Spontaneous and driven cusp dynamics: Optical aurora, particle precipitation, and plasma convection

In this report we describe two different aspects of the dynamical behaviour of the cusp aurora which we refer to as (1) spontaneous and (2) directly driven events, respectively. The first aspect consists of a series of auroral activations/expansions observed during steady interplanetary magnetic field (IMF) and solar wind plasma conditions. This aspect we attribute to reconnection transients at the dayside magnetopause (flux transfer events). Applying a combination of ground and satellite observations, we document the auroral, particle precipitation, plasma convection, and field-aligned current signatures of these events. Emphasis is placed on the spatio-temporal evolution of the auroral and plasma convection events (pulsed ionospheric flows; PIFs) in the cusp ionosphere. Thus, we are able to document the multi-phase association between auroral brightenings/poleward moving auroral forms (PMAFs) and PIFs. Characteristic cusp ion steps (energy versus latitude profile) are observed during north-south traversals of the cusp by spacecraft Polar and DMSP F13 at the time of PMAFs. From the ground-satellite conjunction studies we infer the stepped cusp to be due to the traversal through a sequence of poleward-propagating open flux tubes originating in a pulsed magnetopause reconnection process. The region of the cusp ion steps is shown to be permeated by fine structure in the electron precipitation comprising a broad energy spectrum (30–600 eV). This observation accounts for the long auroral rays, spanning much of the altitude range 120–500 km, which are so typical for the cusp region. The second aspect of cusp dynamics consists of rapid transitions in the auroral configuration which are directly driven by abrupt changes in the external conditions, mainly northward and southward rotations of the IMF vector. Focus is placed on the rapidity of the transitions in the IMF-magnetosphere interconnection topology. The presence of a bifurcated (hybrid) cusp, with 1–5 keV auroral arcs excited at its poleward boundary, is documented during the northward IMF (clock angle range 45–70°) phase of one transition.