Modulation depth of the gyrosynchotron emission as identifier of fundamental sausage modes

Abstract

Aims. We study the intensity, the modulation depth and the mean modulation depth of the gyrosynchotron (GS) radiation as a function of the frequency and the LOS in fast sausage modes.Methods. By solving the 2.5D 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 gyrosynchotron 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, results in a larger and smaller 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 obtained by Reznikova, Antolin, and Van Doorsselaere (2014), 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 a impulsive, nonlinear perturbation, it seems that the behaviour found can be used as observational identifiers of the presence of sausage modes with respect to other QPP features. It can be inferred from this that finite-amplitude sausage modes have the potential to generate the observed deep modulation depths.