Auroral and Magnetotail Dynamics During Quiet-Time STEVE and SAID

Abstract

Although Strong Thermal Emission Velocity Enhancement (STEVE) and subauroral ion drifts (SAID) are often considered in the context of geomagnetically disturbed times, we found that STEVE and SAID can occur even during quiet times. Quiet-time STEVE has the same properties as substorm-time STEVE, including its purple/mauve color and occurrence near the equatorward boundary of the pre-midnight auroral oval. Quiet-time STEVE and SAID emerged during a non-substorm auroral intensification at or near the poleward boundary of the auroral oval followed by a streamer. Quiet-time STEVE only lasted a few minutes but can reappear multiple times, and its latitude was much higher than substorm-time STEVE due to the contracted auroral oval. The THEMIS satellites in the plasma sheet detected dipolarization fronts and fast flows associated with the auroral intensification, indicating that the transient energy release in the magnetotail was the source of quiet-time STEVE and SAID. Particle injection was weaker and electron temperature was lower than the events without quiet-time STEVE. The plasmapause extended beyond the geosynchronous orbit, and the ring current and tail current were weak. The interplanetary magnetic field (IMF) B_z was close to zero, while the IMF B_x was dominant. We suggest that the small energy release in the quiet magnetosphere can significantly impact the flow and field-aligned current system.