Cusp cleft auroral forms and activities in relation to ionospheric convection: Responses to specific changes in solar wind and interplanetary magnetic field conditions


This work is intended to be a first step toward a categorization of dayside auroral responses to various solar wind and interplanetary magnetic field (IMF) conditions and the corresponding states of plasma convection in the dayside magnetosphere. In this paper we relate cusp latitude auroral emissions observed during a 4-hour interval on December 17, 1992, to solar wind data supplemented by ground magnetograms. We focus on different types of variability associated with an interplanetary shock and with an IMF directional discontinuity. We further investigate the quasi-steady conditions related to periods of low activity versus periods of high activity, as well as periods of IMF By-related zonal convection in the cusp region versus periods of IMF BZ-related convection in the north-south direction. To these various conditions the dayside aurora responds differently. These observations are discussed in relation to entry of magnetosheath plasma into the magnetosphere associated with different modes of solar wind-magnetosphere coupling. Two main categories of persistent auroral forms in the cusp/cleft region are observed, a latitudinally wide (≥200 km) zone of weak 630.0-nm emission with no sharp boundaries extending to magnetic latitudes of ∼78°–79° during quiet intervals and another latitudinally narrow (∼100 km) zone of strong red line emission, located equatorward of 75° MLAT during disturbed periods. These forms occur in association with sunward and antisunward convection in the dayside polar cap, respectively. It is suggested that these two signatures of electron precipitation in the noon/near-noon sector reflect plasma entry by lobe reconnection and reconnection at low (subcusp) latitudes, respectively. The observed sudden transition from the former to the latter auroral condition may thus reflect a change of reconnection site. The associated brightening of the cusp/cleft aurora then corresponds to the larger efficiency of magnetosheath plasma entry at low magnetopause latitudes when the subcusp reconnection process is switched on, compared to the more limited plasma entry occurring at high latitude during lobe recönnection. The most intense and latitudinally narrow auroral forms at 630.0 nm are observed during intervals of strong IMF By-related zonal convection and a strong DPY mode of ground magnetic deflection. Information on the ionospheric convection pattern in the vicinity of the magnetic field separatrix is obtained for the different IMF orientations by combining the auroral and magnetic observations in the Arctic sectors of Greenland and Svalbard.

Publication Date


Journal Title

JGR: Space Physics



Digital Object Identifier (DOI)

Document Type