摘要:

We invert high spectral resolution hard X‐ray observations of the 1980 June 27 solar flare to obtain the parent X‐ray‐producing electron population. Assuming that collisions are the dominant loss mechanism, the accelerated electron population is derived as a function of time through the flare, using a continuity equation. We find that there appear to be two separate components in the accelerated electrons: an impulsive component with a spectrum peaked at 50 keV, and a slowly varying component with a power‐law spectrum extending down to 20 keV. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50967

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The photosphere reflects a substantial fraction of hard X‐rays impinging on it. Based on Monte Carlo simulations of hard X‐ray propagation within the photosphere, we derive the shape and brightness of the albedo patch resulting from Compton reflection. We discuss the information that can be obtained from observations of the solar hard X‐ray albedo. When the albedo patch is resolved from the primary hard X‐ray source, we can learn about the angular distribution of the high‐energy electrons that produce the X‐rays as well as the height of the primary X‐ray source above the photosphere. Taking into account the albedo, however, is important even when the patch cannot be resolved, as the Compton backscatter affects the total flux, the spectrum and the spatial distribution of the observable X‐rays. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50968

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We examine the spatial relationship between solar flare hard x‐ray emission sites observed with the HXT aboard Yohkoh and photospheric electric currents observed at Mees Solar Observatory. Canfieldetal. (3) concluded that nonthermal electron precipitation tends to occur at the edges of sites of high vertical current, not at their maxima. They did not, however, have very direct observations of the electron precipitation, since they used H&agr; Stark wing emission. In this work we compare hard x‐ray images and vertical current maps in six large M/X‐class flares. Our results confirm that electron precipitation sites avoid regions of strong photospheric vertical currents in large flares, and that magnetic mirroring strongly influences the relative HXR brightness of conjugate footpoints. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50995

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

I review recent elemental abundance studies from X‐ray flare spectra obtained with Bragg crystal spectrometer experiments on board theSMM,P78‐1, andYohkohspacecraft. Using the line‐to‐continuum method, data from all three satellites indicate an enhancement of the abundance of low‐FIP Ca relative to H. But the average magnitude of the enhancement is somewhat uncertain. Flare‐to‐flare variations in the enhancement are also seen. Fe flare abundances seem to be close to photospheric values, with differing methods giving somewhat differing values. These findings, in conjunction with results forS, leave open the possibility thatHmay behave as an intermediate‐FIP element or that a more complex characterization may apply. Further studies of theYohkohdata, and studies comparing different analysis methods are needed to clarify these issues. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50969

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Solar flare dynamics are reviewed as revealed by the X‐ray Bragg Crystal Spectrometer (BCS) package and the Soft X‐ray Telescope (SXT) on theYohkohspacecraft. Doppler motions are detected by BCS as spectral line intensity asymmetries. Motions can be detected by SXT by studying time sequences of images. High speed upflows (400–800 km s−1) are correlated to within a few seconds with the onset of hard X‐ray bursts. However, for most flares a strong non‐Doppler shifted (stationary) spectral line component exists even at flare onset, which is not predicted by standard 1D numerical simulations of chromospheric evaporation into a low density coronal loop. In about 10% of all flares, an intense blueshifted component is present at flare onset. These flares appear to be morphologically complex, but this conclusion is based on poor statistics. The blueshifted plasma shows a longitude dependence, consistent with radial flow. Low speed upflows (<100 km s−1) appear to occur in some instances before the onset of hard X‐ray bursts. In some cases flare loops appear to either twist, expand, or have footpoints that shift in position after the flare rise phase. These motions are slow, e.g., <50 km s−1. As found from previous missions, during the rise phase of flares, line profiles also reveal nonthermal Doppler broadening that decreases during the rise phase from about 200 km s−1to 60 km s−1or less. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50994

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We solve the energy balance equation for a coronal loop that is heated simultaneously by field‐aligned currents and by runaway electrons accelerated by a DC‐electric field. We combine hard and soft X‐ray observations from theComptonGammaRayObservatoryandYohkoh, respectively, and compute the temporal variation of the DC‐electric field strength during the flare impulsive phase. We find that Joule heating by coronal currents dominates the flare energy balance during the preflare and impulsive phases, while collisional heating by runaway electrons becomes significant only during the hard X‐ray impulsive phase. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50971

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We describe a technique for measuring the polarization of hard X‐rays from solar flares based on the angular distribution of that portion of the flux which is scattered off the top of the Earth’s atmosphere. The scattering cross section depends not only on the scatter angle itself, but on the orientation of the scatter angle with respect to the incident polarization vector. Consequently, the distribution of the observed albedo flux will depend on the direction and the polarization properties (i.e., the level of polarization and polarization angle) of the source. Since the albedo component can represent a relatively large fraction (up to 40%) of the direct source flux, there will generally be sufficient signal for making such a measurement. The sensitivity of this approach is therefore dictated by the effective area and the ability of a detector system to ‘image’ the albedo flux. The 4&pgr; coverage of the BATSE detectors on theComptonGamma‐RayObservatoryprovides an opportunity to measure both the direct and the albedo flux from a given solar flare event. Although the BATSE design (with its large field‐of‐view for each detector) is not optimized for albedo polarimetry, we have nonetheless investigated the feasibility of this technique using BATSE data. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50972

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

There are various observational results which suggest that certain solar bursts have spectral components extending into the submm‐w and infrared range. This is essentially unexplored for burst emissions in the continuum. The short time scales of burst structures (tens of milliseconds) correlated to hard X‐rays bring serious constraints for interpretation. One possibility is to assume the impulsive phase made of explosions of compact synchrotron sources and self inverse‐Compton reducing the electrons energy, producing the correlated spiky hard X‐ray component. A new project for a solar submm‐w telescope is now being developed in a collaboration between groups in Brazil, Switzerland and Argentina. It is expected to bring important observational clues for the understanding of the initial phase of flares. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50945

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have analyzed two intense, extended microwave and hard X‐ray flares which occurred slightly behind the west limb and were accompanied by X‐class, long‐duration events (LDEs) in soft X‐rays. We have found that: (1) Both events have typical soft X‐ray properties of LDEs. (2) A major hard X‐ray source at 23–33 keV and 33–53 keV is located in the high‐temperature region, considerably higher than the corresponding soft X‐ray loop, while a 17 GHz microwave source, where electrons at energy ≥ several hundreds keV mainly contribute to the 17 GHz emission, nearly coincides with the soft X‐ray loop in extent. (3) The electron spectrum, which is derived from the microwave spectrum, is significantly harder than the electron spectrum at energy ≤100 keV, which is derived from the hard X‐ray data. (4) The time profile of total intensity at 17 GHz is delayed by about 25 s with respect to that of the hard X‐rays for one of the events. The above results show evidence that lower‐energy (≤100 keV) electrons with a softer spectrum are accelerated at the top of the cusp, and that higher‐energy (≫100 keV) electrons are accelerated at the top of the soft X‐ray loop and trapped. Probably, the lower‐energy electrons accelerated in the cusp are transferred by a reconnection outflow into the second acceleration region at the top of the soft X‐ray loop, and further accelerated to higher energy. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50973

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have provided evidence that certain coronal X‐ray structures such as flaring X‐ray bright points and X‐ray jets give rise to nonthermal radio emission in the form of metric type III bursts. We have shown an example of a metric type IV/flare continuum being associated with the rupture of a flaring loop‐top and the ejection of X‐ray emitting material. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.50974

出版商:AIP    

年代:1996

数据来源: AIP