Slow Magnetoacoustic Waves in Two-Ribbon Flares

Introduction

The physical processes operating in solar energy releases are still unclear and under very active scrutiny. The understanding of three-dimensional morphology and dynamics of large, eruptive, two-ribbon flares can shed light on this problem.

According to the more or less standard picture of large eruptive flares oppositely directed magnetic field lines being in the form of arcade of magnetic loops are stretched by some non-stationary agent (e.g. by an erupting twisted magnetic flux rope or filament) to form a quasi-vertical current sheet in the corona. Here, in the current sheet, magnetic field lines can reconnect converting free magnetic energy to thermal and kinetic energy of plasma and charged particles causing a multitude of secondary flaring effects.

Unfortunately, the "standard" picture is essentially two-dimensional. Being capable to represent a lot of observational effects in the planes of magnetic loops, it does not represent flare development in the third direction perpendicular to the loop planes - along the flaring arcade axis and the magnetic neutral line. But often large, eruptive, two-ribbon flares develop predominantly just in this direction - in a number of observations energy release is seen to propagate mainly along the flaring arcade axis. What is very exciting here is that still unknown triggering disturbances observed to propagate in this direction at the speed of a few tens km/s, well below the Alfven and sound speeds.