regions.

We discuss the significant implications of three eye-witness drawings of the total solar eclipse on 1706 May 12 in comparison with two on 1715 May 3, for our understanding of space climate change. These events took place just after what has been termed the “deep Maunder Minimum” but fall within the “extended Maunder Minimum” being in an interval when the sunspot numbers start to recover. Maria Clara Eimmert’s image in 1706 is particularly important because she was both a highly accomplished astronomical observer and an excellent artist: it was thought lost and was only re-discovered in 2012. Being the earliest coronal drawings of observational value yet identified, these drawings corroborate verbal accounts a corona without significant streamers, seen at totality of this and another eclipse event in 1652 during the Maunder Minimum. The graphical evidence implies that the coronal solar magnetic field was not lost but significantly weakened and the lack of coronal structure means there was little discernable open flux (either polar or at lower latitudes) even during the recovery phase of the Maunder Minimum. These observations provide evidence for a different state of oscillation of the solar dynamo, and hence behaviour of the Sun, in comparison with that during normal solar cycle minima (when a streamer belt between two polar coronal holes is visible) or near normal sunspot maxima (when coronal structure is caused by coronal holes at all latitudes) even to observers without a telescope.

The elemental composition of the Sun's hot atmosphere, the corona, shows a distinctive pattern that is different than the underlying surface, or photosphere (Pottasch 1963). Elements that are easy to ionize in the chromosphere are enhanced in abundance in the corona compared to their photospheric values. A similar pattern of behavior is often observed in the slow speed (< 500 km/s) solar wind (Meyer 1985), and in solar-like stellar coronae (Drake et al. 1997), while a reversed effect is seen in M-dwarfs (Liefke et al. 2008). Studies of the inverse effect have been hampered in the past because only unresolved (point source) spectroscopic data were available for these stellar targets. Here we report the discovery of several inverse events observed in-situ in the slow solar wind using particle counting techniques. These very rare events all occur during periods of high solar activity that mimic conditions more widespread on M-dwarfs. The detections allow a new way of connecting the slow wind to its solar source, and are broadly consistent with theoretical models of abundance variations due to chromospheric fast mode waves with amplitudes of 8-10 km/s; sufficient to accelerate the solar wind. The results imply that M-dwarf winds are dominated by plasma depleted in easily ionized elements, and lend credence to previous spectroscopic measurements., To be published in The Astrophysical Journal Letters

Solar Wind 15 brought together almost 250 experts from all continents of the world to discuss the current trends and future perspectives of the research on the Sun and its solar wind. The present article collection recaptures some of the highlights of their contributions.

The Sun’s magnetic field drives the solar wind and produces space weather. It also acts as the prototype for an understanding of other stars and their planetary environments. Plasma motions in the solar interior provide the dynamo action that generates the solar magnetic field. At the solar surface, this is evident as an approximately 11-year cycle in the number and position of visible sunspots. This solar cycle is manifest in virtually all observable solar parameters, from the occurrence of the smallest detected magnetic features on the Sun to the size of the bubble in interstellar space that is carved out by the solar wind. Moderate to severe space-weather effects show a strong solar cycle variation. However, it is a matter of debate whether extreme space-weather follows from the 11-year cycle. Each 11-year solar cycle is actually only half of a solar magnetic “Hale” cycle, with the configuration of the Sun’s large-scale magnetic field taking approximately 22 years to repeat. At the start of a new solar cycle, sunspots emerge at mid-latitude regions with an orientation that opposes the dominant large-scale field, leading to an erosion of the polar fields. As the cycle progresses, sunspots emerge at lower latitudes. Around solar maximum, the polar field polarity reverses, but the sunspot orientation remains the same, leading to a build-up of polar field strength that peaks at the start of the next cycle. Similar magnetic cyclicity has recently been inferred at other stars.

International audience; Within the coronae of stars, abundances of those elements with low first ionization potential (FIP) often differ from their photospheric values. The coronae of the Sun and solar-type stars mostly show enhancements of low-FIP elements (the FIP effect) while more active stars such as M dwarfs have coronae generally characterized by the inverse-FIP effect (I-FIP). Here we observe patches of I-FIP effect solar plasma in AR 12673, a highly complex βγδ active region. We argue that the umbrae of coalescing sunspots, and more specifically strong light bridges within the umbrae, are preferential locations for observing I-FIP effect plasma. Furthermore, the magnetic complexity of the active region and major episodes of fast flux emergence also lead to repetitive and intense flares. The induced evaporation of the chromospheric plasma in flare ribbons crossing umbrae enables the observation of four localized patches of I-FIP effect plasma in the corona of AR 12673. These observations can be interpreted in the context of the ponderomotive force fractionation model which predicts that plasma with I-FIP effect composition is created by the refraction of waves coming from below the chromosphere. We propose that the waves generating the I-FIP effect plasma in solar active regions are generated by subphotospheric reconnection of coalescing flux systems. Although we only glimpse signatures of I-FIP effect fractionation produced by this interaction in patches on the Sun, on highly active M stars it may be the dominant process.

We present observations and magnetic field models of an intermediate filament present on the Sun in August 2012, associated with a polarity inversion line that extends from AR 11541 in the east into the quiet sun at its western end. A combination of SDO/AIA, SDO/HMI, and GONG H alpha data allow us to analyse the structure and evolution of the filament from 2012 August 4 23:00 UT to 2012 August 6 08:00 UT when the filament was in equilibrium. By applying the flux rope insertion method, nonlinear force-free field models of the filament are constructed using SDO/HMI line-of-sight magnetograms as the boundary condition at the two times given above. Guided by observed filament barbs, both modelled flux ropes are split into three sections each with a different value of axial flux to represent the non-uniform photospheric field distribution. The flux in the eastern section of the rope increases by 4$\times$10$^{20}$ Mx between the two models, which is in good agreement with the amount of flux cancelled along the internal PIL of AR 11541, calculated to be 3.2$\times$10$^{20}$ Mx. This suggests that flux cancellation builds flux into the filament's magnetic structure. Additionally, the number of field line dips increases between the two models in the locations where flux cancellation, the formation of new filament threads and growth of the filament is observed. This suggests that flux cancellation associated with magnetic reconnection forms concave-up magnetic field that lifts plasma into the filament. During this time, the free magnetic energy in the models increases by 0.2$\times$10$^{31}$ ergs., 17 pages, 10 figures

We analyze the light curve of the M5.5 dwarf Proxima Centauri obtained by the TESS in Sectors 11 and 12. In the $\approx 50$ day-long light curve we identified and analyzed 72 flare events. The flare rate was 1.49 events per day, in total, 7.2% of the data was classified as flaring. The estimated flare energies were in the order of $10^{30}-10^{32}$ ergs in the TESS passband ($\approx4.8\times$ higher in bolometric energies, but in the same order of magnitude). Most of the eruptions appeared in groups. Two events showed quasi-periodic oscillations during their decay phase with a time scale of a few hours, that could be caused by quasiperiodic motions of the emitting plasma or oscillatory reconnection. From the cumulative flare frequency distribution we estimate that superflares with energy output of $10^{33}$ ergs are expected to occur three times per year, while a magnitude larger events (with $10^{34}$ ergs) can occur every second year. This reduces the chances of habitability of Proxima Cen b, although earlier numerical models did not rule out the existence of liquid water on the planetary surface. We did not find any obvious signs of planetary transit in the light curve., Comment: 15 pages, 3 tables, 5 figues; accepted in ApJ

Flares and CMEs can have deleterious effects on their surroundings: they can erode atmospheres of orbiting planets over time and also have high importance in stellar evolution. Most of the CME detections in the literature are single events found serendipitously sparse for statistical investigation. We aimed to gather a large amount of spectral data of M-dwarfs to drastically increase the number of known events to make statistical analysis possible in order to study the properties of potential stellar CMEs. Using archive data we investigated asymmetric features of Balmer-lines, that could indicate the Doppler-signature of ejected material. Of more than 5500 spectra we found 478 with line asymmetries--including nine larger events--on 25 objects, with 1.2-19.6 events/day on objects with line asymmetries. Most events are connected with enhanced Balmer-line peaks, suggesting these are connected to flares similar to solar events. Detected speeds mostly do not reach surface escape velocity: the typical observed maximum velocities are in the order of 100-300km/s , while the typical masses of the ejecta were in the order of $10^{15}-10^{18}$g. Statistical analysis suggests that events are more frequent on cooler stars with stronger chromospheric activity. Detected maximum velocities are lower than those observed on the Sun, while event rates were somewhat lower than we could expect from the solar case. These findings may support the idea that most of the CMEs could be suppressed by strong magnetic field. Alternatively, it is possible that we can observe only an early low-coronal phase before CMEs are accelerated at higher altitudes. Our findings could indicate that later-type, active dwarfs could be a safer environment for exoplanetary systems CME-wise than previously thought, and atmosphere loss due to radiation effects would play a stronger role in exoplanetary atmosphere evolution than CMEs., Comment: 14 pages, 9 figures, 2 tables, accepted in A&A. Abstract was slightly shortened due to arXiv limits

Quiescent solar prominences are observed to exist within the solar atmosphere for up to several solar rotations. Their eruption is commonly preceded by a slow increase in height that can last from hours to days. This increase in the prominence height is believed to be due to their host magnetic flux rope transitioning through a series of neighbouring quasi-equilibria before the main loss-of-equilibrium that drives the eruption. Recent work suggests that the removal of prominence mass from a stable, quiescent flux rope is one possible cause for this change in height. However, these conclusions are drawn from observations and are subject to interpretation. Here we present a simple model to quantify the effect of "mass-draining" during the pre-eruptive height-evolution of a solar flux rope. The flux rope is modeled as a line current suspended within a background potential magnetic field. We first show that the inclusion of mass, up to $10^{12}$~kg, can modify the height at which the line current experiences loss-of-equilibrium by up to 14\%. Next, we show that the rapid removal of mass prior to the loss-of-equilibrium can allow the height of the flux rope to increase sharply and without upper bound as it approaches its loss-of-equilibrium point. This indicates that the critical height for the loss-of-equilibrium can occur at a range of heights depending explicitly on the amount and evolution of mass within the flux rope. Finally, we demonstrate that for the same amount of drained mass, the effect on the height of the flux rope is up to two order of magnitude larger for quiescent than for active region prominences., 13 pages, 6 figures, Accepted for publication in the Astrophysical Journal

Faraday rotation measure at radio wavelengths is commonly used to diagnose large-scale magnetic fields. It is argued that the length-scales on which magnetic fields vary in large-scale diffuse astrophysical media can be inferred from correlations in the observed RM. RM is a variable which can be derived from the polarised radiative transfer equations in restrictive conditions. This paper assesses the usage of RMF (rotation measure fluctuation) analyses for magnetic field diagnostics in the framework of polarised radiative transfer. We use models of various magnetic field configurations and electron density distributions to show how density fluctuations could affect the correlation length of the magnetic fields inferred from the conventional RMF analyses. We caution against interpretations of RMF analyses when a characteristic density is ill-defined, e.g. in cases of log-normal distributed and fractal-like density structures. As the spatial correlations are generally not the same in the line-of-sight longitudinal direction and the sky plane direction, one also needs to clarify the context of RMF when inferring from observational data. In complex situations, a covariant polarised radiative transfer calculation is essential to capture all aspects of radiative and transport processes, which would otherwise ambiguate the interpretations of magnetism in galaxy clusters and larger-scale cosmological structures., MNRAS accepted, 18 pages, 9 figures

We analyse the coronal elemental abundances during a small flare using Hinode/EIS observations. Compared to the pre-flare elemental abundances, we observed a strong increase in coronal abundance of Ca XIV 193.84 {\AA}, an emission line with low first ionisation potential (FIP < 10 eV), as quantified by the ratio Ca/Ar during the flare. This is in contrast to the unchanged abundance ratio observed using Si X 258.38 {\AA}/S X 264.23 {\AA}. We propose two different mechanisms to explain the different composition results. Firstly, the small flare-induced heating could have ionised S, but not the noble gas Ar, so that the flare-driven Alfv\'en waves brought up Si, S and Ca in tandem via the ponderomotive force which acts on ions. Secondly, the location of the flare in strong magnetic fields between two sunspots may suggest fractionation occurred in the low chromosphere, where the background gas is neutral H. In this region, high-FIP S could behave more like a low-FIP than a high-FIP element. The physical interpretations proposed generate new insights into the evolution of plasma abundances in the solar atmosphere during flaring, and suggests that current models must be updated to reflect dynamic rather than just static scenarios., Comment: 14 pages, 6 figures, accepted for publication in The Astrophysical Journal

In this multi-instrument paper, we search for evidence of sustained magnetic reconnection far beyond the impulsive phase of the X8.2-class solar flare on 2017 September 10. Using Hinode/EIS, CoMP, SDO/AIA, K-Cor, Hinode/XRT, RHESSI, and IRIS, we study the late-stage evolution of the flare dynamics and topology, comparing signatures of reconnection with those expected from the standard solar flare model. Examining previously unpublished EIS data, we present the evolution of non-thermal velocity and temperature within the famous plasma sheet structure, for the first four hours of the flare's duration. On even longer time scales, we use Differential Emission Measures and polarization data to study the longevity of the flare's plasma sheet and cusp structure, discovering that the plasma sheet is still visible in CoMP linear polarization observations on 2017 September 11, long after its last appearance in EUV. We deduce that magnetic reconnection of some form is still ongoing at this time - 27 hours after flare onset., Comment: 14 pages, 7 figures; accepted for publication to ApJ

Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the coronae of stars. Understanding the plasma processes involved in CME initiation has applications to space weather forecasting and laboratory plasma experiments. James et al. (Sol. Phys. 292, 71, 2017) used EUV observations to conclude that a magnetic flux rope formed in the solar corona above NOAA Active Region 11504 before it erupted on 14 June 2012 (SOL2012-06-14). In this work, we use data from the Solar Dynamics Observatory to model the coronal magnetic field of the active region one hour prior to eruption using a nonlinear force-free field extrapolation, and find a flux rope reaching a maximum height of 150 Mm above the photosphere. Estimations of the average twist of the strongly asymmetric extrapolated flux rope are between 1.35 and 1.88 turns, depending on the choice of axis, although the erupting structure was not observed to kink. The decay index near the apex of the axis of the extrapolated flux rope is comparable to typical critical values required for the onset of the torus instability, so we suggest that the torus instability drove the eruption., Comment: 8 pages, 5 figures

International audience; Not Available

Since their discovery, upflows at the edges of active regions have attracted a lot of interest, primarily as they could potentially contribute to the slow solar wind. One aspect that has not been studied yet is how the long term evolution of active regions impacts the upflows. In this work, we analyze one active region that survives three solar rotations. We track how the flows change with time. We use local and global modeling of the decaying active region to determine how the age of the active region will impact the extent of the open magnetic fields, and then how some of the upflows could become outflows. We finish with a discussion of how these results, set in a broader context, can be further developed with the Solar Orbiter mission. Fil: Harra, Louise K.. Colegio Universitario de Londres; Reino Unido Fil: Ugarte Urra, Ignacio. Spece Sciences División. Naval Research Laboratory; Estados Unidos Fil: De Rosa, Marc. Lockheed Martin Corporation; Estados Unidos Fil: Mandrini, Cristina Hemilse. Consejo Nacional de Investigaciones Científicas y Técnicas; 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: van Driel Gesztelyi, Lidia. Lesia - Laboratoire D; . Colegio Universitario de Londres; Reino Unido. Centre National de la Recherche Scientifique. Observatoire de Paris; Francia Fil: Baker, Deborah. Colegio Universitario de Londres; Reino Unido Fil: Culhane, J. Leonard. Colegio Universitario de Londres; Reino Unido Fil: Démoulin, Pascal. Lesia - Laboratoire D; . Centre National de la Recherche Scientifique. Observatoire de Paris; Francia

A variety of indices have been proposed in order to represent the many different observables modulated by the solar cycle. Most of these indices are highly correlated with each other owing to their intrinsic link with the solar magnetism and the dominant eleven year cycle, but their variations may differ in fine details, as well as on short- and long-term trends. In this paper we present an overview of the indices that are often employed to describe the many features of the solar cycle, moving from the ones referring to direct observations of the inner solar atmosphere, the photosphere and chromosphere, to those deriving from measurements of the transition region and solar corona. For each index, we summarize existing measurements {\bf and typical use}, and for those that quantify physical observables, we describe the underlying physics.

Since its introduction to astro- and solar physics, the concept of helicity has proven to be useful in providing critical insights into physics of various processes from astrophysical dynamos, to magnetic reconnection and eruptive phenomena. Signature of helicity was also detected in many solar features, including orientation of solar active regions, or Joy’s law. Here we provide a summary of both solar phenomena and consider mutual relationship and its importance for the evolution of solar magnetic fields.

International audience

We investigate first ionization potential (FIP) bias levels in an anemone active region (AR) - coronal hole (CH) complex using an abundance map derived from Hinode/EIS spectra. The detailed, spatially resolved abundance map has a large field of view covering 359″ × 485″. Plasma with high FIP bias, or coronal abundances, is concentrated at the footpoints of the AR loops whereas the surrounding CH has a low FIP bias, ~1, i.e. photospheric abundances. A channel of low FIP bias is located along the AR's main polarity inversion line containing a filament where ongoing flux cancellation is observed, indicating a bald patch magnetic topology characteristic of a sigmoid/flux rope configuration.

Fil: Charbonneau, Paul. University Of Montreal; Canadá Fil: Leibacher, John. National Solar Observatory; Estados Unidos. Universite Paris-sud Xi; Francia 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: van Driel Gesztelyi, Lidia. Colegio Universitario de Londres; Reino Unido. Université Paris Diderot - Paris 7; Francia. Konkoly Observatory Hungarian Academy Of Sciences; Hungría Fil: Wheatland, Michael S.. The University Of Sydney; Australia

Aims. Although the temporal evolution of active regions (ARs) is relatively well understood, the processes involved continue to be the subject of investigation. We study how the magnetic field of a series of ARs evolves with time to better characterise how ARs emerge and disperse. Methods. We examine the temporal variation in the magnetic field distribution of 37 emerging ARs. A kernel density estimation plot of the field distribution was created on a log-log scale for each AR at each time step. We found that the central portion of the distribution is typically linear and its slope was used to characterise the evolution of the magnetic field. Results. The slopes were seen to evolve with time, becoming less steep as the fragmented emerging flux coalesces. The slopes reached a maximum value of ~ -1.5 just before the time of maximum flux before becoming steeper during the decay phase towards the quiet Sun value of ~ -3. This behaviour differs significantly from a classical diffusion model, which produces a slope of -1. These results suggest that simple classical diffusion is not responsible for the observed changes in field distribution, but that other processes play a significant role in flux dispersion. Conclusions. We propose that the steep negative slope seen during the late decay phase is due to magnetic flux reprocessing by (super)granular convective cells.

After more than half a century of community support related to the science of "solar activity", IAU Commission 10 was formally discontinued in 2015, to be succeeded by C.E2 with the same area of responsibility. On this occasion, we look back at the growth of thescientific disciplines involved around the world over almost a full century. Solar activity and fields of research looking into the related physics of the heliosphere continue to be vibrant and growing, with currently over 2,000 refereed publications appearing per year from over 4,000unique authors, publishing in dozens of distinct journals and meeting in dozens of workshops and conferences each year. The size of the rapidly growing community and of the observational and computational data volumes, along with the multitude of connections into otherbranches of astrophysics, pose significant challenges; aspects of these challenges are beginning to be addressed through, among others, the development of new systems of literature reviews, machine-searchable archives for data and publications, and virtual observatories. As customary in these reports, we highlight some of the research topics that have seen particular interest over the most recent triennium, specifically active-region magnetic fields, coronal thermal structure, coronal seismology, flares and eruptions, and the variability of solar activity on long time scales. We close with a collection of developments, discoveries, and surprises that illustrate the range and dynamics of the discipline. Fil: Schrijver, Carolus J.. Lockheed Martin Solar And Astrophysics Laboratory; Estados Unidos Fil: Fletcher, Lyndsay. University of Glasgow; Reino Unido Fil: van Driel Gesztelyi, Lidia. Centre National de la Recherche Scientifique. Observatoire de Paris; Francia Fil: Asai, Ayumi. No especifica; Fil: Cally, Paul S.. No especifica; Fil: Charbonneau, Paul. No especifica; Fil: Gibson, Sarah E.. No especifica; Fil: Gomez, Daniel Osvaldo. 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: Hasan, Siraj S.. No especifica; Fil: Veronig, Astrid M.. No especifica; Fil: Yan, Yihua. No especifica

Commission 10 of the International Astronomical Union has more than 650 members who study a wide range of activity phenomena produced by our nearest star, the Sun. Solar activity is intrinsically related to solar magnetic fields and encompasses events from the smallest energy releases (nano- or even picoflares) to the largest eruptions in the Solar System, coronal mass ejections (CMEs), which propagate into the Heliosphere reaching the Earth and beyond. Solar activity is manifested in the appearance of sunspot groups or active regions, which are the principal sources of activity phenomena from the emergence of their magnetic flux through their dispersion and decay. The period 2008-2009 saw an unanticipated extended solar cycle minimum and unprecedentedly weak polar-cap and heliospheric field. Associated with that was the 2009 historical maximum in galactic cosmic rays flux since measurements begun in the middle of the 20th Century. Since then Cycle 24 has re-started solar activity producing some spectacular eruptions observed with a fleet of spacecraft and ground-based facilities. In the last triennium major advances in our knowledge and understanding of solar activity were due to continuing success of space missions as SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched by the breathtaking images of the solar atmosphere produced by the Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the framework of NASA's Living with a Star program. In August 2012, at the time of the IAU General Assembly in Beijing when the mandate of this Commission ends, we will be in the unique position to have for the first time a full 3-D view of the Sun and solar activity phenomena provided by the twin STEREO missions about 120 degrees behind and ahead of Earth and other spacecraft around the Earth and ground-based observatories. These new observational insights are continuously posing new questions, inspiring and advancing theoretical analysis and modelling, improving our understanding of the physics underlying magnetic activity phenomena. Commission 10 reports on a vigorously evolving field of research produced by a large community. The number of refereed publications containing `Sun', `heliosphere', or a synonym in their abstracts continued the steady growth seen over the preceding decades, reaching about 2000 in the years 2008-2010, with a total of close to 4000 unique authors. This report, however, has its limitations and it is inherently incomplete, as it was prepared jointly by the members of the Organising Committee of Commission 10 (see the names of the primary contributors to the sections indicated in parentheses) reflecting their fields of expertise and interest. Nevertheless, we believe that it is a representative sample of significant new results obtained during the last triennium in the field of solar activity.

Solar variability and its effects on the physical variability of our (space) environment produces complex signals. In the indicators of solar activity at least four independent cyclic components can be identified, all of them with temporal variations in their timescales.Time-frequency distributions (see Kolláth & Oláh 2009) are perfect tools to disclose the “music scores” in these complex time series. Special features in the time-frequency distributions, like frequency splitting, or modulations on different timescales provide clues, which can reveal similar trends among different indices like sunspot numbers, interplanetary magnetic field strength in the Earth's neighborhood and climate data.On the pseudo-Wigner Distribution (PWD) the frequency splitting of all the three main components (the Gleissberg and Schwabe cycles, and an ≈5.5 year signal originating from cycle asymmetry, i.e. the Waldmeier effect) can be identified as a “bubble” shaped structure after 1950. The same frequency splitting feature can also be found in the heliospheric magnetic field data and the microwave radio flux.

Recent studies of NOAA active region 10953, by Okamoto {\it et al.} ({\it Astrophys. J. Lett.} {\bf 673}, 215, 2008; {\it Astrophys. J.} {\bf 697}, 913, 2009), have interpreted photospheric observations of changing widths of the polarities and reversal of the horizontal magnetic field component as signatures of the emergence of a twisted flux tube within the active region and along its internal polarity inversion line (PIL). A filament is observed along the PIL and the active region is assumed to have an arcade structure. To investigate this scenario, MacTaggart and Hood ({\it Astrophys. J. Lett.} {\bf 716}, 219, 2010) constructed a dynamic flux emergence model of a twisted cylinder emerging into an overlying arcade. The photospheric signatures observed by Okamoto {\it et al.} (2008, 2009) are present in the model although their underlying physical mechanisms differ. The model also produces two additional signatures that can be verified by the observations. The first is an increase in the unsigned magnetic flux in the photosphere at either side of the PIL. The second is the behaviour of characteristic photospheric flow profiles associated with twisted flux tube emergence. We look for these two signatures in AR 10953 and find negative results for the emergence of a twisted flux tube along the PIL. Instead, we interpret the photospheric behaviour along the PIL to be indicative of photospheric magnetic cancellation driven by flows from the dominant sunspot. Although we argue against flux emergence within this particular region, the work demonstrates the important relationship between theory and observations for the successful discovery and interpretation of signatures of flux emergence., Comment: 14 pages, 8 figures, accepted for publication in Solar Physics

Properties of the Sun's interior can be determined accurately from helioseismological measurements of solar oscillations. These measurements, however, are in conflict with photospheric elemental abundances derived using 3-D hydrodynamic models of the solar atmosphere. This divergence of theory and helioseismology is known as the $"$solar modeling problem$"$. One possible solution is that the photospheric neon abundance, which is deduced indirectly by combining the coronal Ne/O ratio with the photospheric O abundance, is larger than generally accepted. There is some support for this idea from observations of cool stars. The Ne/O abundance ratio has also been found to vary with the solar cycle in the slowest solar wind streams and coronal streamers, and the variation from solar maximum to minimum in streamers ($\sim$0.1 to 0.25) is large enough to potentially bring some of the solar models into agreement with the seismic data. Here we use daily-sampled observations from the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory taken in 2010--2014, to investigate whether the coronal Ne/O abundance ratio shows a variation with the solar cycle when the Sun is viewed as a star. We find only a weak dependence on, and moderate anti-correlation with, the solar cycle with the ratio measured around 0.2--0.3 MK falling from 0.17 at solar minimum to 0.11 at solar maximum. The effect is amplified at higher temperatures (0.3--0.6 MK) with a stronger anti-correlation and the ratio falling from 0.16 at solar minimum to 0.08 at solar maximum. The values we find at solar minimum are too low to solve the solar modeling problem., Comment: To be published in The Astrophysical Journal. Figure 1 is reduced resolution to meet size limits

Commission 10 deals with solar activity in all of its forms, ranging from the smallest nanoflares to the largest coronal mass ejections. This report reviews scientific progress over the roughly two-year period ending in the middle of 2008. This has been an exciting time in solar physics, highlighted by the launches of the Hinode and STEREO missions late in 2006. The report is reasonably comprehensive, though it is far from exhaustive. Limited space prevents the inclusion of many significant results. The report is divided into the following sections: Photosphere and chromosphere; Transition region; Corona and coronal heating; Coronal jets; flares; Coronal mass ejection initiation; Global coronal waves and shocks; Coronal dimming; The link between low coronal CME signatures and magnetic clouds; Coronal mass ejections in the heliosphere; and Coronal mass ejections and space weather. Primary authorship is indicated at the beginning of each section.

Magnetic reconnection is thought to play an important role in liberating free energy stored in stressed magnetic fields. The consequences vary from undetectable nanoflares to huge flares, which have signatures over a wide wavelength range, depending on e.g. magnetic topology, free energy content, total flux, and magnetic flux density of the structures involved. Events of small energy release, which are thought to be the most numerous, are one of the key factors in the existence of a hot corona in the Sun and solar-like stars. The majority of large flares are ejective, i.e. involve the expulsion of large quantities of mass and magnetic field from the star. Since magnetic reconnection requires small length-scales, which are well below the spatial resolution limits of even the solar observations, we cannot directly observe magnetic reconnection happening. However, there is a plethora of indirect evidences from X-rays to radio observations of magnetic reconnection. I discuss key observational signatures of flares on the Sun and solar-paradigm stellar flares and describe models emphasizing synergy between observations and theory.

A number of coronal bright points and associated plasma jet features were seen in an observation of the South polar coronal hole during 2007 January. The 40″ wide slot was used at the focus of the Hinode EUV Imaging Spectrometer to provide spectral images for two of these events. Light curves are plotted for a number of emission lines that include He II 256A (0.079MK) and cover the temperature interval from 0.4MK to 5.0MK. Jet speed measurements indicate values less than the escape velocity. The light curves show a post-jet enhancement in a number of the cooler coronal lines indicating that after a few minutes cooling, the plasma fell back to its original acceleration site. This behavior has not been previously observed by e.g., the Yohkoh Soft X-ray Telescope due to the comparatively high temperature cut-off in its response. The observations are consistent with the existing models that involve magnetic reconnection between emerging flux and the ambient open field lines in the polar coronal hole. However we do not have sufficient coverage of lines from lower temperature ion species to register the Hα-emitting surge material that is associated with some of these models. © 2007. Astronomical Society of Japan.

Since its introduction to astro- and solar physics, the concept of helicity has proven to be useful in providing critical insights into physics of various processes from astrophysical dynamos, to magnetic reconnection and eruptive phenomena. Signature of helicity was also detected in many solar features, including orientation of solar active regions, or Joy’s law. Here we provide a summary of both solar phenomena and consider mutual relationship and its importance for the evolution of solar magnetic fields.

The evolution of active regions (AR) from their emergence through their long decay process is of fundamental importance in solar physics. Since large-scale flux is generated by the deep-seated dynamo, the observed characteristics of flux emergence and that of the subsequent decay provide vital clues as well as boundary conditions for dynamo models. Throughout their evolution, ARs are centres of magnetic activity, with the level and type of activity phenomena being dependent on the evolutionary stage of the AR. As new flux emerges into a pre-existing magnetic environment, its evolution leads to re-configuration of small-and large-scale magnetic connectivities. The decay process of ARs spreads the once-concentrated magnetic flux over an ever-increasing area. Though most of the flux disappears through small-scale cancellation processes, it is the remnant of large-scale AR fields that is able to reverse the polarity of the poles and build up new polar fields. In this Living Review the emphasis is put on what we have learned from observations, which is put in the context of modelling and simulation efforts when interpreting them. For another, modelling-focused Living Review on the sub-surface evolution and emergence of magnetic flux see Fan (2009). In this first version we focus on the evolution of dominantly bipolar ARs.

A variety of indices have been proposed in order to represent the many different observables modulated by the solar cycle. Most of these indices are highly correlated with each other owing to their intrinsic link with the solar magnetism and the dominant eleven year cycle, but their variations may differ in fine details, as well as on short- and long-term trends. In this paper we present an overview of the indices that are often employed to describe the many features of the solar cycle, moving from the ones referring to direct observations of the inner solar atmosphere, the photosphere and chromosphere, to those deriving from measurements of the transition region and solar corona. For each index, we summarize existing measurements and typical use, and for those that quantify physical observables, we describe the underlying physics.

The Business Meeting of Commission 10 was held as part of the Business Meeting of Division II (Sun and Heliosphere), chaired by Valentin Martínez-Pillet, the President of the Division. The President of Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took the chair for the business meeting of C10. She summarised the activities of C10 over the triennium and the election of the incoming OC.

We correlate the evolution of the mean X-ray flux, emission measure and temperature (Yohkoh SXT & BCS) with the magnetic flux density (SOHO/MDI) in active region NOAA 7978 from its birth throughout its decay, for five solar rotations. We show that these plasma parameters together with other quantities deduced from them, such as the density and the pressure, follow power-law relationships with the mean magnetic flux density (B). We derive the dependence of the mean coronal heating rate on the magnetic flux density. We use the obtained scaling laws of coronal loops in thermal equilibrium to derive observational estimates of the scaling of the coronal heating with B. These results are used to test the validity of coronal heating models. We find that models based on the dissipation of stressed, current-carrying magnetic fields are in better agreement with the observations than models that attribute coronal heating to the dissipation of MHD waves injected at the base of the corona. This confirms, with smaller error bars, previous results obtained for individual coronal loops, as well as for the global coronal emission of the Sun and cool stars.

We present evidence of flaring behavior in a transequatorial loop (TEL) that lights up in soft X-rays on 2000 July 13. The large loop structure connects NOAA Active Regions 9070/9066 in the northern hemisphere and AR 9069/9068 in the southern hemisphere. We follow the loop systems for 2 days and observe several pieces of evidence strongly suggesting flare behavior of the form seen in standard flaring in active regions. These include brightenings of the loop structure, cooling of plasma that is seen both in soft X-rays and in the transition region temperatures, morphological evidence of reconnection inflow, and blueshifts around the footpoint of the TEL suggestive of chromospheric evaporation. We present, to our knowledge for the first time, observations of TEL in the O V emission line.

International audience; Not Available