Magnetospheric multiscale observations of electromagnetic ion cyclotron waves associated with cold ion heating in the Earth’s magnetosphere

Electromagnetic ion cyclotron (EMIC) waves play a significant role in shaping the dynamics of Earth’s magnetosphere. On September 13, 2015, EMIC wave activity within the proton band was detected in the inner magnetosphere through observations made by the Magnetospheric Multiscale mission. These waves are understood to arise due to thermal anisotropy in populations of hot protons. Data indicate that EMIC waves extend across a broad range of L shell values in the aftermath of three successive geomagnetic storms, implying that the free energy source responsible for their generation is most likely linked to the injection of energetic ions during storm time intervals. Consequently, it is important to include EMIC wave effects into radiation belt modeling, especially during extended magnetic storm periods and the substorm recovery phase when electric fields are produced. When EMIC waves are intense enough, cold protons and helium ions with low energy can be activated by them. During one observed event, both perpendicular and parallel heating of hydrogen ions by the hydrogen band EMIC waves were recorded. Furthermore, these hydrogen-band EMIC waves were also found to contribute to the heating of helium ions. As a result of this heating mechanism, ions that were previously below detection thresholds become observable, accompanied by a rise in both number density as well as temperature anisotropy of hydrogen and helium ions within the low-energy range of 1–100 eV.