Abstract
Abstract. Observations on 30 October 1978 show the ISEE-1 spacecraft passing though the high-altitude dayside northern magnetospheric cusp region from roughly 16:00 to 18:30 UT, during a slow solar wind period (~380 km/s). More than two orders of magnitude enhancements of the cusp energetic particle (CEP) fluxes were observed along with a depressed and turbulent local magnetic field. The observed variations of the pitch angle distributions (PAD) provide a unique opportunity to determine the structure of the cusp and the origin of the CEP. Through a boundary sounding technique, the location and orientation of the cusp poleward (or backside) boundary was observed for almost 10 min during which time it appeared initially to be stationary in the GSM/GSE X-direction and then moved sunward about 0.12 Earth radii (RE). The orientation remained approximately perpendicular to the GSM/GSE X-axis until it was observed to rotate by 60 degrees in ~3 min before ISEE-1 was fully inside the cusp cavity. The cavity itself was filled with CEP fluxes displaying large anisotropies, indicative of their source being located below (Earthward) of the satellite location. The spacecraft entered from the backside of the cusp, then traveled ~4 RE through the cavity, and exited through the "top" of the cavity leaving a region of energetic ions below. The PADs demonstrate that the bow shock cannot be the main source of the observed CEPs. The CEP fluxes were measured at about 8.5 h MLT when the IMF had both an 8–10 nT duskward and southward component.
Highlights
The magnetospheric cusp is defined by a combination of (1)a decrease in magnetic field magnitude in the dayside highlatitude region (Farrell and Van Allen, 1990) and (2) a more than one order of magnitude increase in intensity of the 1–keV ions (Chen et al, 1997, 1998; Fung et al, 1997)
We investigate the geometry of the cusp boundary and the evolution of the boundary orientation angle through a boundary sounding technique as ISEE-1 penetrated into the cusp diamagnetic cavity (CDC) while observing intense Cusp Energetic Particles (CEP) fluxes
– ISEE-1 is on open field lines in the dayside cusp during this entire time period, where fluxes measured by the satellite are observed to maintain a large anisotropy along the magnetic field direction with fluxes arriving primarily from below or earthward of the satellite and are not observed to return from above or sunward of the satellite
Summary
A decrease in magnetic field magnitude in the dayside highlatitude region (Farrell and Van Allen, 1990) and (2) a more than one order of magnitude increase in intensity of the 1–. The local magnetic field observed by ISEE-1 (Fig. 1) had a negative Bz component that fluctuated between ∼0 and −30 nT, Bx was roughly nT, and By was positive(∼30 nT). We investigate the geometry of the cusp boundary and the evolution of the boundary orientation angle through a boundary sounding technique as ISEE-1 penetrated into the cusp diamagnetic cavity (CDC) while observing intense CEP fluxes. The locus of particles with pitch angles of 90◦ for the example PAD is traced on the unit sphere, with the magnetic field vector pointing from a PA of 0◦ to 180◦. 15:57 UT, energetic particles peaked at 90◦ are observed with azimuthal angles of roughly 45◦ to 180◦. By 16:08 UT the spacecraft observes strong particle populations peaked at 90 degrees (see Panel 9, 16:09:11 UT) at all azimuthal angles.
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