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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.