Questions In Solar Physics Research Articles

On the Nature of Nonthermal Broadening of Spectral Lines Observed by IRIS

The origin of nonthermal broadening in solar spectra is one of the long-standing questions in solar physics. Various processes have been invoked—including unresolved flows, waves, and turbulent processes—but definitive answers are lacking. To investigate the physical processes responsible for nonthermal broadening, we examine its relation with the angle between the magnetic field and the line of sight in three different closed-field regions above plage regions at different locations on the solar disk. We obtained the nonthermal width of transition-region Si iv 1403 Å spectra observed in active regions by the Interface Region Imaging Spectrograph, after subtraction of the thermal and instrumental line broadening. To investigate the dependence of the measured broadening on the viewing angle between the line of sight and magnetic field direction, we determined the magnetic field direction at transition-region heights using nonlinear force-free extrapolations based on the observed photospheric vector magnetic field taken by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We found that the nonthermal broadening shows a correlation with downward motion (redshifts) and alignment between the magnetic field and the observer’s line-of-sight direction. Based on the observed correlations, we suggest that velocity gradients within plasma flowing down along the magnetic field may lead to a significant portion of the observed nonthermal broadening of transition-region spectral lines in closed fields above plage regions.

Chromospheric Heating from Local Magnetic Growth and Ambipolar Diffusion under Nonequilibrium Conditions

The heating of the chromosphere in internetwork regions remains one of the foremost open questions in solar physics. In the present study, we tackle this old problem by using a very-high-spatial-resolution simulation of quiet-Sun conditions performed with radiative MHD numerical models and interface region imaging spectrograph (IRIS) observations. We have expanded a previously existing 3D radiative MHD numerical model of the solar atmosphere, which included self-consistently locally driven magnetic amplification in the chromosphere, by adding ambipolar diffusion and time-dependent nonequilibrium hydrogen ionization to the model. The energy of the magnetic field is dissipated in the upper chromosphere, providing a large temperature increase due to ambipolar diffusion and nonequilibrium ionization (NEQI). At the same time, we find that adding the ambipolar diffusion and NEQI in the simulation has a minor impact on the local growth of the magnetic field in the lower chromosphere and its dynamics. Our comparison between synthesized Mg ii profiles from these high-spatial-resolution models, with and without ambipolar diffusion and NEQI, and quiet-Sun and coronal hole observations from IRIS now reveal a slightly better correspondence. The intensity of profiles is increased, and the line cores are slightly broader when ambipolar diffusion and NEQI effects are included. Therefore, the Mg ii profiles are closer to those observed than in previous models, though some differences still remain.

(When) Can Wave Heating Balance Optically Thin Radiative Losses in the Corona?

Why the atmosphere of the Sun is orders of magnitudes hotter than its surface is a long standing question in solar physics. Over the years, many studies have looked at the potential role of magnetohydrodynamic (MHD) waves in sustaining these high temperatures. In this study, we use 3D MHD simulations to investigate (driven) transverse waves in a coronal loop. As the boundary-driven transverse waves propagate along the flux tube, the radial density profile leads to resonant absorption (or mode coupling) and phase mixing in the boundaries of the flux tube and the large velocity shears are subject to the Kelvin–Helmholtz instability (KHI). The combination of these effects leads to enhanced energy dissipation and wave heating. Considering both resonant and nonresonant boundary driving as well as different densities for the flux tube, we show that only wave heating associated with a resonant driver in a lower-density loop (with a loop core density ∼5 × 10−13 kg m−3) is able to balance radiative losses in the loop shell. Changing the model parameters to consider a denser loop or a driver with a nonresonant frequency, or both, leads to cooling of the coronal loop as the energy losses are greater than the energy injection and dissipation rates.

Jack B. Zirker (1927–2022)

Zirker explored key questions in solar physics — the solar dynamo, active region formation, coronal heating, the solar cycle, coronal holes, and the generation of the solar wind. He was instrumental in the creation of the National Solar Observatory.

Instrumentation for solar spectropolarimetry: state of the art and prospects

Given its unchallenged capabilities in terms of sensitivity and spatial resolution, the combination of imaging spectropolarimetry and numeric Stokes inversion represents the dominant technique currently used to remotely sense the physical properties of the solar atmosphere and, in particular, its important driving magnetic field. Solar magnetism manifests itself in a wide range of spatial, temporal, and energetic scales. The ubiquitous but relatively small and weak fields of the so-called quiet Sun are believed today to be crucial for answering many open questions in solar physics, some of which have substantial practical relevance due to the strong Sun-Earth connection. However, such fields are very challenging to detect because they require spectropolarimetric measurements with high spatial (sub-arcsec), spectral (<100 mA), and temporal (<10 s) resolution along with high polarimetric sensitivity (<0.001 of the intensity). We collect and discuss both well-established and upcoming instrumental solutions developed during the last decades to push solar observations toward the above-mentioned parameter regime. This typically involves design trade-offs due to the high dimensionality of the data and signal-to-noise-ratio considerations, among others. We focus on the main three components that form a spectro-polarimeter, namely, wavelength discriminators, the devices employed to encode the incoming polarization state into intensity images (polarization modulators), and the sensor technologies used to register them. We consider the instrumental solutions introduced to perform this kind of measurements at different optical wavelengths and from various observing locations, i.e., ground-based, from the stratosphere or near space.

SDO/HMI observations of the average supergranule are not compatible with separable flow models

Aims. Despite extensive studies carried out since its discovery half a century ago, the nature of supergranulation remains an open question in solar physics. Separability of flow models is a common assumption made in the literature to shed light on the properties of supergranules. This paper studies the ability of separable mass-conserving flow models to reproduce photospheric observations from the Helioseismic Magnetic Imager (HMI) on board of the Solar Dynamics Observatory (SDO) spacecraft corresponding to an average supergranule. Methods. For a steady mass-conserving separable flow model to be compatible with the observations, there is an integral relation between the horizontal and vertical components of the flow. We test this relation directly on observations and compare the results with the proportionality relationship for a separable model. Results. Observations of an average supergranule do not satisfy the condition for separability. Selecting a narrower range of horizontal scales of supergranules when performing the average does not change this result. Separable models are not consistent with observations of an average supergranule.

Modelling of proton acceleration in application to a ground level enhancement

Context. The source of high-energy protons (above ~500 MeV) responsible for ground level enhancements (GLEs) remains an open question in solar physics. One of the candidates is a shock wave driven by a coronal mass ejection, which is thought to accelerate particles via diffusive-shock acceleration. Aims. We perform physics-based simulations of proton acceleration using information on the shock and ambient plasma parameters derived from the observation of a real GLE event. We analyse the simulation results to find out which of the parameters are significant in controlling the acceleration efficiency and to get a better understanding of the conditions under which the shock can produce relativistic protons. Methods. We use the results of the recently developed technique to determine the shock and ambient plasma parameters, applied to the 17 May 2012 GLE event, and carry out proton acceleration simulations with the Coronal Shock Acceleration (CSA) model. Results. We performed proton acceleration simulations for nine individual magnetic field lines characterised by various plasma conditions. Analysis of the simulation results shows that the acceleration efficiency of the shock, i.e. its ability to accelerate particles to high energies, tends to be higher for those shock portions that are characterised by higher values of the scattering-centre compression ratio rc and/or the fast-mode Mach number MFM. At the same time, the acceleration efficiency can be strengthened by enhanced plasma density in the corresponding flux tube. The simulations show that protons can be accelerated to GLE energies in the shock portions characterised by the highest values of rc. Analysis of the delays between the flare onset and the production times of protons of 1 GV rigidity for different field lines in our simulations, and a subsequent comparison of those with the observed values indicate a possibility that quasi-perpendicular portions of the shock play the main role in producing relativistic protons.

Chromospheric Heating due to Cancellation of Quiet Sun Internetwork Fields

Abstract The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by the magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global contribution to the chromosphere may be significant due to their ubiquitous presence in quiet Sun regions. In this paper, we study cancellations of IN elements and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope (SST) to identify cancellations of IN magnetic flux patches and follow their evolution. We find that, on average, these events live for ∼3 minutes in the photosphere and ∼12 minutes in the chromosphere and/or transition region. Employing multi-line inversions of the Mg ii h and k lines, we show that cancellations produce clear signatures of heating in the upper atmospheric layers. However, at the resolution and sensitivity accessible to the SST, their number density still seems to be one order of magnitude too low to explain the global chromospheric heating.

The NWRA Classification Infrastructure: description and extension to the Discriminant Analysis Flare Forecasting System (DAFFS)

A classification infrastructure built upon Discriminant Analysis (DA) has been developed at NorthWest Research Associates for examining the statistical differences between samples of two known populations. Originating to examine the physical differences between flare-quiet and flare-imminent solar active regions, we describe herein some details of the infrastructure including: parametrization of large datasets, schemes for handling “null” and “bad” data in multi-parameter analysis, application of non-parametric multi-dimensional DA, an extension through Bayes’ theorem to probabilistic classification, and methods invoked for evaluating classifier success. The classifier infrastructure is applicable to a wide range of scientific questions in solar physics. We demonstrate its application to the question of distinguishing flare-imminent from flare-quiet solar active regions, updating results from the original publications that were based on different data and much smaller sample sizes. Finally, as a demonstration of “Research to Operations” efforts in the space-weather forecasting context, we present the Discriminant Analysis Flare Forecasting System (DAFFS), a near-real-time operationally-running solar flare forecasting tool that was developed from the research-directed infrastructure.

The Direct Effect of Toroidal Magnetic Fields on Stellar Oscillations: An Analytical Expression for the General Matrix Element

Abstract Where is the solar dynamo located and what is its modus operandi? These are still open questions in solar physics. Helio- and asteroseismology can help answer them by enabling us to study solar and stellar internal structures through global oscillations. The properties of solar and stellar acoustic modes are changing with the level of magnetic activity. However, until now, the inference on subsurface magnetic fields with seismic measures has been very limited. The aim of this paper is to develop a formalism to calculate the effect of large-scale toroidal magnetic fields on solar and stellar global oscillation eigenfunctions and eigenfrequencies. If the Lorentz force is added to the equilibrium equation of motion, stellar eigenmodes can couple. In quasi-degenerate perturbation theory, this coupling, also known as the direct effect, can be quantified by the general matrix element. We present the analytical expression of the matrix element for a superposition of subsurface zonal toroidal magnetic field configurations. The matrix element is important for forward calculations of perturbed solar and stellar eigenfunctions and frequency perturbations. The results presented here will help to ascertain solar and stellar large-scale subsurface magnetic fields, and their geometric configuration, strength, and change over the course of activity cycles.

Slow Solar Wind: Observations and Modeling

While it is certain that the fast solar wind originates from coronal holes, where and how the slow solar wind (SSW) is formed remains an outstanding question in solar physics even in the post-SOHO era. The quest for the SSW origin forms a major objective for the planned future missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless, results from spacecraft data, combined with theoretical modeling, have helped to investigate many aspects of the SSW. Fundamental physical properties of the coronal plasma have been derived from spectroscopic and imaging remote-sensing data and in situ data, and these results have provided crucial insights for a deeper understanding of the origin and acceleration of the SSW. Advanced models of the SSW in coronal streamers and other structures have been developed using 3D MHD and multi-fluid equations.

High-resolution, high-sensitivity, ground-based solar spectropolarimetry with a new fast imaging polarimeter

Context. Remote sensing of weak and small-scale solar magnetic fields is of utmost relevance for a number of important open questions in solar physics. This requires the acquisition of spectropolarimetric data with high spatial resolution (0.1 arcsec) and low noise (1e-3 to 1e-5 of the continuum intensity). The main limitations to obtain these measurements from the ground, are the degradation of the image resolution produced by atmospheric seeing and the seeing-induced crosstalk (SIC). Aims. We introduce the prototype of the Fast Solar Polarimeter (FSP), a new ground-based, high-cadence polarimeter that tackles the above-mentioned limitations by producing data that are optimally suited for the application of post-facto image restoration, and by operating at a modulation frequency of 100 Hz to reduce SIC. Results. The pnCCD camera reaches 400 fps while keeping a high duty cycle (98.6 %) and very low noise (4.94 erms). The modulator is optimized to have high (> 80%) total polarimetric efficiency in the visible spectral range. This allows FSP to acquire 100 photon-noise-limited, full-Stokes measurements per second. We found that the seeing induced signals present in narrow-band, non-modulated, quiet-sun measurements are (a) lower than the noise (7e-5) after integrating 7.66 min, (b) lower than the noise (2.3e-4) after integrating 1.16 min and (c) slightly above the noise (4e-3) after restoring case (b) by means of a multi-object multi-frame blind deconvolution. In addition, we demonstrate that by using only narrow-band images (with low SNR of 13.9) of an active region, we can obtain one complete set of high-quality restored measurements about every 2 s.

Solar Science with the Atacama Large Millimeter/Submillimeter Array—A New View of Our Sun

The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere - a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases.

Energetic electrons generated during solar flares

The Sun is a giant particle accelerator. During solar flares, magnetic field energy stored in the corona is suddenly released and transferred to local heating of the coronal plasma, mass motions (e.g. jets) and the generation of energetic particles, i.e. electrons, protons and heavy ions. Basically, a flare occurs as a local enhancement of the emission of electromagnetic radiation from the radio up to the ${\it\gamma}$-ray range on the Sun. That indicates the production of energetic electrons during flares. NASA’s RHESSI mission has the aim to investigate electron acceleration processes by studying the Sun’s X-ray and ${\it\gamma}$-ray emission with high spatial, temporal and spectral resolution, i.e. by means of imaging spectroscopy. A substantial part of the energy released during a flare is carried by these energetic electrons. Apart from them, solar energetic particles, i.e. protons and heavy ions, and coronal mass ejections play an important role in the energy budget of a flare. Here, we focus on electron acceleration. The way in which $10^{36}$ electrons are accelerated up to energies beyond 30 keV is one of the open questions in solar physics. A flare is considered as the manifestation of magnetic reconnection in the solar corona. Which mechanisms lead to the production of energetic electrons in the magnetic reconnection region is discussed in this paper. Two of them are described in more detail.

THE ORIGIN OF NET ELECTRIC CURRENTS IN SOLAR ACTIVE REGIONS

There is a recurring question in solar physics about whether or not electric currents are neutralized in active regions (ARs). This question was recently revisited using three-dimensional (3D) magnetohydrodynamic (MHD) numerical simulations of magnetic flux emergence into the solar atmosphere. Such simulations showed that flux emergence can generate a substantial net current in ARs. Another source of AR currents are photospheric horizontal flows. Our aim is to determine the conditions for the occurrence of net vs. neutralized currents with this second mechanism. Using 3D MHD simulations, we systematically impose line-tied, quasi-static, photospheric twisting and shearing motions to a bipolar potential magnetic field. We find that such flows: (1) produce both {\it direct} and {\it return} currents, (2) induce very weak compression currents - not observed in 2.5D - in the ambient field present in the close vicinity of the current-carrying field, and (3) can generate force-free magnetic fields with a net current. We demonstrate that neutralized currents are in general produced only in the absence of magnetic shear at the photospheric polarity inversion line - a special condition rarely observed. We conclude that, as magnetic flux emergence, photospheric flows can build up net currents in the solar atmosphere, in agreement with recent observations. These results thus provide support for eruption models based on pre-eruption magnetic fields possessing a net coronal current.

Eyeing the Sun. Probing the solar interface region. Introduction.

![Figure][1] NASA/GSFC/CI LAB The Sun has been the subject of human curiosity since the dawn of time. It provides the energy that makes Earth habitable and is also the closest star to Earth. The Sun thus acts as a laboratory that provides detailed views of physical processes that occur in other, much more distant, astrophysical objects. Much progress has been made in understanding how nuclear fusion powers the Sun's 15-million-degree core and the mechanisms that transport this energy to the visible surface, where most of the light that reaches Earth is released. However, major unresolved questions linger about how the heliosphere, the Sun's outer atmosphere in which we live, is shaped and powered. We do not understand the counterintuitive rise of temperature from the 6000-K surface to millions of degrees in the Sun's outer atmosphere or corona. Equally puzzling is the solar wind, a high-speed continuous stream of particles that permeates space around Earth. These are not academic problems: Violent explosions such as flares and coronal mass ejections cause bouts of bad space weather that threaten power grids, satellites, and astronauts. These eruptions originate in and travel through this poorly understood solar atmosphere and wind. An important step in our quest to better understand such violent events is then to explore what drives the quiescent state of the solar atmosphere. In June 2013, NASA launched the Interface Region Imaging Spectrograph (IRIS), an Earth-orbiting small explorer satellite with a 20-cm telescope onboard. IRIS uses gratings to split the Sun's near- and far-ultraviolet light into its constituent wavelengths, in order to remotely probe the physical conditions in the interface region that consists of the chromosphere and transition region. Recent research suggested that it is here, at the interface between surface and corona, that answers to some of the more vexing unresolved questions in solar physics might be found. In this special section of Science , five Reports exploit the high-resolution images and spectra obtained with IRIS to present major advances toward a comprehensive understanding of how the solar atmosphere is energized ([sciencemag.org/special/iris][2]). Testa et al. find compelling evidence for the presence of high-energy particles generated during coronal nanoflares, small-scale heating events long hypothesized to drive coronal heating through the release of energy when magnetic field lines reconnect. These results provide constraints for models of the poorly understood mechanism that accelerates these electrons to such high energies and that probably acts under many other astrophysical conditions. Hansteen et al. reveal the presence of small-scale magnetic loops in high-resolution images of IRIS and advanced three-dimensional numerical models, resolving a long-standing debate about the nature of the transition region emission. These results vindicate the view that much of this emission does not originate in the “classical” transition region between the surface and the hot loops. Rather, the emission occurs in “unresolved fine structure” that has now been spatially resolved, thereby removing a major impediment to the modeling of coronal loops. Peter et al. exploit the power of high-resolution spectroscopy to reveal a solar atmosphere turned upside down: Hot plasma at 100,000 K is found closer to the solar surface than previously imagined, sandwiched by cool plasma both below and above. The hot plasma is heated by “bombs” in which the reconnection of magnetic fields leads to rapid heating. These unexpected results will likely lead to a reassessment of other phenomena in the low solar atmosphere, such as the mysterious Ellerman bombs discovered almost a century ago. De Pontieu et al. describe a chromosphere that is replete with twisting motions on very small scales that are associated with the heating of plasma to transition region temperatures. They are the signature of propagating Alfven wave pulses and provide support for recently developed models of atmospheric heating and dynamics and insight into the transport of helicity in the solar atmosphere. Tian et al. find evidence of high-speed jets at the root of the solar wind, fountains of plasma that appear to undergo rapid heating from chromospheric to transition region temperatures. These observations provide support for recent suggestions that the solar wind does not necessarily originate only from gentle evaporation in funnels rooted in strong field regions. Together, these results provide critical pieces in the still-unsolved puzzle of fully understanding of how the Sun shapes and affects the heliosphere. With solar activity at high levels, more advances from the imaging spectrograph onboard IRIS can be expected, especially with respect to flares and coronal mass ejections. [1]: pending:yes [2]: http://sciencemag.org/special/iris

INVESTIGATING THE DIFFERENTIAL EMISSION MEASURE AND ENERGETICS OF MICROFLARES WITH COMBINEDSDO/AIA ANDRHESSIOBSERVATIONS

An important question in solar physics is whether solar microflares, the smallest currently observable flare events in X-rays, possess the same energetic properties as large flares. Recent surveys have suggested that microflares may be less efficient particle accelerators than large flares, and hence contribute less nonthermal energy, which may have implications for coronal heating mechanisms. We therefore explore the energetic properties of microflares by combining Extreme Ultraviolet (EUV) and X-ray measurements. We present forward-fitting differential emission measure (DEM) analysis of 10 microflares. The fitting is constrained by combining, for the first time, high temperature RHESSI observations and flux data from SDO/AIA. Two fitting models are tested for the DEM; a Gaussian distribution and a uniform DEM profile. A Gaussian fit proved unable to explain the observations for any of the studied microflares. However, 8 of 10 events studied were reasonably fit by a uniform DEM profile. Hence microflare plasma can be considered to be significantly multi-thermal, and may not be significantly peaked or contain resolvable fine structure, within the uncertainties of the observational instruments. The thermal and non-thermal energy is estimated for each microflare, comparing the energy budget with an isothermal plasma assumption. From the multithermal fits the minimum non-thermal energy content was found to average approximately 30% of the estimated thermal energy. By comparison, under an isothermal model the non-thermal and thermal energy estimates were generally comparable. Hence, multi-thermal plasma is an important consideration for solar microflares that substantially alters their thermal and non-thermal energy content.

Signatures of Slow Solar Wind Streams from Active Regions in the Inner Corona

The identification of solar-wind sources is an important question in solar physics. The existing solar-wind models (e.g., the Wang–Sheeley–Arge model) provide the approximate locations of the solar wind sources based on magnetic field extrapolations. It has been suggested recently that plasma outflows observed at the edges of active regions may be a source of the slow solar wind. To explore this we analyze an isolated active region (AR) adjacent to small coronal hole (CH) in July/August 2009. On 1 August, Hinode/EUV Imaging Spectrometer observations showed two compact outflow regions in the corona. Coronal rays were observed above the active-region coronal hole (ARCH) region on the eastern limb on 31 July by STEREO-A/EUVI and at the western limb on 7 August by CORONAS-Photon/TESIS telescopes. In both cases the coronal rays were co-aligned with open magnetic-field lines given by the potential field source surface model, which expanded into the streamer. The solar-wind parameters measured by STEREO-B, ACE, Wind, and STEREO-A confirmed the identification of the ARCH as a source region of the slow solar wind. The results of the study support the suggestion that coronal rays can represent signatures of outflows from ARs propagating in the inner corona along open field lines into the heliosphere.

COMPARISON BETWEEN OBSERVATION AND SIMULATION OF MAGNETIC FIELD CHANGES ASSOCIATED WITH FLARES

It has been a long-standing question in solar physics how magnetic field structure changes with eruptive events (e.g., flares and coronal mass ejections). In this Letter, we present the eruption-associated changes in the magnetic inclination angle, the horizontal component of magnetic field vectors, and the Lorentz force. The analysis is based on the observation of the X3.4 flare on 2006 December 13 and in comparison to the numerical simulation of Fan. Both observation and simulation show that (1) the magnetic inclination angle in the decayed peripheral penumbra increases, while that in the central area close to the flaring polarity inversion line (PIL) deceases after the flare; (2) the horizontal component of magnetic field increases at the lower altitude near the flaring PIL after the flare. The result suggests that the field lines at the flaring neutral line turn to more horizontal near the surface, that is in agreement with the prediction of Hudson et al.

Modelling shock drift acceleration of electrons at the reconnection outflow termination shock in solar flares

Context. The acceleration of electrons to nonthermal energies in solar flares is one of the main unsolved questions in solar physics. One possibility for producing these energetic electrons is acceleration at the reconnection outflow termination shock (TS). Aims. By comparing theoretical results with observations of nonthermal electrons, we determine whether shock drift acceleration (SDA) at the TS is a viable electron acceleration mechanism. Methods. We used radio observations to constrain the characteristics of the TS, and hard X-ray observations provided by RHESSI , INTEGRAL , and HXRS to obtain the characteristics of the injected electrons. Invoking relativistic shock-drift acceleration at the TS, we calculated electron flux spectra, which are then compared with the corresponding observational results from RHESSI . A parametric study of the model allows us to answer the question under which conditions the TS is a viable electron accelerator. Results. SDA at the TS is able to reproduce the required fluxes and kinetic power of nonthermal electrons in solar flares as long as there is significant heating of the outflow jet and a sufficiently large shock area. A prediction of the model is that the flux and power of injected electrons is higher than the values that are usually given by fitting observed spectra, since the low-energy cutoff is generally below 10 keV. The synthetic spectra are consistent with the observed spectral indices up to ≈ 100 keV. Beyond that, they soften too quickly. Possibly this is because we have not yet considered various additional effects, such as multiple reflections at the shock or some form of preacceleration. The observed relation between electron flux and spectral index is reproduced by the model, as well as the temporal evolution of the energetic electrons. We conclude that SDA at the TS is a viable electron acceleration mechanism that deserves further study.