摘要:

We reexamined the issue of continuous acceleration vs. trapping in the 1991 June 11 flare using a much broader data set than was available previously. We consider updated EGRET spark chamber data, high energy continuum and nuclear line data from EGRET/TASC, and 2.22 MeV line data from COMPTEL covering an extended time period. We find that the data indicate the existence of at least three distinct emission phases characterized by changes in the ion spectrum during transitions from phase to phase. By combining the 2.22 MeV and 4.44 MeV line fluxes with the pion decay emission flux in the first two phases, we show that ion spectrum hardened during the transition from the first to the second phase. We derive the ion spectrum in the third phase from a detailed spectral analysis of the EGRET spark chamber data and show that this spectrum is consistent with the 2.22 MeV line‐to‐pion decay flux ratio in this phase. The ion spectrum in the third phase is softer than that in the second phase. Concerning variability within the phases, we find that the ion spectrum probably remained constant during the second and third phases. This implies that the hitherto developed ion transport models are not appropriate for explaining the extended emission observed from the June 11 flare. We discuss a different scenario in which the ions are trapped in the low density coronal portions of loops but produce the gamma rays in the denser subcoronal interaction regions; this model of episodal acceleration and subsequent trapping could be consistent with the data. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50996

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The solar flare on March 26, 1991 presents a unique case when high energy (up to 300 MeV) gamma radiation was registered in both, impulsive and delayed, phases of the flare. The radiation in the delayed phase has been attributed to neutral pions decay (1) analogous to the high energy gamma‐ray emission at the late stages of the solar flares on June 11 and June 15, 1991 (2), (3), (4). On the contrary, spectra of the emission in the impulsive phase of the March 26 flare definitely indicate a bremsstrahlung origin of this emission. From the position of the flare close to the center of the disc we conclude that the high energy gamma‐rays could be radiated only by moving upward electrons. We compare time profiles of the gamma‐ray and the microwave emissions and show that the high and the low energy electrons responsible for these emissions were accelerated in the same acts. We put forward arguments in favour of an acceleration of the electrons in the upper chromosphere or in the transition layer. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50959

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have analyzed data on solar protons, neutrons, electrons, gamma‐ray, optical and microwave emissions for the 1990 May 24 solar flare. Taking into account high energy neutron and gamma‐ray observations, we have suggested two neutron injections occurred during the flare. These two injections are calledf‐ (first) ands‐ (second). Two components of interacting protons correspondingly existed to produce these neutrons at the Sun. The flare gave also a rise to solar cosmic ray event, which was detected by the neutron monitor network and GOES satellites. Two components of protons were observed in the interplanetary medium (p‐ (prompt) andd‐ (delayed) components). A possible spectrum of thes‐component of interacting protons coincided with injection spectrum ofp‐component of interplanetary protons. For this reason,s‐ andp‐ components of protons may be considered as different portions of a single population of accelerated particles in the solar corona. The net result is that three proton components (f‐,p/s‐, andd‐) were accelerated during flare process developing from the Sun to the interplanetary medium. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50960

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The solar neutron event on May 24th 1990 has been analysed with use of Shibata’s response function. It has been found from those solar neutron data that ions are accelerated within one minute with power index &ggr;=2.5. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50961

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Solar flare observations with the Hard X‐ray Telescope (HXT) on boardYohkohare briefly reviewed with an attention to the impulsive phase. Hard X‐rays in the impulsive phase are typically emitted from three sources, namely, two ‘‘footpoint sources’’ and a ‘‘loop‐top source’’, with the former pair usually predominating over the latter. The double footpoint sources vary their fluxes simultaneously to each other with time lags less than a few tenths of a second, and are located in or near the chromosphere at magnetically conjugate footpoints, i.e., at the two ends of a flaring loop seen in soft X‐rays. The loop‐top source is a coronal source, located at an altitude of more than 104km above the photosphere. At least in some cases, this source is located well above the apex of the corresponding soft X‐ray loop; this source may be better named ‘‘above‐the‐loop‐top source’’. Implications of these and related observations are discussed for revealing the site(s) and mechanism(s) of magnetic energy release, particle acceleration, and energy transport in solar flares. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50962

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The soft X‐ray telescope onYohkohhas observed plasma structures at the top of flare loops which are suggestive of reconnection jets. However, these structures are relatively cool compared to the surrounding plasma, and their location within the dense flare loops is not consistent with the location expected of a reconnection jet. Numerical simulations of field line reconnection in a radiative plasma suggest that the observed structures are instead condensations which are formed below a reconnection jet lying at higher altitude. In the simulations the condensation is caused by the increase in density downstream of the fast‐mode shock which terminates the reconnection jet. The increased density locally enhances the radiative cooling and causes the top of the loop to cool faster than the legs. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50963

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Solar flares, almost as their defining property, fill coronal magnetic flux tubes with hot plasma. When the temperature of a significant fraction of this plasma exceeds about 30×106K, we call the event ‘‘superhot’’, following the initial observation of the hard X‐ray continuum of such an event by Linetal. (11). TheYohkohobservations include many examples of similar events, of which three have been published thus far. This paper reports a survey of theYohkohobservations, based mainly on the hard X‐ray spectra obtained by the HXT instrument. While comprehensive conclusions will not be possible until the survey includes the Yohkoh imaging observations, we make tentative suggestions here about the nature of flares with superhot properties. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50964

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Hard X‐ray emission is commonly observed in association with solar flares and it is believed to signify interactions of accelerated electrons with the ambient ions. As a first step to understand what conditions are conducive to acceleration of electrons in solar flares, we have analyzedYohkohX‐ray images for a total of four‐dozen events whose hard X‐ray spectra extend at least to 100 keV. The morphology of soft X‐ray emission relative to hard X‐ray sources shows a wide variety, but it is usually confined in a compact loop, which is sometimes topped with a cusp that is much more diffuse. While eruptive behaviors are often seen in the soft X‐ray images, it is hard to decide whether they trigger or result from the nonthermal processes. We give a case study which indicates that double hard X‐ray sources do not always come from conjugate footpoints of a loop. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50993

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

High‐time resolution (64 ms) hard X‐ray (HXR) data fromBATSE/CGROallow us to study the energy‐dependent timing of acceleration, propagation, energy loss, and trapping of ≳20 keV HXR‐emitting electrons during solar flares. In many flares two different HXR flux components can be distinguished: (1) the fine structure of a HXR time profile (containing sequences of subsecond pulses) exhibits delays of ≊10–100 ms for the low‐energy electrons, while (2) the unmodulated HXR time profile (a smooth lower envelope to the fine structure) shows a delay of opposite sign and much larger magnitude, of typically 1–10 s. We model the timing of various acceleration mechanisms and find that the delay of the HXR pulses is dominated by time‐of‐flight differences rather than by acceleration time scales, while the timing of the unmodulated HXR flux is governed by trapping and collisional time scales. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50965

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The present paper discusses the results of a systematic study of subsecond time variations performed on the PHEBUS observations of more than one hundred solar bursts detected around 100 keV. The analysis shows that: 1) a large percentage (73%) of hard X‐ray bursts observed around 100 keV presents time features with rise times in the 100 ms–1 s range, 2) these time features are less systematically observed in long duration events, 3) there is a continuous distribution (during a single burst or for all the bursts of the sample) of the values of the different timescales derived from the analysis. The results are discussed with respect of the electron acceleration timescales in solar flares as well as in the context of the ‘‘statistical flare’’ scenario. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50966

出版商:AIP    

年代:1996

数据来源: AIP