Spatially resolved microwave pulsations of a flare loop

Abstract

A microwave burst with quasi-periodic pulsations was studied with high spatial resolution using observations with the Nobeyama Radioheliograph (NoRH). We found that the time profiles of the microwave emission at 17 and 34 GHz exhibit quasi-periodic (with two well defined periods P_1= 14-17 s and P_2= 8-11 s) variations of the intensity at different parts of an observed flaring loop. Detailed Fourier analysis shows the P₁ spectral component to be dominant at the top, while the P₂ one near the feet of the loop. The 14-17 s pulsations are synchronous at the top and in both legs of the loop. The 8-11 s pulsations at the legs are well correlated with each other but the correlation is not so obvious with the pulsations at the loop top. For this P₂ spectral component, a definite phase shift, P_2/4≈ 2.2~s, between pulsations in the northern leg and loop top parts of the loop have been found. The length of the flaring loop is estimated as L = 25 Mm (≈ 34 arcsec) and its average width at half intensity at 34 GHz as about 6 Mm (≈ 8 arcsec). Microwave diagnostics shows the loop to be filled with a dense plasma with the number density n₀ ≈ 10¹¹ cm^-3, penetrated by the magnetic field changing from B₀ ≈ 100 G near the loop top up to B₀ ≈ 200 G near the north footpoint. A comparative analysis of different MHD modes of the loop demonstrates the possibility of the simultaneous existence of two modes of oscillations in the loop: the global sausage mode, with the period P_1= 14-17 s and the nodes at the footpoints, and a higher harmonics mode (possibly with the radial wave number l>1), with P_2= 8-11 s.