Energetic Electron Injections Deep Into the Inner Magnetosphere: A Result of the Subauroral Polarization Stream (SAPS) Potential Drop

Abstract

It has been reported that the dynamics of energetic (tens to hundreds of keV) electrons and ions is inconsistent with the theoretical picture in which the large-scale electric field is a superposition of corotation and convection electric fields. Combining one year of measurements by the Super Dual Auroral Radar Network, DMSP F-18, and the Van Allen Probes, we show that subauroral polarization streams (SAPSs) are observed when energetic electrons have penetrated below L = 4. Outside the plasmasphere in the premidnight region, potential energy is subtracted from the total energy of ions and added to the total energy of electrons during SAPS onset. This potential energy is converted into radial motion as the energetic particles drift around Earth and leave the SAPS azimuthal sector. As a result, energetic electrons are injected deeper than energetic ions when SAPSs are included in the large-scale electric field picture, in line with observations.Plain Language Summary The interaction between the Sun and the Earth's magnetic field often transmits significant energy to the electrons and protons present in the magnetospheric tail, a region of space located far from the Earth's nightside. As a result, these particles move closer to the Earth. They are "injected" into the Earth's inner space environment. Since the 1970s, scientists have theorized that electrons and protons with the same initial kinetic energy starting from the same radial location have about the same distance of closest approach to the Earth. Yet recent observations from the Van Allen Probes reveal that this is not the case! Electrons appear to be systematically injected "deeper" than protons. So what happens along the way to make electrons approach Earth at closer distances? To answer this question, we combine observations from both space and the ground. We show that a localized source of electric fields called a subauroral polarization stream (aka a "SAPS") is always present during these injections. We argue that a SAPS acts like a marathon aid station for electrons in that it provides them with additional energy. As a result, electrons "move faster" and approach the Earth at closer distances.

Bifurcated Region 2 Field‐Aligned Currents Associated With Substorms

“…This polarization electric field leads to a strong E x B drift, which forms the fast westward drifting SAPS flow (Kunduri et al, 2017Landry & Anderson, 2018;Lejosne et al, 2018). During the growth phase, the R1/R2 currents progress equatorward as the polar cap expands.…”

Particle Dynamics in the Earth's Radiation Belts: Review of Current Research and Open Questions

“…(These electrons have quite low magnetic moments compared to the electrons in the peak of the outer radiation belt.) For instance, Lejosne et al () showed some of the injections occurring deep into the inner magnetosphere could be due to a potential drop associated with subauroral polarization stream (SAPS) (e.g., Lejosne & Mozer, ). Are these electrons locally accelerated?…”

SAPS in the 17 March 2013 Storm Event: Initial Results From the Coupled Magnetosphere‐Ionosphere‐Thermosphere Model

“…SAPS flow variations are often connected to earthward plasma sheet flow bursts and the plasma pressure distribution in the ring current (e.g., Clausen et al, ; Ebihara et al, ; Gallardo‐Lacourt et al, ; Lyons et al, ). The development of SAPS is suggested to be associated with the injection of plasma sheet ions/electrons, which establishes the polarization electric fields (e.g., Califf et al, ; Garner et al, ; Lejosne et al, ; Makarevich & Dyson, ; Mishin et al, ; Yuan et al, ). It is also demonstrated that the SAPS electric field can erode the outer plasmasphere, which also correlates with the generation of storm enhanced density and plumes of elevated ionospheric total electron content (TEC) on the dayside (e.g., Foster et al, , ; ; Foster & Rideout, ).…”