Multipoint measurements of magnetotail dynamics

Abstract

Multipoint spacecraft measurements have played an important role in documenting the morphology of magnetospheric substorms. Widely separated measurements have quite clearly delineated many large scale features such as, (1) plasma sheet thinning associated with the onset of substorms, (2) energy storage and release in the magnetotail associated with the growth and expansion phase of substorms, (3) the earthward and tailward field aligned currents associated with the dawn and dusk portions of the substorm current wedge and (4) the deep tail response that follows substorms detected near synchronous orbit. Smaller scale phenomena that must be observed on finer time scales and detected in more limited spatial regions have been studied by ISEE 1 and 2 and a number of spacecraft near geosynchronous orbit. Injection fronts that are sometimes thought to propagate earthward toward synchronous orbit are difficult to detect and observed time delays may be due at least partially to longitudinal expansions of the substorm current wedge. Measurements near the equatorial current sheet where substorm onsets are often thought to originate, frequently reveal rapid time variations yet nearby spacecraft are seldom available to help determine whether these variations are due to local turbulence or more coherent behavior on a somewhat larger scale. High time resolution measurements from the Cluster and ISTP programs should help rectify these ambiguities.