Substorm injection of relativistic electrons to geosynchronous orbit during the great magnetic storm of March 24, 1991

The great March 1991 magnetic storm and the immediately preceding solar ene rgetic particle event (SEP) were among the largest observed during the past solar cycle, and have been the object of intense study. We investigate her e, using data from eight satellites, the very large delayed buildup of rela tivistic electron flux in the outer zone during a 1.5-day period beginning 2 days after onset of the main phase of this storm. A notable feature of th e March storm is the intense substorm activity throughout the period of the relativistic flux buildup, and the good correlation between some temporal features of the lower-energy substorm-injected electron flux and the relati vistic electron flux at geosynchronous orbit. Velocity dispersion analysis of these fluxes between geosynchronous satellites near local midnight and l ocal noon shows evidence that both classes of electrons arrive at geosynchr onous nearly simultaneously within a few hours of local midnight. From this we conclude that for this storm period the substorm inductive electric fie ld transports not only the usual (50-300 keV) substorm electrons but also t he relativistic (0.3 to several MeV) electrons to geosynchronous orbit. A s implified calculation of the electron epsilon x B and gradient/curvature dr ifts indicates that sufficiently strong substorm dipolarization inductive e lectric fields (greater than or equal to 10 mV/m) could achieve this, provi ded sufficient relativistic electrons are present in the source region. Con sistent with this interpretation, we find that the injected relativistic el ectrons have a pitch angle distribution that is markedly peaked perpendicul ar to the magnetic field. Furthermore, the equatorial phase space density a t geosynchronous orbit (L = 6.7) is greater than it is at GPS orbit at the equator (L = 4.2) throughout this buildup period, indicating that a source for the relativistic electrons lies outside geosynchronous orbit during thi s time. Earthward transport of the relativistic electrons by large substorm dipolarization fields, since it is unidirectional, would constitute a stro ng addition to the transport by radial diffusion and, when it occurs, could result in unusually strong relativistic fluxes, as is reported here for th is magnetic storm.