The Microstructure of the Magnetopause

Summary The microstructure of the magnetopause is reviewed with particular emphasis on the identification of important problems that require further study. The subject is introduced by considering theoretically the microstructure of the thin boundary layer (∼ 1 km) that exists when a cold, unmagnetized stream of magnetosheath ions and electrons impinges normally on an idealized, one‐dimensional geomagnetic field. This model magnetopause, first examined in detail by Ferraro, indicates how the geomagnetic field can be confined by the impact pressure of the solar wind. The various simplifications and idealizations involved in this theoretical model are discussed critically; it is shown that the structure of the magnetopause can be modified by the presence of ambient thermal plasma in the magnetosphere, which results in a boundary layer with thickness comparable to the proton cyclotron radius (∼ 100 km). Experimental evidence indicating that the thickness of the magnetopause is at least of this order is mentioned briefly, and recent satellite observations of the microstructure are used to assess the relevance of the different theoretical models. The various mechanisms that could contribute to the tangential drag on the magnetopause, including magnetic merging of the terrestrial and interplanetary magnetic fields at the dayside magnetopause, are discussed in detail and it is pointed out that the structure of the magnetopause may depend on field‐aligned currents linking the magnetopause to the polarcusp ionosphere. The relevant properties of the polar cusps (clefts) and the plasma mantle are summarized and it is noted that anomalous resistivity of the magnetosheath plasma in the polar cusps could influence the micro‐structure of the magnetopause. Finally, it is concluded that this micro‐structure can no longer be considered in isolation, but must now be considered in relation to the overall problem of magnetospheric dynamics.