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The evolution of weakly nonlinear Alfvén waves in coronal loops in the density structuring both across and along the field while experiencing dissipation is highlighted. Energy transfer due to shock formation in coronal loops provides the basis for the context of the present study. Coronal loops are modeled analytically using the magnetohydrodynamic (MHD) theory in cylindrical geometry, incorporating stratification and viscosity. The effects of the external medium, along with equilibrium conditions, are considered, with the background magnetic field aligned parallel to the loop axis. The second-order thin flux tube approximation is employed to derive a Cohen–Kulsrud–Burgers type of evolutionary equation that highlights the influences of nonlinear, dissipative, and stratification terms, alongside the effect of the external medium. The Alfvén wave speed in coronal loops reaches its maximum when shocks are experienced. If not due to low amplitudes, the maximum is observed at the other footpoint. The location of shock formation is determined by the ratio of the Alfvén wave amplitude and the background Alfvén wave speed. The existence of energy transfer mechanisms due to MHD shocks at various locations of coronal loops, especially loop footpoints, brings to mind that observed energy transfers in the case of coronal loops or a set of loops not only provide coronal heating but also provide transition region heating subject to conditions. For high plasma- β values at lower altitudes, shock formation is less dependent on the external medium and depends strongly on the loop's internal dynamics. This is contrary to open magnetic structures. [ABSTRACT FROM AUTHOR]

Temporal variations in the number and area of coronal bright points (CBPs) were studied separately in the Northern and Southern hemispheres over more than two solar cycles (years 1996–2022). Hemispheric asymmetry was identified in both the total number and area of CBPs, with differences reaching up to 15 per cent. Statistical analysis using Student's t -test confirms that these differences are statistically significant. The observed asymmetry arises from the asynchronous formation of CBPs in the hemispheres, rather than from the asymmetry of active regions. These variations in asymmetry are explained by the existence of two different types of CBP: 'bright' and 'dim'. Differences in the asymmetry and overall number variations between these two types during solar cycles suggest distinct physical mechanisms underlying their formation. Bright CBPs closely follow the patterns of active regions, supporting the hypothesis that they are miniature active regions formed through similar mechanisms. In contrast, dim CBPs, predominantly located in quiet Sun regions, often display behaviour opposite to sunspots, partially attributable to the visibility effect and influence of active regions. However, the temporal variations in the normalized asymmetry index indicate that the visibility effect alone cannot fully explain the observed patterns. The relatively uniform asymmetry of dim CBPs across 11-yr solar cycles supports the hypothesis of a cycle-independent (or dependent on a longer period) component of the solar dynamo, potentially linked to a small-scale turbulent dynamo. [ABSTRACT FROM AUTHOR]

Cool (⁠|$\approx 10^4$|  K), dense material permeates the hot (⁠|$\approx 10^6$|  K), tenuous solar corona in form of coronal condensations, for example prominences and coronal rain. As the solar atmosphere evolves, turbulence can drive mixing between the condensations and the surrounding corona, with the mixing layer exhibiting an enhancement in emission from intermediate temperature (⁠|$\approx 10^5$|  K) spectral lines, which is often attributed to turbulent heating within the mixing layer. However, radiative cooling is highly efficient at intermediate temperatures and numerical simulations have shown that radiative cooling can far exceed turbulent heating in prominence-corona mixing scenarios. As such the mixing layer can have a net loss of thermal energy, i.e. the mixing layer is cooling rather than heating. Here, we investigate the observational signatures of cooling processes in Kelvin–Helmholtz mixing between a prominence thread and the surrounding solar corona through 2D numerical simulations. Optically, thin emission is synthesized for Si  iv , along with optically thick emission for H  |$\alpha$|⁠ , Ca  ii  K, and Mg  ii  h using Lightweaver. The Mg  ii  h probes the turbulent mixing layer, whereas H  |$\alpha$| and Ca  ii  K form within the thread and along its boundary, respectively. As the mixing evolves, intermediate temperatures form leading to an increase in Si  iv emission, which coincides with increased radiative losses. The simulation is dominated by cooling in the mixing layer, rather than turbulent heating, and yet enhanced emission in warm lines is produced. As such, an observational signature of decreased emission in cooler lines and increased emission in hotter lines may be a signature of mixing, rather than an implication of heating. [ABSTRACT FROM AUTHOR]

Plasmoids (or magnetic islands) are believed to play an important role in the onset of fast magnetic reconnection and particle acceleration during solar flares and eruptions. Direct imaging of flare current sheets and the formation/ejection of multiple plasmoids in extreme-ultraviolet images, along with simultaneous X-ray and radio observations, offers significant insights into the mechanisms driving particle acceleration in solar flares. Here, we present direct imaging of the formation and ejection of multiple plasmoids in flare plasma/current sheets and the associated quasiperiodic pulsations (QPPs) observed at X-ray and radio wavelengths, using observations from the Solar Dynamics Observatory/Atmospheric Imaging Assembly, RHESSI, and the Fermi Gamma-ray Burst Monitor. These plasmoids propagate bidirectionally upward and downward along the flare current sheet beneath the erupting flux rope during two successive flares associated with confined/failed eruptions. The flux rope exhibits evidence of helical kink instability, with the formation and ejection of multiple plasmoids in the flare current sheet, as predicted in an MHD simulation of a kink-unstable flux rope. RHESSI X-ray images show double coronal sources ("looptop" and higher coronal sources) located at both ends of the flare current/plasma sheet. Moreover, we detect an additional transient faint X-ray source (6–12 keV) located between the double coronal sources, which is cospatial with multiple plasmoids in the flare current sheet. X-ray (soft and hard) and radio (decimetric) observations unveil QPPs (periods ≈ 10 s and 100 s) associated with the ejection and coalescence of plasmoids. These observations suggest that energetic electrons are accelerated during the ejection and coalescence of multiple plasmoids in the flare current sheet. [ABSTRACT FROM AUTHOR]

In the solar corona, magnetically sheared structures are unstable to both tearing and thermal instabilities in a coupled fashion. However, how the choice of linear perturbation modes influences the time-scale to achieve the thermal runaway in a coupled tearing–thermal coronal current sheet is not well understood to date. Here, we model a force-free Harris current sheet under solar coronal conditions to investigate this coupling in the linear and non-linear regimes. In the linear regime, we adopt the magnetohydrodynamic spectroscopy code legolas to compare the current sheet under thermal and thermoresistive conditions, after which we initialize non-linear simulations (with mpi-amrvac) with the unstable, linear tearing and thermal perturbations obtained with legolas. It is shown that part of the unstable thermal quasi-continuum adopts tearing properties in the linear stage, but that it is not until the non-linear stage is reached that a true thermal 'runaway' effect leads to condensations inside tearing-induced flux ropes. Hence, the linear stage is governed by the dominant tearing instability whilst condensations form due to tearing–thermal coupling in the non-linear stage. Our results imply that perturbing an equilibrium current sheet with the fastest growing linear mode skips the mode-mixing phase in which the dominant instability traditionally emerges, and significantly reduces the time-scale to enter into the non-linear stage and thermal runaway process from its equilibrium configuration. [ABSTRACT FROM AUTHOR]

Propagating slow magnetoacoustic waves have long been observed in the corona and their use for magnetohydrodynamic seismology is growing. While these waves are thought to be generated by lower atmospheric oscillations, the exact mechanisms behind their generation and propagation remain unclear. This study aims to investigate the fine structure and drivers of slow waves through high-resolution solar observations. We observed slow magnetoacoustic waves with three distinct periodicities along sunspot-anchored coronal fan feathers using data from the Atmospheric Imaging Assembly. The waves were tracked for 3 h in active region 13100 on 2022 September 19. Time–distance analysis was used to determine wave periods and projected phase speeds, and Fourier analysis for period intensity maps. We detect distinct periods of |$3.06 \pm 0.04$|⁠ , |$2.47 \pm 0.02$|⁠ , and |$2.82 \pm 0.02$| min in three feathers of a coronal fan. Increased intensity of chromospheric oscillations with the same distinct periods was observed in the 304 Å channel in the umbral region where the feathers are anchored, which suggests that those locations could be footpoints of the specific feathers. These results indicate that propagating slow waves exhibit fine structuring in their oscillation periods. The specific period is a unique signature of each feather, which can be utilized for seismological diagnostics of the local coronal magnetic geometry, in stereoscopic observations. [ABSTRACT FROM AUTHOR]

The failure of a filament eruption caused by magnetic reconnection between the erupting filament and the overlying magnetic field has been previously proposed in numerical simulations. It is, however, rarely observed. In this study, we report a reconnection between an erupting filament and its overlying coronal loops that results in the failure of the filament eruption. On 2023 September 24, a filament was located in active region 13445. It slowly rose, quickly erupted, rapidly decelerated, then finally stopped, with an untwisting motion. As a failed eruption, the event is associated with an M4.4 flare but no coronal mass ejection. During the eruption, the filament became bright, and the overlying loops appeared first in the high-temperature channels. They have average temperatures of ∼12.8 and ∼9.6 MK, respectively, indicating that both of them were heated. Two sets of new loops, separately connecting the filament endpoints and the overlying loop footpoints, then formed. Subsequently, the heated overlying loops were seen sequentially in the low-temperature channels, showing the cooling process, which is also supported by the light curves. Plasmoids formed and propagated bidirectionally along the filament and the overlying loops, indicating the presence of plasmoid instability. These results suggest that reconnection occurs between the erupting filament and the overlying loops. The erupting filament eventually disappeared, with the appearance of more newly formed loops. We propose that the reconnection between the erupting filament and the overlying loops ruins the filament completely, hence resulting in the failed eruption. [ABSTRACT FROM AUTHOR]

Decayless kink oscillations, characterized by their lack of decay in amplitude, have been detected in coronal loops of varying scales in active regions, the quiet Sun, and coronal holes. Short-period (<50 s) decayless oscillations have been detected in short loops (< 50 Mm) within active regions. Nevertheless, long-period decayless oscillations in these loops remain relatively unexplored and crucial for understanding the wave modes and excitation mechanisms of decayless oscillations. We present the statistical analysis of decayless oscillations from two active regions observed by the Extreme Ultraviolet Imager onboard Solar Orbiter. The average loop length and period of the detected oscillations are 19 Mm and 151 s, respectively. We find 82 long-period and 23 short-period oscillations in these loops. We do not obtain a significant correlation between loop length and period. We discuss the possibility of different wave modes in short loops, although standing waves cannot be excluded from possible wave modes. Furthermore, a different branch exists for active region short loops in the loop length versus period relation, similar to decayless waves in short loops in the quiet Sun and coronal holes. The magnetic fields derived from MHD seismology, based on standing kink modes, show lower values for multiple oscillations compared to previous estimates for long loops in active regions. Additionally, the comparison of period distributions in short loops across different coronal regions indicates that different excitation mechanisms may trigger short-period kink oscillations in active regions compared to the quiet Sun and coronal holes. [ABSTRACT FROM AUTHOR]

We establish a spherically symmetric model of solar atmosphere, which consists of the whole chromosphere and low corona below the 1.25 solar radius. It is a hydrodynamic model with heating in the chromosphere through an artificial energy flux. We performed a series of simulations with our model and found oscillations with a peak frequency of ∼4 mHz in the power spectrum. We confirmed that this resulted from the p-mode excited in the transition region and amplified in a resonant cavity situated in the height range ∼ 4 × 10 3 – 2 × 10 4 km. This result is consistent with global observations of Alfvénic waves in corona and can naturally explain the observational ubiquity of 4 mHz without the difficulty of the p-mode passing through the acoustic-damping chromosphere. We also confirmed that acoustic shock waves alone cannot heat the corona to the observed temperature, and found mass upflows in the height range ∼ 7 × 10 3 – 7 × 10 4 km in our model, which pumped the dense and cool plasma into the corona and might be the mass supplier for solar prominences. [ABSTRACT FROM AUTHOR]

Context. Transition region explosive events are characterized by the non-Gaussian profiles of the emission lines that form at transition region temperatures, and they are believed to be manifestations of small-scale reconnection events in the transition region. Aims. Traditionally, the enhanced emission at the line wings is interpreted as bi-directional outflows generated by the reconnection of oppositely directed magnetic fields. We investigate whether the 2D picture also holds in a more realistic setup of a 3D radiation magnetohydrodynamic (MHD) quiet-Sun model. We also compare the thermal responses in the transition region and corona of different events. Methods. We took a 3D self-consistent quiet-Sun model extending from the upper convection zone to the lower corona calculated using the MURaM code. We first synthesized the Si IV line profiles from the model and then located the profiles which show signatures of bi-directional flows. These tend to appear along network lanes, and most do not reach coronal temperatures. We isolated two hot events (around 1 MK) and one cool event (order of 0.1 MK) and examined the magnetic field evolution in and around these selected events. Furthermore, we investigated why some explosive events reach coronal temperatures, while most remain cool. We also examined the emission of these events as seen in the 174 Å passband of the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter and all coronal passbands of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). Results. The field lines around two events reconnect at small angles (i.e., they undergo component reconnection). The third case is associated with the relaxation of a highly twisted flux rope. All three events reveal signatures in the synthesized EUI 174 Å images. The intensity variations in two events are dominated by variations of the coronal emissions, while the cool component seen in the respective channel contributes significantly to the intensity variation in one case. In comparison, one hot event is embedded in regions with higher magnetic field strength and heating rates while the densities are comparable, and the other hot event is heated to coronal temperatures mainly because of the low density. Conclusions. Small-scale heating events seen in the extreme-ultraviolet (EUV) channels of AIA or EUI might be hot or cool. Our results imply that the major difference between the events in which coronal counterparts dominate or not is the amount of converted magnetic energy and/or density in and around the reconnection region. [ABSTRACT FROM AUTHOR]

The article "Living With a Star: NASA's Mission To Study the Sun and Space Weather" discusses NASA's Solar Dynamics Observatory (SDO) and the Living With a Star program, which aims to study the Sun's dynamics and space weather. The SDO, launched in 2010, provides valuable data on the Sun's activities and has led to significant discoveries in Sun-Earth dynamics. The article also delves into the history of solar observation, the impact of space weather on Earth, and the potential consequences of severe space weather storms on modern technology and infrastructure. [Extracted from the article]

The solar corona is the prototypical example of a low-density environment heated to high temperatures by external sources. The plasma cools radiatively, and because it is optically thin to this radiation, it becomes possible to model the density, velocity, and temperature structure of the system by modifying the MHD equations to include an energy source term that approximates the local heating and cooling rates. The solutions can be highly inhomogeneous and even multiphase because the well-known linear instability associated with this source term, thermal instability, leads to a catastrophic heating and cooling of the plasma in the nonlinear regime. Here we show that there is a separate, much simpler linear instability accompanying this source term that can rival thermal instability in dynamical importance. The stability criterion is the isochoric one identified by Parker (1953), and we demonstrate that cooling functions derived from collisional ionization equilibrium are highly prone to violating this criterion. If catastrophic cooling instability can act locally in global simulations, then it is an alternative mechanism for forming condensations, and due to its nonequilibrium character, it may be relevant to explaining a host of phenomena associated with the production of cooler gas in hot, low density plasmas. [ABSTRACT FROM AUTHOR]

Coronal holes (CHs) are large-scale, low-density regions in the solar atmosphere that may expel high-speed solar wind streams that incite hazardous, geomagnetic storms. Coronal and solar wind models can predict these high-speed streams, and the performance of the coronal model can be validated against segmented CH boundaries. We present a novel method named Sub-Transition Region Identification of Ensemble Coronal Holes (STRIDE-CH) to address prominent challenges in segmenting CHs using extreme-ultraviolet (EUV) imagery. Ground-based, chromospheric He i 10,830 Å line imagery and underlying Fe i photospheric magnetograms are revisited to disambiguate CHs from filaments and quiet Sun, overcome obscuration by coronal loops, and complement established methods in the community which use space-borne coronal EUV observations. Classical computer vision techniques are applied to constrain the radiative and magnetic properties of detected CHs, produce an ensemble of boundaries, and compile these boundaries in a confidence map that quantifies the likelihood of the CH presence throughout the solar disk. This method is a science-enabling one towards future studies of CH formation and variability from a mid-atmospheric perspective. [ABSTRACT FROM AUTHOR]

Studying the dynamics of magnetic plasmas in the presence of pressure gradients, gravity, and Lorentz forces is essential for understanding wave generation and propagation in the solar atmosphere and other magnetic environments. Understanding the sudden increase in the temperature of the sun’s corona is one of the challenges for solar physicists. Various solar phenomena, including sunspots, spicules, and coronal loops, are explored within these environments. We consider them to be waveguides. These waveguides are interpreted as magnetic flux tubes, providing valuable information about wave properties. For study the dissipation in the magnetic flux tubes, two key damping mechanisms are: thermal conductivity, which transfers heat within the coronal plasma, and compressive viscosity, which reveals the plasma's resistance to pressure changes and deformation.The investigation of the dissipation of magnetohydrodynamic waves in hot coronal loops is carried out using a two-dimensional Cartesian model. The findings demonstrate that there is a decrease in pulse amplitude, and dissipation or damping manifests across distinct segments of the magnetic flux tube as time progresses. We find that the considered viscosity in our model can accelerate the wave damping in the coronal loop, which is in agreement with the results reported by Ofman & Wang (2022).

The formation of a S-shaped filament was investigated to determine if and how magnetoacoustic waves in the solar corona can trigger filament excitation. The study investigated how magnetoacoustic waves interact with two magnetic null points in the solar corona. Since the solar corona has a complex magnetic field structure, it is expected that magnetic structures are predominantly responsible for the occurrence of coronal events. Simulation methods are required because of the complex nature of the reconnection problem and the way MHD waves behave. Because of notable progress in the production of supercomputers and the enhancement of techniques, it is now possible to analyze the behaviour of nonlinear plasma near the magnetic null point through numerical methods. This article aims to explore the idea of a 2.5D null point pair and investigate the motion and generation of plasma flow resulting from a solitary magnetoacoustic pulse passing through a particular magnetic structure that contains two null points. In this statement, the PLUTO code, which is a type of code called Godunov, is used to solve a set of equations known as resistive magnetohydrodynamic equations. It was depicted that when transitioning from X-point to O-point, twisted formations are formed, which play a significant role in developing S-shaped filaments. These formations also have a vital function right before the excitement of the filaments. [ABSTRACT FROM AUTHOR]

This paper investigates the features of the generation of fast and slow MHD shock waves in the lower layers of the solar corona. An analytical solution is obtained at the shock front for both types of shock waves and it is shown that the primary role in the formation of anomalous heating belongs to weak MHD shock waves, which do not have a lower speed limit for generation in the upper layers of the solar chromosphere. [ABSTRACT FROM AUTHOR]

Determining the speed of the solar wind emanating from coronal holes near the Sun is a key problem for modeling plasma parameters throughout the heliosphere. Plasma temperature and density, in addition to speed, are the input parameters to global magnetohydrodynamics models, but the estimated temperature and density boundary conditions are calculated from the simulated solar wind speed. In this study, we analyzed how long-term variations in the properties of open magnetic field sources modeled from different series of magnetograph observations are related to the plasma parameters measured by satellite. The analysis also considers coronal holes based on SDO/AIA 193 Å and solar wind speed observations in the Wang-Sheeley-Arge approximation (WSA). We found that during the period 2015–2023, the areas of equatorial coronal holes correlate better with the observed solar wind speed than the results of WSA simulations. Among the three considered series of magnetographic observations (STOP, SDO/HMI, GONG), during the period of minimum solar activity, calculations based on STOP data perform better. [ABSTRACT FROM AUTHOR]

The paper studies the dynamics of high-temperature structures (with a temperature of T ≥ 10 MK) in the corona above active regions (ARs) in quiet temporal intervals, before solar flares of high X-ray classes and during and after individual flare events, and determines the role of electric currents in heating the coronal plasma. In the study, we used data from the Solar Dynamics Observatory (SDO) spacecraft: magnetograms obtained by the Helioseismic and Magnetic Imager (HMI) instrument (used to detect and calculate the magnitude of large-scale electric current) and photoheliograms of the solar corona in ultraviolet radiation 94, 131, 171, 193, 211, and 335 Å channels of the Atmospheric Imaging Assembly (AIA/SDO) instrument (used to construct maps of temperature distribution in the corona above the AR, detect high-temperature structures, and study their evolution). The objects of the study were ARs NOAA 12 192 (October 2014) and 12 371 (June 2015) of the 24th solar activity cycle, which have high absolute values of large-scale electric current. The following results were obtained: (1) The discovered high-temperature structures represent a channel of large-scale electric current at coronal heights. (2) High-temperature structures in the corona above the studied ARs exist over a long (several days) time interval, which indicates the presence of a constant source of plasma heating; the temperature of the structures, the area they occupy, and their spatial orientation change over time. (3) High-temperature structures in the corona consist of individual elements with a cross section of ~108 cm. (4) Several hours before the X-ray flares of classes M and X datected in the studied ARs during their monitoring time, a significant decrease in the area occupied by high-temperature structures was observed, and in some cases, a decrease in temperature to 3–5 MK, which indicates a change in the physical conditions in the corona before powerful flares. [ABSTRACT FROM AUTHOR]

The results of observations of coronal jets on the Sun are briefly reviewed. Data on jets of different types (jets, jetlets) were collected. Their properties are considered, such as lifetime, length, width, velocities, coupling to the magnetic field, and their putative role in hot plasma and energy transfer into the corona. Observational data obtained with ground-based and space telescopes were used. There is growing evidence that jets play a key role in imparting mass to the corona and solar wind and can provide sufficient energy to power the solar wind (see, e.g., (Tian et al., 2014)). Modern observations by the Parker Solar Probe and Solar Orbiter spacecraft will contribute to the understanding of solar jets and related phenomena. [ABSTRACT FROM AUTHOR]

Most earlier studies have been limited to estimating the kinematic evolution of coronal mass ejections (CMEs), and only limited efforts have been made to investigate their thermodynamic evolution. We focus on the interplay of the thermal properties of CMEs with their observed global kinematics. We implement the Flux rope Internal State model to estimate variations in the polytropic index, heating rate per unit mass, temperature, pressure, and various internal forces. The model incorporates inputs of 3D kinematics obtained from the Graduated Cylindrical Shell (GCS) model. In our study, we chose nine fast-speed CMEs from 2010 to 2012. Our investigation elucidates that the selected fast-speed CMEs show a heat-release phase at the beginning, followed by a heat-absorption phase with a near-isothermal state in their later propagation phase. The thermal state transition, from heat release to heat absorption, occurs at around 3(⁠|$\pm$| 0.3) to 7(⁠|$\pm$| 0.7) |$R_\odot$| for different CMEs. We found that the CMEs with higher expansion speeds experience a less pronounced sharp temperature decrease before gaining a near-isothermal state. The differential emission measurement (DEM) analysis findings, using multiwavelength observation from Solar Dynamics Observatory/Atmospheric Imaging Assembly, also show a heat release state of CMEs at lower coronal heights. We also find the dominant internal forces influencing CME radial expansion at varying distances from the Sun. Our study shows the need to characterize the internal thermodynamic properties of CMEs better in both observational and modeling studies, offering insights for refining assumptions of a constant value of the polytropic index during the evolution of CMEs. [ABSTRACT FROM AUTHOR]

Remote sensing observations of the nascent solar wind in the solar corona imply that heavy ions are often faster and hotter than the plasma protons. In situ observations of the near-Earth solar wind with low collisional age show that hotter and faster states of heavy ions usually do not occur at the same time: heavy ions are either faster or hotter than the protons. This discrepancy highlights the need to understand the competing effects of heating, acceleration, Coulomb collisions, and instabilities on heavy ions in the inner heliosphere, from the solar corona to Earth. The Parker Solar Probe (PSP) is currently the closest spacecraft to the Sun and is anticipated to cross the outer boundary of the preferential heating and acceleration zone. Using in situ measurements from PSP, we find that larger drift speeds between α -particles and protons (V α p ) correspond to larger α -to-proton temperature ratios ( T α T p ) in the young solar wind close to the corona. This observation differs from in situ observations farther from the Sun (e.g., with Helios and Wind at 0.3–1 au). We believe that this difference is related to the speed of the solar wind, as well as the distance from the Sun. The anti-correlation between V α p and T α T p in fast wind away from the Sun is potentially related to the energy transfer from the drift kinetic energy of α-particles to proton thermal energy due to drift instabilities. In the young slow wind, heavy ions exhibit signs of both preferential heating and preferential acceleration, but are not affected much by drift instabilities. We identify the outer boundary of the preferential heating and acceleration zone for heavy ions in the young slow wind to be at a distance of around 0.16 au, beyond which the effects of Coulomb collisions weaken the effects of preferential heating and acceleration. [ABSTRACT FROM AUTHOR]

The study of MHD waves in coronal structures is of great importance in coronal seismology. The study of these waves makes it possible to reveal the physical structure and heating mechanism of the solar corona. It is of great interest to calculate the line profile in the emission spectrum of a magneto-sonic wave for various physical parameters, calculate the energy flux and compare them with observations. In this paper, the profiles of the FeIX λ171Å line in the emission spectrum of slow magneto-acoustic waves propagating in coronal loops are calculated for cases of an optically thin layer and the change in density. The line profiles were calculated for the following parameter values: wave velocity amplitude = 10 km/s, coronal loop width 2000 and 5000 km, wavelength Λ = 20 000 and 50 000 km, Doppler width Δλd = 0.01 Å, and at values of the angle of the line of sight and at different phases of the wave. The energy flux density is 622.5 erg/(cm2 s). The calculated values of the energy flux density strongly depend on the angle of the line of sight and on the phase of the wave and range from zero at large values of θ to ~4 × 103 erg/(cm2 s), the values of Doppler velocities and velocities of non-thermal movements at small values of θ have a maximum value of ~13 km/s and decrease almost to zero at large values of θ. At different values of the angle of the line of sight, the asymmetry is almost not noticeable. An interesting result is that the values of the calculated (observed) energy flux can be both much less and much more than the true value: from almost zero at small values of θ. These values depend not only on the angle of the line of sight, but also on the width of the coronal loop and the wavelength. [ABSTRACT FROM AUTHOR]

The existence of continuous cooling and heating processes is an important condition governing the behavior of the solar corona, which is characterized by temperatures of several million Kelvin. These processes can be significantly influenced by small-scale coronal formations, which largely determine the thermal balance of the corona and solar-wind disturbances. High-sensitivity observations of polarized radiation allow us to evaluate the complex structure of magnetic fields that accumulate the energy necessary to excite coronal eruptions, bursts and flares. However, at high altitudes the corona becomes optically thin, and observations of it pose a major challenge, requiring the use of instruments with a large effective area. Many researchers note that the emerging field of coronal magnetometry is hard to develop due to the fact that experimental observations in optical range are limited by the low plasma density in the corona, high temperature, and insufficient sensitivity of the instruments. In contrast, higher sensitivity is achievable in the radio frequency range. In particular, the 1–3 GHz range is optimal for detecting very weak coronal structures of emerging activity, despite limitations in spatial resolution. To carry out radio requency observations of the corona on the RATAN-600 large reflector-type radio telescope, a wide-range spectrometer in the range of 1–3 GHz was created. It has continuous coverage of the entire frequency range with maximum frequency and time resolutions, as well as high sensitivity to radiation flux. The results of the first series of observations of weak coronal structures are presented, and their interpretation in terms of their effect on thermal processes in the corona is discussed. [ABSTRACT FROM AUTHOR]

Solar eruptions, primarily manifested as solar flares, filament eruptions and coronal mass ejections, represent explosive releases of magnetic energy stored in the solar corona, with the potential to drive severe space weather. The initiation of solar eruptions remains an open question, leading to various theoretical models that are inferred from observations. However, these models are subjects of debate due to the absence of direct measurements of the three-dimensional (3D) magnetic fields in the corona. Numerical simulations, based on solving magnetohydrodynamics (MHD) equations that govern the macroscopic dynamics of solar corona, serve as a touchstone for testing these theoretical models. One early proposed model suggested that eruptions could be triggered by reconnection within a single sheared magnetic arcade, which is known as the tether-cutting reconnection model, but it was never confirmed through 3D MHD simulations until very recently. Consequently, two models have gained more popularity: one involving the eruption of a twisted magnetic flux rope (MFR) due to ideal instability (or loss of equilibrium), and the other known as the breakout eruption, which requires a quadrupolar configuration with a delicately located magnetic null point. Other mixed mechanisms, involving both ideal instability and reconnection, are also proposed in association with localized magnetic flux emergence. Now with the validation of the tether-cutting model, the fundamental mechanisms are boiled down to two types of models, one primarily based on the ideal instability of a pre-existing MFR, and the other based on the reconnection of sheared field lines with or without an MFR. Recently, the modelling of the birth of solar eruption using observed data-based MHD simulations has advanced rapidly, becoming a crucial research tool in the study of the initiation mechanisms. These realistic modellings reveal a higher level of complexity compared to all currently available theories and idealized models. [ABSTRACT FROM AUTHOR]

Coronal loops are known to host Alfvén waves propagating in the corona from the lower layers of the solar atmosphere and because of their internal structure, phase mixing is likely to occur. The structure of the coronal loop could be significantly affected by the thermodynamic feedback of the heating generated by phase mixing. However, this phenomenon can be sensitive to the period of the propagating Alfvén waves due to how short period waves can be easily dissipated and the way long-period waves may accumulate considerable energy in resonating coronal loops. Using the Lare2d code, a coronal loop model of a field-aligned thermodynamic equilibrium and a cross-field background heating profile is created, with an additional forcing term added to drive Alfvén waves with coronal amplitudes between |$5{\!-\!}30 \, \mathrm{km} \, \mathrm{s}^{-1}$|⁠. We show that high-frequency waves can generate heating corresponding to a |${\sim} 10~{{\ \rm per\ cent}}$| increase of the initial coronal shell temperature, chromospheric upflows of up to   |$0.6 \, \mathrm{km} \, \mathrm{s}^{-1}$| and a coronal shell mass increase of |${\sim} 15~{{\ \rm per\ cent}}$|⁠. These changes are sufficient to alter and maintain a new coronal loop density structure, broadening the region where efficient phase mixing (and therefore heating) occurs. In contrast, low-frequency waves are unable to be effectively dissipated, resulting in minimal changes to the loop structure. We see little evidence of wave energy accumulation in the corona and are unable to conclude that the dissipation of low-frequency Alfvén waves can be an effective heating mechanism in coronal loops in the setup used in this study. [ABSTRACT FROM AUTHOR]

Space weather models endeavoring to connect remote observations to in-situ measurements at various locations in the heliosphere invariably require a coronal model to connect the photosphere magnetically to the inner heliosphere. The most famous and popular implementation of this connection is a potential field source surface (PFSS) model out to the source surface, typically located at 2.5 solar radii, combined with a Schatten current sheet (SCS) model. While the PFSS model is mostly understood, the SCS has been utilized in heliospheric physics for nearly 50 years with little understanding of it's physical and mathematical underpinnings. In this overview article, I lay out the mathematical formalism of the SCS, describe how it differs from the PFSS, and summarize several techniques used to combine the PFSS and SCS to create a global coronal model from the photosphere to the inner heliosphere. [ABSTRACT FROM AUTHOR]

Magnetic reconnection before flux cancellation in the solar photosphere when two opposite-polarity photospheric magnetic fragments are approaching one another is usually modelled by assuming that a small so-called 'floating current sheet' forms about a null point or separator that is situated in the overlying atmosphere. Here, instead we consider the reconnection that is initiated as soon as the fragments become close enough that their magnetic fields interact. The resulting current sheet, which we term a 'fin sheet' extends up from the null point or separator that is initially located in the solar surface. We develop here non-linear analyses for finite-length models of both fin and floating current sheets that extend the previous models that were limited to short floating current sheets. These enable the length of the current sheet and the rate of heating to be calculated in both cases as functions of the separation distance of the sources and the reconnection rate. Usually, the fin current sheet liberates more energy than a floating current sheet. [ABSTRACT FROM AUTHOR]

Coronal mass ejections (CMEs) from pseudostreamers represent a significant fraction of large-scale eruptions from the Sun. In some cases, these CMEs take a narrow jet-like form reminiscent of coronal jets; in others, they have a much broader fan-shaped morphology like CMEs from helmet streamers. We present results from a magnetohydrodynamic simulation of a broad pseudostreamer CME. The early evolution of the eruption is initiated through a combination of breakout interchange reconnection at the overlying null point and ideal instability of the flux rope that forms within the pseudostreamer. This stage is characterized by a rolling motion and deflection of the flux rope toward the breakout current layer. The stretching out of the strapping field forms a flare current sheet below the flux rope; reconnection onset there forms low-lying flare arcade loops and the two-ribbon flare footprint. Once the CME flux rope breaches the rising breakout current layer, interchange reconnection with the external open field disconnects one leg from the Sun. This induces a whip-like rotation of the flux rope, generating the unstructured fan shape characteristic of pseudostreamer CMEs. Interchange reconnection behind the CME releases torsional Alfvén waves and bursty dense outflows into the solar wind. Our results demonstrate that pseudostreamer CMEs follow the same overall magnetic evolution as coronal jets, although they present different morphologies of their ejecta. We conclude that pseudostreamer CMEs should be considered a class of eruptions that are distinct from helmet-streamer CMEs, in agreement with previous observational studies. [ABSTRACT FROM AUTHOR]

Understanding the global rotational profile of the solar atmosphere and its variation is fundamental to uncovering a comprehensive understanding of the dynamics of the solar magnetic field and the extent of coupling between different layers of the Sun. In this study, we employ the method of image correlation to analyze the extensive data set provided by the Atmospheric Imaging Assembly of the Solar Dynamic Observatory in different wavelength channels. We find a significant increase in the equatorial rotational rate (A) and a decrease in absolute latitudinal gradient (∣ B ∣) at all temperatures representative of the solar atmosphere, implying an equatorial rotation up to 4.18% and 1.92% faster and less differential when compared to the rotation rates for the underlying photosphere derived from Doppler measurement and sunspots respectively. In addition, we also find a significant increase in equatorial rotation rate (A) and a decrease in differential nature (∣ B ∣ decreases) at different layers of the solar atmosphere. We also explore a possible connection from the solar interior to the atmosphere and interestingly found that A at r = 0.94 R ⊙ and 0.965 R ⊙ show an excellent match with 171 Å, 304 Å, and 1600 Å, respectively. Furthermore, we observe a positive correlation between the rotational parameters measured from 1600 Å, 131 Å, 193 Å, and 211 Å with the yearly averaged sunspot number, suggesting a potential dependence of the solar rotation on the appearance of magnetic structures related to the solar cycle or the presence of cycle dependence of solar rotation in the solar atmosphere. [ABSTRACT FROM AUTHOR]

This study measured the polarization of the white-light corona during the total solar eclipse on 20 April 2023, which occurred under high solar activity. The same instrument that was used for the 2017 and 2019 eclipse observations was employed, and despite the short duration of totality, the obtained data could be used for high-accuracy polarization analysis. We derived the brightness and polarization of the K + F corona and estimated the brightness distributions of the K- and F-coronae using polarization information. The polarization data of the corona are the key to estimating the amount of coronal hot plasma and its electron density distribution. Therefore, we examined the consistency between the eclipse data and those taken by the C2 coronagraph of the Large Angle Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory. Consequently, a systematic difference was observed; the polarization measured by LASCO-C2 was approximately 30% smaller than the results from the eclipse. Data from eclipses, which are captured under low background sky brightness and no scattered light due to the Sun's disk, can be a good calibration source of the brightness and polarization of the white-light corona. [ABSTRACT FROM AUTHOR]

Torsional Alfvén waves (TAWs) play a significant role in the dynamics of the solar atmosphere. A detailed study of the TAWs can provide valuable insights into various aspects of the internal structure of the solar atmosphere, and the coronal heating problem. In this paper, the effect of longitudinal structuring (such as gravitational stratification, temperature inhomogeneity, radiative cooling, and background plasma flow) on the characteristics of the standing TAWs in coronal plasma loops, which can be exploited in both temporal and spatial coronal seismology applications, is investigated. The governing equation for TAWs in a dynamic and stratified coronal plasma is reduced to a time-dependent partial differential equation. Analytical dispersion relations of the differential equation are extracted and solved numerically under various scenarios by imposing the necessary and sufficient boundary conditions. The numerical results indicate that the fundamental and first overtone mode frequencies and their ratios and the spatial anti-node shift of the first overtone mode are sensitive functions of gravitational stratification and scaled time of radiative cooling. The magnitude values of these quantities are strongly influenced by the magnitude of the temperature inhomogeneity parameter and are slightly affected by the scaled background plasma flow speed. Tuning the parameters that affect the oscillatory properties of the standing TAWs and matching them with observations can enhance our understanding of the coronal structures and their evolution and serve as a diagnostic tool in coronal seismology. [ABSTRACT FROM AUTHOR]

In the dynamic and complex environment of the solar corona, the interaction between kinetic Alfvén waves (KAWs) and magnetic null points might play a significant role in understanding various plasma processes. Recognizing the potential role of reconnection in coronal heating, our study aims to delve into how different types of null points affect KAW dynamics and ultimately contribute to heating. We investigate the behavior of nonlinear KAWs near the more frequently occurring components-null point with a mean magnetic field in the solar corona. The nonlinearity is attributed to the ponderomotive effects due to density perturbations. We used a three-dimensional model equation that describes the dynamics of KAWs in the presence of components-null point. Numerical methods are employed to solve the model equation for solar coronal parameters. Our simulations reveal that the nonlinear interaction between KAWs and magnetic null points can lead to the generation and amplification of turbulent and chaotic structures. This formation of localized structures, progressively exhibit more chaotic behavior over time, which may efficiently contribute to energy transfer. The power spectrum analysis of these turbulent structures shows a steeper spectrum with a pronounced cascade. Turbulence implies the presence of localized plasma heating, particle acceleration, and magnetic reconnection. These phenomena have significant implications for understanding the energy transport, particle dynamics, and magnetic topology in the solar corona. We also address nonlinearity's role in promoting turbulence. This research offers insights into the dynamics of nonlinear KAWs near null points in the solar corona, suggesting their potential role in energy transfer and current sheet formation. [ABSTRACT FROM AUTHOR]

Context. Simulations and observations of the solar atmosphere often reveal the presence of torsional Alfvén waves and vortices with sufficient power to heat the solar corona and accelerate the solar wind. Aims. We challenge the long-held view that low-frequency Alfvén waves are suppressed due to inhomogeneities and steep spatial gradients in the atmosphere. Alfvén waves and vortices in a stratified solar atmosphere are modelled with the aim of calculating and comparing their energy flux for different field line geometries. Methods. We show that the general problem of linear Alfvén wave propagation along field lines of arbitrary geometry can be reduced to a set of Klein–Gordon equations for the perturbations of the magnetic field and velocity. Solutions and corresponding energy fluxes are constructed for three cases with different expansion rates of the field lines in the lower atmosphere. Results. Expansion rates that are associated with cut-off free propagation in the lower atmosphere suppress the perturbation amplitudes and the corresponding energy flux. These include the uniform field model and the thin flux tube model. A counterexample with an intermediate field line expansion rate and non-vanishing cut-offs exhibits consistently large perturbation amplitudes and unrestricted energy flux across the entire frequency spectrum. Conclusions. Field lines with different expansion rates and geometries in the lower atmosphere can significantly alter the amplitudes of the Alfvén waves and vortices and the extent of the energy flux entering the corona. [ABSTRACT FROM AUTHOR]

Context. The properties of the solar wind measured in situ in the heliosphere are largely controlled by energy deposition in the solar corona, which is in turn closely related to the properties of the coronal magnetic field. Previous studies have shown that long-duration and large-scale magnetic structures show an inverse relation between the solar wind velocity measured in situ near 1 au and the expansion factor of the magnetic flux tubes in the solar atmosphere. Aims. The advent of the Solar Orbiter mission offers a new opportunity to analyse the relation between solar wind properties measured in situ in the inner heliosphere and the coronal magnetic field. We exploit this new data in conjunction with models of the coronal magnetic field and the solar wind to evaluate the flux expansion factor and speed relation. Methods. We use a Parker-like solar wind model, the "isopoly" model presented in previous works, to describe the motion of the solar wind plasma in the radial direction and model the tangential plasma motion due to solar rotation with the Weber and Davis equations. Both radial and tangential velocities are used to compute the plasma trajectory and streamline from Solar Orbiter location sunward to the solar 'source surface' at rss. We then employed a potential field source surface (PFSS) model to reconstruct the coronal magnetic field below rss to connect wind parcels mapped back to the photosphere. Results. We found a statistically weak anti-correlation between the in situ bulk velocity and the coronal expansion factor, for about 1.5 years of solar data. Classification of the data by source latitude reveals different levels of anticorrelation, which is typically higher when Solar Orbiter magnetically connects to high latitude structures than when it connects to low latitude structures. We show the existence of a fast solar wind that originates in strong magnetic field regions at low latitudes and undergoes large expansion factor. We provide evidence that such winds become supersonic during the super-radial expansion (below rss) and are theoretically governed by a positive v–f correlation. We find that faster winds exhibit, on average, a flux tube expansion at a larger radius than slower winds. Conclusions. An anticorrelation between solar wind speed and expansion factor is present for solar winds originating in high latitude structures in solar minimum activity, typically associated with coronal hole-like structures, but this cannot be generalized to lower latitude sources. We have found extended time intervals of fast solar wind associated with both large expansion factors and strong photospheric magnetic fields. Therefore, the value of the expansion factor alone cannot be used to predict the solar wind speed. Other parameters, such as the height at which the expansion gradient is the strongest, must also be taken into account. [ABSTRACT FROM AUTHOR]

Context. In general, large solar flares are more efficient at accelerating high-energy electrons than microflares. Nonetheless, sometimes microflares that accelerate electrons to high energies are observed. Their origin is unclear. Aims. We statistically characterized microflares with strikingly hard spectra in the hard X-ray (HXR) range, which means that they are efficient at accelerating high-energy electrons. We refer to these events as "hard microflares". Methods. We selected 39 hard microflares, based on their spectral hardness estimated from the Solar Orbiter/STIX HXR quicklook light curves in two energy bands. The statistical analysis is built on spectral and imaging information from STIX combined with extreme ultraviolet (EUV) and magnetic field maps from SDO/AIA and SDO/HMI. Results. The key observational result is that all hard microflares in this dataset have one of the footpoint rooted directly within a sunspot (either in the umbra or the penumbra). This clearly indicates that the underlying magnetic flux densities are large. For the events with the classic two-footpoints morphology, the absolute value of the mean line-of-sight magnetic flux density (and vector magnetic field strength) at the footpoint rooted within the sunspot ranges from 600 to 1800 G (1500 to 2500 G), whereas the outer footpoint measures from 10 to 200 G (100 to 400 G), therefore about ten times weaker. In addition, approximately 78% of the hard microflares, which exhibited two HXR footpoints, have similar or even stronger HXR flux from the footpoint rooted within the sunspot. This contradicts the magnetic mirroring scenario. The median footpoint separation, measured through HXR observations, is approximately 24 Mm, which aligns with regular events of similar GOES classes. In addition, about 74% of the events could be approximated by a single-loop geometry, demonstrating that hard microflares typically have a relatively simple morphology. Out of these events, around 54% exhibit a relatively flat flare loop geometry. Conclusions. We conclude that all hard microflares are rooted in sunspots, which implies that the magnetic field strength plays a key role in efficiently accelerating high-energy electrons, with hard HXR spectra associated with strong fields. This key result will allow us to further constrain our understanding of the electron acceleration mechanisms in flares and space plasmas. [ABSTRACT FROM AUTHOR]

Context. Solar eruptive events, including solar flares and coronal mass ejections (CMEs), are typically characterised by energetically significant X-ray emissions from flare-accelerated electrons and hot thermal plasmas. However, the intense brightness of solar flares often overshadows high-coronal X-ray emissions from the associated eruptions due to the limited dynamic range of current instrumentation. Occulted events, where the main flare is blocked by the solar limb, provide an opportunity to observe and analyse the X-ray emissions specifically associated with CMEs. Aims. This study investigates the X-ray and extreme ultraviolet (EUV) emissions associated with a large filament eruption and CME that occurred on February 15, 2022. This event was highly occulted from the three vantage points of Solar Orbiter (∼45° behind the limb), Solar–TErrestrial RElations Observatory (STEREO-A), and Earth. Methods. We utilised X-ray observations from the Spectrometer/Telescope for Imaging X-rays (STIX) and EUV observations from the Full Sun Imager (FSI) of the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter, supplemented by multi-viewpoint observations from STEREO-A/Extreme-UltraViolet Imager (EUVI). This enabled a comprehensive analysis of the X-ray emissions in relation to the filament structure observed in the EUV. We used STIX's imaging and spectroscopy capabilities to characterise the X-ray source associated with the eruption. Results. Our analysis reveals that the X-ray emissions associated with the occulted eruption originate from an altitude exceeding 0.3 R⊙ above the main flare site. The X-ray time profile shows a sharp increase and exponential decay, and consists of both a hot thermal component at 17 ± 2 MK and non-thermal emissions (> 11.4 ± 0.2 keV) characterised by an electron spectral index of 3.9 ± 0.2. Imaging analysis shows an extended X-ray source that coincides with the EUV emission as observed from EUI, and was imaged until the source grew to a size exceeding the STIX imaging limit (180″). Conclusions. Filament eruptions and associated CMEs have hot and non-thermal components, and the associated X-ray emissions are energetically significant. Our findings demonstrate that STIX combined with EUI provides a unique and powerful tool for examining the energetic properties of the CME component of solar energetic eruptions. Multi-viewpoint and multi-instrument observations are crucial for revealing such energetically significant sources in solar eruptions that might otherwise remain obscured. [ABSTRACT FROM AUTHOR]

Stellar X-ray and extreme ultraviolet (XUV) emission drives the heating and chemical reactions in planetary atmospheres and proto-planetary disks, and therefore, a proper estimation of a stellar XUV spectrum is required for their studies. One proposed solution is to estimate stellar atmospheric heating using numerical models, although the validation was restricted to the Sun over a limited parameter range. For this study, we extended the validation of the model by testing it with the Sun and three young, nearby solar-type stars with available XUV observational data (κ1 Ceti, π1 UMa, and EK Dra). We first tested the model with the solar observations, examining its accuracy for the activity minimum and maximum phases, its dependence on the loop length, the effect of loop length superposition, and its sensitivity to elemental abundance. We confirm that the model spectrum is mostly accurate both for the activity minimum and maximum, although the high-energy X-rays (λ < 1 nm) are underestimated in the activity maximum. Applying the model to young solar-type stars, we find that it can reproduce the observed XUV spectra within a factor of 3 in the range of 1–30 nm for stars with a magnetic flux up to 100 times that of the Sun (κ1 Ceti and π1 UMa). For a star with 300 times the solar magnetic flux (EK Dra), although the raw numerical data show a systematically lower spectrum than observed, the spectra are in good agreement once corrected for the effect of insufficient resolution in the transition region. For all young solar-type stars, high-energy X-rays (λ < 1 nm) are significantly underestimated, with the deviation increasing with stellar magnetic activity. Furthermore, our model-based estimation shows performance that is comparable to or surpasses that of previous empirical approaches. We also demonstrate that the widely used fifth-order Chebyshev polynomial fitting can accurately reproduce the actual differential emission measure and XUV spectrum. Our findings indicate that the stellar XUV spectrum can be reasonably estimated through a numerical model, given that the essential input parameters (surface magnetic flux and elemental abundance) are known. [ABSTRACT FROM AUTHOR]

Context. Solar jets are collimated plasma ejections along magnetic field lines observed in hot (extreme-ultraviolet (EUV) jets) and cool (chromospheric surges) temperature diagnostics. Their trigger mechanisms and the relationship between hot and cool jets are still not completely understood. Aims. We aim to investigate the generation of a sequence of active-region solar jets and their evolution from the photospheric to the coronal heights using multithermal observations from ground-based and space-borne instruments. Methods. Using the synergy of high-spatial-resolution and high-temporal-resolution observations by the Swedish 1-m Solar Telescope (SST), along with the Solar Dynamics Observatory (SDO), we analyzed a sequence of solar jets originating in a mixed-polarity region between the leading and following sunspots of an active region. We investigated the kinematics of these jets using the spectra from the SST observations. We used a non-force-free field (NFFF) extrapolation technique to derive the magnetic field topology of the active region. Results. A mixed-polarity region is formed over a long period (24 hours) with persistent magnetic flux emergence. This region has been observed as an arch filament system (AFS) in chromospheric SST observations. In this region, negative polarities surrounded by positive polarities create a fan surface with a null point at a height of 6 Mm detected in the NFFF extrapolation. SST observations in the Hβ spectral line reveal a large flux rope over the AFS moving from north to south, causing successive EUV and cool jets to move in the east–west direction and later towards the south along the long open loops. Conclusions. The high-resolution SST observations (0″.038 per pixel) resolve the dark area observed at the jet base and reveal the existence of an AFS with an extended cool jet, which may be the result of a peeling-like mechanism of the AFS. Based on the combined analysis of SST and AIA observations along with extrapolated magnetic topology, it is suggested that the magnetic reconnection site may move southward by approximately 20 Mm until it reaches a region where the open magnetic field lines are oriented north–south. [ABSTRACT FROM AUTHOR]

Aims. Solar radio type II bursts are often associated with coronal shocks driven by solar eruptions. In this study, we report a type II burst associated with filament–filament interaction. Methods. Combining the high-quality multiwavelength observations from CHASE, SDO, STEREO, and CALLISTO, we conducted a detailed study of the type II burst associated with filament–filament interaction. Results. On 2023 September 11, an erupting filament (F1) likely disturbed a nearby long filament (F2), causing F2 to subsequently erupt. As a result of possible magnetic reconnection between ejective materials from the two filaments, loop-like structures formed perpendicular to them. Subsequently, the expansion of these loop-like structures triggered a strong coronal mass ejection (CME). Interestingly, a type II burst appeared on the solar spectrum around the time when the loop-like structures formed and the CME appeared above the occulting disk of STEREO/COR1. By converting the frequency of the type II burst to the coronal height using polarization brightness data recorded by the COR1 coronagraph and the spherically symmetric polynomial approximation technique, we determined the formation height of the type II burst to be around 1.45 R⊙, with a speed of approximately 440 km s−1. This is comparable to the observed height of the CME (∼1.43 R⊙), although slightly lower in speed (540 km s−1). Conclusions. All these results indicate that the type II burst was closely associated with filament–filament interactions and was possibly excited by the accompanying CME at the flank. We suggest that the filament–filament interactions played an important role in producing the type II burst by acting as a piston to trigger a strong CME. [ABSTRACT FROM AUTHOR]

Context. The new in situ measurements of the Solar Orbiter mission contribute to the knowledge of the suprathermal populations in the solar wind, especially of ions and protons whose characterization, although still in the early phase, seems to suggest a major involvement in the interaction with plasma wave fluctuations. Aims. Recent studies point to the stimulating effect of suprathermal populations on temperature anisotropy instabilities in the case of electrons already being demonstrated in theory and numerical simulations. Here, we investigate anisotropic protons, addressing the electromagnetic ion-cyclotron (EMIC) and the proton firehose (PFH) instabilities. Methods. Suprathermal populations enhance the high-energy tails of the Kappa velocity (or energy) distributions measured in situ, enabling characterization by contrasting to the quasi-thermal population in the low-energy (bi-)Maxwellian core. We use hybrid simulations to investigate the two instabilities (with ions or protons as particles and electrons as fluid) for various configurations relevant to the solar wind and terrestrial magnetosphere. Results. The new simulation results confirm the linear theory and its predictions. In the presence of suprathermal protons, the wave fluctuations reach increased energy density levels for both instabilities and cause faster and/or deeper relaxation of temperature anisotropy. The magnitude of suprathermal effects also depends on each instability's specific (initial) parametric regimes. Conclusions. These results further strengthen the belief that wave-particle interactions govern space plasmas. These provide valuable clues for understanding their dynamics, particularly the involvement of suprathermal particles behind the quasi-stationary non-equilibrium states reported by in situ observations. [ABSTRACT FROM AUTHOR]

Context. Solar flares exhibit complex variations in elemental abundances compared to photospheric values. These abundance variations, characterized by the first ionization potential (FIP) bias, remain challenging to interpret. Aims. We aim to (1) examine the spatial and temporal evolution of coronal abundances in the X8.2 flare on 2017 September 10, and (2) provide a new scenario to interpret the often observed high FIP bias loop top, and provide further insight into differences between spatially resolved and Sun-as-a-star flare composition measurements. Methods. We analyzed 12 Hinode/Extreme-ultraviolet Imaging Spectrometer (EIS) raster scans spanning 3.5 hours, employing both Ca XIV 193.87 Å/Ar XIV 194.40 Å and Fe XVI 262.98 Å/S XIII 256.69 Å composition diagnostics to derive FIP bias values. We used the Markov Chain Monte Carlo (MCMC) differential emission measure (DEM) method to obtain the distribution of plasma temperatures, which forms the basis for the FIP bias calculations. Results. Both the Ca/Ar and Fe/S composition diagnostics consistently show that flare loop tops maintain high FIP bias values of > 2–6, with peak phase values exceeding 4, over the extended duration, while footpoints exhibit photospheric FIP bias of ∼1. The consistency between these two diagnostics forms the basis for our interpretation of the abundance variations. Conclusions. We propose that this variation arises from a combination of two distinct processes: high FIP bias plasma downflows from the plasma sheet confined to loop tops, and chromospheric evaporation filling the loop footpoints with low FIP bias plasma. Mixing between these two sources produces the observed gradient. Our observations show that the localized high FIP bias signature at loop tops is likely diluted by the bright footpoint emission in spatially averaged measurements. The spatially resolved spectroscopic observations enabled by EIS prove critical for revealing this complex abundance variation in loops. Furthermore, our observations show clear evidence that the origin of hot flare plasma in flaring loops consists of a combination of both directly heated plasma in the corona and from ablated chromospheric material; and our results provide valuable insights into the formation and composition of loop top brightenings, also known as EUV knots, which are a common feature at the tops of flare loops. [ABSTRACT FROM AUTHOR]

Context. Coronal dimmings are regions of reduced emission in the lower corona observed in the wake of coronal mass ejections (CMEs), representing their footprints. Studying the lifetime evolution of coronal dimmings helps us to better understand the recovery and replenishment of the corona after large-scale eruptions. Aims. We study the recovery of dimmings on different spatial scales to enhance our understanding of the replenishment and dynamics of the corona after CMEs. Methods. In order to investigate the long-term evolution of coronal dimming and its recovery, we propose two approaches that focus on both the global and the local evolution of dimming regions: the fixed mask approach and the pixel boxes approach. We present four case studies (September 6, 2011; March 7, 2012; June 14, 2012; and March 8, 2019) in which a coronal dimming is associated with a flare/CME eruption. We analyzed each event with the same methodology, using extreme-ultraviolet filtergrams from the Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA) and Solar TErrestrial RElations Observatory's Extreme UltraViolet Imager (STEREO/EUVI) instruments. We identified the dimming region by image segmentation, then restricted the analysis to a specific portion of the dimming and tracked the time evolution of the dimming brightness and area. In addition, we study the behavior of small subregions inside the dimming area, of about 3 × 3 pixels, to compare the recovery in different regions of the dimming. Results. Three out of the four cases show a complete recovery 24 hours after the flare/CME eruption. The primary recovery mechanism identified in the observations is the expansion of coronal loops into the dimming region. The recovery of the brightness follows a two-step trend, with a steeper and quicker segment followed by a slower one. In addition, some parts of the dimming, which may be core dimmings, are still present at the end of the analysis time and do not recover within 3 days, whereas the peripheral regions (secondary dimmings) show a full recovery. Conclusions. The high temporal and spatial resolution of SDO/AIA observations combined with multi-view data of the STEREO/EUV instrument reveal high-situated coronal loops expanding after CME eruptions, which cover dimming regions and gradually increase their intensity. Our developed approaches enable the analysis of dimmings alongside these bright structures, revealing different timescales of recovery for core and secondary or twin dimming regions. Combined with magnetic field modeling, these methods lay the foundation for further systematic analysis of dimming recovery and enhance the knowledge gained from already-analyzed events. [ABSTRACT FROM AUTHOR]

Understanding the evolution of radial sizes and instantaneous expansion speeds of coronal mass ejections (CMEs) is crucial for assessing their impact duration on Earth's environment. We introduce a non-conventional approach to derive the CME's radial sizes and expansion speeds at different instances during its passage over a single-point in situ spacecraft. We also estimate the CME's radial sizes and expansion speeds during its journey from the Sun to 1 au using the 3D kinematics of different CME features, including the leading edge, centre, and trailing edge. The continuous 3D kinematics of the CME is estimated by employing the graduated cylindrical shell and stereoscopic self-similar expansion reconstruction methods on multipoint observations from coronagraphs and heliospheric imagers combined with the drag-based model. We choose the 2010 April 3 CME as a suitable case for our study, promising a more accurate comparison of its remote and in situ observations. We show that the introduced non-conventional approach can provide better accuracy in estimating radial sizes and instantaneous expansion speeds of CMEs at different instances. We examine the aspect ratio of the CME, which influences its expansion behaviour and shows the discrepancy between its value in the corona and interplanetary medium. Our study highlights significant inconsistencies in the arrival time, radial size, and expansion speed estimates obtained from remote and in situ observations. We advocate for future studies leveraging multispacecraft in situ observations and our non-conventional approach to analyse them to improve the comprehension of CME dynamics in the solar wind. [ABSTRACT FROM AUTHOR]

Over the past 20 yr, there has been increasing evidence of the existence of sausage waves in the solar atmosphere. These observations make them useful tools in the context of atmospheric seismology. Here, we study sausage magnetohydrodynamic waves in a magnetic flux tube of non-zero plasma beta with a circular cross-section and a radially inhomogeneous plasma density. Solving numerically the equations of motion for an initial value problem, the spatio-temporal evolution of the velocity perturbations is obtained for different sets of parameters. We show that the ratio of the amplitudes of the longitudinal and radial perturbations is determined by the amount of plasma beta. Additionally, the longitudinal component of the velocity perturbation experiences phase mixing within a layer surrounding the boundary of the flux tube with a rate depending on the amount of plasma beta. The results revealed that in the presence of a non-zero plasma beta, the flux tube exhibits oscillations in both the radial and longitudinal directions, characterized by a combination of two frequencies: one belonging to the slow continuum and the other to the Alfvén continuum. Also, the period of radial oscillation is obtained for different sets of parameters. The dependence of the period of the radial oscillation on the wavenumber confirms the results obtained in previous studies. [ABSTRACT FROM AUTHOR]

Potential field source surface (PFSS) models are widely used to simulate coronal magnetic fields. PFSS models use the observed photospheric magnetic field as the inner boundary condition and assume a perfectly radial field beyond a "source surface" (R ss). At present, total solar eclipse (TSE) white-light images are the only data that delineate the coronal magnetic field from the photosphere out to several solar radii (R ⊙). We utilize a complete solar cycle span of these images between 2008 and 2020 as a benchmark to assess the reliability of PFSS models. For a quantitative assessment, we apply the Rolling Hough Transform to the eclipse data and corresponding PFFS models to measure the difference, Δ θ, between the data and model magnetic field lines throughout the corona. We find that the average Δ θ, 〈Δ θ 〉, can be minimized for a given choice of R ss depending on the phase within a solar cycle. In particular, R ss ≈ 1.3 R ⊙ is found to be optimal for solar maximum, while R ss ≈ 3 R ⊙ yields a better match at solar minimum. Regardless, large (〈Δ θ 〉 > 10°) discrepancies between TSE data and PFSS-generated coronal field lines remain regardless of the choice of source surface. However, implementation of solar-cycle-dependent R ss optimal values does yield more reliable PFSS-generated coronal field lines for use in models and for tracing in situ measurements back to their sources at the Sun. [ABSTRACT FROM AUTHOR]

High-energy electrons carry much of a solar flare's energy. Therefore, understanding changes in electron beam distributions during their propagation is crucial. A key focus of this paper is how the cospatial return current reduces the energy flux carried by these accelerated electrons. We systematically compute this reduction for various beam and plasma parameters relevant to solar flares. Our 1D model accounts for collisions between beam and plasma electrons, return-current electric-field deceleration, thermalization in a warm target approximation, and runaway electron contributions. The results focus on the classical (Spitzer) regime, offering a valuable benchmark for energy flux reduction and its extent. Return-current losses are only negligible for the lowest nonthermal fluxes. We calculate the conditions for return-current losses to become significant and estimate the extent of the modification to the beam's energy flux density. We also calculate two additional conditions that occur for higher injected fluxes: (1) where runaway electrons become significant, and (2) where current-driven instabilities might become significant, requiring a model that self-consistently accounts for them. Condition 2 is relaxed and the energy flux losses are reduced in the presence of runaway electrons. All results are dependent on beam and cospatial plasma parameters. We also examine the importance of the reflection of beam electrons by the return-current electric field. We show that the interpretation of a number of flares needs to be reviewed to account for the effects of return currents. [ABSTRACT FROM AUTHOR]

Plumes are considered to play an important role in the origin of the solar wind. However, an understanding of their thermodynamic evolution is not complete. Here, we perform a detailed study of a plume inside a coronal hole throughout its lifetime, using the observations from the Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager. We find that the plume's formation is preceded by frequent occurrences of small-scale jets and jetlets at its base, leading to the gradual development of plume haze. The plume rapidly developed within the first 6 hr into its well-known morphology. Light curves from all extreme ultraviolet channels exhibit a similar profile, suggesting its multithermal nature and intensity modulation over its lifespan. Moreover, the photospheric magnetic field dynamics at the plume's base are highly correlated with its light curve in 171 Å. We calculate outflow velocities, observed prominently in the 171 Å passband and mildly in the 193 and 211 Å passbands, with median speeds lower in higher temperature bands but occasionally comparable to the respective sound speeds. When data are averaged over larger spatial scales, the plume appears isothermal along its length, with constant temperature throughout its lifetime. However, an analysis of the differential emission measure at full resolution reveals the presence of higher-temperature plasma, indicating internal temperature structures within the plume. These results provide new insights into the formation, dynamics, and thermal properties of coronal plumes, placing tighter constraints on models to understand their thermodynamic evolution and potential role in the solar wind. [ABSTRACT FROM AUTHOR]

In this study, we map the origin, acceleration, and propagation of the high-speed solar wind streams (HSS) and observe their impact on the low-latitude Earth's ionosphere. Data from radio-sounding experiments conducted by the Indian Mars Orbiter Mission (MOM) from 2015 May 9–19 is analysed to understand the solar wind speed's evolution at various helio-centric distances. The slope of the turbulence spectrum from 25 to 35 Rs was in the range of 0.2–0.4, indicative of the underdeveloped turbulence corresponding to the high-flow streams. It coincided with the appearance of the earth-facing coronal holes as observed in the coronal EUV images. The particle bulk velocity at L1 showed that the speeds began to rise from 400 km s−1 on May 11th–12th, reaching a peak of around 800 km s−1 on May13th–14th, followed by a gradual decrease to the average slow speeds. Geomagnetic disturbances during the same period manifested as a dip in the DST index values. The GNSS (Global Navigation Satellite System) data from the InSWIM (Indian network for Space Weather Impact Monitoring) network show an appreciable increase in the VTEC (vertical total electron content) of the ionosphere on disturbed days in entire low-latitude ionospheric region in the Indian sector. All these observed parameters correlate well with the HSS arrival. This is a unique study that connects the propagation of the HSS and its impact on near-Earth's environment from the different vantage points in interplanetary space and proposes the application of Radio beacons to improve space weather forecasting. [ABSTRACT FROM AUTHOR]

Solar eruptions are sudden release of the magnetic free energy accumulated within a quasi-static evolutionary process of the corona. Interestingly, many solar eruptions are preceded by a short-term slow-rise phase, during which the pre-eruption structure rises at a speed significantly larger than that of the quasi-static evolution. Here we suggest an explanation for the slow-rise phase based on a recent high-accuracy magnetohydrodynamic simulation for initiation of solar eruption. The simulation shows that by continuously shearing a bipolar magnetic arcade, an internal current sheet forms gradually, and an eruption begins once magnetic reconnection is triggered at the current sheet. We find in the simulation that the overlying field presents a slow-rise phase before the reconnection sets in. In addition, the rising speed is significantly larger than that of the core field during this phase. This slow rise is a manifestation of the growing expansion of the arcade in the process of approaching a fully open field state, which is inherent to the formation of a current sheet before the eruption. We also show three flare events with slow-rise phases that are highly consistent with these key characteristics in the simulation: an expansion of the overlying coronal loops with speeds much larger than the quasi-static evolution speed, and for those events with filament eruption, the slow rise of filament is much smaller than that of the overlying loops. In this type of events, the eruption might be initiated through the mechanism as shown in the simulation, and the expansion of overlying coronal loops is a better indicator of the slow-rise phase. [ABSTRACT FROM AUTHOR]

Introduction: We aim to examine how magnetic resistivity impacts the movement of magnetoacoustic waves near a magnetic null-point in the solar corona. Method: The resistive, nonlinear MHD simulations are solved by the PLUTO code in 2.5D for different amount of the resistivity. Results and Discussion: Propagation of magnetoacoustic waves in the vicinity of a magnetic null point has the potential to create current sheets with high current density excitation and plasmoid generation. During the entire duration of the simulation, it is discovered that plasma density became significant due to the plasmoid and also current density is high for high resistivity. It is depicted that high resistivity also leads to bigger plasmoids or magnetic islands in comparison to small resistivity. [ABSTRACT FROM AUTHOR]