Physics
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996ssrv...77....1s&link_type=abstract
Space Science Reviews, Volume 77, Issue 1-2, pp. 1-192
Physics
86
Scientific paper
This paper deals with the temporal dynamics of solar flares. It gives a systematic description of solar flare models and tries to link the observations to results of simulations. After a review of the development of ideas on flare structure and on theories on current-loop interaction in flares since the pioneering work by Gold and Hoyle (1960), this paper gives first a synthesis of present-days observationally based views on solar flares, essentially describing the developments since the review by de Jager (1986). We distinguish between confined/impulsive and eruptive/dynamic flares (briefly: confined and eruptive). The main difference between these two types is one of field-line topology: ‘closed’ or ‘open’. The ‘grand instability’ in a field-line system opening to space is basic to the relation between eruptive flares, filament instability, and Coronal Mass Ejections. A fair part of the paper deals with the developments in our understanding of the physical processes during collisions between current-carrying loops. After work by Tajima et al. (1982), who introduced the concept of current-loop coalescence in solar flares, using results obtained from two-dimensional particle simulation, it became clear that the current-loop interaction process includes a rich variety of physical mechanisms associated with rapid magnetic energy conversion through partial or complete magnetic reconnection, such as prompt high-energy particle acceleration, plasma heating, shock formation, plasma-jet formation and plasma radiation. This part of the paper concentrates on the developments since the review by Sakai and Ohsawa (1987), dealing with particle acceleration by magnetic reconnection and shocks during current-loop coalescence in solar flares. Theoretical research performed since the above review paper refers to magnetic reconnection, shock formation, particle acceleration and plasma emission during collisions between current-carrying loops. These theoretical developments are compared with observations. The Yohkoh spacecraft, launched August 30, 1991, observed many evidences of two-loops interaction in active regions and a growing number of examples of flares due to coalescence of loops. These observational evidences are reviewed in a systematic way (Section 6), by relating them to the theoretical studies described before. First we describe flares due to interactions of the Y-, X-, and I-types. These can be either confined or eruptive. A new observation is the occurrence of ‘ongoing partial reconnection’ in a flare. Thereafter, we discuss flares due to single-loop instability (these are confined flares), followed by the newly discovered ‘cusp-type’ flares. These are eruptive flares. We end with a description of a typical eruptive flare. In Section 7 we summarize observational and theoretical evidences for our earlier proposed scheme of three different phases of acceleration in flares. The summary in the last section compares flare observations with flare simulations; it deals briefly with the three phases of acceleration, and outlines the present status and prospects of simulation studies and related theoretical research.
de Jager Cornelis
Sakai Jun-Ichi
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