Large-scale inverted-V channels of upflowing oxygen ions pumped by Alfvén waves

Abstract

Large-scale inverted-V channels of upflowing oxygen ions are frequently identified in data collected by Cluster, at all local times, near the open-closed field line boundary over Earth’s high-latitude ionosphere-occur with downward propagating MHD Alfvén waves which have cascaded into kinetic regimes of plasma. The transverse acceleration of the oxygen ions in the center of these structures is interpreted as the integrated energization by these waves along the channels. Also observed within the channels are upward parallel electric fields, a key characteristic of kinetic Alfvén waves, which may contribute not only to lifting the ions but also to precipitating aurora electrons that might initiate ion upflow in the ionosphere below. Statistics on five-year observations of Cluster show that the channels typically form during geomagnetic perturbations, particularly when solar-wind dynamic pressure is high or highly fluctuated. Near the open-closed field line boundary, the stronger the wave power, the higher the upward oxygen flux and the higher the beam energy, indicating that these waves provide a simple but efficient way to drive oxygen upflows.

Keywords

• oxygen ions
• dispersive alfvén waves
• ion outflow
• parallel electric field