Auroral poleward boundary intensifications (PBIs): Their two‐dimensional structure and associated dynamics in the plasma sheet

Abstract

Auroral poleward boundary intensifications (PBIs) are typically seen both in ground meridian scanning photometers (MSP) and in ground and spacecraft auroral images. They appear as a localized increase in intensity at or near the magnetic separatrix. This increase is often seen to extend equatorward through the ionospheric mapping of the plasma sheet. PBIs are associated with plasma sheet flow bursts and are thus important for the remote monitoring of plasma sheet dynamics. From the study of simultaneous ground MSP observations, IMAGE FUV auroral images, and Geotail plasma sheet data, we find that PBIs correlate well with plasma sheet fast flows observed within the local time sector of the PBIs and that there can be several PBIs over the longitudinal range of fast flows in the tail. We infer that every north‐south PBI is the ionospheric signature of a fast flow channel in the plasma sheet and that many fast flow channels exist simultaneously over a width of the plasma sheet that can comprise the whole width of the plasma sheet or only a part of it. Also, we find that there is a local time dependence on the type of PBI structure. Most PBIs are narrow auroral structures that are not strictly north‐south oriented. Instead, PBIs are tilted counterclockwise away from the north‐south direction, leading to a preferred orientation that is approximately aligned with a line going from 0300 magnetic local time (MLT) to 1700 MLT. This results in PBIs that are closer to north‐south (NS) structures in the postmidnight sector and closer to east‐west (EW) near the dusk sector. In the premidnight sector (2200–0000 MLT), PBIs start as EW arcs and then tilt and become primarily NS structures. We further found for one event that the PBI fast flows have a large Vy component resulting in tail convection that is both earthward and dawnward in the region of Geotail. We suggest that the continuous and strongly positive interplanetary magnetic field (IMF) By may order the two‐dimensional convection as observed, thus offering a possible explanation for the alignment direction of PBIs in the ionosphere under the assumption that fast flow channels align themselves with the background convection. However, the projection of the PBI structures into the tail using the T96 model suggests that all PBIs, both EW and NS, map to radially stretched channels in the tail that do not have a significant dawnward component. Future work is needed to clarify this apparent contradiction. Finally, frequency analysis indicates that the PBI/bursty bulk flow (BBF) period is characterized by oscillations in the velocity and magnetic field with frequencies of ∼0.6 mHz and ∼1.3–1.5 mHz. This oscillation in velocity is superposed on the background strong convection.