Hard X-ray Bursts Research Articles

Dynamic evolution of the source volumes of gradual and impulsive solar flare emissions

This study compares flare source volumes inferred from impulsive hard X-rays and microwaves with those derived from density sensitive soft X-ray line ratios in the O VII spectrum. The data for this study were obtained with the SMM Hard X-ray Burst Spectrometer, Earth-based radio observatories, and the SOLEX-B spectrometer on the P78-1 satellite. Data were available for the flares of 1980 April 8, 1980 May 9, and 1981 February 26. The hard X-ray/microwave source volume is determined under the assumption that the same electron temperature or power law index characterizes both the source of hard X-rays and the source of microwaves. The O VII line ratios yield the density and volume of the 2 x 10 to the 6th K plasma. For all three flares, the O VII source volume is found to be smallest at the beginning of the flare, near the time when the impulsive hard X-ray/microwave volume reaches its first maximum. At this time, the O VII volume is three to four orders of magnitude smaller than that inferred from the hard X-ray/microwave analysis. Subsequently, the O VII source volume increases by one or two orders of magnitude then remains almost constant until the end of themore » flare when it apparently increases again. 29 references.« less

Solar narrowband decimetric spike bursts and associated phenomena

Based on the observational data of solar narrowband decimetric spike bursts on 1981 Nov 12–14 (Enome et al. 1985) and, associated phenomena, we propose a model of two acceleration regions during the impulsive phase of solar flare. One (region A) is formed in the current sheet by magnetic reconnection process, the other (region B), at the top of the flaring loop by turbulence acceleration. Region B is able to generate mildly relativistic electrons and to emit gyrosynchrotron radiation or decimeter waveband microwave bursts. Two accelerated electron streams escape from region A simultaneously. One travels through the outer corona and excites type III bursts, the other, injected into the emerging flaring-loop at a certain pitch angle, will stimulate electron-cyclotron maser with a “hollow beam” distribution and emit narrowband decimetric spike bursts. Utilizing the formulae of “hollon beam” electron cyclotron maser, we have evaluated the maximum growth rate of the 2nd harmonic wave of FX mode and reached the following conclusions. As the energy of the downward electron stream increases, the maximum growth rate, probability of spike emissions, relative bandwidth and brightlness temperature all increase, the linear size of the spike source decreases, and the peak flux remains fixed. In addition, we have clarified the close association between narrow band spike bursts and type III bursts, and hard X-ray bursts.

Properties of an impulsive compact solar flare determined from Solar Maximum Mission X-ray measurements

The authors have analyzed soft X-ray, hard X-ray, magnetogram, and Hα data for an impulsive compact flare which occurred on 1985 May 21. The X-ray observations were from the X-ray Polychromator and the Hard X-ray Burst Spectrometer on board the SMM satellite. The authors derive the evolution of the density and temperature in the flare loop, and compute the energy content of the flare for the impulsive and gradual phases. The simplest interpretation of the event is one in which the source of the soft X-ray flare emission is confined to a thin loop of very high density. The very short time scales, high pressure and density, and small volume for this event place it beyond the extremes of the classical compact flare characteristics.

Temporal correlations between impulsive ultraviolet and hard X-ray bursts in solar flares observed with high time resolution

The impulsive phase of solar flares has been simultaneously observed in the ultraviolet O V line, the UV continuum, and hard X-rays with a time resolution of 0.128 s by the SMM satellite. A close time correspondence between the three impulsive components is found, with the best correlation being at the peak of the impulsive phase. Individual bursts or fast features in the O V and the UV continuum are shown to lag behind the corresponding hard X-ray features. None of the considered energy transport mechanisms (thermal conduction, a nonthermal electron beam, electron hole boring, UV radiation, and Alfven waves) are able to consistently account for the observed temporal correlations.

A 2.2 second period in the 1984 August 5 gamma-ray burst

view Abstract Citations (18) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A 2.2 Second Period in the 1984 August 3 Gamma-Ray Burst Kouveliotou, C. ; Desai, U. D. ; Cline, T. L. ; Dennis, B. R. ; Fenimore, E. E. ; Klebesadel, R. W. ; Laros, J. G. Abstract A time history and Fourier power spectrum are presented for the August 5, 1984 gamma-ray burst (GRB) observed with the SMM Hard X-ray Burst Spectrometer and the Pioneer Venus Orbiter spacecraft. A significant complex feature of the event is identifiable in both data sets: a gradual ripple with a well-defined 2.2-sec period lasting for seven cycles, with a series of narrow spikes, which appear always on the ascending phase of the sinusoidal ripple. This event is unusual in both its temporal and spectral attributes: it exhibits coexistence of periodic and nonperiodic features, and it was also reported as the most intense GRB event observed with the SMM Gamma-Ray Spectrometer, extending up to 100 MeV. Publication: The Astrophysical Journal Pub Date: July 1988 DOI: 10.1086/185214 Bibcode: 1988ApJ...330L.101K Keywords: Fourier Analysis; Gamma Ray Bursts; Periodic Variations; Power Spectra; Pioneer Venus Spacecraft; Ripples; Solar Maximum Mission; Time Measurement; X Ray Spectroscopy; Space Radiation; GAMMA RAYS: BURSTS full text sources ADS | data products SIMBAD (1)

Evidence for two hard X-ray components in double power-law FITS to the 1980 June 7 flare

The June 7, 1980 flare at 0312 UT was analyzed with double power-law fits on the basis of SMM hard X-ray burst spectrometer data. The flare is found to consist of seven peaks of characteristic time scale of about 8 sec followed by seven valleys which may contain significant peak components because of overlap. It is suggested that the possibility of thermal spectra for the peaks is unlikely. An investigation of the double power-law parameters through the third and fourth peaks revealed a hysteresis effect in the fourth peak. The present results have been interpreted in terms of a trap plus precipitation model.

Rapid temporal evolution of radiation from nonthermal electrons in solar flares

Solutions of the time dependent Fokker-Planck equation was found for accelerated electrons undergoing Coulomb collisions in a magnetized, fully ionized plasma. An exact solution was found for arbitrary pitch angle and energy distribution in a uniform background plasma. Then, for an inhomogeneous plasma, a solution was found for particles with small pitch angles. These solutions were used to calculate the temporal evolution of bremsstrahlung x-rays from short bursts of nonthermal electron beams, and these spectra were compared with observed high time resolution spectra of short timescale solar hard x-ray bursts. It is shown that the observed softening in time of the spectra rules out a homogeneous background and therefore the possibility of electrons being confined to the corona either because of converging magnetic field or high densities. The inhomogeneous solution was also applied to a model with constant coronal density and exponentially rising chromospheric density. The spectra are shown to be consistent with that produced by a collimated beam of electrons accelerated in the corona with certain given conditions. These conditions could be violated if large pitch angle electrons are present.

On the increase of solar activity in the southern hemisphere during solar cycle 21

The present paper investigates the north-south asymmetry for major flares (solar cycles 19 and 20), type II radio bursts (solar cycles 19,20 and 21), white light flares (solar cycle 19,20 and 21), and gamma ray bursts, hard X-ray bursts and coronal mass ejections (solar cycle 21). The results are compared with the found asymmetry in favour of the northern hemisphere during solar cycles 19 and 20 in favour of the southern hemisphere during solar cycle 21.

Energetic electrons in impulsive and extended solar flares as deduced from flux correlations between hard X-rays and microwaves

The peak flux relationship between hard X-rays and microwaves from solar flares is studied using about 400 events simultaneously recorded with the hard X-ray burst spectrometer on the SMM satellite and the Nobeyama 17 GHz radiometer. The data indicate that the hard X-ray and microwave peak fluxes correlate best for X-ray energies of less than about 80 keV for impulsive flares and greater than about 360 keV for extended flares. By postulating that electrons responsible for microwave emission at 17 GHz are those emitting hard X-rays at these photon energies, it is concluded that: (1) in impulsive flares, microwaves at about 20 GHz are emitted mainly by electrons of less than about 200 keV from a layer through which the electrons stream down into the thick-target hard X-ray source; and (2) in extended flares, microwaves are emitted mainly by MeV electrons trapped in a coronal loop or loops.

H? line profile observations of solar flares with high temporal resolution

Hα line profile observations of solar flares with high temporal resolution are an important tool for the analysis of the energy transport mechanism from the site of the flare energy release to the chromosphere. A specially designed instrument (imaging spectrograph) allows two-dimensional imaging of an active region simultaneously in 15 spectral channels along the Hα line profile with a temporal resolution of 5.4 s. Two flares have been observed in November 1982. The first one shows Hα signatures which one would typically expect in the case of explosive chromospheric evaporation produced by massive injection of non-thermal electrons. The observations of the other flare indicate that the heating of the upper chromosphere is dominated by thermal conduction, although during the impulsive hard X-ray burst there are also signatures of heating by non-thermal electrons.

SMM hard X-ray observations of the soft gamma-ray repeater 1806-20

view Abstract Citations (72) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS SMM Hard X-Ray Observations of the Soft Gamma-Ray Repeater 1806-20 Kouveliotou, C. ; Norris, J. P. ; Cline, T. L. ; Dennis, B. R. ; Desai, U. D. ; Orwig, L. E. ; Fenimore, E. E. ; Klebesadel, R. W. ; Laros, J. G. ; Atteia, J. -L. ; Boer, M. ; Hurley, K. ; Neil, M. ; Vedrenne, G. ; Kuznetsov, A. V. ; Sunyaev, R. A. ; Terekhov, O. V. Abstract A new type of gamma-ray source has been recently discovered. Six bursts from this source were recorded with the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission during a highly active phase in 1983. High time resolution measurements of one burst show rise and decay times of less than 5 ms, the fastest yet observed from this source. Time profiles of these events are simple, but indicate low-level emission before and after the main peaks. Two transients in the series show no spectral evolution over their durations of fractions of a second. Bursts from soft gamma-ray repeaters (SGRs) appear to form a separate class of events. The characteristic properites of the three known SGRs are briefly reviewed. Publication: The Astrophysical Journal Pub Date: November 1987 DOI: 10.1086/185029 Bibcode: 1987ApJ...322L..21K Keywords: Astronomical Spectroscopy; Gamma Ray Bursts; Solar Maximum Mission; Spaceborne Astronomy; X Ray Spectroscopy; Spatial Distribution; Spectrum Analysis; Temporal Distribution; Temporal Resolution; Astrophysics; GAMMA RAYS: BURSTS full text sources ADS | data products SIMBAD (3) NED (1)

The H-alpha spectral counterparts of solar hard X-ray microflares

view Abstract Citations (35) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The H alpha Spectral Counterparts of Solar Hard X-Rays Microflares Canfield, Richard C. ; Metcalf, Thomas R. Abstract X-ray observations have revealed energetically significant numbers of very small hard X-ray bursts, termed microflares by Lin et al. The authors have simultaneously observed the Hα counterparts of serveral of these microflares. It is found that microflares occur in regions that are also productive of larger flares, suggesting that they may be components of the larger flare event. All but the weakest miroflares show pronounced impulsive-phase red asymmetry in Hα. Their energetics, interpreted using the thick-target nonthermal model of electron transport, implies that these events are substantially underresolved at the authors' spatial resolution and have a true area of 1015- 1016cm-2. Publication: The Astrophysical Journal Pub Date: October 1987 DOI: 10.1086/165654 Bibcode: 1987ApJ...321..586C Keywords: H Alpha Line; Solar Flares; Solar Spectra; Solar X-Rays; Balloon Flight; Electron Energy; Red Shift; Solar Instruments; Time Series Analysis; Solar Physics; SUN: FLARES; SUN: X-RAYS full text sources ADS |

A high-energy solar flare burst complex and the physical properties of its source region

We discuss a solar flare microwave burst complex, which included a major structure consisting of some 13 spikes of 60 ms FWHM each, observed 21 May, 1984 at 90 GHz (3 mm). It was associated with a simultaneous very hard X-ray burst complex. We suggest that the individual spikes of both bursts were caused by the same electron population: the X-bursts by their bremsstrahlung, and the microwave bursts by their gyrosynchrotron emission. This latter conclusion is based on the evidence that the radio turnover frequency was ≤ 150 GHz. It follows that the emission sources were characterized by an electron density of about 1011 cm−3, a temperature of 5 × 108 K and a magnetic field of about 1400–2000 G. They had a size of about 350 km; if the energy release is caused by reconnection the sources of primary instability could have been smaller and in the form of thin sheets with reconnection speed at a fraction of the Alfven velocity and burst-like energy injections of ≈ 1027 erg during about 50 ms each. The energized plasma knots lost their injection energy by saturated convective flux (collisionless conduction) in about 30 ms.

Observation of nonthermal energy distributions during the impulsive phase of solar flares

The Fe XXV resonance line and dielectronic satellite intensities have been measured as functions of time for several flares recorded by the Naval Research Laboratory crystal spectrometer (SOLFLEX) flown on the US Air Force P78-I spacecraft. The intensity ratios of the Fe XXV resonance line, the Fe XXIV n = 2 satellite line j, and the Fe XXIV n = 3 satellite line d13 indicate that nonthermal electron energy distributions occur during the impulsive phase of the flares. For the electron energies at which the j and d13 satellites are formed (4.7 and 5.8 keV, respectively), the electron energy distributions during the impulsive phase are observed to have a bump or to be nearly flat. For all of the flares that were studied, hard X-ray bursts occurred near the time of the nonthermal distributions observed in the SOLFLEX data.

Short solar events with evidence of repetitive structures

Several short (less than 40 s) solar hard X-ray events (greater than 100 keV) have been observed simultaneously with identical instruments on the Venera 11, 12, 13, and 14 and Prognoz 7 (Franco-Soviet Signe experiments) spacecraft. High time resolution (equal to or greater than 2 ms) data were stored in memory when a trigger occurred. The observations reported here were made by Prognoz 7 and V12 which were closer to the sun at the time and provided good statistics. Observations of modulation with a period of 1.6 s for the event on 1978 December 3 are presented. Also presented is evidence for fast time fluctuations from an event on 1979 November 6, observed with Venera 12, and another on 1981 September 6, observed with the Solar Maximum Hard X-Ray Burst Spectrometer. Power spectrum analysis, epoch folding, and Monte Carlo simulation to evaluate the statistical significance of persistent time delays between features. The results are discussed in light of the MHD model proposed by Zaitsev and Stepanov (1982) and Zaitsev, Stepanov, and Chernov (1984).

Analysis of ultraviolet and X-ray observations of three homologous solar flares from SMM

Three homologous flares observed in the UV lines of Fe XXI and O V and in X-rays from the SMM were studied. It was found that: (1) the homology of the flares was most noticeable in Fe XXI and soft X-ray emissions; (2) the three flares shared many of the same loop footprints which were located in O V bright kernals associated with hard X-ray bursts; and (3) in spite of the strong spatial homology, the temporal evolution in UV and X-ray emissions varied from flare to flare. A comparison between the UV observations and photospheric magnetograms revealed that the basic flare configuration was a complex loop system consisting of many loops or bundles of loops.

The 152-day periodicity of the solar flare occurrence rate

The occurrence rate of solar flares exhibits a periodicity of about 152 days. This periodicity was discovered from analyses of flares observed with the Solar Maximum Mission (SMM), and confirmed by analysis of flares observed by other observatories. This periodicity was also discovered from the flare rate of solar cycle 20 (1965–75). The cause of the 152-day periodicity still remains a mystery. But answers to the following questions will enhance our understanding of it. (1) Is the periodicity a local or a global phenomenon? (2) Is the periodicity due to the interaction of 'hotspots' rotating at different rates such that they align with one another once every 152-day period? (3) Is the periodicity due to the interaction of rotating features originating from g-mode oscillations? Here we report our analysis of 'major flares' observed with the Hard X-ray Burst Spectrometer (HXRBS) aboard SMM and we conclude that the 152-day periodicity is a global phenomenon, and that the answers to questions (2) and (3) are negative.

Distribution of flares on the sun - Superactive regions and active zones of 1980-1985

The author analyzed coordinates of energetic solar flares observed with the hard X-ray burst spectrometer during the period from February 1980 through August 1985. A small number of 'superactive regions' produced the majority of the major flares during this period. There exist active zones, in which the flare occurrence rate is much higher. Two active zones are in the northern hemisphere, and two are in the southern hemisphere. The longitudinal separation of the two active zones in the same hemisphere is about 180 deg, but tha active zones of the southern hemisphere are misaligned from those of the northern hemisphere by about 80 deg. The rotation period of the active zones is 26.75 d. Active zones last several years or longer. There is a hint of active centers rotating at a period of 23.7 d.

High spectral resolution measurements of a solar flare hard X-ray burst

Observations are reported of an intense solar flare hard X-ray burst on June 27, 1980, made with a balloon-borne array of liquid nitrogen-cooled Ge detector which provided unprecedented spectral resolution (no more than 1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 0.1-1 billion K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting about 3-15 sec, which have a hard spectrum and a break energy of 30-65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 kev to at least 100 keV through the event. The double power-law shape indicates that DC electric field acceleration, similar to that occurring in the earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission.

Electron acceleration in solar flares

We present observations of an intense solar flare hard X-ray burst on 1980 June 27, made with a balloon-borne array of liquid nitrogen-cooled germanium detectors which provided unprecedented spectral resolution (≲1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 108–109K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting ∼3–15 s, whch have a hard spectrum and a break energy of 30–65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 keV to ≳100 keV through the event. The double power-law shape indicates that acceleration by DC electric fields parallel to the magnetic field, similar to that occurring in the Earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission. The total potential drop required for flares is typically ∼102 kV compared to ∼10 kV for auroral substorms.