Electron-Cyclotron Maser Driven by Charged-Particle Acceleration from Magnetic Field-aligned Electric Fields

We present a detailed description of the auroral kilometric radiation (AKR) source region based on observations from the Fast Auroral SnapshoT (FAST) satellite and discuss how these new results may pertain to solar and stellar radio sources. FAST satellite observations are directly within the AKR source region and have unprecedented spatial and temporal resolution. They confirm many of the fundamental elements of the electron-cyclotron maser mechanism but with substantial modification. The most important modification is that the emissions do not draw their energy from a loss-cone instability; rather, the radiation results from an unstable "horseshoe" or "shell" distribution. The most far-reaching implication is that the electron-cyclotron maser is directly associated with a particular type of charged particle acceleration, a magnetic field-aligned (parallel) electric field in a dipole magnetic field. These findings change several of the characteristics of the electron-cyclotron maser mechanism and may necessitate reanalysis of some astrophysical radio sources. Under the shell instability, radio emissions with brightness temperatures ~10¹⁴ K, the steady state limit of the loss-cone instability, may be continuous. Through observations, we demonstrate that source brightness may be as high as 10²⁰ K in steady state. A moderately or strongly relativistic beam may result in broadband emissions. A loss cone is not required, so the radiation source may be high above the stellar or planetary surface. Although the generation is in the X mode with k_|| = 0, we suggest that the radiation, guided by a density cavity that is created by the parallel electric field, efficiently converts to the R mode, which experiences substantially lower absorption at higher harmonics. These findings also suggest that parallel electric fields may be a fundamental particle acceleration mechanism in astrophysical plasmas.