Weak solar flares with a detectable flux of hard X rays: Specific features of microwave radiation in the corresponding active regions

Abstract

The emission of very weak flares was registered at the Suzaku X-ray observatory in 2005–2009. The photon power spectrum in the 50–110 keV range for a number of these phenomena shows that some electrons accelerate to energies higher than 100 keV. The corresponding flares originate in active regions (ARs) with pronounced sunspots. As in the case of AR 10933 in January 2007 analyzed by us previously (Grigor'eva et al., 2013), the thoroughly studied weak flares in May 2007 are related to the emergence of a new magnetic field in the AR and to the currents that originate in this case. A comparison of the Suzaku data with the RATAN-600 microwave observations indicates that a new polarized source of microwave radiation develops in the AR (or the previously existing source intensifies) one-two days before a weak flare in the emerging flux regions. Arguments in favor of recent views that fields are force-free in the AR corona are put forward. The development of weak flares is related to the fact that the free energy of the currents that flow above the field neutral line at altitudes reaching several thousand kilometers is accumulated and subsequently released.