Phenomena in the unusually long pre-impulsive phase of SOL2011-06-07

From RHESSI Wiki

Introduction

The most energetic flares often start with the rising motion of a filament followed by magnetic reconnection below this filament. The start of the reconnection corresponds to the beginning of the flare impulsive phase. While this impulsive phase has been studied in many papers, the very early phase just after the start of the filament rise and before the impulsive phase is still not well understood. This is due to the fact that this phase is usually short. However, in SOL2011-06-07, associated with the the ejection of an unusually huge filament, this pre-impulsive phase was rather long and enabled us to recognize several phenomena that were not observed up to the present.

1. Continuum with negatively drifting boundary

At the very beginning of the flare we found an unusual radio continuum emission whose starting boundary drifts from high to low frequencies (Figure 1). The emission starts at 870 MHz at about 06:21:40 UT, but at 300 MHz it starts at about 06:24:20 UT, thus giving the mean drift of the edge (the emission onset) of about -3.6 MHz/s. Assuming that this continuum is generated by the plasma emission mechanism on the fundamental frequency, and using a model of coronal density, the velocity of the agent generating the starting boundary of this continuum is about 400 km/s. This velocity is comparable to that of the filament itself, 200-400 km/s (Ref. [1]). Therefore, we propose that this continuum comes from the rising magnetic flux rope in which the filament is embedded.

Figure 1: The radio spectrum observed on 2011 June 7, showing an unusual continuum which starts at 870 MHz at about 06:21:40 UT and whose starting boundary drifts from high to low frequencies. This emission is followed by the drifting pulsation structure (DPS) observed in the 220-450 MHz range at about 06:24:15-06:30 UT (see the box delineated by the dashed line).