Long-duration plasma heating in solar microflares of X-ray class A1.0 and lower

Abstract

Three solar microflares of X-ray class ∼A1.0 and lower observed at solar activity minimum in 2009 have been studied: the April 19, 2009 flare (A0.38), the June 24, 2009 flare (A0.47), and the July 18, 2009 flare (A2.2). A distinguishing feature of these events from other microflares of low X-ray classes was the maintenance of a high plasma temperature (T ≳ 4 MK) at the decay phase well after the flare peak. The lifetimes of the high-temperature emission source in the corona in all three cases exceed considerably its conductive and radiative cooling times, while the thermal energy released at the decay phase of the flare exceeds the energy liberated during its impulsive phase by an order of magnitude. It is hypothesized that a high plasma temperature in microflares of low X-ray classes can be maintained for a long time through magnetic reconnection associated with mass ejections from the flare region. The lifetime of the hot plasma in the corona in this case can exceed appreciably the plasma cooling time. The study has shown that the eruption of material actually occurred in all three investigated microflares.