Hard X-ray Bursts Research Articles

Soft X-ray emission from electron-beam-heated solar flares

Using time-dependent numerical simulations and Solar Maximum Mission observations of a solar flare on 1985 January 23, a study is conducted of the ability of an electron-beam-heating model to reproduce the rise phase of a flare as observed in soft X-ray lines of Ca XIX. The electron beam is parameterized by a peak flux, a low-energy cutoff, and a spectral index, and has a time dependence similar to the observed hard X-ray burst. For a spectral index of 6, only models with a low-energy cutoff of 20 keV reproduce the observed peak emission in the Ca XIX line complex. All models with a low-energy cutoff of 15 keV produce too much emission, while all models with a 25-keV cutoff too little emission. None of the models reproduces the temporal behavior of the soft X-ray emission. The electron-beam-heated component is theorized to only represent a small fraction of the energy released in the impulsive phase of this flare.

Spectroscopy of high-temperature solar flare plasmas

The past decade has seen great improvements in the quality of X -ray spectra of solar flares obtained from spacecraft. Such spectra show lines emitted by highly ionized atoms of abundant elements which make up high-temperature plasma contained within coronal magnetic flux tubes. This plasma is probably energized at or a little before the flare impulsive stage, as revealed by bursts of hard X-rays. Temperature and density conditions can be deduced from ratios of line intensities, as well as element abundances under certain conditions. In this paper, several examples of line ratios to deduce these are given. Analysis shows that there is a wide range of electron temperatures - generally from 2 x 10 6 K to 20 x 10 6 K - though sometimes even higher. Electron densities of around 10 17 -10 18 m -3 have been derived, higher values occurring at the flare peak or just before, and then declining. The physical conditions of the hot plasma are now precisely enough known from X -ray spectroscopy that models of flares which have been constructed in the past can be constrained. The most profitable direction for research in this area in the near future would in fact appear to be for a much better linking of the findings from X -ray spectra and modelling of plasma in flux tubes to understand better the flare process in general.

Energetic particle production in flares

Bursts of energetic ions and electrons are frequently seen in interplanetary space and it is beyond doubt that the majority originate at or near the Sun. Many events are correlated with major solar flares and it is customary to associate such flares with energetic particle production. Possible acceleration mechanisms are presented, and we argue that the particle production occurs following magnetic reconnection relatively high in the corona; the bulk of the energy goes into the ionic, rather than the electronic, component. The principal characteristics of energetic solar particle events are reviewed briefly. In small events only a modest amount of energy is deposited below the transition zone, thereby producing a minimal Hα response. In large events, as the bulk of the energy is carried by protons of less than 1 MeV, only during the impulsive phase is a significant amount of energy deposited below the transition zone. Later in the event, chromospheric ablation enhances the coronal density, inhibiting further energy deposition in the chromosphere. A model is outlined whereby acceleration of the electrons responsible for the impulsive hard X-ray burst occurs in the chromosphere as a result of the interaction of the primary ions. Observations supporting this hypothesis are discussed.

SMM/HXRBS observations of Cygnus X-1 from 1986 December to 1988 April

view Abstract Citations (12) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS SMM/HXRBS Observations of Cygnus X-1 from 1986 December to 1988 April Schwartz, R. A. ; Orwig, L. E. ; Dennis, B. R. ; Ling, J. C. ; Wheaton, W. A. Abstract The Solar Maximum Mission's Hard X-ray Burst Spectrometer made 30 measurements of Cygnus X-1 from December, 1986 to April, 1988, yielding a data set of broad synoptic coverage but limited duration for each data point. The hard X-ray intensity was found to be between the gamma(2) and gamma(3) levels, with a range of fluctuations about the average intensity level. The shape of the photon spectrum was found to be closest to that reported by Ling et al. (1983, 1987) during the time of the gamma(3) level emission, although the spectral shapes reported for the gamma(2) and gamma(1) levels were not precluded. Publication: The Astrophysical Journal Pub Date: July 1991 DOI: 10.1086/170281 Bibcode: 1991ApJ...376..312S Keywords: Solar Maximum Mission; Stellar Spectra; X Ray Astronomy; X Ray Binaries; X Ray Spectra; Light Curve; Satellite-Borne Instruments; Spaceborne Astronomy; Spectrum Analysis; Astrophysics; STARS: INDIVIDUAL CONSTELLATION NAME: CYGNUS X-1; X-RAYS: BINARIES; X-RAYS: SPECTRA full text sources ADS | data products SIMBAD (1)

Temporal and Spatial Characteristics of Solar Flares from Observations

AbstractTemporal and spatial characteristics of solar flares are briefly reviewed in this paper. The global, temporal and spatial behaviours of flares are given first. Besides the 154-day periodicity, an 80-day periodicity of occurrence rate of large hard X-ray bursts for the period 1980 February – 1985 December, and the delay of the peak occurrence rate of large flares are pointed out, then the gregariousness of major flares is shown. In the third section, the time process and spatial structure of individual flares are shown and described according to space and ground-based observations. In the last section two problems on flare properties are discussed. (i) Previous classifications of solar flares are based generally on observations in a single spectral region. A new classification of flares based on observations in multi-spectral regions is given. (ii) Energy released in part of a loop seems to be not enough for a whole flare, and a qualitative model in which the energy is supplied by the untwisting of magnetic fields is proposed.

H? line profile observations of a limb flare with high temporal resolution

We observed in Hσa the impulsive phase of a 1N/M2.0 limb flare on September 8, 1988 with high temporal resolution. With an imaging spectrograph line profiles have been acquired every 2.3 s. The temporal evolution in the blue and red wing has been correlated with hard X-ray data from HXRBS. It is shown that a prominent spike in the blue wing of Hα was delayed with respect to the corresponding hard X-ray burst by 7–8 s. With the exception of the spike in the blue wing of Hα, the temporal development of the line profile is compatible with the predictions for nonthermal electron heating during the impulsive phase.

Particle beams in the solar atmosphere: General overview

An overview of particle beams in the solar atmosphere is separated into discussions of (i) current-carrying beams, (ii) current-neutralized electron beams, and (iii) ion beams. The Alfven-Lawson limit on an electric current implies some severe limitations including the following: the current flowing into the corona cannot exceed about 1012 A; if the current density is near threshold for a current instability then the current must flow in thin layers; and, the primary electrons and ions cannot be accelerated simply by the particles falling down a parallel potential drop. Considerable progress has been made in understanding how electron beams in type III solar radio events propagate in a way that is consistent with the generation of Langmuir waves, but a completely consistent picture has not yet emerged. Such beams, and more importantly the electron beams that generate hard X-ray bursts require current neutralization; how the required return current is set up is still not entirely clear. There is direct evidence for ion beams with energies ≳ 10 MeV per nucleon from γ-ray line emission; there is no unambiguous evidence for ion beams of lower energy. A mechanism is suggested for bulk energization of electrons due to dissipation of a parallel current in solar flares. Some outstanding problems concerning particle beams are identified.

On the production of hard X-rays in solar flares

The problem of producing the hard X-ray burst at the onset of solar flares may be thought of in terms of the problem of producing the non-thermal electrons which emit the X-rays via bremsstrahlung. Electron acceleration to relativistic energies without similar ion acceleration is difficult to achieve, even in an ad hoc theoretical model. Yet from global energetic considerations, it is not feasible to accelerate the electrons as a minor constituent of the total energetic particle population. Therefore, it is necessary to invoke a more sophisticated process for the electron acceleration. In this paper we describe a mechanism for achieving this via an initial acceleration of a neutralized ion beam. When such a beam impacts the chromosphere, the electrons start to scatter while the ions continue downwards, rapidly setting up an electric field which is either cancelled by the inflow of background chromospheric electrons or results in the runaway acceleration of beam electrons. In the former case the result is simply heating, whereas in the latter case much of the ion kinetic energy is transferred into electron kinetic energy. The final electron energy may be similar to the typical energy of the ions. The electrons that are accelerated are those in the neutral beam that experience an electric field greater than the critical Dreicer field. Thus there will be a low-energy cut-off to the electron spectrum which overcomes the well-known energetics problem at low energies with certain other spectral forms.

H-alpha spectra of dynamic chromospheric processes in five well-observed X-ray flares

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Statistical study of the correlation of hard X-ray and type III radio bursts in solar flares

A large number of hard X-ray events which were recorded by the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) during the maximum of the 21st solar cycle (circa 1980) are analyzed in order to study their statistical correlation with type 3 bursts. The earlier finding by Kane (1981) are confirmed qualitatively that flares with stronger hard X-ray emission, especially those with harder spectra, are more likely to produce a type 3 burst. The observed distribution of hard X-ray and type 3 events and their correlations are shown to be satisfactorily described by a bivariate distribution consistent with the assumption of statistical linear dependence of X-ray and radio burst intensities. From this analysis it was determined that the distribution of the ratio of X-ray intensity (in counts/s) to type 3 intensity (in solar flux units) which has a wide range and a typical value for this ratio of about 10. The implications of the results for impulsive phase models are discussed.

Observational clues to the energy release process in impulsive solar bursts

The nature of the energy release process that produces impulsive bursts of hard X-rays and microwaves during solar flares is discussed, based on new evidence obtained using the method of Crannell et al. (1978). It is shown that the hard X-ray spectral index gamma is negatively correlated with the microwave peak frequency, suggesting a common source for the microwaves and X-rays. The thermal and nonthermal models are compared. It is found that the most straightforward explanations for burst time behavior are shock-wave particle acceleration in the nonthermal model and thermal conduction fronts in the thermal model.

Plasma motions in the flare of 1982 June 6 (X12)

This paper analyzes soft X-ray spectra obtained from the Hinotori spacecraft for the investigation of plasma motions during the initial phase of the great flare, 1982 June 6. The wavelength calibration of the scanning spectrometers is determined from information on the spacecraft attitude and from the position of the Fexxv resonance line during the decay phase. Hard X-ray bursts, nonthermal line broadenings and blueshifted components in X-ray lines are temporally correlated with time differences of 0–30 s. The possible contribution of the blueshifted component to the line width decreases more rapidly than the nonthermal broadening, which suggests dominant plasma motions are taking place at higher and higher altitude in the corona, because of the increase of electron density in flaring loops. The evolution of the input kinetic energy content to the thermal plasma inferred from line broadenings in the impulsive phase resembles that of the thermal energy content in the source of the Fexxvi emission, which is different from that deduced for Fexxv source. This suggests that the origins of the nonthermal line broadening and Fexxvi source are closely coupled.

Field‐aligned potential drops in the solar corona

A field‐aligned potential drop of Φ∥ ≤ 100 kV can be produced in the corona due to the loss cone constriction effect on a 0.25 A/m² field‐aligned current driven by the dynamo process in the photosphere under the condition that Nm ≥ 2×104 cm−3 and Bm ≥ 60 G near the top region of a coronal loop. This condition probably can be satisfied during coronal mass ejections when the coronal loop rises to a height greater than one solar radius. Since coronal mass ejection events are often observed to precede solar flares, the proposed model of field‐aligned potential drop can be related to solar flares through coronal mass ejection events. The resulting 100 kV field‐aligned potential drop in the corona can accelerate an electron beam to produce nonthermal hard X‐ray bursts. The magnetic field produced by the field‐aligned current is estimated to be δB(0) ≈ 0.3 T at the foot of an erupting coronal loop. The ratio δB(0)/Bo = 1, which is on the high side, but is probably acceptable for coronal mass ejection events of very large spatial scales.

Magnetic energy release in interacting loops observed in a solar flare from Sylab

We studied the energy release morphology in the 21 January 1974 flare observed in the XUV wavelength from Skylab . The XUV images of the flare show that the magnetic configuration consisted of a large cool loop (10 4 – 10 5 K) and a small low-lying high temperature (> 10 7 K) loop. The interaction between the loops, indicated by a hard X-ray burst, triggered the release of stored energy in the two loops. In the large cooler loop, the energy release was manifested in a possible kink instability, which released the stored energy as kinetic energy of motion. In the low lying loop, the energy release was manifested in the heating of the plasma, thus producing the high temperature emissions as observed in Fe XXIV. The observation illustrates clearly the involvement of a multiloop system in the flare energy release and shows that individual loops may undergo different evolutions due to the different conditions in them. This picture is consistent with the flare energy release scenario deduced from the most recent SMM observations /1/.

Ultraviolet observations of the preimpulsive phase in a solar flare - Enhanced turbulence and heating

The unique UV spectra of the January 21, 1974 flare obtained from Skylab before the onset of an impulsive hard X-ray burst are presented. The observations show that the UV emission lines were much enhanced and their line profiles much widened, indicating large nonthermal turbulence in the transition region plasmas before the impulsive phase. This excitation could be due to the dissipation of enhanced current in the preflare loop. This excitation continued until some parameters that characterized the current flow were exceeded and some plasma instabilities appeared that accelerated the electrons which produced the hard X-ray emission. The heating of the flaring plasmas to high temperatures started before the impulsive phase, as shown by the presence of Fe XXI emission. 21 refs.

Soft X-ray diagnostics of electron-heated solar flare atmospheres

The dependence of the impulsive-phase Ca XIX w line profile on the form of the flare energy input (assumed to be due to the collisional degradation of a beam of high-energy electrons) is considered. The injected flux spectrum has the form of a power law with a low-energy 'knee', and the effects of varying the total energy flux, spectral index, and knee energy on the w line profile during the impulsive phase have been evaluated. Early in the burst, blueshifts of order 400 km/s are noted, and the peak intensity of the blueshifted component together with spatially unresolved hard X-ray burst spectra can be used to determine the beam filling factor.

The spatial scales and rise times of impulsive solar burst sources

During the rise time of an impulsive solar burst of hard X-rays and microwaves, energy release overwhelms losses so that the characteristics of the flare energy release process are best displayed. A sample of 30 bursts was analyzed, and the rise times of the bursts were compared with their source length scales. The result, as in earlier work employing the 'thermal conduction-front' model, is that the burst length scales were linerly proportional to their rise times, and ranged from 5.4 x 10 to the 6th to 3.6 x 10 to the 9th cm. This proportionality places a new and restrictive condition upon the possible energy release processes. The new condition is especially powerful because the proportionality was formerly believed to depend on the thermal source model alone, but now is known to follow from every model in current use.

Studies on post-flare loop prominence of 1981 April 27

By use of the Hα observations of the Astrophysical Observatory in Catania, Italy and the Purple Mountain Observatory in Nanking, China as well as hard X-ray and gamma-ray burst data from the Solar Maximum Mission (SMM) Gamma-Ray Spectrometer (GRS), a major eruptive loop prominence was studied during the limb solar flare event of 1981 April 27. Our preliminary analysis shows that there seems to exist a second abrupt energy release for this event, almost 20 min after the end of the impulsive phase of the flare. This energy release is probably associated with the rapidity in upward motion or activation of the loop prominence. A possible candidate for such a process could be the reconnection of the old magnetic field with a newly emerging magnetic field. A theoretical gross estimate for the energy release and particle acceleration has also been made in this work. It appears that the proposed model for charged particle acceleration is very efficient.

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.

Directionality of continuum gamma rays from solar flares

Using hard X-ray burst spectrometer total counts as calibration, the changes in the brightness of gamma rays above 300 keV as a function of the heliocentric angle of a flare are investigated. The normalized gamma-ray brightness, on the average, increases with the heliocentric angle; a flare at the limb is estimated to be 13 times brighter in beam gamma rays than a similar flare near the central meridian. Both pancakelike electron distributions and downward beam distributions can be adjusted to produce the deduced limb brightening.