On the presence and properties of cold ions near Earth's equatorial magnetosphere

Abstract

AbstractPlasma flows related to magnetospheric convection or ULF waves often increase cold ions' kinetic energy sufficiently above the spacecraft potential energy so that these ions can be detected by charged particle instrumentation. Other detection methods also reveal the presence of cold ions even in the absence of flows. By applying such methods to complementary data sets obtained from particle, electric field, and magnetic field measurements by the multiple Time History of Events and Macroscale Interactions during Substorms spacecraft, we study the occurrence rates of cold ions and properties—composition, densities, and temperatures—of the dominant species (H+, He+, and O+) near the equatorial magnetosphere. As plasma flows occur at all magnetic local time (MLT) sectors, predominantly outside the plasmasphere, they enable studies of cold ion occurrence and properties as functions of geocentric distance (up to 13 RE) and MLT. Of the dominant cold ions, protons are most abundant on the dayside and coldest almost everywhere. By comparison, the heavier ions show evidence of larger abundance on the nightside and higher temperatures at most locations: median nHe+/nH+ is ≤0.1 in the afternoon, ≥1.0 on the nightside, and near 0.5 at dawn; median nO+/nH+ exceeds 1 throughout the nightside and around dawn at smaller geocentric distances but varies from 0.0 to 0.1 in the afternoon; and TO+ > THe+ everywhere. By isolating the convective particle drift component of measured ion flows, we deduce that nightside/dawn/prenoon cold ions are components of the warm plasma cloak while cold ions near noon/afternoon are likely of mixed origin, including nightside/cusp outflow, plasmaspheric plumes, and the solar wind.