Modulation depth of the gyrosynchrotron emission as an identifier of fundamental sausage modes

Abstract

Aims. We aim to study the intensity, the modulation depth, and the mean modulation depth of the gyrosynchrotron (GS) radiation as a function of the frequency and the line of sight (LOS) in fast sausage modes. Methods. By solving the 2.5D magnetohydrodynamics (MHD) ideal equations of a straight coronal loop considering the chromosphere and with typical flaring plasma parameters we analyse the wavelet transform of the density and the GS emission for different radio frequencies and different spatial resolutions, given impulsive and general perturbations with energies in the microflare range. Results. A wavelet analysis performed over the GS radiation emission showed that a fast fundamental sausage mode of ∼7 s with a first harmonic mode of 3 s developed, for all the initial energy perturbations used. For both the high spatial resolution (central pixel integration) and the low spatial resolution (entire loop integration), the larger the radio frequency, the larger the modulation depth. However, high- and low-resolution integrations differ in that the larger the LOS angle with respect to the loop axis, the larger and smaller the modulation depth, respectively. Conclusions. Fast MHD modes triggered by instantaneous energy depositions of the order of a microflare energy are able to reproduce deep intensity modulation depths in radio emission as observed in solar events. As the trends of the GS emission previously obtained for a linear and forced oscillation remain present when analysing a more general context, considering the chromosphere and where the sausage mode is triggered by an impulsive, non-linear perturbation, it seems that the behaviour found can be used as observational identifiers of the presence of sausage modes with respect to other quasi-periodic pulsation features. It can be inferred from this that finite-amplitude sausage modes have the potential to generate the observed deep modulation depths.