Solar Hard X-ray Bursts Research Articles

Parametric Evolution of Power-law Energy Spectra of Flare Accelerated Electrons in the Solar Atmosphere

Abstract It is well known that solar hard X-ray bursts (HXRs) and solar radio bursts (SRBs) from solar flares both are produced directly by fast electron beams (FEBs) traveling through the solar atmosphere. The observed characteristics of HXRs and SRBs sensitively depend on the energy distribution of FEBs, which are believed commonly to have a power-law energy spectrum. When FEBs propagating in the solar atmosphere, however, their energy spectra can considerably vary due to the interaction with the atmospheric plasmas and this may significantly influence the observational characteristics of the producing HXRs and SRBs. In the present paper, based on flare atmospheric models, we investigate the parametric evolution of power-law spectra of FEBs due to their energy losses when propagating along flaring loops. The results show that an initially single power-law spectrum with a lower-energy cutoff can evolve into a more complex double power-law spectrum or a broken power-law spectrum with multi-breaking knees because of the dependence of the energy loss on the initial energies. The possible effects of the energy-spectral evolution of FEBs on observational characteristics of their HXR flares and SRBs are discussed. The present results are helpful to understand the physics of dynamical spectra of HXRs and SRBs from solar flares.

Local re-acceleration and a modified thick target model of solar flare electrons

The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts has become an almost 'Standard Model' of flare impulsive phase energy transport and radiation. However, it faces various problems in the light of recent data, particularly the high electron beam density and anisotropy it involves.} {We consider how photon yield per electron can be increased, and hence fast electron beam intensity requirements reduced, by local re-acceleration of fast electrons throughout the HXR source itself, after injection.} {We show parametrically that, if net re-acceleration rates due to e.g. waves or local current sheet electric (${\cal E}$) fields are a significant fraction of collisional loss rates, electron lifetimes, and hence the net radiative HXR output per electron can be substantially increased over the CTTM values. In this local re-acceleration thick target model (LRTTM) fast electron number requirements and anisotropy are thus reduced. One specific possible scenario involving such re-acceleration is discussed, viz, a current sheet cascade (CSC) in a randomly stressed magnetic loop.} {Combined MHD and test particle simulations show that local ${\cal E}$ fields in CSCs can efficiently accelerate electrons in the corona and and re-accelerate them after injection into the chromosphere. In this HXR source scenario, rapid synchronisation and variability of impulsive footpoint emissions can still occur since primary electron acceleration is in the high Alfv\'{e}n speed corona with fast re-acceleration in chromospheric CSCs. It is also consistent with the energy-dependent time-of-flight delays in HXR features.

Diagnostics of the Low-Cutoff Energy of Nonthermal Electrons in Solar Microwave and Hard X-Ray Bursts

Diagnostics of the Low-Cutoff Energy of Nonthermal Electrons in Solar Microwave and Hard X-Ray Bursts

Solar Hard X-Ray Bursts and Electron Acceleration Down to 8 [CLC]ke[/CLC

In 2 days that were typical of solar maximum (1991 October 1-2), the CGRO/BATSE Spectroscopy Detectors observed more than five solar X-ray bursts per hour above 8 keV [8-13 keV flux greater than 0.4 (cm2 s keV)-1]. Only approximately one-third of these were detected above 25 keV [25-59 keV flux greater than 1.7 × 10-3 (cm2 s keV)-1] by the BATSE Large Area Detectors (LADs). Most of the >8 keV bursts are impulsive and similar to solar hard X-ray bursts observed above 20 keV—the bursts occurred on the most rapid rise of the GOES thermal soft X-ray burst, and the ratio of 8-13 keV to greater than 13 keV flux for the impulsive bursts fitted a power-law spectrum [dJ/d(hν) ~ (hν)-γ] with γ = 3-7. More than half of the bursts that should be detected by the LADs if the power law extends to high energies are not detected, indicating that the spectrum steepens or cuts off above ~25 keV. These findings suggest that the acceleration of electrons occurs for essentially all impulsive energy releases but may only be detectable at low energies in smaller releases. The burst occurrence frequency varies with energy W(>8 keV) in accelerated electrons as dn/dW ~ W-1.6 above W ~ 1029 ergs, with an average rate of energy deposition of ~1026 ergs s-1, about 100 times that above 20 keV. Comparison with the occurrence frequency of the energy content in active region transient brightenings (ARTBs) detected by the Yohkoh soft X-ray telescope suggests that accelerated electrons may provide a significant fraction of the total energy in ARTBs.

Total electron number inferred from solar hard X-ray and microwave burst spectra: Yu Xing-feng & Yao Jin-xing Purple Mountain Observatory, CAS, Nanjing 210008

Total electron number inferred from solar hard X-ray and microwave burst spectra: Yu Xing-feng & Yao Jin-xing Purple Mountain Observatory, CAS, Nanjing 210008

The intensity and temporal characteristics of BATSE gamma-ray bursts and solar hard X-ray bursts

We have investigated the intensity and temporal characteristics of some 100 γ-bursts and some 4000 hard X-ray bursts found by CGRO/BATSE. We found their intensity distributions to be similar, suggesting a similar mechanism, and casting doubt on the cosmological interpretation of the γ-bursts. We found the duration and intensity of the solar bursts are positively correlated, while for the γ-bursts, the correlation is negative. The intensity and duration of the solar bursts showed long-term variations in the course of BATSE monitoring, such variations have also been found in recent studies of γ-bursts.

SOLAR ENERGETIC ION AND ELECTRON LIMITS FROM HIREGS OBSERVATIONS

The HIgh-REsolution Gamma-ray and hard X-ray Spectrometer (HIREGS) consists of an actively shielded array of twelve liquid-nitrogen-cooled germanium detectors designed to provide unprecedented spectral resolution and narrow-line sensitivity for solar gamma-ray line observations. Two long-duration, circumpolar balloon flights of HIREGS in Antarctica (10–24 January, 1992 and 31 December, 1992–10 January, 1993) provided 90.9 and 20.4 hours of solar observations, respectively. During the observations, eleven soft X-ray bursts at C levels and above (largest M1.7) occurred, and three small solar hard X-ray bursts were detected by the Compton Gamma-Ray Observatory. HIREGS detected a significant increase above 30 keV in one. No solar gamma-ray line emission was detected. Limits on the 2.223-MeV line and the hard X-ray emission are used to estimate the relative contribution of protons and electrons to the energy in flares, and to coronal heating. For the 2.223-MeV line, the upper limit fluence is ≲ 0.8 ph cm-2 in the flares, and the upper limit flux is 1.8 × 10-4 ph s-1 cm-2 in the absence of flares. These limits imply that ≲ 6 × 1030 (2σ) protons above 30 MeV were accelerated in the flares, assuming standard photospheric abundances and a thick target model. The total energy contained in the accelerated protons >30 MeV is ≲ 4 × 1026 ergs, but this limit can be more than 1030 ergs if the spectrum extends down to ∽1 MeV. The upper limit on the total energy in accelerated electrons during the observed flares can also exceed 1030 ergs if the spectrum goes down to ∽ 7 keV. Quiet-Sun observations indicate that ≲ 1026erg s-1 are deposited by energetic protons >1 MeV, well below the1027 –1028 erg s-1 required for coronal heating, while <3 × 1027 erg s-1 are deposited by energetic electrons, which does not exclude the possibility of coronal heating by quiet-time accelerated electrons. The quiet-Sun observations also suggest that if protons stored in the corona are to supply the energy for flares, as suggested by Elliot (1964), the proton spectrum must extend down to at least ∽2 MeV. However, collisional losses at typical coronal-loop densities prevent those low-energy protons from being stored for ≳ 104 s. It therefore seems unlikely that the energy for flares could come from energetic protons stored over long periods.

Temporal correlation of solar hard X-ray bursts with chromospheric evaporation

During the impulsive phase of many solar flares, blueshifted emission wings are observed on the soft X-ray spectral lines of highly excited ions that are produced in the flare plasma. This emission has been commonly interpreted as chromospheric evaporation of material from the footpoints of coronal loops by non-thermal particle beams, although the question of whether the bulk of the energy is carried by electrons or ions (protons) has been the subject of much debate. The precise temporal relationship between the onsets of the blueshifted emission and the hard X-ray bursts is particularly important in resolving the mechanism of energy transfer to the hot plasma in the impulsive phase. A sample of flares observed with the Bragg Crystal Spectrometer (BCS) onYohkoh has been analysed for blueshifted emission and the results compared with hard X-ray light turves obtained with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO). In some flares, the blueshifted emission precedes the onset of the hard X-rays by up to 100 s. There is no evidence for a temporal correlation between the maximum energy input to the hard X-ray bursts and the maximum blueshifted intensity. These results lend support to those models favouring protons as the dominant energy carrier in the impulsive phase of flares and are inconsistent with the hypothesis that the bulk of the energy resides in electron beatos, although some other energy input, while unlikely, cannot be completely eliminated.

Fine scale temporal structures in solar hard X-ray bursts observed by PHEBUS

We report here on preliminary results of a systematic study of fast temporal fluctuations in “impulsive” and “extended” solar X-ray bursts observed by PHEBUS at energies around 100 keV. Subsecond timescales are quite common in the “impulsive” events and are not observed in “extended” ones.

The correlation of solar flare hard X-ray bursts with Doppler blueshifted soft X-ray flare emission

We have investigated the temporal correlation between hard X-ray bursts and the intensity of Doppler blueshifted soft X-ray spectral line emission. We find a strong correlation for many events that have intense blueshifted spectral signatures and some correlation in events with modest blueshifts. The onset of hard X-rays frequently coincides to within a few seconds with the onset of blueshifted emission. The peak intensity of blueshifted emission is frequently close in time to the peak of the hard X-ray emission. Decay rates of the blueshifted and hard X-ray emission are similar, with the decay of the blueshifted emission tending to lag behind the hard X-ray emission in some cases. There are, however, exceptions to these conclusions, and, therefore, the results should not be generalized to all flares. Most of the data for this work were obtained from instruments flown on the Japanese Yohkoh solar spacecraft.

Solar and Stellar Flare Observations using Watch

AbstractThe Danish experiment WATCH (Wide Angle Telescope for Cosmic Hard X-rays) is to be flown on board the Soviet satellite GRANAT in middle of 1989. The performence characteristics of the WATCH instrument is described. It is estimated that WATCH can detect about 100 solar hard X-ray bursts per day. WATCH can also detect about 40 energetic stellar soft X-ray flares, similar to the fast transient X-ray emissions detected by the Ariel V satellite.

Rapid temporal evolution of radiation from nonthermal electrons in solar flares

Solutions of the time dependent Fokker-Planck equation was found for accelerated electrons undergoing Coulomb collisions in a magnetized, fully ionized plasma. An exact solution was found for arbitrary pitch angle and energy distribution in a uniform background plasma. Then, for an inhomogeneous plasma, a solution was found for particles with small pitch angles. These solutions were used to calculate the temporal evolution of bremsstrahlung x-rays from short bursts of nonthermal electron beams, and these spectra were compared with observed high time resolution spectra of short timescale solar hard x-ray bursts. It is shown that the observed softening in time of the spectra rules out a homogeneous background and therefore the possibility of electrons being confined to the corona either because of converging magnetic field or high densities. The inhomogeneous solution was also applied to a model with constant coronal density and exponentially rising chromospheric density. The spectra are shown to be consistent with that produced by a collimated beam of electrons accelerated in the corona with certain given conditions. These conditions could be violated if large pitch angle electrons are present.

On the periodicities of sunspots and solar strong hard X-ray bursts

In the present investigation, we have carried out power spectrum analysis of sunspot number and great hard x-ray (GHXR) burst (equal to or greater than 10,000 counts per second) for a period of about 6 years. The GHXR bursts show a periodicity of about 155 days. On the other hand, sunspot numbers do not show any periodicity. The GHXR burst periodicity confirms the existence of a 152–158 days periodicity in the occurrence of solar energetic events. Further, the GHXR bursts are showing periodicity independently indicating that the GHXR bursts are a separate class of X-ray flares.

Coronal oscillations as revealed by solar hard X-ray bursts

During the 21st solar activity cycle the HXRBS aboard SMM satellite and the HXM on HINOTORI spacecraft detected several thousand hard X-ray solar flares. Studies of the temporal properties of these events had revealed hundreds of examples of fast spikes with durations of less than 1 sec. We analysed part of these observations and found that they have four common characteristics. Among these characteristics, quasi-periodic oscillations led us to believe the possibility of oscillations existing in the corona. We have studied the characteristics of the oscillations and derived their periods. The conditions of trapping the oscillations are also discussed.

The dependence of solar hard X-ray burst intensity on magnetic field strength

The measurements of peak intensities of hard X-ray bursts from hot flare plasma electrons as a function of peak frequency of associated microwave radio bursts are discussed. The latter is proportional to the magnetic field strength. The results suggest that the flare hard X-rays are emitted when the electron plasma frequency is comparable to the electron gyrofrequency. Thus, the hard X-ray peak intensity varies as B 4.2, where B is the magnetic field strength.

Dynamics of electron beams in a coronal loop and the hard X-ray burst

A numerical simulation has been made for the dynamics of non-thermal electrons (> 10keV) injected with spatial, temporal and velocity distributions into a model coronal loop. The time variations of the spatial intensity distribution and the spectrum for the expected hard X-rays are computed for many models in order to find the important physical parameters for those characteristics. The most important one is the column density of plasma, CD, along the loop. If CD is smaller than 1020 cm−2, the expected X-rays behave like the solar impulsive hard X-ray bursts, that is the spatial maximum of X-rays shifts to the top of the loop in the later phase of the burst accompanying a spectral softening. On the other hand, if CD is greater than this value, ‘quasi-steady decay’ appears in the later phase. In this case the intensity distribution of X-rays above about 20 keV along the loop shows a broad maximum away from the loop top giving an extended spatial distribution of hard X-rays, and spectral hardness is kept constant. These characteristics are similar to the solar gradual hard X-ray bursts (the so-called extended burst which is not a hot thermal gradual burst).

Solar gradual hard X-ray bursts: Observations and an interpretation

A recent study of solar gradual hard X-ray bursts is summarized. The data are interpreted in terms of a model involving the acceleration and trapping of electrons in post flare loop systems following coronal mass ejections. A controversy about the classification of the metric continuum that typically accompanies gradual hard X-ray events is addressed.

The Coronal Oscillations Revealed by Solar Hard X-ray Bursts

The Coronal Oscillations Revealed by Solar Hard X-ray Bursts

Solar hard X-ray bursts

Solar hard X-ray bursts

Possible evidence for stochastic acceleration of electrons in solar hard X-ray bursts observed by SMM

Etude des spectres RX durs, dynamiques d'evenements du 29 mars et 7 juin 1980 observes par le Solar Maximum Mission. On note une anticorrelation du flux de photons et de la pente spectrale