Observation of the impulsive phase of a simple flare

Abstract

We present a broad range of complementary observations of the onset and impulsive phase of a fairly large (1 B, M 1.2) but simple two-ribbon flare. The observations consist of hard X-ray flux measured by the SMM HXRB S, high-sensitivity measurements of microwave flux at 22 GHz from Itapetinga Radio Observatory, sequences of spectroheliograms in UV emission lines from Ov (T~ 2 x 105 K) and FexxI (T~ 1 • 107 K) from the SMM UVSP, Hot and HeID 3 cine-filtergrams from Big Bear Solar Observatory, and a magnetogram of the flare region from the MSFC Solar Observatory. From these data we conclude:(1) The overall magnetic field configuration in which the flare occurred was a fairly simple, closed arch containing nonpotential substructure.(2) The flare occurred spontaneously within the arch; it was not triggered by emerging magnetic flux.(3) The impulsive energy release occurred in two major spikes. The second spike took place within the flare arch heated in the first spike, but was concentrated on a different subset of field lines. The ratio of Ov emission to hard X-ray emission decreased by at least a factor of 2 from the first spike to the second, probably because the plasma density in the flare arch had increased by chromospheric evaporation.(4) The impulsive energy release most likely occurred in the upper part of the arch; it had three immediate products:(a) An increase in the plasma pressure throughout the flare arch of at least a factor of 10. This is required because the Fe xxI emission was confined to the feet of the flare arch for at least the first minute of the impulsive phase.(b) Nonthermal energetic (~25 keV) electrons which impacted the feet of the arch to produce the hard X-ray burst and impulsive brightening in Ov and D 3. The evidence for this is the simultaneity, within + 2 s, of the peak O v and hard X-ray emissions.(c) Another population of high-energy (~ 100 keV) electrons (decoupled from the population that produced the hard X-rays) that produced the impulsive microwave emission at 22 GHz. This conclusion is drawn because the microwave peak was 6 + 3 s later than the hard X-ray peak.

Solar flare hard X-ray observations

“…Individual hard X-ray and microwave spikes are in general either in coincidence or the microwave peak is delayed by a short time ranging up to a second or two. Tandberg-Hanssen et al (1984) reported an unusually long delay of 6 + 3 s. Very few cases, if any, have been observed in which the microwave emission precedes the hard X-ray emission. Much longer delays are seen in type C flares and an example is shown in Figure 25.…”

Energy release in solar flares

“…Recent studies by Takakura et al (1983) have shown that the time curves at various microwave frequencies and at hard X-rays exhibit little correlation on time-scales of seconds, except when fast quasiperiodicity is evident, in which case the periodicity appears at all frequencies, possibly indicating that the same energy-release process is at work. In their analysis of other flares, Marsh et al (1981) and Tandberg-Hanssen et al (1984) have inferred that the population of energetic electrons that produce the impulsive microwave emission is distinct from the population of electrons that produce the hard X-ray burst.…”

“…Although Kiepenheuer (1964) argued some time ago that filament activity and flares should be regarded as different aspects of one complex process, it is still usual to regard filament eruption and flares as separate but related phenomena (Van Hoven et al, 1980). A gradual onset phase preceding the impulsive phase can also occur in flares having no visible filament eruption; an especially well observed example is the flare of November 1, 1980, studied by Tandberg-Hanssen et al (1984).…”

“…It was found that '(a) on time scales of several seconds, the time structures are poorly correlated at different microwave frequencies and between microwaves and hard X-rays; (b) on subsecond time scales, however, the more rapidly repeating time structure are well correlated at two different microwave frequencies and between microwaves and hard X-rays. ' Tandberg-Hanssen et al (1984) also found a lack of correlation between microwave and X-ray burst structures on time scales of several seconds. One intense spike-like burst observed at microwaves and hard X-rays, with half-power duration of about 5 s, was shown to be composed of a 'slow' component with a time-scale of about 1 s and to display, at its peak, a repeating microwave subsecond structure with a time-scale of about 30-60 ms .…”

On the role of energetic particles in solar flares

“…This is in good agreement with the model proposed here since the stability of the region (studied by Martens and Kuperus, 1983) is restricted to substantial changes in the trapped plasma as well as in the intensity and geometry of the magnetic field. That this is not the only mechanism, however, has been shown by Tandberg-Hansen et aL (1984), who, for the large 1 November, 1980, two-ribbon flare, found no evidence of emergence of new flux (see also Holman et al, 1984). This latter possibility has been observed with hard X-ray imaging by Hoyng et al (1981), Woodgate et al (1981 and de Jager et aL (1983).…”