Atmospheric heating and quiescent radio emission in active stars

Surveys of magnetically active stars of K to M spectral classes have revealed an interrelationship between their "quiescent" soft X-ray (SXR) and microwave luminosities. While the quiescent SXR emission has a thermal, corona-like nature, the quiescent microwave emission is usually nonthermal gyrosynchrotron radiation. This correlation is discussed in terms of the ability of heating mechanisms to accelerate the required number of electrons to mildly relativistic energies. We analyze the observational signatures of two models involving heating and electron acceleration by electric currents. The first model proposes that the energetic electrons are accelerated by the electric field in classical current sheets within the stellar coronae. The second considers MHD turbulence cascading down to smaller scales in the presence of current sheets, exciting ion-acoustic waves. These waves enhance the heating and particle acceleration in the stellar transition zone. Differences between RS CVn and Sun-like stars are discussed in terms of these models. Explicit expressions for the SXR-microwave relationship are derived and discussed.