Modulation of gyrosynchrotron emission in solar and stellar flares by slow magnetoacoustic oscillations

Gyrosynchrotron emission generated by non-thermal electrons in solar and stellar coronal flares can be efficiently modulated by slow magnetoacoustic oscillations in the flaring loops. The modulation mechanism is based upon perturbation of the efficiency in the Razin suppression of optically thin gyrosynchrotron emission. Modulation of the emission is in anti-phase with the density perturbation in the slow wave. The observed emission modulation depth can be up to an order of magnitude higher than the slow wave amplitude. This effect is more pronounced at lower frequencies. Observations with spatial resolution, together with analysis of the modulation frequency, are shown to be sufficient for providing the information needed to identify the mode

Types of microwave quasi-periodic pulsations in single flaring loops

Quasi-periodic pulsations (QPP) of microwave emission generated in single flaring loops observed with the Nobeyama Radioheliograph (NoRH) and Nobeyama Radio Polarimeters (NoRP) are studied. Specific features of the time profiles, i.e. the visible presence or absence of QPPs, are not accounted for in the selection. The time evolution of the periods of the QPPs is examined using wavelet and correlation analyses. In ten out of twelve considered events, at least one or more significant spectral components with periods from 5 - 60 s have been found. The quality of the oscillations is rather low: Q = pi N, where N is the number of cycles, mostly varies in the range 12 to 40, with an average of 25. We suggest that the detected QPPs can be classified into four types: i) those with stable mean periods (e. g. of 15 - 20 s or 8 - 9 s, the prevailing type); ii) those with spectral drift to shorter periods (mostly in the rise phase of the microwave emission); iii) those with drift to longer periods (mostly in the decay phase); iv) those with multiple periods showing an X-shaped drift (e. g. in the range from 20 - 40 s in the rise phase)