Ionospheric Outflow During the Substorm Growth Phase: THEMIS Observations of Oxygen Ions at the Plasma Sheet Boundary

Abstract

Ionospheric outflow is an important plasma source that feeds the near‐Earth magnetotail with heavy oxygen ions. Because these ions can significantly alter the structure and stability of the magnetotail current sheet, the characteristics of this outflow are important for accurate magnetosphere modeling, including modeling of substorms—a key element of magnetosphere dynamics. Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft measurements in the magnetotail (around the plasma sheet boundary), we investigate characteristics oxygen outflows observed during substorm growth phases. The observed oxygen ion temperature and flow energy indicate that the outflow is marginally stable to ion acoustic wave generation: The oxygen temperature is slightly lower than the electron temperature and slightly higher than the oxygen flow energy. Moreover, the observed outflows are accompanied by low frequency electrostatic waves that may contribute to outflow thermalization. The oxygen bulk velocity has a significant component directed toward the equatorial plane, originating from the cross‐field drift in the convection electric field. The estimated radial distances at which oxygen ions reach the plasma sheet are ∼20–40RE downtail, that is, outflows during substorm growth phases can alter current sheet characteristics around the potential magnetic reconnection region.

Thermodynamics of the Magnetotail Current Sheet Thinning

“…The enhanced ionosphere outflow during the CS thinning (i.e., substorm growth phase) can be driven by hot plasma sheet electron and ion precipitations due to strong curvature scattering (Delcourt et al., 1996; Yahnin et al., 1997). Indeed, plasma sheet lobe observations show such enhanced outflows for the near‐Earth CS thinning (Artemyev et al., 2020; Kistler et al., 2005). Moreover, the analysis of the ion(Artemyev, Angelopoulos, Runov, & Petrukovich, 2019) and electron (Artemyev, Angelopoulos, Liu, & Runov, 2017) energy spectrum evolution in the thinning CS shows that temperature decrease is generally associated with the decrease of high‐energy fluxes and growth of low‐energy (subthermal) population.…”

Stability of the Magnetotail Current Sheet With Normal Magnetic Field and Field‐Aligned Plasma Flows

“…First, the formation of a thin current sheet is often associated with enhanced precipitations of hot plasma sheet electrons into the ionosphere, and these precipitations drive the ionospheric outflow consisting of cold oxygen and hydrogen ions (Keika et al., 2013; Kronberg et al., 2015; Maggiolo & Kistler, 2014). Outflow ions shape fast beams moving along magnetic field lines (Artemyev, Angelopoulos, Runov, & Zhang, 2020; Kistler et al., 2005; Sauvaud et al., 2004) and contribute to the stress balance (Eastwood, 1972, 1974; Hill, 1975). Although such beams forming in the south and north hemispheres should be generally balanced (i.e., there is a stress balance without a net flow), the precise balance between them is not guaranteed, and there could be a net flow across the current sheet.…”

Stability of the magnetotail current sheet with normal magnetic field and field-aligned plasma flows

“…First, formation of thin current sheet is often associated with enhanced precipitations of hot plasma sheet electrons into ionosphere, and these precipitations drive the ionospheric outflow consisting of cold oxygen and hydrogen ions (Keika et al, 2013;Maggiolo & Kistler, 2014;Kronberg et al, 2015). Outflow ions shape fast beams moving along magnetic field lines (Sauvaud et al, 2004;Kistler et al, 2005;Artemyev, Angelopoulos, Runov, & Zhang, 2020) and contributing to the stress balance p xz (Eastwood, 1972(Eastwood, , 1974Hill, 1975). Although such beams forming in the south and north hemispheres should be generally balanced (i.e., there is a stress balance without a net flow), the precise balance between them is not guaranteed, and there could be a net flow across the current sheet.…”