Elementary Flare Bursts Research Articles

The Evolution of Reconnection along an Arcade of Magnetic Loops

RHESSI observations of a solar flare showing continuous motions of double hard X-ray sources interpreted as footpoints of magnetic loops are presented. The temporal evolution shows many distinct emission peaks of duration of some tens of seconds (elementary flare bursts). Elementary flare bursts have been interpreted as instabilities or oscillations of the reconnection process leading to an unsteady release of magnetic energy. These interpretations based on two-dimensional concepts cannot explain these observations, showing that the flare elements are displaced in a third dimension along the arcade. Therefore, the observed flare elements are not a modulation of the reconnection process but originate as this process progresses along an arcade of magnetic loops. Contrary to previous reports, we find no correlation between footpoint motion and hard X-ray flux. This flare apparently contradicts the predictions of the standard translation invariant 2.5-dimensional reconnection models.

On the Fast Fluctuations in Solar Flare Hα Blue Wing Emission

Fine temporal structures in hard X-ray and microwave emissions of solar flares have been known for many years. Recent observations with high time and spatial resolution revealed that emissions in the wings of Hα could also exhibit fast (subsecond) fluctuations. We argue that such fluctuations are physically related to the small-scale injection of high-energy electrons. We explore this through numerical calculations. The energy equation and the equations for energy-level populations in hydrogen, in particular including the nonthermal collisional excitation and ionization rates, are solved simultaneously for an atmosphere impacted by a short-lived electron beam. We determine the temporal evolution of the atmospheric temperature, the atomic level populations, and the Hα line intensity. We find that although the background Hα wing emission is mainly formed in the photosphere, the fast fluctuations are probably produced in the chromosphere, which is penetrated by ~20 keV electrons. To yield Hα wing fluctuations of amplitude comparable to the observations, a mean energy flux of ~ × 1011 ergs cm-2 s-1 is required for the electron beam, if one adopts a Gaussian macrovelocity of 25 km s-1. Such a burst contains a total energy of 1025-1026 ergs. These parameters are compatible with elementary flare bursts.

Fast discrete emissions at hard X-rays and sub-mm-IR waves in the impulsive phase of solar bursts

The time profiles of electromagnetic fluxes at hard X-rays and short microwaves are signatures of the energy conversion mechanisms at the origin of solar flares. The distinction between continuum and discrete energy production brings drastic conceptual consequences for the interpretation of the energy conversion processes. As more sensitive detectors were used on measurements with higher time resolution, the notion of continuum energy release in the impulsive phase is being replaced by the concept of repetitive energy production or Elementary Flare Bursts manifested at hard X-rays and by rapid time structures in microwave emissions. These discrete time structures are now known to be as short as tens of milliseconds, and part of their emissions are possibly produced by the same populations of accelerated electrons. Fast spikes, with mm-wave emission fluxes increasing for shorter wavelengths, simultaneous with hard X-rays, bring severe constraints for interpretation. This problem is reviewed, with the suggestion of a possible significant burst emission component in the sub-mm-IR range, due to primeval short-lived explosive compact sources, for which there are still no diagnostics.

Quasi-periodic pulsation and the elementary flare bursts of the radio spike event of 1989 August 17

Quasi-periodic pulsation and the elementary flare bursts of the radio spike event of 1989 August 17

Solar Simple Bursts Observed with High Spectral Resolution in the 18-23 GHz Range

AbstractFor the first time, solar bursts in the frequency range of (18-23) GHz have been observed with high-time (0.6-1.2 s) and high-frequency resolution (1 GHz), by using the Itapetinga 13.7m diameter antenna. Here, we investigate the microwave type “simple low level (< 10 SFU) bursts” associated with the impulsive phase of solar flares. Observed properties of these simple bursts are: rise time tr ~3 s, decay time td ~ 5 s and spectral index ranging between −1 and −4. These bursts were found to be associated with SF or SN flares as seen in Hα. The above properties suggest that they are likely to be a microwave counterpart of elementary flare bursts. In the majority of the cases the spectral evolution is soft-hard-soft. This suggests a nonthermal gyrosynchrotron mechanism for generating these elementary flare bursts. Estimated parameters of these simple burst sources are height (h ~ 2400 km), electron density (Nє < 8.8 × 109 cm−3 ), and magnetic field (B ~ 300 G).Subject headings: radiation mechanisms: nonthermal — Sun: radio radiation

Elementary flare bursts explained by explosive coalescence

The observed burst durations of Elementary Flare Bursts (5–25 s), as well as the related other flare characteristics, such as the temperature (30–50 MK), the electron density (log ne = 10 to 11) and the magnetic field strengths (100 G to 200 G) can be explained quantitatively by the mechanism of explosive coalescence.

Evidence for beamed electrons in a limb X-ray flare observed by HXIS

X-ray images taken by the Hard X-Ray Imaging Spectrometer (HXIS) aboard SMM during the 1980, November 18 limb flare are analysed. The temporal and spatial evolutions of the X-radiation are described. They differ significantly for hard and soft X-rays. During the elementary flare bursts energetic photons are predominantly emitted from a region close to the solar limb. In contrast, the soft X-ray sources are situated higher in the solar atmosphere. The observed X-ray spectra, in particular those emitted from small source regions at various altitudes, were fitted to power laws. Analysis of the spatial variation of the spectral index shows that there is a systematic tendency of the spectra to get harder with decreasing source altitude, especially during the elementary flare bursts. This fact is in agreement with the existence of nonthermal electron beams precipitating from the corona towards the denser layers of the solar atmosphere.

Interaction of a collisionless conduction front with the chromosphere and solar hard X-ray bursts

view Abstract Citations (43) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interaction of a collisionless conduction front with the chromosphere and solar hard X-ray bursts Smith, D. F. ; Harmony, D. W. Abstract The interaction of a collisionless conduction front with the transition region and chromosphere is investigated in a one-dimensional fluid approach as an explanation of elementary flare bursts in hard X-rays. It is shown that, for finite energy injection times, material boiled off of the chromosphere rises into the corona and eventually quenches the X-ray emission. This provides an acceptable explanation of elementary flare bursts with many testable predictions. Softer (10-20 keV) X-rays should come primarily from near the chromosphere, while harder (90-100 keV) X-rays should come primarily from higher in the corona. Ion heating to 108 K and upward mass motions to 1000 km s-1 should be observed. Limitations of the analysis resulting from its fluid character and implications for testable predictions are discussed. It is emphasized that, although the efficiency of this thermal model is only slightly larger than a nonthermal model for an initial density of 2.6 × 1010 cm-3, it is much easier from the point of view of plasma physics to heat a plasma than to accelerate a significant fraction of its electrons. Publication: The Astrophysical Journal Pub Date: January 1982 DOI: 10.1086/159602 Bibcode: 1982ApJ...252..800S Keywords: Chromosphere; Collisionless Plasmas; Plasma Interactions; Solar Flares; Solar X-Rays; Bursts; Electron Acceleration; Ion Production Rates; Ion Temperature; Plasma Heating; Plasma Physics; Radiative Transfer; Stellar Models; X Ray Astronomy; Solar Physics full text sources ADS |

VLA Observations of Solar Active Regions - Part Two - Solar Bursts

view Abstract Citations (21) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA Observations of Solar Active Regions. II. Solar Bursts Lang, K. R. ; Willson, R. F. ; Felli, M. Abstract The VLA has been used at wavelengths of 6 cm and 20 cm to detect seven solar bursts from two active regions with flux densities between 0. 1 and 6.0 sfu. The regions of burst energy release were resolved with angular sizes between 6" and 12" and brightness temperatures between 1 and 2 × 107 K. The angular sizes of the radio bursts are comparable to those of the soft X-ray kernels, and the brightness temperatures are comparable to the temperatures of the plasma which gives rise to soft X-ray emission. This suggests that we are detecting the radio wavelength counterpart of the flares seen at soft X-ray wavelengths. The two solar bursts observed at 6 cm wavelength occurred within a plage-associated "coronal condensation" in regions away from sunspots and near magnetic neutral lines. The Hα emission associated with one burst was, within the uncertainties of measurement, spatially coincident with the 6 cm burst. The five solar bursts detected at 20 cm wavelength were composed of individual events which had a duration of between 20 s and 60 s, but our time resolution of 10 s precluded the detection of components with shorter duration. Although thermal energies normally detected at soft X-ray wavelengths are inferred from their brightness temperatures, the train of events which make up the 20 cm bursts are similar to the elementary flare bursts detected at hard X-ray wavelengths in other active regions. The observed bursts at 20 cm wavelength can be explained by either the bremsstrahlung or the gyroresonant radiation of a hot, dense plasma with an electron temperature Te ≲ 2 × 107 K and an electron density of Ne ≍ 1010 cm-3. The gyroradiation easily explains the high observed circular polarization while indicating magnetic field strengths of H 90 G in the emitting region; while the bremsstrahlung requires polarization due to propagation effects at frequencies near the gyrofrequency in magnetic fields of strength H ≍ 500 G. We also discuss the implications of VLA observations for studies of the physical nature of the burst emitting sources. Publication: The Astrophysical Journal Pub Date: July 1981 DOI: 10.1086/159042 Bibcode: 1981ApJ...247..338L full text sources ADS |

On the seats of elementary flare bursts

We suggest to identify the elementary flare bursts with the excitation of the small kernels that occur in flare loops that are observed in soft X-ray pictures of flares. We stress the need of simultaneous observations of spatial structure and time variations of hard X-ray bursts sources in various wavelength regions.

Properties of elementary flare bursts

From a study of eight hard X-ray flares, all with durations of less than five minutes, it is found that these flares can be completely decomposed into short-lived bursts, called Elementary Flare Bursts (EFB). For each one flare the individual EFB's have approximatively the same Full Width at Half Maximum (FWHM); average values range between 4 s (± 1 s) and 24 s (± 5 s) for the different flares. Yet there are significant differences between the FWHM's for the individual EFB's of a flare. The EFB's are slightly asymmetric; rise time is approx. 0.9 of the decay time. Their half-widths decrease with increasing photon energy, proportional to E-0.69±0.05.