A STUDY OF AN EXPANDING INTERPLANETARY MAGNETIC CLOUD AND ITS INTERACTION WITH THE EARTHS MAGNETOSPHERE - THE INTERPLANETARY ASPECT

In a series of three interlinked papers we present a study of an inter planetary magnetic cloud and its interaction with the Earth's magnetos phere on January 14/15, 1988. This first paper is divided into three p arts describing the principal results concerning the magnetic cloud. F irst, by applying the cylindrically symmetric, magnetic flux rope mode l to the high time resolution magnetic field and plasma data obtained by the IMP-8 spacecraft, we show that the axis of the magnetic cloud i n question is approximately in the ecliptic and orthogonal to the Eart h-Sun line. We note the presence of pulsations of approximately 5-hour period in the bulk flow speed which are superimposed on an otherwise monotonically falling bulk speed profile. Second, we apply ideal MHD t o model the self-similar, radial expansion of a magnetic cloud of cyli ndrical geometry. As initial condition for the magnetic field we choos e a constant-alpha, force-free magnetic configuration. We demonstrate that the theoretical velocity profile for the free expansion of a magn etic cloud is consistent with observations made during the January 14/ 15, 1988, magnetic cloud encounter. Comparing model with data, we infe r that prior to the start of observations at 1 AU the magnetic cloud h ad been expanding for 65.4 hours; the radius of the magnetic cloud at the time it arrived at Earth was 0.18 AU; and its expansion speed at 1 AU was approximately 114 km/s. Third, we discuss energetic (approxima tely 1 MeV) ion data, also from instrumentation on IMP-8. We highlight the appearance of a sharp enhancement in the intensity of approximate ly 0.5-MeV ions while IMP-8 was inside the cloud. These ions travel as a collimated, field-aligned beam from the west of the Sun. This is an ''impulsive'' solar event in which particles accelerated at a magneti cally well-connected solar flare arrive promptly at the spacecraft. Th e observation of solar flare particles inside the cloud suggests that field lines within the magnetic cloud remained connected to the Sun. T he observation is, however, inconsistent with the supposition that the cloud is formed of closed magnetic field loops disconnected from the Sun.