Magnetospheric Ion Evolution Across the Low‐Latitude Boundary Layer Separatrix

Abstract

On 20 September 2015, the Magnetospheric Multiscale (MMS) spacecraft crossed the dusk magnetopause after a compression of the magnetosphere. Enhanced densities and fluxes of both colder (≤10 eV) and hotter (>1 keV) magnetospheric and magnetosheath heavy ion species were observed reaching the magnetopause. The evolution of the velocity distributions for H+, He+, and O+ measured by the Hot Plasma Composition Analyzer on MMS during this magnetopause crossing is presented. In particular, this study focuses on the changes in the species' distribution functions as MMS passes from the magnetosphere through the electron edge of the low‐latitude boundary layer (LLBL) separatrix and then into the LLBL. Two types of processes are suggested to play a role in the heating of colder magnetospheric ions across the LLBL separatrix in the region between the separatrix and the electron and ion edges of the LLBL. One mechanism leads to the formation and enhancement of ring distributions in this layer of the LLBL as the magnetospheric ions propagate across the separatrix. A second mechanism leading first to perpendicular heating and then to parallel heating of colder protons may arise from a possible two‐stream instability as the magnetospheric ions first encounter the warmer magnetosheath electrons in the electron layer and then the warmer magnetosheath ions between the electron and ion edges of the LLBL separatrix. Perpendicular heating of He+ and O+ is seen more so in the main reconnection exhaust, due to nonadiabatic behavior of these ions as they are accelerated up to the bulk flow speed.

MMS, Van Allen Probes, GOES 13, and Ground‐Based Magnetometer Observations of EMIC Wave Events Before, During, and After a Modest Interplanetary Shock

“…Also, seeing low fluxes of He ++ in the outer magnetosphere at thermal to suprathermal energies (tens of keV) is quite common: He ++ has been observed in the magnetosphere at low levels (a few percent of the proton population at the same energies) since at least 1980 (e.g., Ipavich and Scholer, ). A recent example of when there is both a real He ++ population at tens of keV in the outer magnetosphere, as well as clear proton contamination in the other species observed by HPCA, can be found in Figure 2 of Vines et al (): the contamination signatures were in the magnetosheath, where the bulk density was nearly 50/cm 3 , well above that for this event. Thus, while the He ++ population shows somewhat similar features to the protons at >10 keV/e, that population is a real signature of He ++ that has persisted at low levels in the magnetosphere, rather than being an effect of proton contamination.…”

Internal Versus External Sources of Plasma at Saturn: Overview From Magnetospheric Imaging Investigation/Charge‐Energy‐Mass Spectrometer Data

“…The other possible source of solar wind ions within the magnetosphere is from Dungey‐cycle related reconnection. Dayside reconnection allows for solar wind‐originating ions to enter the magnetosphere and can provide energization of those particles (Allen et al, ; Allen, Livi, & Goldstein, ; Allen, Livi, Vines, et al, ; Vines et al, ). Consistent with this occurring at Saturn, the relative abundance and group abundance fractions of He ++ in the noon quadrant increases with radial distance (Figures f and j) at a rate faster than the dawn, dusk, and midnight quadrants.…”

“…However, for typical conditions in the dayside magnetosheath and at the magnetopause of Saturn, diamagnetic drift arising from large asymmetries in the plasma beta across the magnetopause leads to the dayside reconnection site to be more spatially confined to higher latitudes than compared to Earth (Fuselier et al, ; Masters et al, ). Where reconnection is favorable, ions can experience a mass‐dependent heating and energization when entering a reconnection exhaust (Drake et al, ; Vines et al, ). This process can lead to energization of ions up to the energy range of CHEMS, particularly for high mass‐to‐charge ratio ion species.…”

Observational Evidence of Large‐Scale Multiple Reconnection at the Earth's Dayside Magnetopause

“…Magnetic field lines in the MSBL thread the magnetopause current layer, allowing energetic magnetospheric ions to escape from the LLBL. In addition, there is a population of magnetosheath ions that are reflected off the magnetopause in the MSBL, and this population has higher energy than the ion population in the magnetosheath proper (e.g., Cowley, ; Fuselier et al, ; Vines et al, ; Vines, Fuselier, Trattner, et al, ). Thus, the presence of an MSBL with >10‐keV protons is one piece of evidence of reconnection at the magnetopause.…”

“…In particular, in the magnetosphere it appears as a single, cold population. However, in the LLBL, it appears as a double‐lobe structure in 2‐D cuts and is therefore a ring beam in 3‐D (see Vines, Fuselier, Trattner, et al, ). The ring beam in Figure has a low, but variable parallel velocity.…”