Multiwavelength diagnostics of the precursor and main phases of an M1.8 flare on 2011 April 22

Abstract

We study the temporal, spatial and spectral evolution of the M1.8 flare, which occurred in the active region 11195 (S17E31) on 2011 April 22, and explore the underlying physical processes during the precursor phase and their relation to the main phase. The study of the source morphology using the composite images in 131Å wavelength observed by the Solar Dynamics Observatory/Atmospheric Imaging Assembly and 6–14 keV [from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)] revealed a multiloop system that destabilized systematically during the precursor and main phases. In contrast, hard X-ray emission (20– 50 keV) was absent during the precursor phase, appearing only from the onset of the impulsive phase in the form of foot-points of emitting loops. This study also revealed the heated looptop prior to the loop emission, although no accompanying foot-point sources were observed during the precursor phase.We estimate the flare plasma parameters, namely temperature (T), emission measure (EM), power-law index (γ ) and photon turn-over energy (to), and found them to be varying in the ranges 12.4–23.4MK, 0.0003–0.6 × 10⁴⁹ cm−³, 5–9 and 14–18 keV, respectively, by forward fitting RHESSI spectral observations. The energy released in the precursor phase was thermal and constituted ≈1 per cent of the total energy released during the flare. The study of morphological evolution of the filament in conjunction with synthesized T and EM maps was carried out, which reveals (a) partial filament eruption prior to the onset of the precursor emission and (b) heated dense plasma over the polarity inversion line and in the vicinity of the slowly rising filament during the precursor phase. Based on the implications from multiwavelength observations, we propose a scheme to unify the energy release during the precursor and main phase emissions in which the precursor phase emission was originated via conduction front that resulted due to the partial filament eruption. Next, the heated leftover S-shaped filament underwent slow-rise and heating due to magnetic reconnection and finally erupted to produce emission during the impulsive and gradual phases.