For many decades, Alfvén waves have been considered to be a source of energy in the solar coronal plasma and supersonic wind. It’s important to understand the possible dissipation mechanism of the Alfvén waves in the heterogeneously structured corona. Magnetic singularities allow mode conversion of the MHD waves from the vicinity of the equipartition layer (Cs≈CA). In our study, we have performed a 2.5D MHD numerical simulation of the Alfvén wave packet generation through the mode conversion when a fast wave interacts with the localized magnetic null in the corona. Subsequently, the out of the plane velocity (i.e., Vz) and magnetic field fluctuations (i.e., bz) are generated from the equi-partition layer (Cs≈CA) and seen to propagate towards the large scale heterogeneous corona. Also, the Alfvén wave packet shows decay in amplitude on the curved magnetic field, owing to the phase mixing due to the gradient of local Alfvén speed across the magnetic field. Moreover, the strength of the phase mixing depends on the amplitude of the local transverse gradient of bz, which then helps to form localized current layers. So, our results demonstrate that Alfvén waves can be generated from the magnetic null through MHD mode conversion, which undergoes damping due to phase mixing and propagates towards another null in the vicinity. We finally infer that realistic physical conditions are required to establish the extent of phase mixing and dissipation in the structured solar corona.
Abhishekh K. Srivastava (IIT BHU, Varanasi, India); T.V. Zaqarashvili (University of Graz, Graz, Austria); Juan Martinez-Sykora (SETI Institute, Mountain View, CA, USA); Bart D. Pontieu (LMSAL, Palo Alto, USA), Ding Yuan (HIT Shenzhen, China); Sripan Mondal (IIT BHU, Varanasi, India); P. Bourdi (University of Graz, Graz, Austria); Ryun-Young Kwon (KASI, Republic of Korea); Astrid Veronig (University of Graz, Graz, Austria)
]]>Non-thermal electron acceleration is one of the earliest observable signatures of magnetic reconnection in solar flares and eruptions, and is key to understanding how it triggers and drives such events. In recent decades, modelling, as well as spatially integrated hard X-ray (HXR) observations, suggest that this process occurs on timescales less than or equal to 1s. However, further probing the acceleration process requires characterising the spatial and spectral evolution of the electrons with HXR imaging spectroscopy. Yet, this has not previously been available on such fast timescales.
In this talk, we discuss the first 1-s HXR spectroscopic imaging of non-thermal flare-accelerated electrons. Thanks to Solar Orbiter/STIX observations of one of the most intense solar flares ever observed (SOL2024-05-20T05:14L171C109, estimated at X16.5 class), we find physical changes in the non-thermal HXR source morphologies on timescales at least as fast as 1 second. This is despite slower evolution of the spatially integrated HXR spectrum.
This may suggest a coherent driver between different acceleration events within the flare whose spectrum varies on timescales greater than one second. Alternatively, it may be an illusion of the spatial averaging in the spectral analysis. Either way, these results show that sub-second electron acceleration in solar flares can be associated with morphological dynamics on similar timescales. This highlights the importance of high spatial resolution sub-second HXR imaging spectroscopy for elucidating non-thermal electron acceleration, and how it drives and sustains solar flares and eruptions.
Alfvenic waves are prevalent throughout the solar atmosphere and are believed to play an essential role in coronal heating, classified as alternating current (AC) heating in contrast to direct current (DC) heating associated with quasi-static magnetic field line braiding. The relative importance of AC versus DC heating depends on the details of the photospheric driver and on the configuration of the magnetic field. Moreover, even if AC heating prevails, several wave dissipation mechanisms have been proposed, and which of them dominates remains unclear, as its efficiency depends on plasma compressibility and density inhomogeneity. We address these issues by performing three-dimensional radiative magnetohydrodynamic (MHD) simulations of a coronal loop spanning from the upper convection zone to the corona, which self-consistently capture many relevant physical processes. We find that the corona is predominantly heated by AC heating, with Alfven wave turbulence providing the primary contribution, accounting for at least 80% of the entire coronal heating in the present simulation. Our results strongly support the use of Alfven wave turbulence-based models employed in space weather and stellar activity research, such as the Alfven Wave Solar Model (AWSoM) and the Magnetohydrodynamic Algorithm outside a Sphere (MAS).
The strong correlation between energy injection and mid-infrared (mid-IR) emission observed during solar flares can be used to probe energy deposition throughout the chromosphere, since the IR tracks prompt flare-induced changes in electron density. Despite its diagnostic value, solar mid-IR observations are relatively recent, with sporadic campaigns over the last decade resulting in only a few recorded flares. Earlier studies found time lags between mid-IR emissions from spatially resolved footpoints, offering clues about flare energy transport. Building on this, we analyse the time lags between emissions at two wavelengths (5.2 μm and 8.2 μm) for each footpoint. Using a local cross-correlation function, we show for the first time that the 8.2 μm emission channel peaks 0.3 s–0.45 s before the 5.2 μm channel. We investigate the origin of this lag, obtaining infrared emission estimates using results from the RADYN radiation hydrodynamics code. The theoretical lag values fall within the range of the observed ones. Variations in opacity—primarily due to flare-induced ionization—explain the wavelength-dependent temporal shift between emission maxima. In particular, longer wavelengths exhibit a smaller lag between the peak of energy injection and peak of intensity. These results contribute to a better understanding of how energy deposition during a flare affects the chromospheric layers of the atmosphere. Future observations with higher temporal resolution could exploit measurements of these time lags to more fully characterize the dynamics of energy deposition during solar flares, opening a new avenue for studying heating and energy transport processes in the solar atmosphere
Small-scale extreme-ultraviolet (EUV) brightenings are ubiquitous in the solar corona and may represent the low-energy counterparts of larger solar flares, with important implications for coronal heating. A key observational signature of solar and stellar flares is the presence of quasi-periodic pulsations (QPPs), which are thought to reflect fundamental processes of impulsive energy release. QPPs can therefore provide a key diagnostic for determining whether EUV brightenings share the same underlying mechanisms as larger-scale flares or instead represent a fundamentally distinct regime.
Using the high spatial and temporal resolution of the Solar Orbiter/Extreme Ultraviolet Imager, we present the first clear evidence that QPPs are present in EUV brightenings. We characterise their properties over a wide range of spatial and temporal scales and show that their periods extend from a few seconds to several minutes, comparable to those observed in larger flares.
We further demonstrate that the relationship between QPP damping time and period follows a common power-law scaling for both EUV brightenings and EUV solar flares. When combined with previously reported X-ray QPPs from solar and stellar flares, all events align with a single unified scaling trend spanning many orders of magnitude in scale. The universality of this scaling strongly suggests that QPPs in small-scale brightenings, solar flares, and stellar flares are governed by a broadly common underlying physical mechanism.
The plasma composition in the solar corona is variable, with a strong dependency on the first ionisation potential (FIP) of elements. In flaring regions, plasma composition has been shown to have significant spatial and temporal variations, likely driven by dynamical processes triggered by energy release at the reconnection site. The origin of these variations and their impact on flare loop dynamics are not yet fully understood. In this work, we use high cadence Hinode EIS spectroscopic observations of the M-class flare peaking at 13:56 UT on 2 April 2022, alongside simulations from the 0D EBTEL hydrodynamic model, to investigate the role of plasma composition in modulating radiative losses in solar flare loops. We identify two regions along the flare loop arcade, with distinct FIP bias values as well as cooling rates, suggesting that spatial variations in plasma composition may play a key role in influencing flare loop cooling. In this framework, I will also discuss the potential of high resolution spectropolarimetric observations from the upcoming IBIS 2.0 instrument, currently under installation at the THEMIS telescope, particularly for advancing studies of the physical mechanisms driving plasma composition variations, flare dynamics and the coupling between the two.
Magnetohydrodynamic waves are often regarded as efficient pathways for magnetic energy transfer from the solar interior. Consequently, they are key to not only understand the physical processes behind coronal heating and/or solar wind acceleration but also serve as useful diagnostic tools. While Alfvén wave modes are notoriously difficult to detect directly by remote sensing – due to their incompressive nature – magnetoacoustic wave modes can perturb the density and temperature as they propagate. More recently, observations from the Metis coronagraph on board the Solar Orbiter have shown the ubiquitous presence of propagating density fluctuations in a helmet streamer as well as in a pseudo-streamer with a 5-minute period. Using 2.5D MHD simulations, we find that such density fluctuations can be generated either from (non-linear) Alfvén waves or directly from magnetoacoustic waves. Furthermore, density fluctuations generated from Alfvén waves have a doubled frequency with respect to the (driven) Alfvén wave frequency because of non-linear ponderomotive forces. This feature might be a useful diagnostic for the (in-situ) distinction between density fluctuations generated from these different mechanisms.
Flare ribbons are a quintessential part of a flare. They are extensively used to understand the overlying reconnection mechanism and overall flaring process. Further, the reconnection rate is a key parameter in the magnetic reconnection process. But it is difficult to estimate the rate from direct observations at higher heights in the solar atmosphere. However, flare ribbons are believed to be one of the major energy deposition sites and the imprints of reconnection occurring at higher altitudes. Utilizing this advantage to map the reconnection rate at the corona, Qiu et al. (2002, 2006) had provided an approach to estimate the flux and flux rate in a flare.
In the talk, I will discuss about the application of this method to multiple wavelengths in the solar atmosphere using high resolution data from ground-based Multi Application Solar Telescope (MAST)/Udaipur Solar Observatory in Ca II 8542 line, and data in UV and EUV channels from the Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). I will highlight the interesting profiles of reconnection flux and associated rates for all the channels in ribbon (sub)structures, along with the probable nature of reconnection occurring there. The work also explains the triggering mechanism behind the reconnection in 3D using the non-force-free-field (NFFF) extrapolation model.
Recent advances in solar physics increasingly rely on automated identification of coronal structures using machine learning. Yet most studies emphasise scientific performance without evaluating feasibility for onboard deployment to prioritise downlink observations. We investigate the automated identification of active regions and coronal holes by applying segmentation and detection techniques to Solar Dynamics Observatory (SDO) data. We compare three approaches: SCSS-Net, a deep learning model for semantic segmentation; YOLOv8n, a lightweight object detector; and a traditional pipeline based on basic computer vision operations (BCVO). Each method is assessed for its scientific accuracy and its suitability for deployment in future resource-limited missions. While no direct hardware benchmarking has been performed yet, we assess the feasibility of an onboard implementation based on the number of trainable parameters, the architecture, and the associated hardware requirements. Training and evaluation are first conducted on well-calibrated SDO images. We then extend the evaluation to raw and uncalibrated SDO images affected by instrumental artefacts. Performance is mainly measured using the Intersection over Union (IoU) and Dice score. Results show that while SCSS-Net achieves the highest segmentation quality, YOLOv8n offers a strong balance between accuracy and efficiency. The BCVO pipeline remains viable under strict hardware limitations. Interestingly, our models retain compatibility on Level-0 observations. This is the first study comparing these widely used methods from the perspective of onboard deployment. Our findings provide a foundation for designing frameworks tailored to onboard hardware configurations.
This work takes advantage of new observational data collected by the High-resolution Coronal Imager (Hi-C) at the highest spatial and temporal resolutions available. This most recent Hi-C dataset is the first to make use of the hot 129 Å channel and the first to consist of targeted observations of a solar flare, rather than the quiescent corona. Images from Hi-C are analysed and compared with complementary EUV observations from SDO/AIA. The widths of the loop strands are measured, and the resulting populations analysed. Preliminary results suggest the majority of loop strands have a width in the range 500-2,000 km, but there is some variation in the loop strand widths observed in plasma at different temperatures. There is some evidence that there are smaller structures present which are partially resolved by Hi-C, with the very smallest smallest loop strands of a similar order of magnitude to the spatial resolution of the Hi-C images. Further results will be presented, as well as a discussion of future work, investigating individual events within the Hi-C observations, making use of the high cadence available to track the dynamic behaviour of the corona.