A multiwavelength study of a double impulsive flare

Abstract

Extensive data from the Solar Maximum Mission (SMM) and ground-based observatories are presented for two flares; the first occurred at 12 : 48 UT on 31 August, 1980 and the second just 3 rain later. They were both compact events located in the same part of the active region. The first flare appeared as a typical X-ray flare: the CaxIx X-ray lines were broadened (-= 190 + 40 km s-1) and blue shifted ( -= 60 + 20 km s -1) during the impulsive phase, and there was a delay of about 30 s between the hard and soft X-ray maxima. The relative brightness of the two flares was different depending on the spectral region being used to observe them, the first being the brighter at microwave and hard X-ray wavelengths but fainter in soft X-rays. The second flare showed no significant mass motions, and the impulsive and gradual phases were almost simultaneous. The physical characteristics of the two flares are derived and compared. The main difference between them was in the pre-flare state of the coronal plasma at the flare site: before the first flare it was relatively cool (3 x 106 K) and tenuous (4 x 109 cm-3), but owing to the residual effects of the first flare the coronal plasma was hotter (5 x 106 K) and more dense (3 x 1011 cm -3) at the onset of the second flare. We are led to believe from these data that the plasma filling the flaring loops absorbed most of the energy released during the impulsive phase of the second flare, so that only a fraction of the energy could reach the chromosphere to produce mass motions and turbulence.A simple study of the brightest flares observed by the SMM shows that at least 43 % of them are multiple. Thus, the situation studied here may be quite common, and the difference in initial plasma conditions could explain at least some of the large variations in observed flare parameters. We draw a number of conclusions from this study. First, the evolution of the second flare is substantially affected by the presence of the first flare. Secondly, the primary energy release in the second event is in the corona. Thirdly, the flares occur in a decaying magnetic region, probably as a result of the interaction of existing sheared loops; there is no evidence of emerging magnetic flux. Also, magnetic structures of greatly varying size participate in the flare processes. Lastly, there is some indication that the loops are not symmetrical or stable throughout the flares, i.e. the magnetic field does not act as a uniform passive bottle for the plasma, as is often assumed in flare models.

Observation of the impulsive phase of a simple flare

“…For example, a subsonic upflow from the 107 K feet of order 102 km s -1 carrying any density n (greater than the initial coronal density in the arch) would increase the density throughout the arch to of order n in a time of order 102 s. A substantial increase in the coronal density before the second spike is consistent with the observation that the ratio of O v flux to hard X-ray flux was lower in the second spike than in the first by at least by a factor of 2: enhanced coronal density would cause more of the energetic electrons to dissipate in the corona and not reach the feet to produce the heating for the Ov emission. Other evidence for this effect has been found by Strong et al (1984) in another double impulsive flare.…”

An Observational Overview of Solar Flares

“…The X-ray and γ-ray observations of several flares, including the major γ-ray flare SOL1972-08-04, were used to show that the kinetic energy of the accelerated electrons constituted a surprisingly large fraction of the total flare energy, perhaps as high as 10 to 50% of the ∼10 32 erg released during the flare (Lin & Hudson 1976). Two flares within the same active region on 1980 August 31 provided the energy content in thermal plasma, non-thermal electrons, and hydrodynamic mass motions of non-ejected material (Strong et al 1984), while the energy content in radiative, thermal, non-thermal electron, and non-CME associated plasma ejected was calculated for SOL2002-02-26T10:27 (C9.6) (Saint-Hilaire & Benz 2002). RHESSI X-ray observations were used determine the energy in accelerated electrons and in the hot plasma for nine medium-sized flares (GOES class C6 to M8), with the conclusion that despite the large uncertainties, the energies in these two components were of the same magnitude in each case .…”

Millisecond radio spikes

“…The understanding of spikes may thus yield information on the primary energy release in flares. Soft X-ray observations of the 31 August, 1980 flares by Strong et al (1984) yield preflare densities with plasma frequencies in the range of the frequencies of the spikes observed later during the flare (Benz, 1985). It is generally believed that spike emission occurs at a frequency which is within a factor of two of the local plasma frequency.…”