Seminario: Francesco Malara (UniCal)

Speaker: Francesco Malara (UniCal)

Titolo del talk: “MHD waves and turbulence in plasmas of the solar corona”

“The solar corona is made of extremely tenuous plasmas having different physical conditions, e.g., temperature, density, and magnetic fields. In response to perturbations, restoring forces determined by pressure and the local magnetic field can determine the formation and propagation of magnetohydrodynamic (MHD) waves. Coronal loops sustain the propagation of different types of MHD with a broad range of periods and amplitude, with the most easily observed and studied one being the kink mode. Kink waves in loops, usually excited in impulsive events (e.g. flares) with amplitude of 4-5 Mm and periods of 3-5 min, are strongly damped within few wave periods. However, high-resolution observations from different space missions in the last decade have shown the omnipresence of so-called “decayless fluctuations”, i.e., kink oscillations with a small amplitude (< 1 Mm) and no damping. Their discovery in the solar corona has posed new questions about their driver and the mechanisms for energy transfer in the solar atmosphere. In fact, the dissipation of the fluctuation energy is one of the possible mechanisms that are believed to be resposible for the heating of the solar corona. In an extremely high Reynolds/Lundquist number plasma, such as in the corona, a key role is played by fluctuation-inhomogeneity couplings, as well as by nonlinear effects that generate a turbulent cascade. In the last few decades such effects have been studied in great detail. Here we present a numerical investigation aiming at clarifying the interplay of these two mechanims in the dissipation of Alfvén waves. We find that, in a regime representative of coronal magnetic structures, fluctuation-inhomogeneity couplings and turbulence synergically work to significantly speed up the conversion of fluctuating kinetic/magnetic energy into thermal energy. Their potential contribution to solving the coronal heating problem is emphasized, as well as the potentialities offered by the MUSE mission in studying the formation and evolution of MHD waves in the corona.”