Observation of microwave oscillations with spatial resolution

V. Reznikova

Melnikov V.F., Shibasaki K., Nakariakov V.M.

We provide an observational proof of the existence of the sausage mode in a flaring loop studying a microwave burst 12-Jan-2000 with quasi-periodical pulsations observed with the Nobeyama Radioheliograph (NoRH). We have found that the time profiles of the microwave emission at 17 and 34 GHz exhibit synchronous quasi-periodical variations of the intensity in different parts (loop top and footpoints) of the corresponding flaring loop. The length of the flaring loop is estimated as $L = 25$~Mm ($\\approx 34\"$) and its width at half intensity at 34~GHz as about 6~Mm ($\\approx 8\"$). These estimations are consistent with Yohkoh/SXT images taken on the late phase of the flare. The loop is estimated to be filled by a dense plasma with density $n_0 \\approx 10^{11}$~cm$^{-3}$ penetrated by the magnetic field of the strength $B_0 \\approx 50-100$~G. Detailed comparison of the corresponding Fourier spectra shows the pulsations at the legs to be almost synchronous with the quasi-period $P_2=$~8-11~s. At the loop apex the synchronism with the legs pulsations is not so obvious but definitely exists on the larger time scale, $P_1=$~14-17~s. These properties of pulsations indicate the possibility of the simultaneous existence of two modes of sausage oscillations in the loop: the global one, with the period $P_1=$~14-17~s and the nodes at the footpoints, and the harmonic with $P_2 =$ 8-11 s.