Topics in Active Region Oscillations

A. COSTA

Warwick

Workshop; MHD Coronal Seismology; 2020

Center for Fusion, Space and Astrophysics

I will present a review of recent works on active region oscillations. In one case we numerically analyze the capability of different type of perturbations -associated with usual environment energy fluctuations of the solar corona- to excite slow and sausage modes in solar flaring loops. We find that local loop energy depositions of typical microflares are prone to drive slow shock waves that induce slow mode patterns. The slow mode features are obtained for every tested local energy deposition inside the loop. Meanwhile, to obtain an observable sausage mode pattern a global perturbation, capable of modifying instantaneously the internal loop temperature, is required, i.e. the characteristic conductive heating time must be much smaller than the radiative cooling one.In other case, assuming a thin flux tube approximation, the cut-off periods of slow-mode magneto-acoustic-gravity waves that travel from the photosphere to the corona are analytically obtained considering a non-isothermal gravitationally stratified atmosphere in hydrostatic equilibrium; we also include the variation of the mean atomic weight. We discuss the cut-off results varying the intensity and the magnetic scale height of the magnetic field for different transition region profiles. We show that the cut-off periods obtained are consistent with observational results of a particular active region.Within the limitations of our model, we show that slow monochromatic oscillations of the solar atmosphere are the atmospheric response, at its natural frequency, to random impulsive perturbations, and not a consequence of the forcing from the photosphere. A final reflection about the relation between coronal and chromospheric mutual influences is offered.