The location of the site of energy release in a solar X-ray subflare

Self Cite

125

A survey of soft X-ray images from Skylab has revealed a class of large-scale transient X-ray enhancements in the lower corona which are typically associated with the disappearance of Ha filaments away from active regions. Contemporary with the Ha filament disappearance, X-r@ emitting structures appeared at or near the filament location with shape and size resembling the filament. Eventually these structures faded, but the filament cavity was no longer obvious. Typically the peak of the X-ray event lagged the end of the filament disappearance by tens of minutes. The durations of the coronal X-ray enhancements were considerably longer than the associated Ha filament disappearances. Major flare effects, such as chromospheric brightenings, typically were not associated with these X-ray events.One event analyzed quantitatively had a peak temperature between 1.8 and 2.7 x 106 K, achieved a peak density of ~ 109 cm -3 and resulted in an enhancement in the plasma pressure over the conditions of the preexisting coronal cavity of at least a factor of 7. The mass of the coronal X-ray emitting material was about 10% that of the preexisting filament and the thermal energy of the coronal event was on the order of 1029 erg, about 10% of the mechanical energy of the Ha filament eruption. The event appeared to cool by radiative losses and not by thermal conduction. It is likely that the coronal enhancements are caused by heating of an excess of previously cooler material, either from the filament itself, or by compression of coronal material by a changing magnetic field.

Section: Summary and Discussionsupporting

confidence: 54%

Coronal X-ray enhancements associated with H? filament disappearances

Webb¹,

Krieger²,

Rust³

1976

Self Cite

125

“…Similarly, in the McMath 12507 flare of 6 September 1973, the intense X-ray emission came from small loops which straddled the photospheric magnetic neutral line separating the leader and follower polarities of the active region. Petrasso et al (1975) using the AS & E X-ray telescope found similar results for the flare of McMath 12511 of 1 September, 1973.…”

Section: The Small Flare Energy Sourcesupporting

confidence: 53%

“…Most of the X-ray, EUV, and UV radiation from flares is emitted by very compact structures presumably loops (or acades of loops) some 4 to 8 Mm wide and 4 to 20 Mm long, with finer structure by no means ruled out Brueckner, 1975;Petrasso et al, 1975;Vorpahl et al, 1975). Low heights of 2 to 10 Mm are inferred for these loops.…”

Section: The Small Flare Energy Sourcementioning

confidence: 99%

Source of the solar flare energy

Altschuler

1976

Recent Skylab and magnetograph observations indicate that strong photospheric electric currents underlie small flare events such as X-ray loops and surges. What is not yet certain, because of the non-local dynamics of a fluid with embedded magnetic field, is whether flare emission derives from the energy of on-site electric currents or from energy which is propagated to the flare site through an intermediary, such as a stream of fast electrons or a group of waves. Nevertheless, occurrences of: (1) strong photospheric electric currents beneath small flares; (2) similar magnetic fine structure inside and outside active regions; (3) eruptive prominences and coronal white light transients in association with big flares; and, (4) active boundaries of large unipolar regions suggest the possibility that all phenomena of solar activity are manifestations of the rapid ejection and/or gradual removal of electric currents of various sizes from the photosphere. The challenge is to trace the precise magnetofluid dynamics of each active phenomenon, particularly the role of electric current build-up and dissipation in the low corona. The ProblemA solar flare is a vaguely defined complex of numerous energetic transient phenomena some of which may be present or absent on any particular occasion. Flare-related events are observed over a wide range of heights and scale sizes. Over the height range, we observe on various occasions: (1) enhanced Ha and other line emission in the chromosphere; (2) X-rays, radio pulses, and various Ha ejecta in the low corona; (3) radio bursts and white light transients high up in the corona out to several solar radii; and, (4) white light emission in localized photospheric regions. Some, perhaps all, of these phenomena may occur (at least with low intensity) considerably before the chromospheric flash phase of the flare, and many are closely correlated in time with one another during the flare.Studies of flare parameters, flare case histories, and photospheric magnetic fields provide overwhelming evidence that the energy released in the manifold activity of a solar flare is initially stored in non-potential magnetic fields (curl B r 0), or equivalently, in electric currents. I assume, therefore, that flare energy accumulates in electric currents or current sheets and restrict my comments to where these pre-flare electric currents (non-potential magnetic fields) originate and amplify.The enormous range in scale size which characterizes flare-associated phenomena probably means that the non-potential magnetic field distribution which is the source of the flare energy has both small-scale structure and large-scale organization. I think it unlikely, for example, that a small flare event 1 to 10Mm in size (such as an X-ray flare kernel or a surge from a satellite sunspot), could be the sole energy source (or even the sole trigger) for global-scale events exemplified by type IV radio bursts, erupting prominences, and coronal white light transients. Rather, I suspect the existence of an underlying large-scal...

“…Brightening of a loop structure before the flare onset has also been identified in another X-ray flare observed by the S-054 experiment (Petrasso et al, 1975). The lack of spectral infocmation at the time the brightening took place does not allow us to distinguish between temperature and density enhancements.…”

Section: Morphological Analysis and Interpretationmentioning

confidence: 87%

“…They felt that collisional losses of energetic electrons were inadequate to account for the bulk of the flare heating. Petrosian (1975) has disagreed with this conclusion; one reason he cites is that the time of peak thermal energy of the flare is only a short time later than the end of the impulsive X-ray burst. Implicit in this argument is the assumption that particle acceleration occurs only through the duration of the hard X-ray event (or the impulsive microwave burst) and that both heating and cooling are taking place in a single volume throughout the flare.…”

Section: Discussionmentioning

confidence: 99%

Spatial structure and temporal development of a solar X-ray flare observed from Skylab on June 15, 1973

Pallavicini

Vaiana

Kahler³

et al. 1975

Self Cite

A solar flare on June 15, 1973 has been observed with high spatial and temporal resolution by the S-054 grazing-incidence X-ray telescope on Skylab. Both morphological and quantitative analyses are presented. Some of the main results are: (a) the overall configuration of the flare is that of a compact region with a characteristic size of the order of 30" at the intensity peak, (b) this region appears highly structured inside with complex systems of loops which change during the event, (c) a brightening over an extended portion of the active region precedes the fare onset, (d) the impulsive phase indicated by the non-thermal radio emission is a period during which a rapid brightening occurs in loop structures, (e) the X-ray emission is centered over the neutral line of longitudinal magnetic field, and the brightest structures at the flare onset bridge the neutral line, (f) loop systems at successively increasing heights form during the decay phase, finally leading to the large loops observed in the postflare phase, (g) different parts of the flare show distinctly different light curves, and the temporal development given by full disk detectors is the result of integrating the different intensity vs time profiles.The implications of these observations for mechanisms of solar flares are discussed. In particular, the flux profiles of different regions of the flare give strong evidence for continued heating during the decay phase, and a multiplicity of flare volumes appears to be present, in all cases consisting of loops of varying lengths.