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

Large-scale inverted-V channels of upflowing oxygen ions are frequently identified out of the data of Cluster at all the local time near the open-closed field line boundary over Earth’s high-latitude ionospheres. These energy spectrum structures generally cooccurred with downward propagating MHD Alfvén waves which had cascaded down into kinetic regime of plasma, and the transverse acceleration of the oxygen ions in the center of the structures are interpreted as the integrated energization by these waves along the channels. Upward parallel electric fields, a key characteristic of kinetic Alfvén waves, are also observed within the channels, which may not only contribute to lift the ions but also precipitate aurora electrons that might initiate the 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 the oxygen upflows.