Hard X-ray Bursts Research Articles

Searching for hard X-ray directivity during the rise, peak, and decay phases of solar flares

We have identified 72 large solar flares (peak counting rates more than 1000 counts/s) observed by Hard X-ray Burst Spectroscopy (HXRBS) on-board the Solar Maximum Mission (SMM). Using a database of these flares, we have studied hard X-ray (50-850 keV) spectral center-to-limb variation and its evolution with time. The major results are the following: (1) During the rise phase, the center-to-limb spectral variation is small, with a hardness of delta delta = 0.02 +/- 0.25, and a statistical significance of 0.1 sigma. (2) During the peak phase, the center-to-limb variation is delta delta = 0.13 +/- 0.13, with a statistical significance of 1 sigma. (3) During the decay phase, the center-to-limb variation changes to softening. The softness is relatively large with delta delta = -0.25 +/- 0.21, and a statistical significance of 1.2 sigma. (4) The linear least-squares fits to the spectral center-to-limb variations do not have slopes significantly different from zero during all those three phases. (5) The center events and limb events spectral distributions are shown to be not different by using Kolmogorov-Smirnov two-samples test. (6) The fraction of events detected near the limb is marginally consistent with that expected from isotropically emitting flares. (7) On average, flares evolve as soft-hard-soft. These results suggest that there is no statistically significant evidence for hard X-ray directivity during the rise, peak, and decay phases of solar flares. The hard X-ray radiation pattern at those energies is almost isotropic during all those phases. This lack of directivity (or anisotropy) found in this study is not in agreement with the results discovered by Vestrand et al. (1987) in which they found energetic photon source is anisotropic, using SMM Gamma-Ray Spectrometer (GRS) data at a much higher energy band of 0.3-1 MeV. If we want to interpret the results of Vestrand et al. (1987) and our present results in a self-consistent way, we must conclude that at energies of greater than 50 keV, the Compton backscattering suppresses the hard X-ray anisotropy sufficiently so that no significant directivity can be measured during the rise, peak, and decay phases of solar flares. during the rise, peak, and decay phases of solar flares.

A Mechanism for Bulk Energization in the Impulsive Phase of Solar Flares: MHD Turbulent Cascade

We propose that the large production rate (approximately 10(exp 36)/s) of energetic electrons (greater than or approximately equal to 25 keV) required to account for the impulsive-phase hard X-ray burst in large flares is achieved through MHD turbulent cascade of the bulk kinetic energy of the outflows from many separate reconnection events. Focusing on large two- ribbon eruptive flares as representative of most large flares, we envision the reconnection events to be the driven reconnection of oppositely directed elementary flux tubes pressing into the flare-length current-sheet interface that forms in the wake of the eruption of the sheared core of the preflare bipolar field configuration. We point out that, because the outflows from these driven reconnection events have speeds of order the Alfven speed and because the magnetic field reduces the shear viscosity of the plasma, it is reasonable that the outflows are unstable and turbulent, so that the kinetic energy of an outflow is rapidly dissipated through turbulent cascade. If the largest eddies in the turbulence have diameters of order the expected widths of the outflows (10(exp 7)-10(exp 8)cm), then the cascade dissipation of each of these eddies could produce approximately 10(exp 26) erg burst of energized electrons (approximately 3 x (10(exp 33) 25 keV electrons) in approximately 0.3 s, which agrees well with hard X-ray and radio sub-bursts commonly observed during the impulsive phase. Of order 10(exp 2) simultaneous reconnection events with turbulent outflow would produce the observed rate of impulsive-phase plasma energization in the most powerful flares (approximately 10(exp 36) 25 keV electrons/ s); this number of reconnection sites can easily fit within the estimated 3 x 10(exp 9) cm span of the overall current-sheet dissipation region formed in these large flares. We therefore conclude that MHD turbulent cascade is a promising mechanism for the plasma energization observed in the impulsive phase of solar flares.

The 1992 January 5 Flare at 13.3 UT: Observations from YOHKOH

We discuss X-ray spectra and soft X-ray images of an M1.9 flare that occurred on 1992 January 5 near 13.3 UT. These data were obtained with instrumentation on the Japanese Yohkoh spacecraft. They cover the entire rise phase of the flare. To supplement these data we have ground-based magnetograms and Halpha spectroheliograms. We calculate the electron temperature and emission measure of the flare as a function of time during the early rise phase using X-ray spectral line intensities and line ratios. Using spectral line widths, line profile asymmetries, and wavelength shifts due to the Doppler effect, we calculate the dynamical properties of the flare. The time development of the morphology of the flare, as revealed by the soft X-ray images and the Halpha spectroheliograms, and the physical quantities inferred from the X-ray spectra, are compared with chromospheric evaporation models. There is an enhancement of blueshifted emission that is closely correlated with the hard X-ray bursts. Heating of one loop in the flare is consistent with a conduction-evaporation model, but heating is found in several structures that do not appear to be physically associated with each other. No standard evaporation model can adequately explain all of the observations.

Temperature and Density Structure of the 1991 November 2 Flare Observed by the YOHKOH Soft X-Ray Telescope and Hard X-Ray Telescope

We measure temperature and density structure for a soft X-ray solar flare observed by the Soft X-Ray Telescope (SXT) on board the Yohkoh satellite. For a flare observed by the SXT on 1991 November 2, we have calculated temperature and emission measure as functions of space and time. The plasma density is calculated from the emission measure using simple geometrical assumptions (for example, cylindrical or spherical symmetry). The energy in the soft X-ray plasma also is calculated. Initial results from this analysis show the following: (1) The flare plasmas range in temperature from several million K up to 25 million K; (2) the flare has impulsive soft X-ray emission from a loop footpoint, which is also the location of the impulsive hard X-ray burst observed by the Yohkoh HXT; and (3) the plasma density, pressure, and energy all increase during the flare

Quasi-periodic Particle Injection into Coronal Loops

view Abstract Citations (40) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Quasi-periodic Particle Injection into Coronal Loops Aschwanden, Markus J. ; Benz, Arnold O. ; Dennis, Brian R. ; Gaizauskas, Victor Abstract We present observations of the flare of 1989 June 22, 1445 UT (in active region NOAA 5555), obtained with the Hard X-Ray Burst Spectrometer (HXRBS) on SMM at energies >52 keV, and with the broad-band radio spectrometer PHOENIX at ETH/Zurich in the frequency range of 100-2800 MHz. The radio emission is dominated by a ≲100% polarized decimetric continuum at 400-1400 MHz, peaking at 750 MHz. The decimetric radio flux is highly correlated with the 50-150 keV hard X-ray flux but is delayed by 3.5-5.4 s with respect to the hard X-rays. The HXR emission shows an excess of ≳10 fast (≳100 ms) spikes (according to Poisson statistics). The radio emission exhibits weak fine structure, consisting of ≍45 quasi-periodic pulses with a mean period of 1.6 s. The frequency-time drift pattern of this fine structure is found to be consistent with segments of inverted-U type bursts, suggesting quasi-periodic injection of electron beams into a loop system. The loop system has an average height of 68,000 km and expands with a velocity of 200 km s-1 due to flare heating. Chromospheric evaporation enhances the electron density near the footpoints. The type III- exciting electrons have a mean velocity of υ/c = 0.30±0.10 (22 keV) and propagate along inverted-U burst trajectories with a mean duration of 2.5 s. For those electrons which reach the mirror point near the opposite footpoint of the loop system, we calculate (from the density and loop length) a low energy cutoff of ≥ 8 keV due to collisional deflection, yielding a propagation velocity of v/c = 0.18 and a propagation delay of 5.1±1.0 s, which agrees well with the observed delay of 5.16 s between the cross-correlated HXR and radio flux. The ≥ 8 keV electrons provide free energy for a loss cone instability near the secondary footpoint, which is observed as decimetric continuum polarized in the same sense of circular polarization as the type III bursts. The constraints from the Hα flare position and the magnetic potential field extrapolation indicate that the loss cone emission is produced in the diverging field region above the umbra of the leading sunspot, which has a photospheric field strength of -1600 G. This flare allows us to deconvolve quasi-periodic particle injection and subsequently triggered coherent radio emission from trapped particles in flare-associated loops. It demonstrates that quasi-periodic modes of particle acceleration, particle dynamics in mirror loops, and the resulting plasma instabilities can be efficiently diagnosed from correlated hard X-ray and radio signatures. Publication: The Astrophysical Journal Pub Date: October 1993 DOI: 10.1086/173283 Bibcode: 1993ApJ...416..857A Keywords: SUN: CORONA; SUN: FLARES; SUN: PARTICLE EMISSION; SUN: RADIO RADIATION; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

The fine scale temporal structure of hard X-ray bursts

We report on the preliminary analysis of fast temporal fluctuations observed in hard X-ray bursts, as recorded by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory.

High spectral resolution, high sensitivity microwave and associated hard X-ray bursts

We have carried out mm-wavelength (18 – 23) GHz observations of solar bursts in June 1989, in Brazil. Nine of the bursts were observed simultaneously with the Hard X-ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM). The Owens Valley Radio Observatory (OVRO) observed five of these events from 1 to 18 GHz. To date, we have analyzed six of these events and the preliminary results are: (i) The turnover frequency of all of these bursts were in the frequency range of 7 – 11 GHz and the radio spectral index ranged from −0.3 to −5.3; (ii) In three bursts, dominant features of high energy (≥ 200 keV) X-rays coincided in time with mm-wavelength peaks, suggesting that the high energy electrons are responsible for the mm-wavelength emission.

Frequency distributions of solar X-ray flare parameters

We have determined frequency distributions of flare parameters from the over 12,000 solar flares that were recorded with the Hard X-ray Burst Spectrometer (HXRBS) on SMM. These parameters include the total flare duration, peak counting rate, the peak hard X-ray flux, the total energy in electrons, and the peak hard X-ray flux in electrons (the latter two computed assuming a thick-target model). The energies were computed above a threshold energy between 25 and 50 keV. All the distributions can be represented by power-laws over a range of several orders of magnitude, above the HXRBS sensitivity threshold. Power-laws can be taken as evidence for self-organized criticality in solar flares. Correlations among these parameters are determined from linear regression fits as well as from the slopes from the frequency distributions. Variations of the frequency distributions were also investigated with respect to the solar cycle.

Flare precursors and triggering

Large solar flares are unpredictable. Some degree of successful forecasting is possible for the time of occurrence, but even with this other properties of the flare will still be a mystery. One aspect of this problem is the identification and interpretation of precursors. We review the various coronal and photospheric/chromospheric events which precede flares, which include the radio, Hα, UV and X-ray regions of the spectrum, together with the magnetograph data. We show how these observations constrain the various models for the energy release in flares. There is evidence that the triggering of the energy release is not coincident with the impulsive hard X-ray burst. This provides constraints both on the nature of the primary energy transfer from the magnetic field and on its location in the solar atmosphere.

The Neupert effect: What can it tell us about the impulsive and gradual phases of solar flares?

The Neupert Effect is the name given to the correlation obsrved in many flares between the impulsive time profile of the microwave and hard X-ray emissions and the time derivative of the soft X-ray profile. We have used data collected between 1980 and 1989 from the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) and the soft X-ray detector on GOES to determine which events show this correlation and which do not. We have found that of 66 HXRBS events observed in 1980 with a peak rate of >1000 counts/s, 58 (80%) showed good correlation with peaks in the GOES time derivative plot corresponding to peaks in the HXR plots to within ±20 s. The more gradually varying X-ray events that are commonly referred to as Type C flares and that are often accompanied by eruptive flares, tend to show poorer correlation between the SXR time derivative and the HXR time profile. In seve al cases studied, the later, more gradually varying, peaks either did not register at all in the SXR time derivative plots or resulted in very broad peaks that, in one ase on 1981 April 26, was delayed by 13 minutes.

Frequency distributions and correlations of solar X-ray flare parameters

We have determined frequency distributions of flare parameters from over 12000 solar flares recorded with the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) satellite. These parameters include the flare duration, the peak counting rate, the peak hard X-ray flux, the total energy in electrons, and the peak energy flux in electrons (the latter two computed assuming a thick-target flare model). The energies were computed above a threshold energy between 25 and 50 keV. All of the distributions can be represented by power laws above the HXRBS sensitivity threshold. Correlations among these parameters are determined from linear regression fits as well as from the slopes of the frequency distributions. Variations of the frequency distributions were investigated with respect to the solar activity cycle. Theoretical models for the frequency distribution of flare parameters depend on the probability of flaring and the temporal evolution of the flare energy build-up. Our results are consistent with stochastic flaring and exponential energy build-up, with an average build-up time constant that is 0.5 times the mean time between flares. The measured distributions of flares are also consistent with predicted distributions of flares from computer simulations of avalanche models that are governed by the principle of self-organized criticality.

Solar Flares: Current Dissipation or Magnetic Annihilation?

Solar flares involve the explosive release of energy, 1022_1025 J in 102-103 s, in the solar corona. A substantial fraction of this energy goes into energetic (~10 keY) electrons, and these produce most of the familiar signatures of flares, such as Ha emission, hard X-ray bursts and type III radio bursts. Solar flares occur in magnetic flux tubes in which the fields are highly stressed with associated large currents, I ~ 1012 A, flowing into the corona. Despite the enormous variety of detailed data on solar flares, there is no wide consensus on the essential theoretical ingredients in an acceptable flare model. Since the first detailed flare models were proposed in the 1940s, there have been two competing types: models based on an electric-current viewpoint and models based on a magnetic-field viewpoint. In principle these are equivalent, but in practice they have led to different and seemingly incompatible models. In this paper the theory of solar flares is reviewed, comparing and contrasting these two viewpoints. It is argued that all models, as presently formulated, contain serious deficiencies. One feature that is unsatisfactory is the treatment of energy propagation into a flare kernel. A specific model for such energy propagation is outlined.

Statistical Characteristics of Solar Proton Flares

AbstractAn investigation of the statistical behaviour of solar flares responsible for the proton events detected at the earth orbit would be of significance for solar physics and sun-earth research. Based on the data given by Kunches (1992) and other relevant data, we study the statistical behaviour of solar proton flares. The asymmetry of distributions, special property of hard X-ray bursts and relation to the large scale mean magnetic field of the proton flares are given in this paper.

A statistical analysis of hard X-Ray solar flares

In this study we perform a statistical study on, 8319 X-Ray solar flares observed with the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission satellite (SMM). The events are examined in terms of the durations, maximum intensities, and intensity profiles. It is concluded that there is no evidence for a correlation between flare intensity, flare duration, and flare asymmetry. However, we do find evidence for a rapid fall-of in the number of short-duration events.

The coevolution of decimetric millisecond spikes and hard X-ray emission during solar flares

Results are presented of an analysis of a comprehensive data set of 27 solar flares with decimetric millisecond spikes between 1980 and 1989, simultaneously observed with the Zuerich radio spectrometers and the Hard X-ray Burst Spectrometer on the SMM spacecraft. Two contradictory relationships of the coevolution of hard X-ray and spiky radio emissions during flares are found: the temporal evolution of both emissions reveals a close functional dependence, but there is a substantial time delay between the two emissions. Five possible scenarios for the hard-X-ray-associated radio spike emission which may account for both their detailed coevolution and their substantial intervening time delay are discussed. All five scenarios are able to explain both the close coevolution of hard X-ray and radio emission as well as their mutual delay to some degree, but none of them can explain all observational aspects in a simple way.

Stochastic acceleration of electrons. I - Effects of collisions in solar flares

Stochastic acceleration of thermal electrons to nonrelativistic energies is studied under solar flare conditions. We show that, in turbulent regions, electron-whistler wave interactions can result in the acceleration of electrons in times comparable to or shorter than the Coulomb collision time. The kinetic equation describing the evolution of the electron energy distribution including stochastic acceleration by whistlers and energy loss via Coulomb interactions is solved for an initial thermal electron energy spectrum. In general, the shape of the resulting electron distributions are characterized by the energy E(c) where systematic energy gain by turbulence equals energy loss due to Coulomb collisions. For energies less than E(c), the spectra are steep (quasi-thermal) whereas above E(c), the spectra are power laws. We find that hard X-ray spectra computed using the electron distributions obtained from our numerical simulations are able to explain the complex spectral shapes and variations observed in impulsive hard X-ray bursts. In particular, we show that the gradual steepening observed by Lin et al. (1981) could be due to a systematic increase in the density of the plasma (due to evaporation) and the increasing importance of collisions instead of the appearance of a superhot thermal component.

Impulsive phase Fe K-alpha emission in a flare of 1989 March

view Abstract Citations (15) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Impulsive Phase Fe K alpha Emission in a Flare of 1989 March Zarro, D. M. ; Dennis, B. R. ; Slater, G. L. Abstract Evidence for enhanced Fe K-alpha line emission that is coincident (to within 15 s) of an intense hard X-ray (greater than 50 keV) burst is presented on the basis of observations of the Fe K-alpha soft X-ray line made with the SMM. Three different models for explaining the K-alpha enhancement are investigated, namely, photoexcitation by soft X-ray thermal bremsstrahlung radiation from an isothermal source, collisional excitation by nonthermal thick-target electrons, and photoexcitation by a nonthermal hard X-ray flux distribution which extends with a power-law spectrum down to the Fe K-alpha ionization threshold at 7.1 keV. It is found that, to within the limits of isothermal temperature and emission measure set by soft X-ray observations, the thermal photoexcitation model cannot reproduce satisfactorily the intensity of enhanced K-alpha emission during the hard X-ray impulsive phase. The impulsive phase K-alpha flux exceeds that predicted by the thermal photoexcitation model by about 3 sigma. The implications of these results for the nonthermal interpretation of impulsive hard X-ray bursts in solar flares are discussed. Publication: The Astrophysical Journal Pub Date: June 1992 DOI: 10.1086/171395 Bibcode: 1992ApJ...391..865Z Keywords: Iron; K Lines; Solar Flares; Solar X-Rays; Thermal Radiation; Bremsstrahlung; Gamma Ray Astronomy; Photoexcitation; Solar Maximum Mission; Solar Radio Bursts; Solar Physics; RADIATION MECHANISMS: THERMAL; SUN: FLARES; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Search for soft gamma repeaters in the SMM/HXRBS data

The triggered fast memory of the hard X-ray burst spectrometer (HXRBS) on board the SMM is used to describe the results of a search for short transients resembling soft gamma repeater (SGR) bursts. Memory data for a total of about 4000 burst triggers, out of which only a very few could be considered as valid SGR candidate events, are analyzed. The search methodology is outlined, the HXRBS exposure and sensitivity to SGR bursts are calculated, and the criteria which constrain the number of candidate events are described. An upper limit is given for the SGR source number density. This limit, combined with results from other relevant observations and the assumption of a neutron star origin, are applied to obtain a constraint on SGR-active lifetimes.

Stereoscopic observations of a solar flare hard X-ray source in the high corona

The vertical structure of the impulsive and gradual hard X-ray sources in high coronae and the characteristics of the impulsive soft X-ray emission are investigated on the basis of PVE, ICE, and GOES observations of the energetic flare on February 16, 1984. The average photon spectra observed by these instruments during the impulsive and gradual hard X-ray bursts are summarized. A comparison of these unocculted and partially occulted spectra shows that the sources of the impulsive hard X-ray (greater than about 25 keV) and impulsive soft X-ray (2-5 keV) emissions in this flare extended to coronal altitudes greater than about 200,000 km above the photosphere. At about 100 keV, the ratio of the coronal source brightness to the total source brightness was 0.001 during the impulsive phase and less than about 0.01 during the gradual hard X-ray burst. The sources of the gradual hard X-ray burst and gradual soft X-ray burst were almost completely occulted, indicating that these sources were located at heights less than 200,000 km above the photosphere.

Hard X-ray characteristics of solar flares using electron Monte-Carlo calculations and beamed thick target model

The experimental results of X-ray bursts with spectral characteristics, spatial distribution, fast time variations, polarization, and directivity measurements carried out with Intercosmos, PVO/ISEE-3 spacecrafts, imaging instrument observations of Hard X-ray (HXIS) and Hard X-ray Burst Spectrometers (HXRBS) during solar maximum mission have been reviewed. The observed results about the above characteristics are discussed in detail in terms of non-thermal and thermal models. It is shown that the results can be interpreted in terms of beamed thick-target model in which electrons stream down to the loop footpoints and produce hard X-rays through electron-ion bremsstrahlung.