The causes of convection in the Earth's magnetosphere: A review of developments during the IMS

Abstract

The physical processes which give rise to convection in the earth's magnetosphere have been the subject of active debate for many years. Most of the discussion has centered on two basic pictures, one in which closed magnetospheric flux tubes are transported from dayside to nightside in a boundary layer around the flanks of the magnetosphere by a ‘viscouslike’ process occurring at the magnetopause and the other in which open flux tubes are transported over the poles of the earth after reconnection has taken place with the interplanetary magnetic field. These processes may coexist on a continuous basis, and the question then arises as to their relative contributions to usual total cross‐magnetospheric voltages of ∼40–100 kV. The first detailed observations of plasmas and fields in the vicinity of the dayside magnetopause have recently been made during the International Magnetospheric Study by the ISEE 1 and 2 spacecraft and are discussed in this paper in relation to this question. Observations which relate to the occurrence of ‘quasi‐steady’ as well as impulsive flux transfer event (FTE) reconnection are reviewed in detail in the paper, together with measurements of the properties of the boundary layer on the magnetospheric flanks. Particular emphasis is given to the interpretation of these data in terms of the physical processes occurring. It is argued that the ISEE observations of quasi‐steady reconnection are indeed compatible with the process playing a major role in magnetospheric dynamics. The observed frequency of these events at the magnetopause indicates that they are associated either with infrequent intervals of intense magnetospheric convection or with more frequent contributions of lesser intensity (i.e., ∼40 kV whenever IMF Bz is negative). The latter seems the more likely situation at the present time. In addition, a rough estimate of ∼20 kV is made for the contribution due to FTE's, this figure having the nature of a lower limit. It is therefore argued that the in situ ISEE observations pertaining to reconnection processes are consistent with the view that these provide a major contribution to magnetospheric flows. A preliminary picture is suggested from our inferences based on ISEE data in which quasi‐steady subsolar reconnection occurs essentially continuously in a band ∼2 hours LT wide when IMF Bz ≤ 0, centered often near the noon meridian, while at other local times, reconnection also occurs but in an unsteady, sporadic manner. Examination of the unsteady flows observed in the boundary layers on the flanks of the magnetosphere yields voltages of typically ∼5–20 kV for dusk and dawn layers combined. Similar values are obtained from studies of low‐altitude spacecraft observations, although there is some disagreement in the literature concerning the exact identification of the boundary layer in such data. This latter topic is also reviewed. It is also argued that part, at least, of the boundary layer flow could occur on open field lines and indeed that some of the boundary layer observations correspond closely to what one would expect for the magnetospheric counterpart of magnetosheath FTE's. Finally, recent analyses of the variation of the transpolar voltage (measured at low altitudes) with solar wind conditions suggest an average ∼30‐kV contribution from the boundary layers, a rather higher value than is indicated by the in situ measurements. However, all these estimates suggest that the layers generally provide a small but sometimes significant contribution to the magnetospheric voltage. We conclude that dayside reconnection is the dominant contributor under usual conditions, in agreement with conclusions reached previously on the basis of less direct information.