Modeling the Effect of a Plasmaspheric Drainage Plume on ULF Wave Power Accessibility and Radiation Belt Electron Energization in Earth's Magnetosphere.

Abstract:

The effect of a plasmaspheric drainage plume on the distribution of ULF wave power is studied by means of an MHD wave model for the magnetosphere. Understanding this effect is interesting because the drainage plume occurs in the afternoon sector where ULF waves and energetic electrons interact via drift-resonance. The wave model includes magnetic-field day/night asymmetry, and extends to a parabolic dayside magnetopause, from which ULF waves are launched. The drainage plume density is evolved using an advection model driven by a (Volland-Stern) convection electrostatic field, and monochromatic (1.5 mHz) ULF waves are launched during the plume development. We find that the plume structure significantly alters the field line resonance (FLR) location, and the turning point for MHD fast waves, introducing strong asymmetry in the ULF wave distribution across the noon meridian. For example, a dusk side bulge in the plasmasphere moves the FLR location to lower L, and significantly reduces its amplitude in the afternoon sector. As the bulge evolves into a plume and extends towards the afternoon magnetopause, it appears that the plume structure creates a waveguide for MHD fast waves, such that an eigenmode (in L and MLT) develops within the plume. Examples shown in the figure, contrast the amplitude of wave field components E_r E_Φ and b_// for cases without (left) and with (right) the plume (white contours indicate plasma density at 50,100,200,500 and 1000 amu/cc). The increased density within the plume then enables penetration of ULF wave power to much lower L (magenta contours) than is possible without the plume, affecting electron energization. This is shown using a test-kinetic advection model for equatorially mirroring electrons driven by the ULF wave model.