Linear Mode Conversion Theory Beaming Formula for Non-thermal Continuum Radiation

Because the non-thermal continuum (NTC) and kilometric continuum (KC) radiations are a primary observable in Earth, understanding physical processes leading to wave emissions is fundamental to interpreting observations of the plasmasphere. At present, generation mechanism of the NTC and KC radiations is not well understood, and in particular, it is not clear whether linear mode conversion (LMC) theory, which is the strongest and most widely quoted generation mechanism for NTC and KC radiation, is relevant or not. The LMC theory involves energetic electrons generating electrostatic upper hybrid resonance (UHR) waves, which then propagate into a higher electron density region in the UHR/Z-mode. When the UHR/Z-mode subsequently reaches the location where the wave frequency matches the local electron plasma frequency (fpe), these waves are linearly converted to the free-space LO-mode waves. The LMC theory predicts that free space radiation will be emitted from the radio window at the equatorial plasmapause with a beaming angle of Theta (a function of fpe and fge the local electron cyclotron frequency). In this presentation, we first perform numerical simulations of the NTC radiation using a high-resolution state-of-art full wave simulation, Petra-M. We launched Z-mode waves near the source and determine how the mode-conversion occurs and how the mode-converted waves propagate to the space. A statistical study covering a large range of geomagnetic conditions, thus plasmapause radial locations, of wave beaming angle has been also examined. Statistical analysis of the complement of the beaming angle derived from Van Allen Probes-A High Frequency Receiver observations over the entire mission qualitatively reproduces an inverted V pattern, which is predicted by the LMC theory, in magnetic latitude between frequencies of 17 and 100 kHz, and the relationship between Theta and the Kp index is qualitatively consistent with changing plasmapause location. However, the observed beaming angle is much larger than that of the theory for observations near the magnetic equator. For frequencies from 100 kHz to 400 kHz the emissions are strongly beamed with the complement of Theta near 90 degrees making it difficult to explain this strong beaming using LMC theory.