Broadband microwave sub-second pulsations in an expanding coronal loop of the 2011 August 10 flare

¹ Astronomical Institute of the Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic
e-mail: hana@asu.cas.cz
² Astronomical Institute of the Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovak Republic
³ Institute of Solar-Terrestrial Physics SB RAS, 664033 Irkutsk, Russia
⁴ Special Astrophysical Observatory of RAS, 196140 St. Petersburg Department, Russia

Received: 30 December 2015
Accepted: 6 July 2016

Abstract

Aims. We studied the characteristic physical properties and behavior of broadband microwave sub-second pulsations observed in an expanding coronal loop during the GOES C2.4 solar flare on 2011 August 10.

Methods. The complex microwave dynamic spectrum and the expanding loop images were analyzed with the help of SDO/AIA/HMI, RHESSI, and the STEREO/SECCHI-EUVI data processing software, wavelet analysis methods, the GX Simulator tool, and the NAFE method.

Results. We found sub-second pulsations and other different burst groups in the complex radio spectrum. The broadband (bandwidth about 1 GHz) sub-second pulsations (temporal period range 0.07−1.49 s, no characteristic dominant period) lasted 70 s in the frequency range 4−7 GHz. These pulsations were not correlated at their individual frequencies, had no measurable frequency drift, and zero polarization. In these pulsations, we found the signatures of fast sausage magnetoacoustic waves with the characteristic periods of 0.7 and 2 s. The other radio bursts showed their characteristic frequency drifts in the range of −262−520 MHz s^-1. They helped us to derive average values of 20−80 G for the coronal magnetic field strength in the place of radio emission. It was revealed that the microwave event belongs to an expanding coronal loop with twisted sub-structures observed in the 131, 94, and 193 Å SDO/AIA channels. Their slit-time diagrams were compared with the location of the radio source at 5.7 GHz to realize that the EUV intensity of the expanding loop increased just before the radio source triggering. We reveal two EUV bidirectional flows that are linked with the start time of the loop expansion. Their positions were close to the radio source and propagated with velocities within a range of 30−117 km s^-1.

Conclusions. We demonstrate that periodic regime of the electron acceleration in a model of the quasi-periodic magnetic reconnection might be able to explain physical properties and behavior of the sub-second pulsations. The depolarization process of the microwave emission might be caused by a plasma turbulence in the radio source. Finally, the observed EUV flows might be linked with reconnection outflows.