High-energy X-rays and
γ
-rays from solar flares were discovered just over fifty years ago. Since that time, the standard for the interpretation of spatially integrated flare X-ray spectra at ...energies above several tens of keV has been the collisional thick-target model. After the launch of the
Reuven Ramaty High Energy Solar Spectroscopic Imager
(
RHESSI
) in early 2002, X-ray spectra and images have been of sufficient quality to allow a greater focus on the energetic electrons responsible for the X-ray emission, including their origin and their interactions with the flare plasma and magnetic field. The result has been new insights into the flaring process, as well as more quantitative models for both electron acceleration and propagation, and for the flare environment with which the electrons interact. In this article we review our current understanding of electron acceleration, energy loss, and propagation in flares. Implications of these new results for the collisional thick-target model, for general flare models, and for future flare studies are discussed.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The hard X-ray time profiles of most solar eruptive events begin with an
impulsive phase
that may be followed by a
late gradual phase
. In a recent article (Aurass
et al.
in
Astron. Astrophys.
555
, ...A40, 2013), we analyzed the impulsive phase of the solar eruptive event on November 3, 2003 in radio and X-ray emission. We found evidence of magnetic breakout reconnection using the radio diagnostic of the common effect of the flare current sheet and, at heights of ±0.4 R
⊙
, of a coronal breakout current sheet (a source site that we called X).
In this article we investigate the radio emission during the late gradual phase of this event. The work is based on 40 – 400 MHz dynamic spectra (
Radio Spectrograph
, Observatorium Tremsdorf, Leibniz Institut für Astrophysik Potsdam, AIP) combined with radio images obtained by the French
Nançay Multifrequency Radio Heliograph
(NRH) of the Observatoire de Paris-Meudon. Additionally, we use
Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) hard X-ray (HXR) flux records, and
Solar and Heliospheric Observatory
(SOHO)
Large Angle and Spectrometric Coronagraph
(LASCO) and
Extreme ultraviolet Imaging Telescope
(EIT) images.
The analysis shows that the late gradual phase is subdivided into two distinct stages. Stage 1 (lasting five minutes in this case) is restricted to reoccurring radio emission at source site X. We observe plasma emission and an azimuthally moving source (from X toward the NE; speed≈1200 km s
−1
) at levels radially ordered
against
the undisturbed coronal density gradient. These radio sources mark the lower boundary of an overdense region with a huge azimuthal extent. By the end of its motion, the source decays and reappears at point X. This is the onset of stage 2 traced here during its first 13 minutes. By this time, NRH sources observed at frequencies≤236.6 MHz radially lift off with a speed of ≈ 400 km s
−1
(one third of the front speed of the coronal mass ejection (CME)) as one slowly decaying broadband source. This speed is still observable in SOHO/LASCO C3 difference frames in the wake of the CME four hours later. In stage 2, the radio sources at higher frequencies appear directly above the active region with growing intensity.
We interpret the observations as the transit of the lower boundary of the CME body through the height range of the coronal breakout current sheet. The relaxing global coronal field reconnects with the magnetic surroundings of the current sheets that still connect the CME in its wake with the Sun. The accelerated particles locally excite plasma emission, but can also escape toward the active region, the CME, and the large-scale solar magnetic field. The breakout relaxation process may be a source of reconnection- and acceleration rate modulations.
In this view, the late gradual phase is a certain stage of the coronal breakout relaxation after the release of the CME. This article is, to our best knowledge, the first observational report of the coronal breakout recovery. Our interpretation of the radio observations agrees with some predictions of magnetic breakout simulations (
e.g.
Lynch
et al.
in
Astrophys. J.
683
, 1192, 2008). Again, combined spectral and imaging radio observations give a unique access to dynamic coronal processes that are invisible in other spectral ranges.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Context. During solar flares a large amount of energy is suddenly released and partly transferred into energetic electrons. They are of special interest since a substantial part of the energy ...released during a flare is deposited into the energetic electrons. RHESSI observations, e.g. of the 2003 October 28 solar event, show that 1036 electrons with energies >20 keV are typically produced per second during large flares. They are related to a power of about 1022 W. It is still an open question in which way so many electrons are accelerated up to high energies during a fraction of a second. Aims. Within the framework of the magnetic reconnection scenario, jets appear in the outflow region and can establish standing fast-mode shocks if they penetrate the surrounding plasma with super-Alfvénic speed. It is our aim to show that this shock can be the source of the energetic electrons produced during flares. Methods. The electrons are regarded as energized by shock drift acceleration. The process is necessarily treated in a fully relativistic manner. The resulting distribution function of accelerated electrons is a loss-cone one and it allows to calculate the differential electron flux, which can be compared with RHESSI. Results. The theoretically obtained fluxes of energetic electrons agree with the observed ones as demonstrated for the 2003 October 28 solar event.
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Context. Magnetic reconnection is understood to be fundamental to energy release in solar eruptive events (SEEs). In these events reconnection produces a magnetic flux rope above an arcade of hot ...flare loops. Breakout reconnection, a secondary reconnection high in the corona between this flux rope and the overlying magnetic field, has been hypothesized. Direct observational evidence for breakout reconnection has been elusive, however. Aims. The aim of this study is to establish a plausible interpretation of the combined radio and hard X-ray (HXR) emissions observed during the impulsive phase of the near-limb X3.9-class SEE on 2003 November 03. Methods. We study radio spectra (AIP), simultaneous radio images (Nançay Multi-frequency Radio Heliograph, NRH), and single-frequency polarimeter data (OAT). The radio emission is nonthermal plasma radiation with a complex structure in frequency and time. Emphasis is on the time interval when the HXR flare loop height was observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to be at its minimum and an X-ray source was observed above the top of the arcade loops. Results. Two stationary, meter-wavelength sources are observed radially aligned at 0.18 and 0.41 R⊙ above the active region and HXR sources. The lower source is apparently associated with the upper reconnection jet of the flare current sheet (CS), and the upper source is apparently associated with breakout reconnection. Sources observed at lower radio frequencies surround the upper source at the expected locations of the breakout reconnection jets. Conclusions. We believe the upper radio source is the most compelling evidence to date for the onset of breakout reconnection during a SEE. The height stationarity of the breakout sources and their dynamic radio spectrum discriminate them from propagating disturbances. Timing and location arguments reveal for the first time that both the earlier described above the flare loop top HXR source and the lower radio source are emission from the upper reconnection jet above the vertical flare CS.
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At the Sun, shock waves are produced either by flares and/or by coronal mass ejections (CMEs) and are regarded as the source of solar energetic particle events. They can be able to generate solar ...type II radio bursts. The non-radial propagation of a disturbance is considered away from an active region through the corona into the interplanetary space by evaluating the spatial behaviour of the Alfvén speed. The magnetic field of an active region is modelled by a magnetic dipole superimposed on that of the quiet Sun. Such a magnetic field structure leads to a local minimum of the Alfvén speed in the range 1.2–1.8 solar radii in the corona as well as a maximum of 740 km s-1 at a distance of 3.8 solar radii. The occurrence of such local extrema has important consequences for the formation and development of shock waves in the corona and the near-Sun interplanetary space and their ability to accelerate particles. It leads to a temporal delay of the onset of solar energetic particle events with respect to both the initial energy release (flare) and the onset of the solar type II radio burst.
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We report observations of a propagating disturbance in the solar corona observed in emission in soft X-ray images from the Yohkoh Soft X-ray Telescope (SXT). The disturbance was associated with a ...flare which began at about 09:04 UT on 1997 November 03. This flare was associated with a type II radio burst observed at decimetric-dekametric wavelengths by the Astrophysikalisches Institut Potsdam Radio Spectrograph. Hα data from Kanzelhöhe Solar Observatory show that a Moreton wave was associated with this event. Moreover, Solar and Heliospheric Observatory Extreme Ultraviolet Imaging Telescope (EIT) 195 Å data show an `EIT wave' associated with this event. Extrapolations of the leading edge of the propagating soft X-ray disturbance show a close association with both of these wave features. The soft X-ray disturbance is observed to travel with a speed of about 546 km s-1. Using Nançay Radioheliograph data we directly determine the source locations of the type II radio burst. These are found to be located close to the soft X-ray disturbance and show motions consistent with the soft X-ray motions. These results lead us to conclude that the “SXT wave” is the coronal counterpart of a Moreton wave, analogous to EIT waves, i.e., it is the first confirmed direct observation of a solar coronal shock wave in X-rays.
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We review recent progress on our understanding of radio emission from solar flares and coronal mass ejections (CMEs) with emphasis on those aspects of the subject that help us address questions about ...energy release and its properties, the configuration of flare – CME source regions, coronal shocks, particle acceleration and transport, and the origin of solar energetic particle (SEP) events. Radio emission from electron beams can provide information about the electron acceleration process, the location of injection of electrons in the corona, and the properties of the ambient coronal structures. Mildly relativistic electrons gyrating in the magnetic fields of flaring loops produce radio emission via the gyrosynchrotron mechanism, which provides constraints on the magnetic field and the properties of energetic electrons. CME detection at radio wavelengths tracks the eruption from its early phase and reveals the participation of a multitude of loops of widely differing scale. Both flares and CMEs can ignite shock waves and radio observations offer the most robust tool to study them. The incorporation of radio data into the study of SEP events reveals that a clear-cut distinction between flare-related and CME-related SEP events is difficult to establish.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
During solar flares a large amount of nonthermal electromagnetic radiation up to the γ-ray range is emitted from the corona, which implies that energetic electrons are generated. Within the framework ...of the magnetic reconnection scenario, jets appear in the outflow region and can establish standing fast-mode shocks if they penetrate the surrounding plasma at super-Alfvénic speed. These shocks can be a source of energetic electrons. During the solar event on October 28, 2003, an enhanced flux of hard X- and γ-rays up to 10 MeV as observed by the INTEGRAL spacecraft indicates the generation of relativistic electrons. The radio signature of a standing shock wave appeared simultaneously with the enhanced hard X- and γ-ray fluxes. Here, we assume this shock is the source of the highly energetic electrons needed for the hard X- and γ-ray, as well as for the nonthermal radio radiation. The electrons are energized by shock drift acceleration, which is treated in a fully relativistic manner. After acceleration, the electrons travel along the magnetic field lines towards the denser chromosphere, where they emit hard X- and γ-ray radiation via bremsstrahlung. The observed photon fluxes in the range 7.5-10 MeV are explained by these theoretical results that adopt the coronal conditions found for the event on October 28, 2003.
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