Charge-Exchange Byproduct Cold Protons in the Earth’s Magnetosphere

Abstract

Owing to the spatial overlap of the ion plasma sheet (ring current) with the Earth’s neutral-hydrogen geocorona, there is a significant rate of occurrence of charge-exchange collisions in the dipolar portion of the Earth’s magnetosphere. During a charge-exchange collision between an energetic proton and a low-energy hydrogen atom, a low-energy proton is produced. These “byproduct” cold protons are trapped in the Earth’s magnetic field where they advect via E×B drift. In this report, the number density and behavior of this cold-proton population are assessed. Estimates of the rate of production of byproduct cold protons from charge exchange are in the vicinity of 1.14 cm−3 per day at geosynchronous orbit or about 5 tons per day for the entire dipolar magnetosphere. The production rate of cold protons owing to electron-impact ionization of the geocorona by the electron plasma sheet at geosynchronous orbit is about 12% of the charge-exchange production rate, but the production rate by solar photoionization of the neutral geocorona is comparable or larger than the charge-exchange production rate. The byproduct-ion production rates are smaller than observed early time refilling rates for the outer plasmasphere. Numerical simulations of the production and transport of cold charge-exchange byproduct protons find that they have very low densities on the nightside of geosynchronous orbit, and they can have densities of 0.2–0.3 cm−3 at geosynchronous orbit on the dayside. These dayside byproduct-proton densities might play a role in shortening the early phase of plasmaspheric refilling.

Geospace: The naturally occurring plasma laboratory

“…For example, the Farley-Buneman instability in the E-region is a modified two-stream instability caused by neutrals dragging ions in a different direction than electrons. While the presence of neutrals has always been emphasized in ionospheric and chromospheric studies, the contribution of the geocorona to the cold-plasma in the magnetosphere is starting to be understood (Borovsky et al, 2022).…”

Geospace: The naturally occurring plasma laboratory

“…For example, the Farley-Buneman instability in the E-region is a modified two-stream instability caused by neutrals dragging ions in a different direction than electrons and cannot be simulated with MHD. While the presence of neutrals has always been emphasized in ionospheric and chromospheric studies, the contribution of the geocorona to the cold plasma in the magnetosphere is starting to be understood (Borovsky et al, 2022).…”

A New Mechanism for Early‐Time Plasmaspheric Refilling: The Role of Charge Exchange Reactions in the Transport of Energy and Mass Throughout the Ring Current—Plasmasphere System

“…While the cold‐particle populations are sourcing a significant portion of the magnetospheric hot plasma, hot‐particle populations can also become the source of cold plasma via charge exchange processes with regional neutral atoms (e.g., Borovsky et al., 2022 ). Due to the spatial overlap of the ring current with the Earth's neutral exosphere (Carruthers et al., 1976 ; Rairden et al., 1986 ), charge exchange processes between the low‐energy exospheric neutral hydrogen atoms and the ring current ions, which allow for an energetic ion to pick up the orbital electron of a cold exospheric neutral hydrogen atom, lead to the formation of Energetic Neutral Atoms (ENAs) accompanied by low‐energy H + .…”

“…Therefore, the charge exchange process not only represents an important loss mechanism for the ring current energy density (Ilie & Liemohn, 2016 ; Ilie et al., 2013 ; Kistler et al., 1989 ; Liemohn & Kozyra, 2003 , 2005 ; Liemohn et al., 1999 ; Smith & Bewtra, 1978 ), but also leads to unstable hot‐ion distributions in the ring current region (Cornwall, 1977 ; Thomsen et al., 2011 , 2017 ), and may also shorten the early‐phase of the plasmaspheric refilling (Denton & Borovsky, 2014 ; Obana et al., 2010 ; Sojka & Wrenn, 1985 ; Su et al., 2001 ). While the density of exospheric neutral hydrogen decreases exponentially with increasing distance away from the Earth (e.g., Borovsky et al., 2022 ; Chamberlain, 1963 ; Ilie et al., 2013 ), the details of cold H + production via charge exchange depend on the energy profile, equatorial temperature distribution, convection drift pattern, and ion composition of the hot plasma. Due to the different charge exchange cross sections for reactions involving various ring current species with neutral hydrogen, changes in the regional ion composition can lead to changes in the cold H + population formed via the charge exchange interaction.…”