Mission research Corporation, Nashua, New Hampshire

This study of dayside electrodynamics provides a first view of quasi-dc electric fields measured by Polar at middle altitudes. Three high-latitude passes chosen for analysis, occurred while Polar was in the midday sector and the interplanetary magnetic field (IMF) maintained a steady northward orientation at clock angles near 40° in the Y_GSM - Z_GSM plane. At different times during the passes, Polar encountered electron and ion fluxes from the central plasma sheet (CPS), the boundary plasma sheet, the polar rain, and various magnetospheric boundary regions. Although electric fields measured within the boundary layers were highly variable, we derived average components of plasma convection. The results compare favorably with convection characteristics of four-cell patterns observed at ionospheric altitudes with northward IMF. We show that Polar encountered lobe cells of both positive and negative polarity in the early postnoon sector. The negative potential extends well into the prenoon sector. Inverse, energy-dispersed fluxes of He⁺⁺ were detected in regions of sunward convection. Sunward convection extended into the region of polar rain north of the merging line's projection. The magnetic merging signatures indicate that the process is both time varying and patchy. Particle and field measurements are consistent with the afternoon, auroral convection cell closing in the low-latitude boundary layer, which extend many tens of R_E tailward of the dusk terminator. Taken in a larger context, the Polar measurements provide new insight concerning the evolution of high-latitude convection from distorted two-cell patterns at large IMF clock angles to four cells at low clock angles.

J. Geophys. Res., 103, A1, 29-45, 1998