The quiet time polar cap: DE 1 observations and conceptual model

Abstract

Auroral activity increases over the polar caps during quiet times, which are associated with northward interplanetary magnetic field (IMF) components. Polar cap auroras (Sun‐aligned arcs, theta auroras, and horse collar auroras) occur under these conditions. DE 1 data show that the theta and horse collar auroras are generally characterized by sunward convection, closed field lines, auroral hiss, and inverted‐V events. These phenomena are similar to those observed along the auroral oval but have somewhat lower electron energies. Adjacent dark regions of the polar caps appear to contain open field lines and antisunward convection. A conceptual northward IMF merging model containing lobe cells, merging cells, and viscous cells is shown to be consistent with the observations. As the IMF becomes more northward, the polar arc configuration changes from the “horse collar” pattern to the theta aurora pattern in the model, and this is shown to be generally true for the set of published data on these phenomena. The model involves dayside merging both at high latitudes on open field lines and at lower latitudes on closed field lines. The ratio between the merged flux produced by the high‐latitude merging to that produced by the lower‐latitude merging increases as the IMF becomes more northward. Two types of open field lines, equator‐crossing and non‐equator‐crossing, are produced by the higher‐ and lower‐latitude merging, respectively. The equator‐crossing field lines have a strong azimuthal component of convection as they flow around the magnetopause, while the non‐equator‐crossing field lines can convect more or less directly across the polar cap, leading to an antisunward flow channel across the central polar cap. This antisunward flow region grows as the IMF becomes less northward, causing dual polar cap arcs to spread out into the horse collar configuration. The sunward flow segments of the lobe cells are associated with induced electric fields, thus accounting for the particle energization leading to auroral forms.