A new look at the Sun's corona as an active medium: implications for coronal seismology*

The hot, 1 million K solar corona exists because of a balance between radiative and conductive energy losses and some yet unknown coronal heating mechanism, which remains one of the major puzzles in solar physics. Such a thermal equilibrium can be readily perturbed by magnetoacoustic waves which are omnipresent in the corona, causing a misbalance between heating and cooling processes. In a series of recent works, it has been shown to lead to a back-reaction causing the wave to either lose or gain energy from the plasma. Thus, the corona acts as an active medium for magnetoacoustic waves (akin to burning gases or gain medium in lasers). In this talk, the recently understood importance of this thermodynamic activity of the corona for the magnetoacoustic wave dynamics and its implication for seismological diagnostics of the enigmatic solar coronal heating function are discussed. For a broad range of coronal conditions, the characteristic timescales of thermal misbalance are shown to be from several to a few tens of minutes, i.e. about the oscillation period of slow magnetoacoustic waves observed in the corona. It causes strong dispersion of slow waves through the modification of the effective coronal adiabatic index and the wave speed. This new dispersion is not connected with the waveguiding effects traditionally considered in the corona. The observed frequency-dependent damping of slow waves in typical thermally stable coronal plasma structures is used for constraining possible coronal heating mechanisms.