Comparative analysis of microwave flare pulsations at low and high frequencies

V.E. Reznikova$^1$, V.F. Melnikov$^1$, Y.N. Su$^2$, G.L. Huang$^2$

$^1$Radiophysical Research Institute (NIRFI), Nizhny Novgorod, 603950, Russia
$^2$Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210008, China

Wavelet analysis of the flare intensity variations has been conducted for the solar flare observed with Nobeyama Radioheliograph at frequencies 17 and 34 GHz and with the spectrometer of Purple Mountain Observatory at frequencies 4.5-7.5 GHz. It was found that signals contain well-pronounced periodicity with period P=14-17 s and stable in-phase behavior at frequencies higher and lower than the peak frequency. We simulated the modulation of the gyrosynchrotron emission by fast sausage magnetoacoustic oscillations for the conditions of low and high plasma density in the radio source. It is shown that the synchronism of pulsations at high and low frequencies can be realized only in the case of high plasma density, when the low frequency turnover of microwave spectrum is caused by the Razin effect, but not by the self-absorption. Independent radio and X-ray diagnostics confirms high value of number density ( $n_0 \sim 10^{11}$ cm$^{-3}$) in the flare loop.