共查询到20条相似文献，搜索用时 281 毫秒
1.
Second-step acceleration of nonrelativistic protons and ions in impulsive solar flares is discussed extending our earlier calculations for relativistic electrons. We derive the relevant particle transport equation, discussing in detail the influence of the particle's effective charge and mass number on the various momentum gain (stochastic acceleration, diffusive shock wave acceleration) and loss (Coulomb interactions, particle escape) processes. Analytical solutions for the ion-momentum spectra in the hard-sphere approximation are given. The inclusion of Coulomb losses modify the particle spectra significantly at kinetic energies smaller than E
B
= 0.64(
e
/5.0) MeV nucl.–1 from the well-known Bessel function variation in long-duration flares. For equal injection conditions this modification explains the observed much smaller ion fluxes from impulsive flares at high energies as compared to long-duration flares. We also calculate the 3He/4He-isotope variation as a function of momentum in impulsive flares in the hard-sphere approximation and find significant variations near E
m
= 0.38(T
e
/2 × 106 K) MeV nucl.–1, where T
e
is the electron temperature of the coronal medium. 相似文献
2.
Recent observations show that the rapid fluctuations in radio, hard X-ray, and H emissions are closely associated with type III and microwave (or decimetric) bursts during the impulsive and/or preimpulsive phases of solar flares.In order to clarify the physical processes of these observed phenomena, this paper proposes a tentative model of two acceleration regions A (source of type III bursts) and B (source of microwave or decimetric bursts) formed in the neutral sheet and at the top of a flaring loop, respectively; and also suggests that the electron beams streaming from region A and/or region B downward to the chromosphere are responsible for the rapid fluctuations in the different emissions mentioned above during the impulsive and/or pre-impulsive phases of solar flares. 相似文献
3.
Yuri E. Litvinenko 《Solar physics》2002,205(2):341-349
Competition between stochastic energy gains and collisional energy losses is known to lead to preferential acceleration of heavy ions in flare loops. Ion acceleration in a reconnecting current sheet is shown to mitigate the influence of collisional energy losses on stochastic particle acceleration in impulsive solar flares. This effect decreases the sensitivity of the resulting abundance ratios on initial ion charge states. The resulting abundances are determined by the fact that the energy loss rate falls off rapidly with increasing energy. As an example, the expected Fe/O enhancement ratios are computed and shown to be comparable with those observed with ACE SEPICA in several impulsive flares in 1998. One consequence of the model is that the preferential acceleration of heavy ions can occur only when the plasma gas pressure is large enough, m
e/m
p, which may explain the observed correlation between the heavy ion enrichment and selective 3He acceleration in impulsive flares. 相似文献
4.
Recent observations have provided much real information about the acceleration of particles in solar flares. High-reliability data about accelerated particles have been obtained for an impulsive phase of some flares of the activity cycle XXII. Therefore, it seems reasonable to re-estimate the amount of Li atoms produced in the upper photospheric layers by – reactions. A value of 5 × 10 29 nuclei during the largest impulsive solar events has been found from calculations for the thick-target model. This agrees with observations of the line of lithium. In conclusion, the probability of enhanced Li absorption observed after large impulsive flares in the sunspot penumbra is discussed. 相似文献
5.
Jun-Ichi Sakai 《Solar physics》1992,140(1):99-119
We present a model for high-energy solar flares to explain prompt proton and electron acceleration, which occurs around moving X-point magnetic fields during the implosion phase of the current sheet. We derive the electromagnetic fields during the strong implosion of the current sheet, which is driven by the converging flow toward the center of the magnetic arcade. We investigated a test particle motion in the strong electromagnetic fields derived from the MHD equations. It is shown that both protons and electrons can be promptly (within 1 s) accelerated to 70 and 200 MeV, respectively. This acceleration mechanism can be applicable for the impulsive phase of the gradual gamma-ray and proton flares (gradual GR/P flare), which have been called two-ribbon flares. 相似文献
6.
Energetic solar electrons in the interplanetary medium 总被引：3，自引：0，他引：3
R. P. Lin 《Solar physics》1985,100(1-2):537-561
ISEE-3 measurements extending down to 2 keV energy have provided a new perspective on energetic solar electrons in the interplanetary medium. Impulsive solar electron events are observed, on average, several times a day near solar maximum, with 40% detected only below 15 keV. The electron energy spectra have a nearly power-law shape extending smoothly down to 2 keV, indicating that the origin of these events is high in the corona. These coronal flare-like events often produced 3He-rich particle events.In large solar flares which accelerate electrons and ions to relativistic energies, the electron spectrum appears to be modified by a second acceleration which results in a double power-law shape above 10 keV with a break near 100 keV and flattening from 10–100 keV. Large flares result in long-lived (many days) streams of outflowing electrons which dominate the interplanetary fluxes at low energies. Even in the absence of solar activity, significant fluxes of low energy electrons flow out from the Sun.Solar type-III radio bursts are produced by the escaping 2–102 keV electrons through a beam-plasma instability. The detailed ISEE-3 measurements show that electron plasma waves are generated by the bump-on-tail distribution created by the faster electrons running ahead of the slower ones. These plasma waves appear to be converted into radio emission by nonlinear wave-wave interactions. 相似文献
7.
S. Kahler N. R. Sheeley Jr. R. A. Howard M. J. Koomen D. J. Michels 《Solar physics》1984,93(1):133-141
We attempt to study the origin of coronal shocks by comparing several flare characteristics for two groups of flares: those with associated metric type II bursts and coronal mass ejections (CMEs) and those with associated metric type II bursts but no CMEs. CMEs accompany about 60% of all flares with type II bursts for solar longitudes greater than 30°, where CMEs are well observed with the NRL Solwind coronagraph. H flare areas, 1–8 Å X-ray fluxes, and impulsive 3 cm fluxes are all statistically smaller for events with no CMEs than for events with CMEs. It appears that both compact and large mass ejection flares are associated with type II bursts. The events with no CMEs imply that at least many type II shocks are not piston-driven, but the large number of events of both groups with small 3 cm bursts does not support the usual assumption that type II shocks are produced by large energy releases in flare impulsive phases. The poor correlation between 3 cm burst fluxes and the occurrence of type II bursts may be due to large variations in the coronal Alfvén velocity.Sachs/Freeman Associates, Inc., Bowie, MD 20715, U.S.A. 相似文献
8.
The new class of -ray spectra from impulsive flares without nuclear -ray lines is compared with bremsstrahlung spectra of energetic electrons undergoing stochastic acceleration, Coulomb and synchrotron losses. The remarkable agreement of both the produced -spectra from the precipitated electrons and the electron spectra measured in the interplanetary space leads to the conclusion that seed population and acceleration process are identical for both classes of electrons. A new estimate of the electron bremsstrahlung contribution in -spectra of impulsive solar flares seems to be necessary. 相似文献
9.
We summarize ERNE/SOHO observations of solar energetic particle events associated with impulsive soft X-ray flares and LASCO coronal mass ejections (CMEs). The new observational data support an idea that the >10 MeV proton acceleration may be initiated at different coronal sources, operating in the flaring active region and on the global coronal scale, in concert with CME development. However, the particle acceleration continues beyond the coronal scales and may culminate at the interplanetary CME well after the flare. We emphasize the importance of CME liftoff/aftermath processes in the solar corona and the possible role of seed particle re-acceleration, which may explain the existence of hybrid solar energetic particle events. 相似文献
10.
A. O. Benz T. Kosugi M. J. Aschwanden S. G. Benka E. L. Chupp S. Enome H. Garcia G. D. Holman V. G. Kurt T. Sakao A. V. Stepanov M. Volwerk 《Solar physics》1994,153(1-2):33-53
Particle acceleration is intrinsic to the primary energy release in the impulsive phase of solar flares, and we cannot understand flares without understanding acceleration. New observations in soft and hard X-rays, -rays and coherent radio emissions are presented, suggesting flare fragmentation in time and space. X-ray and radio measurements exhibit at least five different time scales in flares. In addition, some new observations of delayed acceleration signatures are also presented. The theory of acceleration by parallel electric fields is used to model the spectral shape and evolution of hard X-rays. The possibility of the appearance of double layers is further investigated.Report of Team 3, Flares 22 Workshop, Ottawa, May 25–28, 1993. 相似文献
11.
Among all the signatures of solar ejecta in interplanetary space, magnetic clouds are particularly interesting. We have shown that they are associated with solar mass ejections that involve not only coronal heights, but also chromospheric heights and so, they are almost always associated with low-altitude solar activity such as H flares or filament eruptions. As a magnetic cloud is a very large structure, and not all the ejecta found in the interplanetary medium are clouds, it is interesting to investigate the characteristics of the large-scale coronal magnetic structures in the regions where the activity leading to a cloud takes place. In this paper we use Hoeksema's potential field model of the solar magnetosphere to obtain the magnetic structure of the site of the solar events associated with 35 interplanetary magnetic clouds. The position of the related solar activity was determined from the location of the near-surface solar explosive events (flares and filament eruptions) associated with each cloud, obtained in our previous study. We find that the solar activity associated with interplanetary magnetic clouds occurs in regions of low-altitude, magnetically closed structures lying between higher helmets, or between the highest helmets and coronal holes, where the magnetic field lines are longitudinally oriented. 相似文献
12.
We use H, X-ray, and kilometric radio data to examine the solar coronal activity associated with energetic (1 MeV/nucl–1) 3He-rich particle events observed near Earth. The basis of the study is the 12 3He-rich events observed in association with impulsive 2 to 100 keV electron events reported by Reames et al. (1985). We find that when H and X-ray brightenings can be associated with 3He/electron events, they have onsets coinciding to within 1 min of that of the associated metric type III bursts. In three or four events we found no associated H or X-ray flares, and in two events even the metric type III bursts were weak or absent. The measured low-energy (2 keV) electron spectra for these events show no evidence of a flattening due to Coulomb collisional losses. These results and several other recent findings are consistent with the idea that the 3He/electron events are due to particle acceleration in the corona well above the associated H and X-ray flares. 相似文献
13.
We have analyzed five solar energetic particle (SEP) events observed aboard the SOHO spacecraft during 1996–1997. All events
were associated with impulsive soft X-ray flares, Type II radio bursts and coronal mass ejections (CMEs). Most attention is
concentrated on the SEP acceleration during the first 100 minutes after the flare impulsive phase, post-impulsive-phase acceleration,
being observed in eruptions centered at different solar longitudes. As a representative pattern of a (nearly) well-connected
event, we consider the west flare and CME of 9 July 1996 (S10 W30). Similarities and dissimilarities of the post-impulsive-phase
acceleration at large heliocentric-angle distance from the eruption center are illustrated with the 24 September 1997 event
(S31 E19). We conclude that the proton acceleration at intermediate scales, between flare acceleration and interplanetary
CME-driven shock acceleration, significantly contributes to the production of ≳10 MeV protons. This post-impulsive-phase acceleration
seems to be caused by the CME lift-off. 相似文献
14.
The two major candidates for proton acceleration in impulsive -ray producing flares, shock and stochastic acceleration, are considered in light of recent observations of mass motions and turbulence in flares. Starting with the basic problem of energies required, energy storage and the currents which must be involved, it is concluded that the primary energy release must occur close to the temperature minimum region. It is shown that energy can propagate upwards in the form of fast magnetosonic waves which become evanescent in the transition region, converting a large fraction of their energy to mass motions and turbulence. Present observations are mostly of rather coarse (7000 km) spatial resolution and it is quite possible that significantly higher velocities than those observed were present. Using the results of recent simulations of parallel shocks and the well tested theory of Lee (1983) for parallel shock acceleration in the interplanetary medium, it is shown that shock acceleration is a viable candidate at velocities slightly higher than present observations. It is also shown that shocks must be driven by a mass of material which would be visible in coronal lines such as Caxix for them to be energetically important in proton acceleration.Stochastic acceleration is examined using the hypothesis that there is an equipartition of energy between observed turbulence and magnetic field fluctuations. It is shown that this is a viable acceleration mechanism within a large range of presently observed turbulence provided that the above equipartition hypothesis is valid and the turbulent elements are of small scale (1–200 km). Since turbulence is observed in many flares without any evidence of -rays, one of the above conditions must not be satisfied in general. It is concluded that although present observations favor stochastic acceleration, no definitive conclusion can be made without higher spatial resolution observations and additional theoretical work. 相似文献
15.
A. Krüger J. Hildebrandt B. Kliem H. Aurass J. Kurths V. V. Fomichev I. M. Chertok L. Křivský 《Solar physics》1991,134(1):171-186
Behind-the-limb flares provide a unique opportunity for the study of vertical source structures of microwave bursts and dynamic flare processes. Based on complex observational data related to the outstanding solar proton event on 16 February, 1984, the development of burst emission at a height z 200000 km above the photosphere has been investigated. A comparison with the associated X-ray emission measured aboard various spacecraft yields a time lag of about 1 min between the onset of the unocculted impulsive HXR-emission and the onsets of the X-ray and microwave emissions occulted by the solar limb. The lag corresponds to a range of speeds of the propagation of the flare volume of about 3000–5000 km s–1. Considering competing transport agents that could account for such expansion of the source volume, a qualitative model of shock-wave activation of loops successively reaching into larger coronal heights is proposed.From a discussion of the possible emission processes involved, conclusions about the magnetic field, electron density, and particle energies have been obtained. 相似文献
16.
In this study we analyse the positions of major flares from 1978 and 1979, with respect to the magnetic structure of the solar corona, as described by a potential field model. We find that major flares exhibit no strong association with the neutral line at the chromospheric level. However, when we calculate the neutral line's position at higher and higher altitudes in the corona, we find that major flares show an increasing tendency to be found close to these high-altitude coronal neutral lines. The correlation between flares and higher-altitude coronal neutral lines reaches a maximum at an altitude of 0.35R
, and thereafter decreases as the neutral line is moved out to the source surface at an altitude of 1.50R
. This indicates that major flares are strongly associated with coronal structure at the 0.35R
level ( 250 000 km) - an altitude surprisingly high in the corona. This reinforces the idea that flares are associated with large-scale coronal magnetic fields and also indicates that the region of coronal magnetic topology important to solar flare processes may be larger than previously thought. 相似文献
17.
The physical parameters for the kernels of three solar X-ray flare events have been deduced using photographic data from the S-054 X-ray telescope on Skylab as the primary data source and 1–8 and 8–20 Å fluxes from Solrad 9 as the secondary data source. The kernels had diameters of 5–7 and in two cases electron densities at least as high as 3 × 1011 cm–3. The lifetimes of the kernels were 5–10 min. The presence of thermal conduction during the decay phases is used to argue: (1) that kernels are entire, not small portions of, coronal loop structures, and (2) that flare heating must continue during the decay phase.We suggest a simple geometric model to explain the role of kernels in flares in which kernels are identified with emerging flux regions. The flare is triggered at the neutral sheet between the EFR and a larger loop structure. We associate the X-ray kernels with H kernels, which previously associated (incorrectly, we believe) with the nonthermal impulsive phases of flares.Most of this work was completed while the author was a Visiting Scientist at the Center for Astrophysics, Cambridge, Massachusetts 02138. 相似文献
18.
Observations of impulsive solar flare X-rays 10 keV by the OGO-5 satellite and the measurements of energetic solar electrons made with the Explorer-35 and Explorer-41 (IMP-5) satellites during the period March 1968–September 1969 have been analyzed in order to determine the ion density in the X-ray source region as well as the location of the electron acceleration region in the solar atmosphere. If we assume that the efficiency of escape of the accelerated electrons into interplanetary space is 10–3, the observations are found to be consistent with the following interpretation: (i) the ion density in the X-ray source region varies from event to event and lies between 109 and 1011 ions cm–3 for those events in which the impulsive X-ray emission could be detected; (ii) for those events in which no impulsive emission was detected above threshold, the ion density in the X-ray source was < 109 ions cm–3; (iii) at least in some small solar flares the region where the electrons are accelerated during the flash phase is located in the lower corona. 相似文献
19.
We re-examine observations bearing on the origin of metric type II bursts for six impulsive solar events in November 1997. Previous analyses of these events indicated that the metric type IIs were due to flares (either blast waves or ejecta). Our point of departure was the study of Zhang et al. (2001) based on the Large Angle and Spectrometric Coronagraphs C1 instrument (occulting disk at 1.1 R0) that identified the rapid acceleration phase of coronal mass ejections (CMEs) with the rise phase of soft X-ray light curves of associated flares. We find that the inferred onset of rapid CME acceleration in each of the six cases occurred 1–3 min before the onset of metric type II emission, in contrast to the results of previous studies for certain of these events that obtained CME launch times 25–45 min earlier than type II onset. The removal of the CME-metric type II timing discrepancy in these events and, more generally, the identification of the onset of the rapid acceleration phase of CMEs with the flare impulsive phase undercuts a significant argument against CMEs as metric type II shock drivers. In general, the six events exhibited: (1) ample evidence of dynamic behavior [soft X-ray ejecta, extreme ultra-violet imaging telescope (EIT) dimming onsets, and wave initiation (observed variously in H, EUV, and soft X-rays)] during the inferred fast acceleration phases of the CMEs, consistent with the cataclysmic disruption of the low solar atmosphere one would expect to be associated with a CME; and (2) an organic relationship between EIT dimmings (generally taken to be source regions of CMEs) and EIT waves (which are highly associated with metric type II bursts) indicative of a CME-driver scenario. Our analysis indicates that the broad (90 to halo) CMEs observed in the outer LASCO coronagraphs for these impulsive events began life as relatively small-scale structures, with angular spans of 15 in the low corona. A review of on-going work bearing on other aspects (than timing) of the question of the origin of metric type II bursts (CME association; connectivity of metric and decametric-hectometric type II shocks; spatial relationship between CMEs and metric shocks) leads to the conclusion that CMEs remain a strong candidate to be the principal/sole driver of metric type II shocks vis-à-vis flare blast waves/ejecta. 相似文献
20.
Particle Acceleration and Propagation in Strong Flares without Major Solar Energetic Particle Events
Solar energetic particles (SEPs) detected in space are statistically associated with flares and coronal mass ejections (CMEs).
But it is not clear how these processes actually contribute to the acceleration and transport of the particles. The present
work addresses the question why flares accompanied by intense soft X-ray bursts may not produce SEPs detected by observations
with the GOES spacecraft. We consider all X-class X-ray bursts between 1996 and 2006 from the western solar hemisphere. 21
out of 69 have no signature in GOES proton intensities above 10 MeV, despite being significant accelerators of electrons,
as shown by their radio emission at cm wavelengths. The majority (11/20) has no type III radio bursts from electron beams
escaping towards interplanetary space during the impulsive flare phase. Together with other radio properties, this indicates
that the electrons accelerated during the impulsive flare phase remain confined in the low corona. This occurs in flares with
and without a CME. Although GOES saw no protons above 10 MeV at geosynchronous orbit, energetic particles were detected in
some (4/11) confined events at Lagrangian point L1 aboard ACE or SoHO. These events have, besides the confined microwave emission,
dm-m wave type II and type IV bursts indicating an independent accelerator in the corona. Three of them are accompanied by
CMEs. We conclude that the principal reason why major solar flares in the western hemisphere are not associated with SEPs
is the confinement of particles accelerated in the impulsive phase. A coronal shock wave or the restructuring of the magnetically
stressed corona, indicated by the type II and IV bursts, can explain the detection of SEPs when flare-accelerated particles
do not reach open magnetic field lines. But the mere presence of these radio signatures, especially of a metric type II burst,
is not a sufficient condition for a major SEP event. 相似文献