IMF control of polar cusp and cleft auroras

The dayside auroral luminosity can be separated in components with different location and spectral characteristics, corresponding to the separate plasma source regions projecting to the dayside high-latitude ionosphere. Recent results on the morphology and dynamics of polar cusp and cleft auroras are reviewed, with emphasis on the IMF influence, and the relationship with convection related currents, i.e. the merging and lobe cells. The poleward boundary of the cusp/cleft aurora shows stronger response to IMF north-south transitions than the equatorward boundary, leading to larger cusp/cleft widths when B2 changes from negative to positive value. Effects of IMF variations and magnetospheric substorm activity on the cusp location are found to be separable. Characteristic “breakups” of discrete auroral forms during IMF Bz < 0 intervals, with associated poleward expansions (∼ 1 km/-sec) and fast east- or westward moving structures, are focused. These transient forms appear at the equatorward boundary of the pre-existing cusp arc. They typically occur as alongated arcs or arc-fragments with internal rayed structures, and are disappearing near the cusp/cleft poleward boundary or even farther north. The dynamical evolution seems to depend on IMF Bγ and the local time of occurrence around midday. Ground magnetic signatures of poleward propagating east-west aligned Hall current (monopolar H-deflection), as well as paired sheets of field-aligned currents (satellite-data) are observed to be associated with the discrete cusp auroras in such cases. The ground magnetic effect depends on the energy of the precipitating electrons, i.e. ∼ 1 keV is needed in order to ionize the E-layer, and is therefore rather infrequent.

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