Excitation mechanisms of the storm-time Pc5 ULF waves by the drift-bounce resonance with ring current ions based on the Magnetosphere-Ionosphere coupled model

Abstract:

Storm-time Pc5 ULF waves can be excited by ring current ions injected from the magnetotail during substorms. The excitation mechanism of Pc5 waves is a key to understand dynamic variation of radiation belt, since they can drive radial diffusion of radiation belt electrons [e.g. Elkington et al., JGR, 2003]. The drift-bounce resonance [Southwood, JGR, 1976] is considered to be the candidate excitation mechanism. Recently, Yamakawa et al. [GRL, 2019] confirmed the drift resonance excitation of Pc5 waves by an isotropic PSD ion injection with no north-south asymmetry based on the drift-kinetic model. However, drift-bounce resonance was not detected in the symmetric PSD case. This study aims at investigation of excitation conditions for ULF waves by the drift-bounce resonance.

We performed a kinetic simulation for ring current ions using GEMSIS-RC model [Amano et al., JGR, 2011], in which 5D drift-kinetic equation for PSD of ions and Maxwell equations are solved self-consistently. In order to simulate ion injection from the plasma sheet, we set a localized high pressure region around midnight consisting of protons. We put asymmetric butterfly like pitch angle distribution in addition to isotropic (T = 16 keV) Maxwellian distribution with loss cones [Yamakawa et al., JGR, submitted]. We have identified the excitation of poloidal Pc3 waves due to the drift-bounce resonance in the dusk sector in addition to the Pc5 waves by the drift resonance. Pc5 and Pc3 poloidal mode waves are excited in the region of positive growth rate resultant from the positive PSD gradients at 90 degrees and oblique pitch angle, respectively. However, the simulated ULF waves have small amplitudes compared to observation results. In order to simulate ion injection under more realistic condition, we also have made Magnetosphere-Ionosphere coupling with GEMSIS-POT model [Nakamizo et al., JGR, 2012]. GEMSIS-POT is an ionospheric model, which solves 2-D electric potential. We use FAC from GEMSIS-RC as an input to GEMSIS-POT for the Region 2 current. The resultant electric field potential is then used as inner boundary condition of GEMSIS-RC. The coupled model enables us to simulate the ion injection from the plasma sheet into the inner magnetosphere. We will report on the details of effects of the M-I coupling on the excitation of ULF waves.