Radio Evidence for Nonthermal Magnetic Activity on Main-Sequence Stars of Late Spectral Type

Abstract

The Very Large Array (VLA) and other radio interferometric arrays provide information about the magnetic fields, electrons, temperatures and both thermal and nonthermal particle acceleration on the Sun and other nearby magnetically active stars of late spectral type. The long-lived, nonthermal radio emission of these stars is correlated with their thermal X-ray luminosity. The nonthermal particle acceleration process is probably related to coronal heating in these stars, and to magnetic activity resulting from internal rotation and convection. Thermal radiation dominates the quiescent radio output of the Sun, which is radio underluminous when compared to other radio-underluminous active stars. However, different thermal and nonthermal radio structures are detected at different wavelengths and in different places on the Sun. Peculiar nonthermal radio sources and nonthermal noise storms, are respectively found above solar active regions and in more extended large-scale coronal loops; they may both be fed by global reservoirs of high-energy particles. High-velocity electrons can be nearly continuously accelerated in the magnetically confined coronae of nearby active stars of late spectral type, providing nonthermal incoherent gyrosynchrotron or synchrotron radiation and/or nonthermal coherent radiation.