Flare evolution and polarization changes in fine structures of solar radio emission in the 2013 April 11 event

The measurement of positions and sizes of radio sources in observations is important for un- derstanding of the flare evolution. For the first time, solar radio spectral fine structures in an M6.5 flare that occurred on 2013 April 11 were observed simultaneously by several radio instruments at four different observatories： Chinese Solar Broadband Radio Spectrometer at Huairou （SBRS/Huairou）, Ondrejov Radio Spectrograph in the Czech Republic （ORSC/Ondrejov）, Badary Broadband Microwave Spectropolarimeter （BMS/Irkutsk）, and spectrograph/IZMIRAN （Moscow, Troitsk）. The fine structures included microwave zebra patterns （ZPs）, fast pulsations and fiber bursts. They were observed during the flare brightening lo- cated at the tops of a loop arcade as shown in images taken by the extreme ultraviolet （EUV） telescope onboard NASA＇s satellite Solar Dynamics Observatory （SDO）. The flare occurred at 06：58-07：26 UT in solar active region NOAA 11719 located close to the solar disk center. ZPs appeared near high frequency boundaries of the pulsations, and their spectra observed in Huairou and Ondrejov agreed with each other in terms of details. At the beginning of the flare＇s impulsive phase, a strong narrowband ZP burst occurred with a moderate left-handed circular polarization. Then a series of pulsations and ZPs were observed in almost unpolarized emission. After 07：00 UT a ZP appeared with a moderate right-handed polarization. In the flare decay phase （at about 07：25 UT）, ZPs and fiber bursts become strongly right-hand polarized. BMS/Irkutsk spectral observations indicated that the background emission showed a left-handed circular polarization （similar to SBRS/Huairou spectra around 3 GHz）. However, the fine structure appeared in the right-handed polarization. The dynamics of the polarization was associated with the motion of the flare ex- citer, which was observed in EUV images at 171 A and 131 A by the SDO Atmospheric Imaging Assembly （AIA）. Combining magnetograms observed by the SDO Helioseismic and Magnetic Imager （HMI） with the homologous assumption of EUV flare brightenings and ZP bursts, we deduced that the observed ZPs correspond to the ordinary radio emission mode. However, future analysis needs to verify the assumption that zebra radio sources are really related to a closed magnetic loop, and are located at lower heights in the solar atmosphere than the source of pulsations.