Magnetic Field Fluctuations Coinciding With Substorm Events

Abstract Using the Electric and Magnetic Field Instrument Suite (EMFISIS) onboard the Van Allen Probes spacecraft, the present study investigates the magnetic field fluctuations corresponding to substorms associated with magnetic field dipolarization. We examine thirteen substorm occurrences with L < 6.6 and a range of MLT spanning dawn, midnight, and dusk during the year 2018. Utilizing the 64Hz magnetic field data, our investigation reveals significant magnetic field fluctuations exhibiting power from gyrofrequency of O+ extending up to gyrofrequency of H+. Notably, in all the events studied, there is the absence of a helium stop band which is unique. Through the wave polarization analysis, we speculate that the observed magnetic field fluctuations represent Electromagnetic Ion Cyclotron (EMIC) waves. The proton pitch angle distribution is analyzed to detect particle anisotropy, and resonance energy corresponding to the fluctuations is computed using the cold plasma approximation and is consistent with observations. The plasma and magnetic pressure are computed to understand the role of resonant protons associated with the magnetic field fluctuations in maintaining the pressure balance. The inverse correlation between these two pressures may be the result of the injected hot protons. Our findings suggest that the disappearance of stop bands may be attributed to a combination of heavy ion density, temperature, and the presence of hot proton anisotropy. Additionally, the observed self-sustaining magnetic fluctuations hint at the potential influence of nonlinear theory in this phenomenon.