Our views of the solar chromosphere, transition region, and corona are rapidly changing with the advent of increasingly advanced numerical simulations of the dominant physical processes in the coupled solar atmosphere and the availability of high-resolution observations from both ground-based (e.g., SST, DKIST) and space-based telescopes (e.g., IRIS, Solar Orbiter/EUI). To gain a deeper understanding of which physical processes dominate the dynamics and heating in the solar atmosphere, and in order to further develop and advance our numerical models, it is key to confront the models with high-resolution observations by focusing on existing discrepancies between observed and synthetic spectra. This workshop is focused on such a critical comparison by providing an overview of the most recent observational results and models. The meeting will also feature discussions on the missing physics in current models and the most promising approaches to increase the realism of current simulations, with an eye towards future missions such as MUSE and EUVST, which will be launched later this decade.
Given the limited number of hotel rooms and conference space, participation is limited to participants that were selected by the SOC on the basis of their contributions. The main selection criterion was the relevance of the contribution to the goals of the workshop. All participants have been notified and at this time it is not possible to submit additional abstracts. The abstract submission form is open for invited speakers only.
Realism of models has two components: sophistication of implemented physics and realism of the overall setup. Current 3D radiation MHD simulations of the coupled solar atmosphere are presenting a compromise on a middle ground. Domains are at best large enough to capture the extent of small active regions, but the commonly adapted use of periodic boundary conditions leads in many setups to an unrealistic magnetic field connectivity. Relevant physics in terms of radiation transport, including non-equilibrium treatment and ion-neutral effects are implement in a few commonly used simulation codes, but in many of larger domain models the resolution is just barely enough to start resolving processes of interest. More detailed physics better describing reconnection and energy dissipation can be only studied in dedicated simulations that focus on either small domains or reduce the dimensionality of the domain. In this talk I will touch on these challenges and present a few future directions: (1) We need simulations in local domains that are better informed by the global magnetic field structure of the corona. (2) We need higher resolution and reduction of numerical diffusivity and adoption of sub-grid models that better capture underlying physics. (3) We need a better quantification of model errors. Any combination of these improvements will come with a high cost and will require utilization of the latest computing platforms, including the use of GPUs. Data analysis is developing into a major bottle neck and will require a change in how we conduct and share large numerical simulations.
Task-based computing is offering a break-through in our capabilities to model astrophysical phenomena, and in particular in the context of solar modeling there are several routes along which we can take advantage of this general methodology. In the context of global modeling of the solar interior, we can utilize the ability to tune resolution as a function of depth, and by engaging very low diffusion low-Mach number Riemann solvers we can model the solar interior over time-scales sufficient to cover solar cycles. Given such global models, with large extents in space-time, we can then use static and adaptive mesh refinement to zoom-in on smaller (active) regions of space-time. With initial and boundary conditions from the larger scale models, and with BIFROST-compatible physics added, we can realistically model active region phenomena, first with (non-ideal) magnetohydrodynamics, and ultimately with particle-in-cell modeling of charged particle dynamics and particle acceleration in even smaller regions of space-time. An important technical development is needed to fully utilize these opportunities; a large fraction of the near-future supercomputing capacity is based on GPU-offloading, and we need to continue to develop and integrate accelerated solvers for magnetohydrodynamics and charged particle dynamics, while also staying on top of advances in hardware, firmware, and programming language standards related to offloading and hierarchical memory architecture.
Instrumentation for solar observations always represents a trade-off between spatial resolution, spectral resolution, field-of-view, time-coherence, calibration, and available money. The current state-of-the art for optical and near-infrared wavelengths consists of Fabry-Perot interferometers, and various types of imaging spectrographs.
In this talk I will discuss some aspects of these considerations, and review current and upcoming instrumentation at DKIST, SST, and other observatories.
In this talk I will provide a brief overview of the upcoming solar missions focused on the Sun's corona: MUSE and Solar-C/EUVST, scheduled for launch in 2027 and 2028, respectively. The Multi-slit Solar Explorer (MUSE) is a NASA MIDEX mission, composed of a multi-slit EUV spectrograph (in three spectral bands around 171Å, 284Å, and 108Å) and an EUV context imager (in two passbands around 195Å and 304Å). MUSE will provide spectral and imaging diagnostics of the solar corona at high spatial (~0.5 arcseconds), and temporal resolution (0.5 seconds). By obtaining spectra in 4 bright EUV lines covering a wide range of transition region and coronal temperatures along 37 slits simultaneously, MUSE will be able to "freeze" (at a cadence as short as 10 seconds) with a spectroscopic raster the evolution of the dynamic coronal plasma over a wide range of spatial scales. Solar-C is a JAXA-led international mission with the EUV High-throughput Spectroscopic Telescope (EUVST) as its main payload. EUVST is a high-resolution (0.4 arcsecond), high-cadence (0.5s) single-slit spectrograph that includes multiple spectral passbands in the EUV and FUV, providing temperature coverage from the chromosphere to the flaring corona without any significant gaps. EUVST includes slit-jaw imaging in the photosphere, low chromosphere and upper chromosphere. I will describe the strong synergy between these missions and provide some examples of how the constraints on the properties of the solar atmosphere from these instruments can discriminate between predictions from various current numerical models for coronal heating, solar flares, and coronal mass ejections.
Solar magnetic fields are essential ingredients for the energetics and dynamics of the lower solar atmosphere. After emergence, they continually interact with convective flows and with each other. The resulting field line braiding is believed to trigger magnetic reconnection in the chromosphere and above, generating a wide variety of features and contributing to atmospheric heating, both locally and globally. However, the exact conditions leading to magnetic reconnection are not yet well understood. Also, the origin of the field and the process of flux emergence are poorly known, particularly on the smallest scales.
Determining the topology and energy injected by the fields in the lower solar atmosphere is key to understanding these processes. Significant advances have been achieved through multi-line spectropolarimetric observations at ever increasing spatial resolution. Specific examples will be discussed here. However, the efforts have been hampered by insufficient polarimetric sensitivity, which makes it difficult to follow the evolution of the fields or even detect the weaker ones, particularly in the chromosphere. Thus, a direct confirmation of the scenarios suggested by numerical simulations is not possible in general. The lack of tools to infer the vector magnetic field from spectropolarimetric measurements has impeded progress, too. These limitations will soon be overcome, giving access to the elusive chromospheric fields. The interpretation of the new observations will require simulations of the entire solar atmosphere covering larger fields of view. Together, they will clarify the origin of solar magnetic fields, their structure, and their role in the heating of the solar atmosphere.
In this talk I will look at the scales at which photospheric motions generate waves and changes in the magnetic field topology. I will then discuss how these scales change throughout the solar atmosphere.
Nowadays, solar spectra are routinely analyzed to understand the physical mechanisms that trigger different physical phenomena. These spectral lines are modeled using approaches of increasing complexity, ranging from a simple Gaussian model to complex non-LTE radiative transfer calculations. In practice, these data are also affected by telescope degradation, may include some instrumental artifacts, and some signals may be buried under noise. During these years, different approaches have been developed to solve specific problems, proving solutions under various assumptions. Since many of these models are differentiable (and those that are not differentiable can be emulated with neural networks), we could use deep learning frameworks as a general infrastructure to implement these models and infer the parameters of interest in a flexible way. We can combine and incorporate different models, nonlinear transformations, spatial degradation (telescope PSF), spatial coherence (spatial smoothing), artifacts (e.g., fringes, blends with other lines), and physical constraints (e.g., magnetic field vector divergence). By construction, we can run these codes on GPUs with no extra modification, speeding up all computations. In this talk, I will present different examples with IRIS and SST data with an eye toward future missions such as MUSE and EUVST, highlighting the strengths and limitations of this novel approach.
Recent observational studies on abundances reveal the need to expand models by considering different fluids and/or species to be constrained with these observations and interpret them. Similarly, radiative MHD models fail to reproduce observations of the coldest parts of the chromosphere and some properties of the Mg II profiles. Mg II is formed where interactions between multiple ionized and neutral species prevent an accurate MHD representation. In addition, those regions are where MHD seems to break down, and microphysics may need to be considered, such as a meter-scale electrostatic plasma instability, the Thermal Farley-Buneman Instability (TFBI), which develops in these regions and efficiently converts kinetic energy into electron heating. In this presentation, I will summarize some of the advances in multi-fluid modeling, especially focusing on the multi-fluid Ebysus code and the need to validate and constrain those with existing and future observations.
Solar chromosphere has been at the focus of solar physics studies for decades, but its heating mechanisms are still unknown. Chromospheric plasma is strongly stratified, weakly ionized and not completely collision coupled. In this talk I will overview our recent results of the modeling of solar chromosphere, comparing a more standard single-fluid approach and a more novel multi-fluid approach. I will describe the challenges posted by the multi-fluid modeling, that still need to be overcome, and will talk about possible observables for the multi-fluid effects.
Our understanding of magnetic reconnection (MR) under chromospheric conditions remains limited. Recent observations have demonstrated the important role of ion-neutral interactions in the dynamics of the chromosphere. Furthermore, the comparison between spectral profiles and synthetic observations of reconnections suggest that single-fluid MHD approaches appear to be inconsistent with observations. Indeed, collisions and multi-thermal aspects of the plasma, hydrogen and helium ionization effects play a major role in the energy balance of the chromosphere.
This work investigates multi-fluid/multi-species (MFMS) effects on MR in upper chromospheric conditions. We compare an MFMS approach based on a helium-hydrogen mixture with a two-fluid MHD model based on hydrogen only. We study the evolution of the MR and compare the two approaches including the decoupling of the particles, the evolution of the heating mechanisms, and the composition.
The simulations have been performed in the same numerical code Ebysus (Wargnier et al. 2022) which can solve any MFMS model for any species and/or ionized/excited level as desired. A numerical strategy based on a partitioned implicit-explicit orthogonal Runge-Kutta method has been considered. This algorithm allows an optimization of the timestep while estimating the error of the various terms involved in these models and guaranteeing reasonable computational costs.
Our results show that the presence of helium species leads to more efficient heating mechanisms than the two-fluid case. The different dynamics between helium and hydrogen species could lead to chemical fractionation and enrichment of helium. This could be of significance for recent observations of helium enrichment in switchbacks or CMEs.
Recent observations performed by space missions (SDO, Hinode) prove the existence of mini-filament eruptions within the solar chromosphere that could be connected to the formation of chromospheric jets and spicules. The growth of the helical kink instability within these structures is responsible of the onset of magnetic reconnection and lead to explosive events occurring ubiquitously in the lower solar atmosphere. Numerical studies of low-lying loops neglect critical physics in the form of partial ionization, which can lead to enhanced reconnection rates and a more efficient heating and particle acceleration. Here we perform high resolution multi-fluid simulations of kink unstable flux ropes to elucidate the role of partial ionization in modifying the kink process. The complex charge-neutral coupling in our model includes ionization, recombination and radiative losses. Partial ionization leads to a faster onset of the non-linear phase of the kink instability, whose reconnection rate could be consistent with those estimated by observations of the smaller chromospheric filaments. The magnetic energy lost with reconnection is distributed differently between fully ionized plasmas and partially ionized plasma cases, with a larger increase of internal energy and temperature associated to partial ionization and additional heating terms, such as the frictional heating, resulting from multi-fluid dynamics. Partial ionization effects lead to a faster kink instability and the release of larger quantities of thermal energy, which is reflected by a more explosive chromospheric flux rope dynamics.
The chromospheric heating terms cannot be inferred directly from observational datasets. Furthermore, even estimating the thermodynamical state of the plasma usually involves complex NLTE inversion calculations. Therefore, it has been very difficult to quantify in which proportion different heating mechanisms could be operating at different locations of the chromosphere.
One of the most crucial constraints that we can set a any given location are the chromospheric radiative losses, as they represent the energy that (at least) must be replenish in the chromosphere at any given time.
In this talk I will tackle how the spatio-temporal estimates of the radiative losses and other proxies can be used to discriminate what heating mechanisms could potentially be at work from very high spatial-resolution observations of the solar chromosphere.
The MURaM radiation-magnetohydrodynamics code has long been applied to simulate near-surface magnetoconvection, ranging from quiet sun conditions to complex active regions. The code includes the physics required to treat the convection zone, and the solar atmosphere from the photosphere to the corona. Until now, these simulations have been limited to a local-thermodynamic equilibrium treatment of the chromosphere, limiting it's realism. We have extended the MURaM code to include NLTE effects following the prescriptions used in the Bifrost code. In this work, we summarize the improvements made to the code. We study an initial model of the chromosphere, representing an enhanced network region. Comparing synthetic Mg II h&k spectra to IRIS observations we discuss the implications of the new model towards understanding the physics necessary to model the chromosphere.
This study presents a comparison of the high frequency wave power found in
3D numerical MHD models of the solar atmosphere (Bifrost and MURaM)
with real observations of chromospheric lines. We
also discuss the systematics originating from using different
models to calculate the acoustic wave flux in the solar chromosphere.
In particular, we synthesize from the MHD models spectral lines sampling the
lower chromosphere (Mn I 280.1 nm, Na D1), middle chromosphere
(Ca II 854.2 nm) and the upper chromosphere (Mg II h&k) with
the RH15D code. We compare the synthetic observations with data from the IRIS
observatory and the IBIS instrument at the DST.
We also study the emerging lack of high frequency phase differences
between the observed velocity diagnostics and how is this phenomenon affected
by the rapidly changing line formation height. Based on these results, we investigate
the systematics of inferring acoustic wave flux in the solar chromosphere.
We find that the main uncertainty in determining the wave flux in the chromosphere
is due to the changing height of formation and the associated changes in the
plasma density. This effect leads to uncertainties of the inferred flux on the order of magnitude.
Furthermore, the dynamic time-dependent chromospheric models also show significantly
lower attenuation of the detected wave signals, compared with the previously used 1D
semi-empirical models. The main takeaway from this study is that
we need numerical chromospheric models that resemble closely the solar
wave properties, to be able to infer the acoustic wave flux accurately.
The chromosphere is a very dynamic part of the solar atmosphere. With short time scales and small spatial scales at which fundamental physical processes are taking place, it is a challenge to get a clear observational view of the chromosphere. The CRISP and CHROMIS tunable filter instruments at the Swedish 1-m Solar Telescope (SST) on La Palma are capable of fast wavelength sampling while operating near the diffraction limit of the telescope. In this talk, I will present some of the best observations of the chromosphere that we have acquired with the SST. Over the past decade, we have been running a program of multiple annual campaigns in coordination with IRIS. This long term effort has resulted in a database that covers an extensive range of different types of targets and allows us to concentrate on data that were acquired under the best conditions. These datasets help us to find constraints on what are the main mechanisms that drive the dynamics of the chromosphere.
Recent magnetohydrodynamic models show that many of the chromospheric features and their dynamics can be reproduced to some degree. The models demonstrate that the overall appearance of synthetic chromosphere depends mostly on the field configuration and the treatment of the small-scale dynamics. In this talk, the properties of these models are presented, as well as their comparison to observations. These results are then linked with outstanding questions, both in modeling and observations.
The solar corona is continuously studied through observations and numerical modelling due to its extreme temperatures. One of the prime candidates in understanding these temperatures is nanoflares, which are small-scale events associated with magnetic reconnection in the solar atmosphere. Observations of small-scale events with nanoflare energies are rare because signatures from non-thermal electrons are typically below the detection threshold. We investigate signatures in synthetic observables that arise from non-thermal electrons accelerated by magnetic reconnection in a 3D Bifrost simulation. Our analysis includes the transition region (TR) Si IV lines and the upper chromospheric Mg II h and k lines, which can be observed by space-based telescopes (e.g., IRIS). We also include the Ca II H and K and Ca II 854.2 nm lines that form deeper in the atmosphere and are readily accessible by ground-based telescopes (e.g., SST, DKIST). Our goal is to explore the impact of nanoflare events on the synthetic spectra and investigate the diagnostic potential of TR and chromospheric emission from small-scale events.
Recent observations revealed loop-like structures at very small scales visible in observables that sample transition region (TR) and coronal temperatures. Their formation remains unclear.
We study an example of a bipolar system in realistic magnetohydrodynamic simulations and forward synthesis of spectral lines to investigate how these features occur.
Computations are done using the MURaM code to generate model atmospheres. The synthetic Hα and Si IV spectra are calculated at two angles (μ=1, μ=0.66) using the Multi3D code. Magnetic field lines are traced in the model and the evolution of the underlying field topology is examined.
The synthetic Hα dopplergrams reveal loops that evolve dramatically within a few minutes. The synthetic Hα line profiles show observed asymmetries and doppler shifts in the line core. They, however, also show strong emission peaks in the line wings, even at the slated view. The synthetic Si IV emission features partly coincide with structures visible in Hα dopplergrams and partly follow separate magnetic field threads. Some are even visible in the emission measure maps for the lg(T/K)=[5.0, 5.5] temperature interval. The emission areas trace out the magnetic field lines rooted in opposite polarities in a bipolar region.
We find that our results largely reproduce the observed features and their characteristics. A bipolar system with footpoints undergoing rapid movement and shuffling can produce many small-scale recurrent events heated to high temperatures. The morphology and evolution of the resulting observable features can vary depending on the viewing angle.
Numerical simulations based on 3D MHD models have been used create and sustain a hot upper atmosphere of the Sun for various solar features, e.g. quiet Sun, bright points, or active regions. These models provide self-consistent explanations for quite a range of observational features e.g. for Ellerman bombs or UV bursts. They allow to follow the changes of the magnetic field, e.g. while field lines are braided, and by this show where and when energy is dissipated and what the consequences are for (synthesised) line profiles from the transition region and corona. However, many questions remain open. For example, numerical models usually show very high contrast in synthesized emission, while on the real Sun the majority of the emission is originating from a diffuse background. Is this background seen in the observations just a mixture of small-scale unresolved features, or are there some processes at work that we so far do not capture by numerical models? Coronal imaging from Solar Orbiter at an unprecedented spatial resolution do show not only features smaller than before, but also show, e.g., diffuse patches almost the size of a supergranule or thick loops that are stable for surprisingly long times. For all these features MUSE will provide information on profiles of coronal emission lines at a spatial resolution roughly matching Solar Orbiter EUV imaging. This will provide new constraints and challenges for our understanding of the upper solar atmosphere.
The High-Resolution Coronal Imager (Hi-C) instrument has been launched twice from White Sands Missile Range, each time capturing the highest resolution coronal images ever obtained, first in the 193 Angstrom passband and then in the 171 Angstrom passband. These two rocket flights, which collectively have yielded only 10 minutes of data, have generated over 80 refereed publications. In this talk, I will discuss how these observations have advanced our knowledge of coronal heating and paved the way for the next generation of high resolution instruments, like the Multi-Slit Explorer (MUSE).
Chaotic photospheric motions progressively shuffle and braid the magnetic field confining plasma in coronal loops. The stressed field can suddenly lose equilibrium and develop instabilities, candidate to release magnetic energy into heat. There is long experience in modeling impulsive energy releases in coronal loops, from a purely hydrodynamic approach (e..g.,Reale+2000,Testa&Reale2020), to a full-MHD approach (Guarrasi+2014,Reale+2016,DePontieu+2022). In the MHD model an initial stratified atmosphere, including the steep transition region between the chromosphere and the corona, is in equilibrium with a loop-like magnetic field, which is progressively twisted by photospheric motions and can reconnect and release energy through anomalous diffusivity. The time-dependent 3D-MHD equations are solved numerically with the accurate Godunov-based PLUTO MHD code, including gravity, plasma radiative losses, and thermal conduction.
In this configuration we are currently investigating MHD kink instabilities in twisted magnetic strands (e.g.,Hood+2009). The initial helical current sheet progressively fragments in a turbulent way into smaller scale sheets, whose dissipation is similar to a nanoflare storm. The unstable loop expands and can disrupt nearby stable loops (e.g.,Tam+2015), thus triggering an MHD avalanche (Hood+2016).
Another target is reconnection jets, also called nanojets, observed in coronal loops, and linked to nanoflares (Antolin+2019).
We study them modeling the reconnection of two tilted coronal loops.
For all this work in progress, the combination of magnetic configuration and full loop atmosphere allows us to isolate the critical processes, still maintaining a high level of realism to address specific signatures in imaging and spectroscopic observations foreseeable with the MUSE mission.
The dynamical evolution of the solar magnetic field(s) is a key ingredient in understanding the ubiquitous observed activity in the Sun. A fundamental process, which is responsible for this dynamical evolution, is the emergence of magnetic flux from the solar interior to the outer solar atmosphere. The cost of running realistic numerical models is no longer prohibitive in studying the rising of magnetic flux through the last 10-20 Mm of the solar convection zone and into the outer atmosphere. We review observational constraints on the flux emergence process and current and future modelling efforts using high performance computational resources and techniques. We will especially concentrate on the comparison between model generated synthetic observables against observed spectral signatures. We will discuss how MUSE observations of flux emergence will improve our understanding of solar and stellar magnetic fields and their role in producing the coronal field, igniting flares, and energising chromospheres and coronae and we will discuss what developments in modelling are necessary to form a coherent picture of these complex phenomena.
The Sun’s atmosphere is powered by the complex convective motions which continuously churn the solar surface and stress the atmospheric magnetic field. However, describing the specifics of the resulting energy cycle, including the processes which ultimately drive energy dissipation and atmospheric heating remains a significant challenge. With this in mind, the community is continuously developing sophisticated MHD simulations to examine the atmospheric energy and mass cycles in more detail.
In this talk, I will review recent results from coronal heating simulations which show how a variety of processes (e.g. reconnection, MHD waves, turbulence) can contribute to maintaining atmospheric conditions. As the location, frequency and magnitude of energy release differs between proposed models, so does the plasma response. This includes the evolution of coronal temperatures and densities, and the generation of field-aligned plasma flows. Since these differences are potentially observable (e.g. with sufficient resolution and cadence), they can provide distinguishing signatures which can constrain the prevalence of each of these heating mechanisms in the Sun’s atmosphere. Using synthetic emission derived from numerical models, I will discuss how signatures of elementary heating events may manifest in current and future observational datasets. In particular, I will consider how well the true nature of simulated plasma is reflected in synthetic intensities and spectral diagnostics, and thus detail what inferences can be made from real observations. By focussing on the observational capabilities of the upcoming MUSE mission, I will discuss which features of atmospheric evolution and heating it will unveil for the first time.
The MUSE instrument will provide information on intensity and flows in the corona with unprecedented spatial and temporal resolutions. High-resolution 3D MHD models of coronal loops are thus timely and crucial to investigate the connection between heating events and resulting spectral diagnostics.
We carried out high-resolution simulations of a straightened coronal loop that is self-consistently heated and sustained by magnetoconvection. From the model, we synthesized spectral profiles in temperature ranges of 2–3 MK that the MUSE instrument could observe.
The resulting nonthermal linewidths are compatible with the observed values. Due to the high spatial resolution, our simulations partially resolved the energy cascade to small scales within the loop interior. A significant part of the injected Poynting flux is associated with flows on short timescales and small spatial scales, such as vortices propagating from the photosphere to the corona. Our model allows us to disentangle the contribution of motions perpendicular to the axial field of the loop and evaporative upflows that occur in response to coronal heating events.
In this talk, I will discuss the response of the coronal emission line profiles to energy injection and release and whether signatures of propagating vortices could be observed with MUSE.
Coronal Bright Points (CBPs) are ubiquitous structures in the solar atmosphere composed of hot small-scale loops observed in EUV or X-Rays. They are key elements to understand the heating of the corona; nonetheless, basic questions regarding their energization, heating mechanisms, the chromosphere underneath, or the effects of
flux emergence in these structures remain open.
We have used the Bifrost code to carry out a numerical experiment in which a coronal-hole magnetic nullpoint configuration evolves perturbed by realistic granulation. To compare with observations, synthetic SDO/AIA, Solar Orbiter EUI-HRI, IRIS images have been computed, in addition to synthetic MUSE observables to show the potential diagnostics capabilities of the future mission.
The experiment shows the self-consistent creation of a CBP through the action of the stochastic granular motions alone, mediated by magnetic reconnection in the corona. The reconnection is intermittent and oscillatory, and it leads to coronal and transition-region temperature loops that are identifiable in our EUV/UV observables. During the CBP lifetime, convergence and cancellation at the surface of its underlying opposite polarities takes place. The chromosphere below the CBP shows a number of peculiar features concerning its density and the spicules in it. The final stage of the CBP is eruptive: magnetic flux emergence at the granular scale disrupts the CBP topology, leading to different ejections, such as UV bursts, surges, and EUV coronal jets.
Apart from explaining observed CBP features, our results pave the way for further studies combining simulations and coordinated observations in different atmospheric layers.
In recent years, a renaissance has occurred for wave heating mechanisms,
because of the plethora of wave observations in the corona since 10-20 years. This renewed interest in wave heating modelling has brought models from the 1D and cartoon level to full 3D wave heating models. It has been realised that the waves naturally induce the formation of small scales through turbulence, leading to an attractive pathway to heating. Despite this interesting pathway and continued modelling efforts, heating was only achieved in quiescent loops, and not in active region loops. Moreover, coupling to the lower atmosphere remains largely unexplored.
In the current talk, I will give an update on the latest models and progress in this field. I will offer a critical survey, point out problems and pathways to potential solutions.
The solar corona is shaped and mysteriously heated to millions of degrees by the Sun’s magnetic field. It has long been hypothesised that the heating results from a myriad of tiny magnetic energy outbursts called nanoflares, driven by the fundamental process of magnetic reconnection. This theory recently received significant support through the observational discovery of nanojets - very fast (>100 km/s) and bursty (<20 s) jet-like structures with energies in the nanoflare range, uniquely characterised by being transverse to the loop and, in most cases unidirectional from the reconnection point. We have interpreted the nanojet as the telltale sign of small-angle reconnection leading to nanoflares. It can occur isolated or clustered, with large ensembles showing signatures of an MHD avalanche-like progression, leading to the formation of hot coronal loops. We have since observed nanojets in various structures in which dynamic instabilities such as Kelvin-Helmholtz play a role. Using state-of-the-art numerical simulations, we demonstrate that the nanojet is a consequence of the slingshot effect from the magnetically tensed, braided or curved magnetic field lines reconnecting at small angles. We further show that Alfvén waves can play a binding effect as reconnection triggers. This talk will discuss the open questions related to nanojets and their potential role in coronal heating. We show how next-generation instrumentation, such as MUSE and state-of-the-art simulations, can help elucidate this fascinating phenomenon.
High-resolution, high-cadence EUV observations over the past decade have led to the discovery of a decay-less regime of kink oscillations in coronal loops. The means of excitation and sustaining such oscillations over many wave periods against energy dissipation mechanisms such as phase mixing and turbulence is still an unknown. Therefore, identifying the true nature of these decay-less oscillations is not only essential for their role as diagnostic tools for coronal seismology, but also for understanding their contribution to heating of the solar atmosphere. To that end, we will be presenting results of 3D magnetohydrodynamic simulations for continuously driven transverse waves in models of straight flux tubes in coronal conditions. Different driving mechanisms will be considered, from monochromatic transverse drivers, to oscillations driven by vortex shedding. We will focus on the manifestation of KH instability-induced turbulence in the cross-section of our simulated coronal loops, and the observational signature of the out-of-phase motions in synthetic data targeting instruments like SDO/AIA, Hinode/EIS and future missions such as MUSE. Parallels will be driven between our numerical models and those of impulsively oscillating loops, multi-stranded loops and loops driven by more complex drivers. Finally, the energy content of the driven oscillations of our turbulent loops will be discussed, showing how the underlying energy fluxes from low amplitude decay-less kink oscillations can potentially be of the order of the radiative losses for the Quiet Sun.
I will discuss how high resolution current and future observations of the solar atmosphere (e.g., with IRIS, SDO, Hinode, MUSE and EUVST), help us advance our understanding of the role of different physical processes -- including, e.g., braiding, Alfven waves, accelerated particles resulting from magnetic reconnection -- in heating the solar corona.. In particular I will focus on the synergy between high resolution spectroscopic observations and state-of-the-art models, and will discuss how future transition region and coronal observations from MUSE, EUVST, and other observatories, are expected to test and constrain coronal heating models.
Relaxation of braided coronal magnetic fields through reconnection is thought to be a source of energy to heat plasma in active region coronal loops. However, observations of active region coronal heating associated with untangling of magnetic braids remain sparse. One reason for this paucity could be the lack of coronal observations with sufficiently high spatial and temporal resolution to capture this process in action. Using new high spatial resolution (250–270 km on the Sun) and high cadence (3–10 s) observations from the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter we observed untangling of small-scale coronal braids in different active regions. The untangling is associated with impulsive heating of the gas in these braided loops. We assess that coronal magnetic braids overlying cooler chromospheric filamentary structures are perhaps more common. Furthermore, our observations show signatures of both spatially coherent and intermittent coronal heating during relaxation of magnetic braids. Our study reveals the operation of gentle and impulsive modes of magnetic reconnection in the solar corona. In this talk, we present these new EUI observations and discuss the implications for magnetic braiding associated coronal heating.
It remains unclear which physical processes are responsible for the dramatic increase with height of the temperature in stellar atmospheres, known as the chromospheric ($\sim$10,000 K) and coronal (several million K) heating problems. Statistical studies of sun-like stars reveal that chromospheric and coronal emissions are correlated on a global scale, constraining, in principle, theoretical models of potential heating mechanisms. However, so far, spatially resolved observations of the Sun have surprisingly failed to show a similar correlation on small spatial scales, leaving models poorly constrained. Here we use unique coordinated high-resolution observations of the chromosphere (from the Interface Region Imaging Spectrograph or IRIS satellite) and the low corona (from the Hi-C 2.1 sounding rocket) and machine-learning based inversion techniques to show a strong correlation on spatial scales of a few hundred km between heating in the chromosphere and low corona for regions with strong magnetic field (``plage"). These results are compatible with recent advanced 3D radiative magnetohydrodynamic simulations in which the dissipation of current sheets formed due to the braiding of the magnetic field lines deep in the atmosphere is responsible for heating the plasma simultaneously to chromospheric and coronal temperatures. Our results provide deep insight into the nature of the heating mechanism in active solar regions.
The majority of the Sun is covered by a system of relatively weak magnetic fields called the Quiet Sun (QS) which, despite being far weaker than active regions, plays an important role in energizing the solar atmosphere. With new generations of simulations and instrumentation, it is becoming feasible to understand the dynamics of the QS with more precision than before. Using Bifrost, we have analysed a simulated QS heating event that generates coronal temperatures up to 1.47 MK and is caused by the reconnection of a magnetic arcade and twisted flux rope with an overlying, nearly anti-parallel horizontal field in the corona. Understanding the magnetic topology and field evolution of this event have been the main goals of our fundamental studies, but we move forward now to synthetic observables in order to establish an observational context for this type of event. Synthetic observables of the simulated reconnection event reveal strong signals in SiIV, FeIX and FeXII; all of which are observable with IRIS and/or MUSE among other instruments such as SDO AIA, Hi-C, and the EUI onboard the Solar Orbiter. First results indicate strong emissions during the reconnection event as expected, with characteristics consistent with magnetic braiding and fast jets emanating from the reconnection site. We present this simulation as a case study for QS reconnection and introduce preliminary comparison studies between synthetic and actual observables, providing a baseline for future collaborations and studies on QS activity.
The solar coronal mass ejection (CME) is a global phenomenon that not only disrupts the solar atmosphere but also leads to hazardous space weather events when propagating through the heliosphere. The forecast capability of the CME impacts depends critically on our understanding about the plasma environment of the CME source region, and the physical processes involved when CME interacts with the ambient corona and solar wind. The upcoming MUSE mission with its large FOV and high temporal/spatial resolution will provide crucial measurements on these topics therefore shed new light on the evolution and propagation of CMEs and their effects on the surrounding corona. In this talk, I would like to highlight several aspects of CME impact in the form of global EUV waves, sympathetic eruptions, coronal dimmings, CME-driven shocks, and solar energetic particles (SEPs), as well as the current global modeling efforts and challenges. More importantly, I would like to discuss the unique measurements provided by MUSE, when combined with existing remote-sensing and in-situ observations (e.g., SDO, Solar Orbiter, Parker Solar Probe), will significantly improve the data-constrained CME modeling, which will eventually lead to a better space weather forecast capability.
The source regions of the solar wind, and their drivers and acceleration mechanisms, remain key topics of study in heliophysics with many open questions. One of the major challenges is to connect heliospheric measurements of the solar wind and solar energetic particles with possible source regions in the solar atmosphere, such as active region outflows and coronal holes, and there are now unprecedented opportunities with Parker Solar Probe (PSP) and Solar Orbiter (SO) in operation. There has been some recent success not only in connection science, but also in understanding the properties of these source regions using spectroscopic measurements from Hinode/EIS and IRIS, and high resolution imaging from Hi-C. I will give a brief overview of some of the recent developments, including results from PSP and SO/SPICE, and will outline some of the progress that can be expected from future high spatial and temporal resolution imaging spectroscopy from MUSE and Solar-C_EUVST. Finally, I discuss some advances in supporting numerical modeling that would aid in the interpretation of the observations.
Magnetic Flux Ropes (MFRs) are free-energy-carrying, three-dimensional magnetized plasma structures characterized by twisted magnetic field lines and are widely considered the core structure of Coronal Mass Ejections (CMEs) propagating in the interplanetary space. The way MFRs form remains unclear as different theories predict that either MFRs form during the initiation of the CME or pre-exist the onset of the CME. The term "pre-existing structure" is synonymous with "filament channels." On the one hand, the theories predicting on-the-fly MFR formation require Sheared Magnetic Arcades (SMAs; low twist but stressed magnetic structures) for the filament channel/pre-existing magnetic structure of CMEs. On the other hand, a growing number of works using SDO/AIA observations (combined with non-linear force-free extrapolations; NLFFF) suggest that MFRs may be the form of filament channels, therefore pre-existing the CME eruption. However, due to the inability to routinely measure the 3D magnetic field in the solar atmosphere, we cannot unambiguously interpret optical and EUV imaging observations as projected on the plane of the sky. Therefore, a raging debate on the nature of the pre-eruptive structure continues. It is also possible that the filament channel/pre-eruptive structure evolves from SMA to MFR slowly, further complicating the distinction between these two types of structures in the solar observations. This work presents realistic simulated EUV observations synthesized on a time-evolving radiative MURaM MHD model along the slow evolution of an SMA converting to an MFR. We discuss the implications of our results in the context of filament channel formation and CME initiation theory.
I will present recent additions to -- and applications of -- our open-source MPI-AMRVAC software (http://amrvac.org), designed to solve generic partial differential equations on any-dimensional, block grid-adaptive mesh hierarchies [2018, ApJS 234, 30 ; 2021, CaMWa 81, 316]. The MPI-AMRVAC 3.0 release is ready to go, and features various modules of direct interest to solar physicists, such as a novel plasma-neutral module to investigate solar chromospheric dynamics [2022, A&A 664, A55], or functionality for time-dependent data-driven applications [2021, ApJ 919, 39]. The ERC-funded project PROMINENT sets forth to study the `coolest' part of the million-degree solar atmosphere: the prominence condensations formed by thermal instabilities. These represent state-of-the-art magnetohydrodynamic simulations, where the process of runaway condensations due to radiative losses is studied in unprecedented detail [2022, NatAstro 6, 942]. Our MPI-AMRVAC simulation toolkit shows that grid-adaptivity is essential to zoom in on details that may be resolved by future observing facilities. I will present an overview of ongoing and planned research activities to unravel the intricate multi-phase structure of the solar corona.
A major coronal heating theory based on magnetic reconnection relies on the existence of braided magnetic field structures in the corona, where numerical simulations of stress-induced reconnection in braided loop-like structures have shown to invariably lead to low-amplitude transverse MHD waves. In this small-angle reconnection scenario, the reconnected magnetic field lines are driven sideways by magnetic tension but overshoots from their new rest position; thereby leading to transverse waves. This provides an efficient mechanism for transverse MHD wave generation in the corona, and also constitutes substantial direct evidence of reconnection from braiding. However, this wave-generation mechanism has never been directly observed. For the signature of small-angle reconnection, this has been identified through the recent discovery of nanojets. Nanojets are small, short-lived and fast jet-like bursts in the nanoflare range transverse to the guide-field. As for the waves, magnetic tension has been invoked to explain their characteristic transverse directionality. We present for the first time IRIS and SDO observations of transverse MHD waves in a coronal loop that directly results from braiding-induced reconnection identified by the presence of nanojets. This discovery provides major support to existing theories that transverse MHD waves can be a signature of reconnection and the coronal reconnection scenario identified by nanojets. Additionally, we will also review the latest observations from the IRIS nanojet observing programme, which suggests that this phenomenon is more common than expected and that the reconnection process has an energy flux on the same order as the necessary AR energy balance requirements.
Data-constrained magnetohydrodynamics (MHD) simulations initialised with magnetic field extrapolations based on photospheric magnetograms have been quite successful in capturing many aspects of energetic events in the solar corona, like flare reconnections and coronal mass ejections. On the other hand, radiative-MHD codes like Bifrost initialised with analytical inputs have provided very realistic simulations of the solar atmosphere by including the physics of non-equilibrium states and radiative transfer. This work aims at setting up data-constrained simulations with the Bifrost code to study energetic events in the solar corona. As a first step, we perform MHD simulations using the Bifrost code, with the bottom boundary set at the photosphere. This allows us to take high-resolution photospheric magnetograms from the Swedish Solar Telescope (SST) directly as input for the lower boundary. To fine-tune the code parameters, we perform two sets of simulations initialised with: (a) analytical non-force-free-field (NFFF) input having a sheared-arcade geometry, which is known to produce magnetic flux rope through reconnections in other MHD simulations, (b) magnetic field obtained from NFFF extrapolations based on a photospheric magnetogram observed from SST having a dipolar geometry. We show that in the analytical case we can reproduce the flux rope formation, while the NFFF-initiated case shows a self-consistent evolution which is comparable to the SST chromospheric observations. We thus conclude that NFFF-initiated MHD simulations based on photospheric magnetograms can be very helpful in understanding coronal dynamics and needs to be developed further.
At restaurant Huset in Longyearbyen. About 2.2 km from the hotel. Bus transport will be provided from the hotel to the restaurant, but NOT for the return journey.
Understanding the solar atmosphere, which connects to the heliosphere via radiation, the solar wind and coronal mass ejections, and energetic particles is pivotal for establishing the conditions for life and habitability in the solar system. SOLAR-C (EUVST) (EUV High-Throughput Spectroscopic Telescope) is designed to comprehensively understand the energy and mass transfer from the solar surface to the solar corona and interplanetary space, and to investigate the elementary processes that take place universally in cosmic plasmas. In order to interpret the observation results obtained by Solar-C and to understand the physics process in more detail, comparison with advanced numerical simulations is considered very important. The SOLAR-C team plans to work systematically to facilitate comparisons with numerical simulations.
Numerical simulation efforts have some issues such as radiative transfer. In this talk we will discuss the modeling results of non-equilibrium ionization during solar flares and discuss the importance of non-equilibrium ionization in the interpretation of SOLAR-C observations. In particular, when estimating physical quantities in the magnetic reconnection region, we have assumed the ionization equilibrium so far. Since SOLAR-C can observe various emission lines at the same time, it is expected that analysis can be performed without assuming ionization equilibrium. We believe that these models are useful not only for interpreting solar observations, but also for interpreting other astronomical remote sensing observations.
The 3D extensions to the Standard model of solar flares have been succcessfull in explaining various observed phenomena. Among them, there are (1) hot cores (sigmoids), (2) apparent slipping motion of flare looops, (3) saddle-shaped flare arcades, as well as (4) reconnection of the drifting flux rope with the surrounding corona or itself during the eruption. We review the properties of the 3D reconnection geometries and focus on predicted future observables in the hot flare plasma and its dynamics, especially those that the present instrumentation is insufficient to capture.
During solar flares, the impulsive release of magnetic energy drives plasma heating, fast flows, and intense brightening across the spectrum. Current models of solar flares are able to accurately reproduce many key observables, including the speed of chromospheric evaporation flows, plasma densities and atomic line intensities. However, after the cessation of impulsive heating, the models predict time scales for slowing flows and cooling to quiescence that are an order of magnitude faster than observed. Such a discrepancy indicates missing ingredients in the models. As shown in recent work, turbulence suppresses thermal conduction and is a likely candidate for explaining long duration cooling times. Even more, turbulent velocities produce broadened atomic line profiles, which have been observed in numerous flares. Here we report on our recent work modeling the response of the solar atmosphere to flare heating including turbulent thermal conduction suppression and its effects on atomic line profiles. Comparing model predictions with observations of a C-class flare, we find a moderate amount of turbulence best reproduces observed velocities and line widths.
To understand the trigger of solar flares and eruptions it is necessary to obtain an accurate description of the 3D pre-eruptive coronal magnetic configuration. The latter is not directly observable and one must rely either on static modelling/extrapolation from 2D photospheric measurements, and/or on relatively idealized time-evolution of a magnetic model from numerical simulations. The limitations of both approaches has not permitted to produce magnetic models sufficiently reliable to solve the eruption trigger issue.
Meanwhile, flares are characterized by brightenings in the Ultraviolet and X-ray domains. The distribution of these brightenings is not random, and are, according to the standard model for flares, tightly related to the magnetic field structure. Such a link has received strong confirmations by studies of the magnetic topology, which positively correlated the spatial distribution of UV and X-ray emission with magnetic structures.
Based on a detailed study of a confined circular flare, we will show how the flare emissions can provide a critical information in order to properly model the magnetic configuration and how it can help to have an insight on the trigger mechanism of flare. We will address how critical the measure of vertical electric currents is for the proper modelling of the magnetic field. We will finally discuss the hope that new high-resolution instruments as well as new observation strategies, and in particular stereoscopic magnetic field measurement with SolO/PHI, will provide key information such as fine flare dynamics and next-generation current distribution maps, for advance model of the eruptive magnetic system.
Solar flares are transient yet dramatic events in the atmospheres of the Sun, during which vast amounts of magnetic energy is liberated. This energy is subsequently transported through the solar atmosphere or into the heliosphere, and together with coronal mass ejections flares comprise a fundamental component of space weather. Thus, understanding the physical processes at play in flares is vital. That understanding often requires the use of forward modeling in order to predict the
hydrodynamic and radiative response of the solar atmosphere. Those predictions must then be critiqued by observations to show
us where our models are missing ingredients. While flares are of course 3D phenomenon, simulating the flaring atmosphere including an accurate chromosphere with the required spatial scales in 3D is largely beyond current computational capabilities, and certainly performing parameter studies of energy transport mechanisms is not yet tractable in 3D. Therefore, field-aligned 1D loop models that can resolve the relevant scales have a crucial role to play in advancing our knowledge of flares. In recent years flare loop models have revealed many interesting features of flares. For this review I highlight some important results that illustrate the utility of attacking the problem of solar flares with a combination of high quality observations, and state-oft-the-art flare models, demonstrating: (1) how models help to interpret flare observations, (2) how those observations show us where we are missing physics from our models, and (3) how the ever increasing quality of solar observations drives model improvements.
Solar flares are amongst the most energetic events in our solar system. Accompanied by intense UV and X-ray emissions, energetic particles and coronal mass ejections can be injected into the interplanetary medium during flares. As these various aspects can have a large impact on solar system bodies and a detrimental effect on human activities, there is a strong interest to gain a deeper knowledge of these events.
Over the past decades, ground and space solar observatories and the variety of observations available (from imaging to plasma and particle diagnostics and magnetic field measurements), aided by numerical modelling and theory, have helped us refine a standard model for eruptive flares. At the core of such a model lies the dynamical evolution of magnetic fields that shape and power them. In particular, understanding how flux ropes become unstable, where and how reconnection takes place (especially in complex 3D structures), has shed some light in the understanding of flare processes at large.
Because of the intrinsically transient nature of solar flares and the energy range covered by such events, some questions are still pending: where and how is the energy deposited, how the non local magnetic field topology actual shapes eruptions, how to join the various aspects of flares, from particles to CMEs? This talk will aim at looking at how previous works on flare models are still challenged by new observations, and what are the steps that are needed to continue working towards a (more) complete understanding of solar flares.
Solar coronal jets are observed as collimated plasma flows with high velocity along magnetic field lines in a wide wavelength range, from X-rays to EUV. Occasionally these hot jets are closely related to cool surges, which are chromospheric ejections that emerge in the form of unwrinkled threads. Though these phenomena have been studied over the past few decades with different instruments and models, their physical origin is still actively debated. To have a deeper understanding of the origin and driving of solar coronal jets, we analyze several jet and surge events with very high-resolution observations from the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS). Deeper physical understanding is developed with the aid of radiative MHD models using the Bifrost code. The different data sets constrain the numerical models as they provide details on different aspects of the origin and propagation of jets at different heights from the photosphere to the corona. It is anticipated that the unprecedented capabilities of MUSE will provide new insights in the physics of solar coronal jets.
The overall paradigm of flare energy release is well-known. An energy-bearing coronal magnetic field relaxes via magnetic reconnection to a lower energy state, and the energy released is converted and dissipated in the radiation flash that is a solar flare. But what does that energy conversion and energy dissipation involve? There are strong and long-standing pointers to an important role for heating by non-thermal particles, but also observational hints that waves and turbulence have important roles. Furthermore, how is the energy release into the closed field of the lower corona - resulting in the flare - connected to the energy release into the wider corona that can result in a CME? Diagnosing waves and turbulence requires spectroscopic information, flares require high cadence particularly in their earliest phases, and probing the link to the early evolution of CMEs requires observation over a large field of view. MUSE can provide all of these in the EUV and, particularly when employed together with other facilities such as Solar-C and new ground-based observatories, is certain to fill many of the gaps in our understanding.
Magnetic reconnection governing energy release in solar flares takes place in the corona; the lower atmosphere responds rapidly to energy transfer from the corona, generating prominent radiation and dynamic signatures that help us infer properties of energy release and transfer. Fine-scale structures embedded in the generally curvilinear-shaped flare ribbons indicate the global organization of patchy reconnection events, each of them manifesting a packet of energy release. Recent observations have revealed some intriguing evolution timescales of the radiation and dynamic signatures on flare ribbons, and efforts have been made to quantify the amount of energy released over these timescales in the packets of spatial scales up to the instruments' resolving capability. It has not been clear what determines these timescales, what is the magnetic structure of these packets, and what physical mechanisms convert free magnetic energy to particle or plasma energies during or after magnetic reconnection. Crucial connections between the coronal and chromospheric signatures can be clarified in future observations and advanced numerical models.
According to our current understanding, solar flares are driven by magnetic energy stored in the solar corona being rapidly released through a process involving magnetic reconnection. This scenario was originally proposed on the basis of classic observations including radio and hard X-ray emission from non-thermal electrons accompanying rising emission from hot thermal plasma. Over the past decade complementary observations have offered novel ways to constrain, quantify, and model, flare reconnection and the energy conversion it initiates. High-resolution studies of chromospheric flare ribbons, including their downward velocity (condensation), allow inference of the structure and evolution of coronal reconnection. Multi-band EUV imaging of the high-density, high-temperature plasma sheet formed around the current sheet shows the degree of heating and plasma compression which must accompany reconnection. Similar imaging shows flux tubes retracting through the sheets (SADs), pointing to the location and patchy nature of that reconnection. These myriad, novel observational constraints can be accommodated by a theoretical model in which magnetic energy is released as flux tubes retract through the current sheet following their creation by localized reconnection episodes. The measured, global rate of reconnection reflects their rate of production rather than the local electric field within one episode. Global flare properties, comparable to observation, can be reproduced by convolving the response to a single retraction with that production rate.
Coronal mass ejections (CMEs) are the largest scale eruptions of plasmas in the solar corona. Many observations show that pre-eruptive CMEs always appear as bright structures in EUV high-temperature bands and rise slowly when approaching the onset of their eruption. However, the mechanisms behind these phenomena are still puzzling. In this work, we aim to explore these by combining observations and numerical simulations. Based on the observation of an eruptive event, we find that a moderate magnetic reconnection, evidenced by the weak thermal-dominated hard X-ray emission, occurs at the center of an X-shaped plasma sheet before the eruption. This reconnection forms the hot M-shaped threads and cusp-shaped loops, the former of which merges with the pre-eruptive CME and contributes to its heating and slow rise. More details of the heating and the slow rise phenomena are revealed by performing a thermal 3D MHD simulation of a pre-eruptive CME by MPI-AMRVAC. It is shown that the Ohmic heating, which is related to a weak magnetic reconnection, mainly contributes to the heating in the hyperbolic flux tube (HFT) and quasi-separatrix layers wrapping around the pre-eruptive CME. The slow rise of the pre-eruptive CME is also mainly driven by the reconnection in the HFT. All of the above results give us a better understanding of how the pre-eruptive CME gradually transitions from the quasi-static state to the eruption.
We discuss the current state of MHD modeling of solar flares and eruptions. We focus on models that yield synthetic observables accessible to current and future generations of remote sensing capabilities, such as MUSE, EUVST and ground-based observatories. A critical assessment of the successes and limitations of current models with be presented, as well as suggestions for paths going forward.
Solar active regions are thought to be formed by the emergence of magnetic flux from the deep convection zone and, therefore, it is important to use a large computational domain covering the entire convection zone to understand the physics behind. However, the high acoustic speed makes it difficult to conduct magnetohydrodynamic simulations in such a deep domain. The R2D2 code overcomes this difficulty by implementing the reduced sound of speed technique, which allows us to conduct the simulations of active region formation in a much more realistic way. In this presentation, we will discuss how the large-scale convections in the deep layers affect the flux emergence process and contribute to the formation of complex-shaped active regions, which are prone to produce massive solar flares and coronal mass ejections.
Unresolved mass motions are frequently detected in flares from extreme ultraviolet (EUV) observations, which are often regarded as turbulence. Non-thermal broadening of EUV emission lines caused by turbulence can be found at the entire flare region including flare loop top, legs, footpoints and the region above the looptop. Peaks in non-thermal velocity values tend to show up above the high density flare loops, reaching 100-200 km s^−1, while footpoints have lower non-thermal velocities of a few tens km s^−1. However, how this turbulence forms during the flare is still largely a mystery. Using a three-dimensional (3D) magnetohydrodynamic (MHD) simulation, we demonstrate how turbulent motions widely distribute throughout a flaring region, and can originate from a single source. The turbulence forms as a result of an intricate non-linear interaction between the reconnection outflows and the magnetic arcades below the reconnection site, in which the shear- flow driven Kelvin-Helmholtz Instability (KHI) plays a key role for generating turbulent vortices. The turbulence is produced above high density flare loops, and then propagates to chromospheric footpoints along the magnetic field as Alfvénic perturbations. The simulated strength and spatial distribution of the volume-filling turbulent motions show excellent agreement with observational results as revealed by synthetic views in EUV and by fitted Hinode-EIS spectra.