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In the standard model of solar flares, a large-scale reconnection current sheet is postulated to be the central engine for powering the flare energy release 1 – 3 and accelerating particles 4 – 6 . However, where and how the energy release and particle acceleration occur remain unclear owing to the lack of measurements of the magnetic properties of the current sheet. Here we report the measurement of the spatially resolved magnetic field and flare-accelerated relativistic electrons along a current-sheet feature in a solar flare. The measured magnetic field profile shows a local maximum where the reconnecting field lines of opposite polarities closely approach each other, known as the reconnection X point. The measurements also reveal a local minimum near the bottom of the current sheet above the flare loop-top, referred to as a ‘magnetic bottle’. This spatial structure agrees with theoretical predictions 1 , 7 and numerical modelling results. A strong reconnection electric field of about 4,000 V m −1 is inferred near the X point. This location, however, shows a local depletion of microwave-emitting relativistic electrons. These electrons instead concentrate at or near the magnetic bottle structure, where more than 99% of them reside at each instant. Our observations suggest that the loop-top magnetic bottle is probably the primary site for accelerating and confining the relativistic electrons. Observations of the X8.2 solar flare, which happened on 2017 September 10, could spatially resolve the distribution of the energetic electrons along the reconnection current sheet. More than 99% of them are concentrated at the bottom of the current sheet, not at the reconnection X point.

Solar flares–the most powerful explosions in the solar system–are also efficient particle accelerators, capable of energizing a large number of charged particles to relativistic speeds. A termination shock is often invoked in the standard model of solar flares as a possible driver for particle acceleration, yet its existence and role have remained controversial. We present observations of a solar flare termination shock and trace its morphology and dynamics using high-cadence radio imaging spectroscopy. We show that a disruption of the shock coincides with an abrupt reduction of the energetic electron population. The observed properties of the shock are well reproduced by simulations. These results strongly suggest that a termination shock is responsible, at least in part, for accelerating energetic electrons in solar flares.

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides hard X-ray spectral observations with ≈ 1 keV resolution to study flare-accelerated ( > 10 keV ) electrons through their bremsstrahlung emission. Here we report on a statistical study of RHESSI flares with emission above 150 keV, focusing on the spectral shape at the hard X-ray peak. Spectral parameters are derived by fitting the photon spectrum with a broken power law and by the standard thick-target fit. Consistent with previous studies, the most common spectral shape of the photon spectrum (52 out of 65 events) is a double power law with a downward break (“knee”), with ten events showing a single power law and three events having an upward break (“ankle”). The spectral breaks occur typically around 55 keV and the difference of the spectral index above and below the break, γ 2 and γ 1 , is typically between 0.3 and 1. We show correlations between the downward break parameters. The most prominent correlation, with a rank order coefficient of ρ = 0.92 , is between the power-law indices above and below the break: γ 1 = ( 0.74 ± 0.04 ) γ 2 + ( 0.34 ± 0.14 ) . Applying a thick target fit to the photon spectrum, a similar correlation is also found for the flare-accelerated electron spectra with δ 1 = ( 0.85 ± 0.08 ) δ 2 − ( 0.3 ± 0.3 ) ( ρ = 0.67 ). Spectral breaks could be a property of the acceleration mechanism itself or they could be a secondary effect produced by particle transport or wave-particle interactions. Any theoretical models should be consistent with these correlations. In addition, we find that one upward and 23 (49%) downward breaks are consistent with nonuniform ionization within the thick target.

The processes that heat the solar and stellar coronae to several million kelvins, compared with the much cooler photosphere (5,800 K for the Sun), are still not well known 1 . One proposed mechanism is heating via a large number of small, unresolved, impulsive heating events called nanoflares 2 . Each event would heat and cool quickly, and the average effect would be a broad range of temperatures including a small amount of extremely hot plasma. However, detecting these faint, hot traces in the presence of brighter, cooler emission is observationally challenging. Here we present hard X-ray data from the second flight of the Focusing Optics X-ray Solar Imager (FOXSI-2), which detected emission above 7 keV from an active region of the Sun with no obvious individual X-ray flare emission. Through differential emission measure computations, we ascribe this emission to plasma heated above 10 MK, providing evidence for the existence of solar nanoflares. The quantitative evaluation of the hot plasma strongly constrains the coronal heating models. The authors detect hard X-ray emission produced by plasma heated at ≥10 million kelvin from a quiescent active region of the Sun, providing clear observational evidence of plasma heating by nanoflares, and hinting at their important role in coronal heating.

HXI on ASO-S and STIX onboard Solar Orbiter are the first simultaneously operating solar hard X-ray imaging spectrometers. ASO-S’s low Earth orbit and Solar Orbiter’s periodic displacement from the Sun–Earth line enables multi-viewpoint solar hard X-ray spectroscopic imaging analysis for the first time. Here, we demonstrate the potential of this new capability by reporting the first results of 3D triangulation of hard X-ray sources in the SOL2023-12-31T21:55 X5 flare. HXI and STIX observed the flare near the east limb with an observer separation angle of 18°. We triangulated the brightest regions within each source, which enabled us to characterise the large-scale hard X-ray geometry of the flare. The footpoints were found to be in the chromosphere within uncertainty, as expected, while the thermal looptop source was centred at an altitude of 15.1 ± 1 Mm. Given the footpoint separation, this implies a more elongated magnetic-loop structure than predicted by a semi-circular model. These results show the strong diagnostic power of joint HXI and STIX observations for understanding the 3D geometry of solar flares. We conclude by discussing the next steps required to fully exploit their potential.

We provide a mathematical description of the imaging concept of the Spectrometer/Telescope for Imaging X-rays (STIX) onboard Solar Orbiter . Specifically, we describe the STIX indirect-imaging technique, which is based on spatial modulation of the X-ray photon flux by means of tungsten grids, and we show that each of 30 STIX imaging sub-collimators measures a Fourier component of the flaring X-ray source corresponding to a specific two-dimensional angular frequency. We also provide details about the count-distribution model, which describes the relationship between the photon flux and the measured pixel counts. The derived imaging model is the fundamental starting point both for the interpretation of STIX data and for the description of the data-calibration process. Finally, we provide an overview of the algorithms implemented for the solution of the imaging problem and a comparison of the results obtained with these different methods in the case of the SOL2022-03-31T18 flaring event.

The Spectrometer/Telescope for Imaging X-rays (STIX) is one of six remote sensing instruments on-board Solar Orbiter. The telescope applies an indirect imaging technique that uses the measurement of 30 visibilities, i.e., angular Fourier components of the solar flare X-ray source. Hence, the imaging problem for STIX consists of the Fourier inversion of the data measured by the instrument. In this work, we show that the visibility amplitude and phase calibration of 24 out of 30 STIX sub-collimators has reached a satisfactory level for scientific data exploitation and that a set of imaging methods is able to provide the first hard X-ray images of solar flares from Solar Orbiter. Four visibility-based image reconstruction methods and one count-based are applied to calibrated STIX observations of six events with GOES class between C4 and M4 that occurred in May 2021. The resulting reconstructions are compared to those provided by an optimization algorithm used for fitting the amplitudes of STIX visibilities. We show that the five imaging methods produce results morphologically consistent with the ones provided by the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory (SDO/AIA) in UV wavelengths. The χ 2 values and the parameters of the reconstructed sources are comparable between methods, thus confirming their robustness.

The temperature reached by solar flares is a key parameter to understanding the physical process that causes the energy release. In this work, we analysed data from a Hinode Observing Programme that focused on high cadence measurement of the flaring plasma. This was carried out when the X-ray imager and spectrometer (STIX) on Solar Orbiter was observing. We analysed 3 small microflares, and determined their evolution and temperature. The temperature of the B2.8 microflare reached 16 MK. There was evidence in the smaller B1.4 flare of Fe  xxiv emission, indicating that hot plasma of 15 MK can be reached.

In this paper, we propose that the accelerated electrons in the quiet Sun could collide with the solar atmosphere to emit Hard X-rays (HXRs) via non-thermal bremsstrahlung, while some of these electrons would move upwards and escape into the interplanetary medium, to form a superhalo electron population measured in the solar wind. After considering the electron energy loss due to Coulomb collisions and the ambipolar electrostatic potential, we find that the sources of the superhalo could only occur high in the corona (at a heliocentric altitude ≳ 1.9 R ⊙ (the mean radius of the Sun)), to remain a power-law shape of electron spectrum as observed by Solar Terrestrial Relations Observatory (STEREO) at 1 AU near solar minimum (Wang et al. in Astrophys. J. Lett. 753 , L23, 2012 ). The modeled quiet-Sun HXRs related to the superhalo electrons fit well to a power-law spectrum, f ∼ ε − γ in the photon energy ε , with an index γ ≈ 2.0 – 2.3 (3.3 – 3.7) at 10 –  100 keV, for the warm/cold-thick-target (thin-target) emissions produced by the downward-traveling (upward-traveling) accelerated electrons. These simulated quiet-Sun spectra are significantly harder than the observed spectra of most solar HXR flares. Assuming that the quiet-Sun sources cover 5 % of the solar surface, the modeled thin-target HXRs are more than six orders of magnitude weaker than the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) upper limit for quiet-Sun HXRs (Hannah et al. in Astrophys. J. 724 , 487, 2010 ). Using the thick-target model for the downward-traveling electrons, the RHESSI upper limit restricts the number of downward-traveling electrons to at most ≈ 3 times the number of escaping electrons. This ratio is fundamentally different from what is observed during solar flares associated with escaping electrons where the fraction of downward-traveling electrons dominates by a factor of 100 to 1000 over the escaping population.

On November 4th, 2015 secondary air traffic control radar was strongly disturbed in Sweden and some other European countries. The disturbances occurred when the radar antennas were pointing at the Sun. In this paper, we show that the disturbances coincided with the time of peaks of an exceptionally strong (∼10 5 Solar Flux Units) solar radio burst in a relatively narrow frequency range around 1 GHz. This indicates that this radio burst is the most probable space weather candidate for explaining the radar disturbances. The dynamic radio spectrum shows that the high flux densities are not due to synchrotron emission of energetic electrons, but to coherent emission processes, which produce a large variety of rapidly varying short bursts (such as pulsations, fiber bursts, and zebra patterns). The radio burst occurs outside the impulsive phase of the associated flare, about 30 min after the soft X-ray peak, and it is temporarily associated with fast evolving activity occurring in strong solar magnetic fields. While the relationship with strong magnetic fields and the coherent spectral nature of the radio burst provide hints towards the physical processes which generate such disturbances, we have so far no means to forecast them. Well-calibrated monitoring instruments of whole Sun radio fluxes covering the UHF band could at least provide a real-time identification of the origin of such disturbances, which reports in the literature show to also affect GPS signal reception.