Although much work has been done to characterize the global response of Earth's upper atmosphere to geomagnetic storms, much is unknown regarding the response on mesoscales (similar to 30-500 km). In order to understand how the nightside, high-latitude ionosphere responds on these scales during storms, we have characterized widths, velocities, and occurrence rates of equatorward and poleward mesoscale flows. We characterized them according to main phase versus recovery phase as well as coronal mass ejection (CME) versus high-speed stream (HSS) storms. Some results include the following: Mesoscale flows are faster during the main phase of the storms. Faster flows >400 m/s are more frequent and probable during CME storms as compared to HSS storms, but more flows occur during HSS storms. Polar cap flows are wider during CMEs than during HSSs. There is a postmidnight preference for polar cap mesoscale flows during storms, especially during recovery phase and during HSSs. Plain Language Summary Geomagnetic storms are the Sun's most effective way to transport energy from the solar wind into the Earth system. The type of storm and the phase of the storm (its developing vs. its recovering phase) drive Earth's system differently and therefore create different types of disturbances to Earth's nearby space and upper atmosphere. Our study characterizes and contrasts one type of disturbance-30-500 km wide, nightside upper atmosphere plasma flows-during two types of storms and during the two storm phases. We found the flow characteristics are different between the types and phases of storms. For example, more flows occur during one type of storm, but faster flows occur during the other. Plasma flows at these scale sizes have not been studied until recently, but their effects may be very important to the global Earth system. Our results can be input into global models to improve them and to better understand and predict Earth's response to storms.

Solar-wind-magnetosphere coupling causes plasma exchange across the magnetopause. Using ARTEMIS observations, we investigated dawn-dusk asymmetries and responsible coupling processes for two distinct populations in the midtail (r similar to 60 R-E): (1) mantle plasma resulting from magnetosheath plasma coming through the open magnetopause and (2) bursty hot electron enhancements (HEEs) in the magnetosheath resulting from magnetosphere electrons coming out with transient magnetopause deformation. Mantle plasma appears more frequently in the postmidnight (premidnight) sector above the current sheet when the IMF B-y is positive (negative). Good agreement between the observations and global MHD simulations indicates that the dawn-dusk asymmetry is caused by the open magnetopause moving to the opposite sides of the magnetosphere above and below the current sheet as the IMF B-y becomes more dominant. The HEE occurrence rates are two to three times higher on the dawn side. HEEs correlate more strongly than magnetosheath plasma with sharper transient changes in IMF direction and magnetosheath density and with quasiparallel bow shock. These correlations suggest that perturbations created at the quasiparallel bow shock can possibly cause magnetopause deformation and HEEs in the midtail, contributing to the HEE asymmetry because the quasiparallel bow shock is more often on the dawn side.

Auroral zone observations often show significant ULF power. We have analyzed auroral and plasma sheet observations during two prolonged periods of strongly southward and relatively steady interplanetary magnetic field (IMF). We find evidence that auroral poleward boundary intensifications (PBIs), which have large intensity and occur repetitively throughout such periods, may be a manifestation of a large‐scale ULF oscillation mode that strongly perturbs the plasma sheet and the auroral ionosphere. If this is correct, then ULF modes would be a major component of tail dynamics, of magnetosphere coupling to the ionosphere, and of auroral zone disturbances during periods of enhanced convection. They would simultaneously affect a large region of the nightside, extending along auroral zone field lines from the ionosphere to the equatorial plasma sheet and extending from field lines that lie near the magnetic separatrix to, at times, as close to the Earth as synchronous orbit. They would also occasionally have amplitudes as large as the changes that occur in association with other auroral zone disturbances such as substorms. Here we have found peak‐to‐peak amplitudes as high as several hundred nanoteslas in ground X, an order of magnitude in synchronous energetic proton fluxes, ∼20–40 nT in synchronous magnetic field components, ∼20 nT in tail magnetic field components, ∼1000 km/s in tail flow speeds, and ∼400 m/s in ionospheric flow speed. We find evidence for significant power at 0.5–0.7 mHz (∼25–30 min period), significant power at a possible second harmonic (∼1.1–1.3 mHz), and power at frequencies that could be higher harmonics simultaneously within the auroral ionosphere and within the nightside plasma sheet.