AURORAL IONOSPHERIC SIGNATURES OF THE PLASMA SHEET BOUNDARY-LAYER IN THE EVENING SECTOR

We report on particles and fields observed during Defense Meteorologic al Satellite Program (DMSP) F9 and DE 2 crossings of the polar cap/aur oral oval boundary in the evening MLT sector. Season-dependent, latitu dinally narrow regions of rapid, eastward plasma flows were encountere d by DMSP near the poleward boundary of auroral electron precipitation . Ten DE 2 orbits exhibiting electric field spikes that drive these pl asma flows were chosen for detailed analysis. The boundary region is c haracterized by pairs of oppositely-directed, field-aligned current sh eets. The more poleward of the two current sheets is directed into the ionosphere. Within this downward current sheet, precipitating electro ns either had average energies of a few hundred eV or were below polar rain flux levels. Near the transition to upward currents, DE 2 genera lly detected intense fluxes of accelerated electrons and weak fluxes o f ions, both with average energies between 5 and 12 keV. fn two instan ces, precipitating ions with energies >5 keV spanned both current shee ts. Comparisons with satellite measurements at higher altitudes sugges t that the particles and fields originated in the magnetotail inside t he distant reconnection region and propagated to Earth through the pla sma sheet boundary layer. Auroral electrons are accelerated by paralle l electric fields produced by the different pitch angle distributions of protons and electrons in this layer interacting with the near-Earth magnetic mirror. Electric field spikes driving rapid plasma flows alo ng the poleward boundaries of intense, keV electron precipitation repr esent ionospheric responses to the field-aligned currents and conducti vity gradients. The generation of field-aligned currents in the bounda ry layer may be understood qualitatively as resulting from the differe nt rates of earthward drift; for electrons and protons in the magnetot ail's current sheet.