Flare Plasma Dynamics

Abstract

A solar flare is a catastrophic event that is triggered by an instability of the underlying magnetic field configuration (§ 10) and evolves then into a more stable state by changing and reconnecting the magnetic topology. This change in magnetic topol-ogy provides free magnetic energy that is released in the form of currents that spawn primary plasma heating and particle acceleration (§ 11). In most flare models these primary processes take place in the corona, in the immediate environment of magnetic reconnection points and associated magnetic separator lines or separatrix surfaces. In a second step, the accelerated particles and thermal conduction fronts propagate to the chromosphere where they heat up the chromospheric plasma, which is a secondary heating process, driven by the energy loss of the precipitating particles or by thermal conduction of the impinging ion-acoustic waves. This chromospheric heating process triggers a third step, an upflow of heated chromospheric plasma, which is called chro-mospheric evaporation (or physically more correctly, chromospheric ablation). This third step fills up what appears as prominent flare loops in soft X-ray wavelengths. In principle, also the primary coronal heating process could be detected in soft X-rays, but is usually outshone by the much brighter upflows of chromospheric plasma, due to its higher density and emission measure. The second step of chromospheric heating is most prominently observed in gamma-rays (§ 13) and hard X-rays (§ 14), and sometimes even in UV and white light. Thus we have to keep in mind that most of the soft X-ray observations document only the third step in this chain reaction, and we still have very insufficient diagnostic about the first step of how the flare is initiated. Once the flare passes its peak in soft X-ray emission, plasma cooling processes start to dominate over heating. When the plasma cools down from the initial 10−30 MK temperatures at the peak of the flare down to 1 − 3 MK, the postflare loop system becomes prominently detectable in EUV, showing the beautiful fractal structures of postflare arcades seen in high-resolution TRACE movies (Plate 17). Once the temperature drops below 0.5 MK, instabilities occur in the postflare loops that cause a rapid break-up and precipitation of the cooling plasma, visible in UV and Hα. These five steps may occur in successive order for a simple single-loop flare, but usually occur parallel and time-overlapping in multi-loop flares, and thus are hard to disentangle. In this section we focus on the various heating and cooling processes of the flare plasma.