Flare Core Research Articles

Early signatures of large-scale field line opening

A fast halo-type coronal mass ejection (CME) associated with a two-ribbon flare, GOES class M 1.3, was observed on February 8, 2000. Soft X-ray and EUV images revealed several loop ejections and one wave-like moving front that started from a remote location, away from the flare core region. A radio type-II burst was observed near the trajectory of the moving soft X-ray front, although association with the CME itself cannot be ruled out. Large-scale dimmings were observed in EUV and soft X-rays, both in the form of disappearing transequatorial loops. We can pinpoint the time and the location of the first large-scale field-line opening by tracing the electron propagation paths above the active region and along the transequatorial loop system, in which large-scale mass depletion later took place. The immediate start of a type-IV burst (interpreted as an upward moving structure) which was located over a soft X-ray dimming region, confirms that the CME had lifted off .W e compare these signatures with those of another halo CME event observed on May 2, 1998, and discuss the possible connections with the magnetic breakout model.

Properties of Remote Flare Ribbons Associated with Coronal Mass Ejections

To understand the large-scale structure of flares and coronal mass ejections, we studied the motion and properties of remote flare ribbons of three major events: 2003 May 27 X1.3, 2000 November 24 X1.8, and 2000 November 25 X1.9 flares. The remote ribbons are in addition to the typical two-ribbon pairs in the flare cores. We found two common properties of these three events. First, the flare core occurs at the edge of a major sunspot, associated with flux emergence parallel to the edge. Such a flux emergence formed complicated magnetic channel structure. Second, three flare ribbons are visible for each event. The first two make up a typical two-ribbon structure near the flare core. The third ribbon (remote ribbon) moves away from the flare site at a speed between 30 and 100 km s-1, several times larger than the nominal two-ribbon separation speed of flares. It is interesting to note that for the 2000 November 25 event, the remote ribbon first moved away in the same manner as the other two events; it then retracted after the flare emission peaked. We compared such an expansion/retraction motion with a limb event observed on 2001 April 15. We interpret the observed motion of remote ribbons by the interaction between the erupting flux rope in the flare core region and the overlying large-scale magnetic fields. For the 2000 November 25 event, the overlying field and flare core fields interacted twice, during the outward and inward motions.

Soft X-ray observation of a large-scale coronal wave and its exciter

Recent extreme ultraviolet (EUV) observations from SOHO have shown the common occurrence of flare-associated global coronal waves strongly correlated with metric type II bursts, and in some cases with chromospheric Moreton waves. Until now, however, few direct soft X-ray detections of related global coronal waves have been reported. We have studied Yohkoh Soft X-ray Telescope (SXT) imaging observations to understand this apparent discrepancy, and describe the problems in this paper. We have found good X-ray evidence for a large-scale coronal wave associated with a major flare on 6 May 1998. The earliest direct trace of the wave motion on 6 May consisted of an expanding volume within 20 Mm (projected) of the flare-core loops, as established by loop motions and a dimming signature. Wavefront analyses of the soft X-ray observations point to this region as the source of the wave, which began at the time of an early hard X-ray spike in the impulsive phase of the flare. The emission can be seen out to a large radial distance (some 220 Mm from the flare core) by SXT, and a similar structure at a still greater distance by EIT (the Extreme Ultraviolet Imaging Telescope) on SOHO. The radio dynamic spectra confirm that an associated disturbance started at a relatively high density, consistent with the X-ray observations, prior to the metric type II burst emission onset. The wavefront tilted away from the vertical as expected from refraction if the Alfven speed increases with height in the corona. From the X-ray observations we estimate that the electron temperature in the wave, at a distance of 120 Mm from the flare core, was on the order of 2–4 MK, consistent with a Mach number in the range 1.1–1.3.

The physical relationship between flares and surges observed in the extreme ultraviolet

A search was made for EUV surges among the EUV flares recorded by the Harvard spectroheliometer on ATM. Out of a large set of partial observations of such flares, a subset of 24 ‘complete’ events was chosen. More than 24 associated surges were found, many of them multiple events. The flare-surge correlation is therefore considerably higher in the EUV than in Hα, presumably because EUV surges generally appear in emission, and in high contrast compared to Hα. In over 70% of the cases, the surges were found to grow out of the flare structure. Making reasonable assumptions, it was possible to infer the magnitude of the gas pressure gradient from the flare core into the surge by using the EUV intensity gradient. The inferred pressure gradient appears sufficient to drive the surge, although higher resolution observations will be required to corroborate this, and rule out the importance of magnetic Lorentz force.

Observations of limb flares with a soft X-ray telescope

The structure and evolution of 26 limb flares have been observed with a soft X-ray telescope flown on Skylab. The results are: (1) One or more well defined loops were the only structures of flare intensity observed during the rise phase and near flare maximum, except for knots which were close to the resolution of the telescope in size (≈2 arc seconds) and whose structure can therefore not be determined. (2) The flare core features were always sharply defined during the rise phase. (3) For the twenty events which contain loops, the geometry of the structure near maximum was that of a loop in ten cases, a loop with a spike at the top in four cases, a cusp or triangle in four cases, and a cusp combined with a spike in another two cases. (4) Of the fifteen cases in which sufficient data were available to allow us to follow a flare's evolution, five showed no significant geometrical deviation from a loop structure, one displayed little change except for a small scale short-lived perturbation on one side of the loop 10 seconds before a type III radio burst was observed, eight underwent a large scale deformation of the loop or loops on a time scale comparable to that of the flare itself and one double loop event changed in a complex and undetermined manner, with reconnection being one possibility. Based on observation of the original film, it is suggested that the eight flares which underwent large scale deformations had become unstable to MHD kinks. This implies that these flares occurred in magnetic flux tubes through which significant currents were flowing.It is suggested that the high energy electrons responsible for type III bursts accompanying these flares could have been accelerated by the V x B electric field induced by a small scale short-lived perturbation of parts of a flaring flux tube, similar to the one perturbation which was observed having these characteristics.

On some results of calculations of the chromospheric flare initial phase model according to the strong gasodynamic explosion theory

It is shown that while comparing the observed dependences of radius, front and plasma velocities at the time of the very initial flare phase with the family of the theoretical curves of these functions obtained according to strong explosion theory with the set of the initial density ρ 0 and estimated from the observations of the total flare energy E 0, one can determine ρ 0 in the flare core. Knowing ρ 0 and E 0, densities in the filaments responsible for the flare radiation and the effective and spectroscopic radiation volumes as well, one can determine the effective mass and the density between the filaments. The estimated mass, electron temperature, front and plasma velocities and densities both in the filaments and in the intervals are in satisfactory agreement with the modern concept of flares.

Rocket Observation of AR Xii-Xvi CA Xiv-Xviii and fe XIV, xv, XXIV in the Extreme-Ultraviolet Spectrum of a Solar Flare

view Abstract Citations (48) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Rocket Observation of Ar XII-XVI Ca XIV-XVIII and Fe XIV, XV, XXIV in the Extreme-Ultraviolet Spectrum of a Solar Flare Purcell, J. D. ; Widing, K. G. Abstract The lithiumlike doublets of S xiv, Ar XVI, and Ca xviii are identified in the 300-450 A region by extrapolation of solar wavelengths and term intervals in the isoelectronic sequence. A possible observation of Fe xxiv in the flare is discussed. The solar blend at 417 A involving S xiv and the Fe xv inter- system line is resolved. The singlet resonance lines of S xiii, Ar xv, and Ca xvii are also found in the flare spectrum by isoelectronic extrapolation. Ca xiv and Ca xv are identified on the basis of laboratory wavelengths. The problem of the identification of boron-like Ar xiv and Ca xvi is discussed. The weakness of argon lines relative to calcium lines in the flare is consistent with a solar abundance of argon less than calcium. An electron density of 3 >( 1010 in the flare core is estimated from the intensity ratio of the Fe xiv lines at 211 and 219 A. Publication: The Astrophysical Journal Pub Date: August 1972 DOI: 10.1086/151626 Bibcode: 1972ApJ...176..239P full text sources ADS |