摘要：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.
关键词：Sun: flares;Sun: corona;Sun: radio radiation;Sun: UV radiation;Sun: oscillations
