Planet. Space Sci, 1974, Vol. 22, pp. 79 to 94. Pergazrton Press. Printed in Northern Ireland THERMAL PROTON FLOW IN THE PLASMASPHERE: THE MORNING SECTOR PETER M. BANKS* Radioscience Laboratory, Stanford University, Stanford, California 94305, U.S.A. and JOE R. DOUPNIK Department of Applied Physics and Information Science, University of California, San Diego, La Jolla, California 92037, U.S.A. (Received 28 May 1973) Abstract---Vertical profiles of electron density obtained in the vicinity of the plasmapause using the Alouette-II topside sounder have been analyzed to assess the presence of H ÷ flow in the topside ionosphere. The observations in the midnight sector show clearly the presence of the plasmapause; i.e. there is a sharp boundary separating the poleward regions of polar wind H + flow and the more gentle conditions of the plasmasphere where light ions are present in abundance. In contrast, in the sunlit morning sector upwards H + flow is deduced to be present to invariant latitudes as low as 48°(L = 2-2) in the regions normally known to be well inside the plasmasphere. The upwards H ÷ flux is sufficiently large (3 x 10 s ions cm-2 sec -1) that the plasmapause cannot be seen in the latitudinal electron density contours of the topside ionosphere. The cause for this flow remains unknown but it may be a result of a diurnal refilling process. 1. INTROI)UCTION During the past several years there has been an increasing appreciation of the dy- namical coupling between the ionospheric F.z-region and the magnetosphere. In the regions exterior to the plasmasphere both the O + of the F2-1ayer and the H + of the topside ionosphere are exposed to the general magnetospheric electric field. As a consequence, thermal plasma in this region undergoes an E × /~ drift which leads to plasma loss at the magnetopause and consequently to the establishment of large scale upward polar wind flows of H +. For magnetically quiet conditions, the plasmapause represents the boundary between plasma which attempts to co-rotate with the Earth and plasma which is exposed to the magnetospheric electric field. (A recent review has been given by Chappell, •972.) Within the plasmasphere the behavior of H + and He + is governed by ion flow along mag- netic field lines in response to changes in plasma pressure and chemical composition. Such changes occur at low latitudes as a result of F2-region ionization and thermal pro- cesses and at high latitudes as a consequence of E × /~ plasma drift associated with the partial penetration of the magnetospheric convection electric field inside the plasmapause boundary. The effects of the E × /~ drift are especially important in the afternoon and evening sectors of the plasmasphere where plasma motions across field lines normally occur and lead to substantial plasma pressure gradients along the lines of magnetic force. The idea of diffusive equilibrium between O + in the F2-region and the light ions of the topside ionosphere has been explored in a number of theoretical studies (Mange, 1960; Kockarts and Nicolet, 1963; Bauer, 1966, Rush and Venkateswaren, 1965). Outside the plasmasphere, however, measurements provide evidence for the existence of a much more dynamic state with high speed flows occurring at high altitudes acting to replenish regions depleted of ionization by/~ x /9 drifts (Brinton et al., 1971; Hoffman, 1969; Banks, * On leave of absence from the University of California, San Diego, U.S.A. 79