M. Tomczak

Investigation of Quasi-periodic Variations in Hard X-rays of Solar Flares

The aim of the present paper is to use quasi-periodic oscillations in hard X-rays (HXRs) of solar flares as a diagnostic tool for the investigation of impulsive electron acceleration. We have selected a number of flares which showed quasi-periodic oscillations in hard X-rays and their loop-top sources could be easily recognized in HXR images. We have considered MHD standing waves to explain the observed HXR oscillations. We interpret these HXR oscillations as being due to oscillations of magnetic traps within cusp-like magnetic structures. This is confirmed by the good correlation between periods of the oscillations and the sizes of the loop-top sources. We argue that a model of oscillating magnetic traps is adequate to explain the observations. During the compressions of a trap, particles are accelerated, but during its expansions plasma, coming from chromospheric evaporation, fills the trap, which explains the large number of electrons being accelerated during a sequence of strong pulses. The advantage of our model of oscillating magnetic traps is that it can explain both the pulses of electron acceleration and quasi-periodicity of their distribution in time.

Energy release and transport in arcade flares

Abstract This work is based on hard and soft X-ray observations from the YOHKOH satellite. We investigate an example of an arcade flare, for which the arcade channel is seen in soft X-rays as a long bright filament. We have found that: 1. (1) Energy can efficiently flow along the arcade channel from the very beginning of a flare. 2. (2) During flare evolution a few kernels of hard X-ray emission develop along the arcade channel. Clearly, they are new, additional sources of the flare energy release. A probable scheme of formation of such hard X-ray kernels is briefly discussed.

Investigation of the energy release in flare kernels

Abstract We have investigated the heating rate, E H , in flare kernels for a number of M-class flares. We have found that: 1) The heating rate decreases with the altitude of kernel. We explain this as a result of magnetic field decrease with the height in the corona, i.e. the decrease of the available magnetic energy H 2 /8π, per unit volume. 2) However, the temperature of the investigated kernels does not decrease with the altitude. We explain this as being the result of decreasing conductive losses from the kernel with increasing height, which compensates the decrease of E H . We have divided the investigated flares into three groups: low, intermediate and higher loops, and calculated the median values of the heating rate, E H , and of other parameters for each group. The low loops (compact flares) have high heating rates E H .

Investigation of the energy release in long-duration flares

We investigate an example of long-duration arcade flare of 14/15 May 1993. Its spatial structure can be reliably determined from the Yohkoh X-ray images and Kitt Peak magnetograms. Temperatures, densities and heights of the loop-top flare kernels have been estimated from these data and also the kernel heating has been investigated in detail. We have found that (1) The energy release in the loop-top flare kernels occurs also during the whole flare decay phase; (2) The kernel evolution during the decay phase is quasi-steady-state, i.e. it is determined by the slow decrease of the heating rate, EH(t).

Structure and energetics of filament-like SXR flares

Abstract We investigate an example of flare having the shape of a long, bright filament in soft X-rays (SXR). Another characteristic feature of the flare and of other similar flares is that a few emission kernels, best seen in hard X-rays (HXR), develop along the filament. The kernels are new sites of the flare energy release. It is shown that the kernels develop at the crossings of perpendicular magnetic flux tubes. The flare energy release, and the flow of heat and plasma along the filament are carefully investigated.

What are the bright loop-top kernels in soft X-ray flares?

A major puzzle in the study of solar X-ray flares is the presence of a bright, persistent ‘kernel’ at or near the tops of flaring loops in soft X-ray images from Yohkoh. Here we argue that they can be explained by a tangled magnetic field geometry with unresolved current sheets continually being formed. Hot (20 MK) plasma is formed near the current sheets and is surrounded by plasma which has cooled to ∼ 10 MK plasma so that the two temperatures appear to co-exist.

Investigation of impulsive soft X-ray brightening in solar flares

Abstract Four multi-loops or arcade flares showing strong impulsive soft X-ray brightenings on Yohkoh /SXT frames have been selected. By inspection of light curves of individual pixels, the areas of brightening have been localised. Evidences that non-thermal electron beams easily penetrate through whole flaring structures have been found. In some footpoints of the flaring structures during the impulsive phase the evidence of the chromospheric evaporation driven by non-thermal electron beams has been detected. The velocities of the upflowing plasma have been estimated. Derived values are in a wide range among 220 and 750 km/s. The SXT images of the investigated flares have been compared with the Yohkoh /HXT images. Generally good spatial and temporal coincidence between soft and hard X-ray emission from footpoints of flaring structures during the impulsive phase have been found but some exceptions occur. An explanation of the reported exceptions based on the magnetic field configuration has been proposed.