Ionosphere-Magnetosphere Coupling via Energized Ion Outflow

Abstract:

Ion outflow from the Earth’s polar region is an important ionosphere-magnetosphere coupling mechanism. The outflow consists of both light thermal ions (H⁺ and He⁺) and several energized ions (NO⁺, O₂⁺, N₂⁺, O⁺, N⁺, He⁺, and H⁺). Energized O⁺ ions have been observed in several regions of the magnetosphere, including the plasma sheet, lobe, and distant tail, and during geomagnetic storms and substorms O⁺ can become the dominant ion in these magnetospheric regions. The field-aligned ion outflow occurs in association with magnetospheric convection, which causes the plasma to drift into and out of sunlight, cusp, polar cap, nocturnal oval, and main trough regions (Figure). Because the field-aligned and horizontal motions are both important, 3-dimensional time-dependent models of the ionosphere-polar wind are needed to properly describe the flow.

It is also important to include all of the relevant fluid and kinetic processes that can energize the escaping ionospheric ions. On the dayside, elevated electron temperatures and escaping photoelectrons are important energy sources for escaping ions. In the cusp and aurora, unstable field-aligned currents can excite waves that can then accelerate ions both parallel and perpendicular B. The resulting ion beams and conics have sufficient energy to escape the ionosphere. The convection of the ion beams and conics into the polar cap can then lead to unstable plasma conditions as they pass through the slower moving background polar wind. Also, the interaction of the cold polar wind and hot polar rain electrons can result in field-aligned double-layer electric fields (~ 4000 km), which can energize the escaping ions. At higher polar cap altitudes (~ 6000 km), electromagnetic wave turbulence can affect the ion outflow through the perpendicular wave heating that is associated with wave-particle interactions. Furthermore, centrifugal acceleration acts to increase ion escape velocities as the plasma convects across the polar cap.