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1.
The generation of lower-hybrid waves by cross-field currents is applied to reconnection processes proposed for solar flares. Recent observations on fragmentation of energy release and acceleration, and on hard X-ray (HXR) spectra are taken into account to develop a model for electron acceleration by resonant stochastic interactions with lower-hybrid turbulence. The continuity of the velocity distribution is solved including collisions and escape from the turbulence region. It describes acceleration as a diffusion process in velocity space. The result indicates two regimes that are determined by the energy of the accelerating electrons which may explain the double power-law often observed in HXR spectra. The model further predicts an anticorrelation between HXR flux and spectral index in agreement with observations. 相似文献
2.
Katsuo Tanaka 《Astrophysics and Space Science》1986,118(1-2):101-113
Extensive observations of solar flares made in high energy bands during the maximum of the present solar cycle are discussed with a special reference to the results from HINOTORI, and with attention to the relevant flare models. The hard X-ray (HXR) images from HINOTORI showed mostly coronal emission at 20–25 keV suggesting that the HXR is emitted from multiple coronal loops, consistent with the non-thermal electron beam model in a high density corona. The thermal HXR model seems to be inconsistent with some observations. Three types of flares which have been classified from the Hinotori results are described, along with newly discovered hot thermal component of 30–40 million K which contributes thermal HXR emission. A summary is given for the characteristics of the energy release in an impulsive burst; and an empirical model is described, which explains simultaneous energy releases in multiple loops and successive movements of the release site as suggested from the HXR morphology. The discovery of large blue-shifted hot plasma from the soft X-ray line spectrum leads to some quantitative arguments for the evaporating flare model. An electron-heated flare atmosphere appears to explain various observations consistently.Invited paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. 相似文献
3.
Heating and acceleration of electrons in solar impulsive hard X-ray (HXR) flares are studied according to the two-stage acceleration model developed by Zhang for solar 3He-rich events. It is shown that electrostatic H-cyclotron waves can be excited at a parallel phase velocity less than about the electron thermal velocity and thus can significantly heat the electrons (up to 40 MK) through Landau resonance. The preheated electrons with velocities above a threshold are further accelerated to high energies in the flare-acceleration process. The flare-produced electron spectrum is obtained and shown to be thermal at low energies and power law at high energies. In the non-thermal energy range, the spectrum can be double power law if the spectral power index is energy dependent or related. The electron energy spectrum obtained by this study agrees quantitatively with the result derived from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) HXR observations in the flare of 2002 July 23. The total flux and energy flux of electrons accelerated in the solar flare also agree with the measurements. 相似文献
4.
In a sample of strong RHESSI M-class flares we have made a study of the relationship between the `hardness' of the HXR spectrum
and the intensity in the 30–50 keV energy range. In all events we find clear evidence for a `soft–hard–soft' pattern of correlation
between hardness and flux, on time scales as short as 10 s. We investigate whether or not this pattern is intrinsic to the
acceleration mechanism. The RHESSI images in this energy range are dominated by footpoint brightenings, and we have searched
for a correlation between footpoint separation velocity and spectral hardness, to be compared qualitatively with theoretical
flare models. We find quite systematic footpoint motions, and also note that episodes in which footpoint separation varies
rapidly often correspond with episodes of significant change in the flare spectral index, though not as the simplest flare
models would predict. We report also on one of our events, on 14 March 2002, which exhibits highly sheared HXR footpoint ribbons
extending over a scale of 100 arc sec. For this flare we find a correlation between footpoint motion and hard X-ray flux.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022479610710 相似文献
5.
Neupert效应的定性描述是耀斑中脉冲分量(硬X射线、微波暴)与渐变分量(软X射线发射)之间存在的因果关系,即耀斑最初的能量是以加速粒子的形式释放,加速的电子在大气传输过程中产生非热硬X射线轫致辐射,并加热大气,耀斑软X射线发射是高能粒子注入大气的响应.根据经典Neupert效应的定量描述,硬X射线发射(表征非热电子注入)结束时软X射线应该立刻达到极大,但以往的观测发现一些耀斑软X射线峰值时间(t2)明显晚于硬X射线结束时间(t1)(τ=t2–t1,τ 0),热与非热辐射之间存在明显的偏离经典Neupert效应的情况.为了研究偏离经典Neupert效应的事件,在2002—2015年间的RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager)和GOES (Geostationary Operational Environmental Satellites)耀斑列表中,按照在25–50 keV范围内光变较简单、软X射线有对应发射峰等判据,共选择276个耀斑样本,统计了这些耀斑的τ分布、环长d (用双足点源之间的距离来表征)与τ的关系.结果显示:(1)有227个耀斑τ 0,即有约82%的耀斑偏离经典Neupert效应;(2)τ与d之间存在一定的线性相关,即环越长,软X射线极大的时间越延后;(3)似乎存在一个临界距离,当环长小于临界距离时,经典Neupert效应成立.这些结果印证了修正Neupert效应的必要性,并对其物理意义进行了讨论. 相似文献
6.
The Neupert effect is the name given to the correlation observed in many flares between the time-integrated microwave and hard X-ray emissions and the soft X-ray emission light curve. We have used hard X-ray data from the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) and soft X-ray data from the detector on GOES to determine what fraction of all events show this correlation and how the correlation changes from the impulsive to the gradual phase. We have found that of 66 HXRBS events observed in 1980 with a peak rate of > 1000 counts s-1, 58 (80%) showed good correlations with peaks in the GOES time derivative plot corresponding to peaks in the hard X-ray (HXR) plots to within ±20 s. In 20 of these good-correlation cases (30%), the soft X-ray (SXR) time derivative stays high after the HXR emission has decreased suggesting that the later emissions result from energy release in a loop already affected by the initial energy release. In 8 of the 13 flares that showed poor correlation, the SXR time derivative shows no peak corresponding to the initial HXR impulsive peak that has structure on a time scale of 1 s. This suggests that in these events the initial impulsive energy release results primarily in electron acceleration, and that the secondary plasma heating from the accelerated electrons contributes relatively little compared to the more gradual plasma heating already taking place at the same time. The more gradually varying events, commonly referred to as type C flares, tend to show poorer correlation between the SXR time derivative and the HXR time profile. Of 26 GOES X1 or greater flares recorded between 1980 and 1989 with HXR peaks lasting over 10 rain, 13 (50%) showed poor correlation with the gradual HXR peaks either not registering at all in the SXR time derivative plots or showing up as very broad peaks. In one case, on 1981 April 26, the SXR time derivative peak was delayed by 13 rain. Only 17 (65%) of the 26 X-flares had an earlier, impulsive component and of those, 12 (71%) showed good correlation between the impulsive peaks. 相似文献
7.
The flattening at the low energy end of the hard X-ray (HXR) photon spectrum of solar flares was generally thought to be due to a cutoff of nonthermal electrons in flares. However, some authors have suggested that inverse Compton scattering (i.e., the albedo effect) or certain other reaction of flare photons with the lower atmosphere can also lead to the flattening. This paper adopts the method of deriving the cutoff proposed by Gan et al. [12–14], and makes a statistical analysis on 100 flares observed by the satellite Ramaty High Energy Solar Spectroscopy Imager (RHESSI) in 2002–2005. We found that after the albedo correction, the HXR photon spectra of 18 flares can be fitted with single powerlaw spectra, and those of 80 flares, with double power-law spectra. Besides, 21 flares can be directly interpreted with a single power-law electron spectrum plus a low energy cutoff. The range of the low energy cutoff is 20–50 keV and the mean value is approximately 30 keV. Some other possible interpretations are also investigated. 相似文献
8.
In our recent paper (Jakimiec and Tomczak, Solar Physics
261, 233, 2010) we investigated quasi-periodic oscillations of hard X-rays during the impulsive phase of solar flares. We have come to the
conclusion that they are caused by magnetosonic oscillations of magnetic traps within the volume of hard-X-ray (HXR) loop-top
sources. In the present paper we investigate four flares that show clear quasi-periodic sequences of the HXR pulses. We also
describe our phenomenological model of oscillating magnetic traps to show that it can explain the observed properties of the
HXR oscillations. The main results are the following: i) Low-amplitude quasi-periodic oscillations occur before the impulsive
phase of some flares. ii) The quasi-periodicity of the oscillations can change in some flares. We interpret this as being
due to changes of the length of oscillating magnetic traps. iii) During the impulsive phase a significant part of the energy
of accelerated (non-thermal) electrons is deposited within a HXR loop-top source. iv) The quick development of the impulsive
phase is due to feedback between the pressure pulses by accelerated electrons and the amplitude of the magnetic-trap oscillation.
v) The electron number density and magnetic field strength values obtained for the HXR loop-top sources in several flares
fall within the limits of N≈(2 – 15)×1010 cm−3, B≈(45 – 130) gauss. These results show that the HXR quasi-periodic oscillations contain important information about the energy
release in solar flares. 相似文献
9.
Properties of electron acceleration in flares, especially the density structure in the acceleration region, are deduced from a correlation study between decimetric type III, spike, and hard X-ray (HXR) bursts. The high association rate found (71%) strongly suggests that spikes also originate from energetic electrons. Spikes and type III bursts have been found to be easily identified by their different polarizations. The two types of emission generally do not overlap in frequency. A reliable lower limit to the density is derived from the starting frequency of type III and U bursts. The spike emission very likely yields an upper limit. The density inhomogeneity in the acceleration region spans more than one order of magnitude and is more than one order of magnitude larger in the associated type U sources. A peak-to-peak correlation does not always exist between type III, spike and HXR bursts. This discrepancy can be interpreted in terms of the different source conditions and propagation properties. Whereas spikes need special conditions to become visible, type III and peaks of HXR may be the product of many elementary accelerations.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985. 相似文献
10.
We investigate temporal and spatial correlations in solar flares of hard X-rays (HXR) and decimetric continuum emissions,
ejecta, and CMEs. The focus is on three M-class flares, supported by observations from other flares. The main conclusions
of our observations are that (1) major hard X-ray flares are often associated with ejecta seen in soft X-rays or EUV. (2)
Those ejecta seem to start before HXR or related decimetric radio continua (DCIM emission). (3) DCIM occurring nearly simultaneously
with the first HXR peak are located very close to the HXR source. Later in the flare, DCIM generally becomes stronger, drifts
to lower frequency and occurs far from the HXR source. Thus the positions at high frequency are generally closer to the HXR
source. DCIM emission consists of pulses that drift in frequency. The very high and sometimes positive drift rate suggests
spatially extended sources or type III like beams in an inhomogeneous source. Movies of selected flares used in this study
can be found on the CD-ROM accompanying this volume.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1026194227110 相似文献
11.
The aim of the present paper is to use quasi-periodic oscillations in hard X-rays (HXRs) of solar flares as a diagnostic tool
for the investigation of impulsive electron acceleration. We have selected a number of flares which showed quasi-periodic
oscillations in hard X-rays and their loop-top sources could be easily recognized in HXR images. We have considered MHD standing
waves to explain the observed HXR oscillations. We interpret these HXR oscillations as being due to oscillations of magnetic
traps within cusp-like magnetic structures. This is confirmed by the good correlation between periods of the oscillations
and the sizes of the loop-top sources. We argue that a model of oscillating magnetic traps is adequate to explain the observations.
During the compressions of a trap, particles are accelerated, but during its expansions plasma, coming from chromospheric
evaporation, fills the trap, which explains the large number of electrons being accelerated during a sequence of strong pulses.
The advantage of our model of oscillating magnetic traps is that it can explain both the pulses of electron acceleration and
quasi-periodicity of their distribution in time. 相似文献
12.
Photospheric Compton backscatter (albedo) makes a significant contribution to observed hard X-ray (HXR) spectral fluxes over
the RHESSI energy range and should be allowed for in HXR spectral interpretation. The full correction problem is nonlinear
and messy but we offer a simple approximate first-order correction procedure for global HXR spectra based upon empirical fits
to published albedo simulations. We also illustrate the impact of this correction on inferred electron spectra for the thin-
and thick-target models. 相似文献
13.
Fast electrons in the solar atmosphere are detected by their hard X-ray bremsstrahlung and by type III radio bursts caused
by ‘bump-on-tail’ plasma wave generation. This paper investigates empirically the effect of wave generation on the HXR spectrum.
Purely collisional propagation of an electron beam generates a bump in the distribution function, due to stopping of low-velocity
electrons. The consequent positive gradient means there is a possibility of wave generation, production of type III radio
bursts, and energy redistribution of the electron beam. We have represented this relaxation parametrically and calculated
the global bremsstrahlung HXR emission spectrum. We show that for a range of relaxed forms, with different local electron
spectral shapes, the bremsstrahlung spectrum integrated over the whole target is identical in shape to the purely collisionally
evolved beam. Our results show that spatially integrated HXR spectral measurements would be unable to distinguish between
the presence or absence of relaxation effects. Only spatially resolved hard X-ray spectra, such as anticipated from the HESSI
mission, will be able to remove this ambiguity in HXR diagnostics of beam relaxation. 相似文献
14.
V. K. Verma 《Astrophysics and Space Science》1991,183(2):317-321
We find that gamma-ray line (GRL) emissions start later than the hard X-ray (HXR) emissions during impulsive and extended solar flares. Starting delay is more in the case of extended solar flares suggesting a slow acceleration of electrons and ions, in comparison to impulsive solar flares which indicate different acceleration mechanism for impulsive and extended solar flares. We further infer that during solar flares, electrons and ions are accelerated simultaneously and the delay between HXR and GRL emissions results mainly due to differences in acceleration times of electrons and ions to attain energies required for producing HXR emissions for electrons and GRL emissions for ions. Therefore, we are of view that a single step acceleration mechanism may work in solar flares. 相似文献
15.
Using RHESSI and some auxiliary observations we examine possible connections between the spatial and temporal structure of
nonthermal hard X-ray (HXR) emission sources from the two-ribbon flares of 29 May 2003 and 19 January 2005. In each of these
events quasi-periodic pulsations (QPP) with time period of 1 – 3 minutes are evident in both hard X rays and microwaves. The
sources of nonthermal HXR emission are situated mainly at the footpoints of the flare arcade loops observed by TRACE and the
SOHO/EIT instrument in the EUV range. At least one of the sources moves systematically during and after the QPP phase in each
flare. The sources move predominantly parallel to the magnetic inversion line during the 29 May flare and along flare ribbons
during the QPP phase of both flares. By contrast, the sources start to show movement perpendicular to the flare ribbons with
velocity comparable to that along the ribbons’ movement after the QPP phase. The sources of each pulse are localized in distinct
parts of the ribbon during the QPP phase. The measured velocity of the sources and the estimated energy release rate do not
correlate well with the flux of the HXR emission calculated from these sources. The sources of microwaves and thermal HXRs
are situated near the apex of the flare loop arcade and are not stationary either. Almost all of the QPP as well as some pulses
of nonthermal HXR emission during the post-QPP phase reveal soft – hard – soft spectral behavior, indicating separate acts
of electron acceleration and injection. In our opinion at least two different flare scenarios based on the Nakariakov et al. (2006, Astron. Astrophys.
452, 343) model and on the idea of current-carrying loop coalescence are suitable for interpreting the observations. However,
it is currently not possible to choose between them owing to observational limitations. 相似文献
16.
The analysis of 315 hard X-ray bursts (HXR) producing solar flares observed by Hinotori satellite shows that the HXR bursts occur most prominently at 110°, 140°, 290°, and 320° longitude, respectively. These longitudes are not only prolific in producing flares in number but also in producing flares with large photon counts. 相似文献
17.
The radio emission during 201 selected X-ray solar flares was surveyed from 100 MHz to 4 GHz with the Phoenix-2 spectrometer of ETH Zürich. The selection includes all RHESSI flares larger than C5.0 jointly observed from launch until June 30, 2003. Detailed association rates of radio emission during X-ray flares are reported. In the decimeter wavelength range, type III bursts and the genuinely decimetric emissions (pulsations, continua, and narrowband spikes) were found equally frequently. Both occur predominantly in the peak phase of hard X-ray (HXR) emission, but are less in tune with HXRs than the high-frequency continuum exceeding 4 GHz, attributed to gyrosynchrotron radiation. In 10% of the HXR flares, an intense radiation of the above genuine decimetric types followed in the decay phase or later. Classic meter-wave type III bursts are associated in 33% of all HXR flares, but only in 4% are they the exclusive radio emission. Noise storms were the only radio emission in 5% of the HXR flares, some of them with extended duration. Despite the spatial association (same active region), the noise storm variations are found to be only loosely correlated in time with the X-ray flux. In a surprising 17% of the HXR flares, no coherent radio emission was found in the extremely broad band surveyed. The association but loose correlation between HXR and coherent radio emission is interpreted by multiple reconnection sites connected by common field lines. 相似文献
18.
The Electron Energy Spectrum from Large Solar Flares 总被引：2，自引：0，他引：2
G. M. Simnett 《Solar physics》2006,237(2):383-395
We report on the differential electron spectrum for intense transient events seen at one AU by the EPAM instrument on the
Advanced Composition Explorer (ACE) spacecraft. Over an observing period from September 1997 to September 2005, there were 45 major events that could be
reliably identified with a source flare on the Sun. In the ∼40 – 300 keV energy range, the electron spectral index was between
one and three for all but two of the events. Twenty-five of the events were associated with Geostationary Operational Environmental
Satellites (GOES) X-ray class X flares. We compare this result with the spectral index measured from electron pulse events,
lasting approx. one hour or less, where the spectral index is typically much softer than three. This suggests that the measured
spectral index of near-relativistic electrons at one AU may be a reliable indicator of the source. We also examine the likelihood
that fast coronal mass ejections (CMEs) are responsible in themselves for accelerating near-relativistic electrons and conclude
that they do not. 相似文献
19.
V. K. Verma 《Astrophysics and Space Science》2011,334(1):83-102
We present study of relationship of GSXR flares with Hα flares, hard X-ray (HXR) bursts, microwave (MW) bursts at 15.4 GHz, type II/IV radio bursts, coronal mass ejections (CMEs),
protons flares (>10 MeV) and ground level enhancement (GLE) events we find that about 85.7%, 93%, 97%, 69%, 60%, 11.1%, 79%,
46%, and 23%% GSXR flares are related/associated with observed Hα flares, HXR bursts, MW bursts at 15.4 GHz, type II radio bursts, type IV radio bursts, GLE events, CMEs, halo CMEs, and proton
flares (>10 MeV), respectively. In the paper we have studied the onset time delay of GSXR flares with Hα flares, HXR, and MW bursts which shows the during majority GSXR flares SXR emissions start before the Hα, HXR and MW emissions, respectively while during 15–20% of GSXR flares the SXR emissions start after the onset of Hα, HXT and MW emissions, respectively indicating two types of solar flares. The, onset time interval between SXR emissions
and type II radio bursts, type IV radio bursts, GLE events CMEs, halo CMEs, and protons flares are 1–15 min, 1–20 min, 21–30 min,
21–40 min, 21–40 min, and 1–4 hrs, respectively. Following the majority results we are of the view that the present investigations
support solar flares models which suggest flare triggering first in the corona and then move to chromospheres/ photosphere
to starts emissions in other wavelengths. The result of the present work is largely consistent with “big flare syndrome” proposed
by Kahler (1982). 相似文献
20.
Slow long-duration events (SLDEs) are flares characterized by the long duration of their rising phase. In many such cases the impulsive phase is weak without typical short-lasting pulses. Instead, smooth, long-lasting hard X-ray (HXR) emission is observed. We analyzed hard X-ray emission and morphology of six selected SLDEs. In our analysis we utilized data from the RHESSI and GOES satellites. The physical parameters of HXR sources were obtained from imaging spectroscopy and were used for the energy balance analysis. The characteristic decay time of the heating rate, after reaching its maximum value, is very long, which explains the long rising phase of these flares. 相似文献