A document by Kyungguk Min. Click on the document to view its contents.

We performed a statistical study of electromagnetic ion cyclotron (EMIC) wave distributions and their coupling with energetic protons in the inner magnetosphere using the Arase satellite data from May 2017 to December 2020. We investigated the energetic proton pitch-angle distributions and partial thermal pressures associated with EMIC waves using inter-calibrated proton data in the energy range of 30 eV/q-187 keV/q. With a cold plasma approximation, we computed the proton minimum resonance energies using the observed EMIC wave frequency and plasma density values. We found that the EMIC waves had left-handed polarization near the magnetic equator close to the threshold of proton cyclotron instability, and propagated to higher latitudes along the field line with polarization reversal. H-EMIC waves showed two peak occurrence regions in the morning and noon sectors at L=7.5-9 outside the plasmasphere. The flux enhancements associated with morning side H-EMIC waves appeared at E<1 keV/q among all pitch angles, while H-EMIC waves in the noon sector exhibited flux enhancement in field-aligned directions at E=1-100 keV/q. He-EMIC waves showed a broad occurrence region from 12 to 20 magnetic local time at L=5.5-8.5 inside the plasmasphere with strong flux enhancements at all pitch-angle ranges at E=1-100 keV/q. The proton minimum resonance energy using the obtained central frequency was consistent with the observed flux enhancements at different peak occurrence regions. We conclude that the free energy sources of EMIC waves in different geomagnetic environments drive the two different types of EMIC waves, and they interact with energetic protons at different energy ranges.

Two-dimensional hybrid particle-in-cell (PIC) simulations are carried out on a constant L-shell (or drift shell) surface of the dipole magnetic field to investigate the generation process of near-equatorial fast magnetosonic waves (a.k.a equatorial noise; MSWs hereafter) in the inner magnetosphere. The simulation domain on a constant L-shell surface adopted here allows wave propagation and growth in the azimuthal direction (as well as along the field line) and is motivated by the observations that MSWs propagate preferentially in the azimuthal direction in the source region. Furthermore, the equatorial ring-like proton distribution used to drive MSWs in the present study is (realistically) weakly anisotropic. Consequently, the ring-like velocity distribution projected along the field line by Liouville’s theorem extends to rather high latitude, and linear instability analysis using the local plasma conditions predicts substantial MSW growth up to +- 27deg latitude. In the simulations, however, the MSW intensity maximizes near the equator and decreases quasi-exponentially with latitude. Further analysis reveals that the stronger equatorward refraction at higher latitude due to the larger gradient of the dipole magnetic field strength prevents off-equatorial MSWs from growing continuously, whereas MSWs of equatorial origin experience little refraction and can fully grow. Furthermore, the simulated MSWs exhibit a rather complex wave field structure varying with latitude, and the scattering of energetic ring-like protons in response to MSW excitation occurs faster than the bounce period of those protons so that they do not necessarily follow Liouville’s theorem during MSW excitation.

Although some plasma waves exhibit the largest growth rate and amplitude at 90deg wave normal angle (WNA), particle scattering by these waves in a quasilinear (QL) sense has not been examined previously. Using test-particle calculation and QL theory, the present study investigates the proton scattering by equatorial fast magnetosonic waves (MSWs; a.k.a equatorial noise) with varying WNAs including 90deg. Comparison with the diffusion coefficients in momentum space obtained from the test-particle approach indicates that the QL diffusion coefficients given by, e.g., Kennel and Engelmann (1966) are valid up to 90deg WNA, provided that MSWs described conform to the usual QL theory assumptions. The test-particle dynamics due to MSWs at 90deg WNA are examined in detail. Although in the QL picture, protons are only supposed to resonate with MSWs of integer harmonic frequencies at perpendicular propagation, the presence of slightly off-integer harmonic modes as part of a narrowband discrete spectrum of incoherent MSWs plays an important role in making the proton scattering stochastic. Considering the recent test-particle result of bounce-averaged resonance of energetic protons, non-zero wave power at the WNAs >~ 89.5deg typically excluded in QL diffusion can be important for ring current proton dynamics.