Particle distributions, waves, dc electric fields and magnetic fields were measured by two sounding rockets at altitudes of 950 and 430 km through an energetic (>5 keV) narrow (~10 km) stable discrete arc. Although the payloads' magnetic footprints were separated by only 50 km, differences in the arc's structure were observed including the spatial width, peak energy, and characteristic spectra. The energetic electron precipitation included both slowly varying isotropic fluxes that formed an inverted-V energy-time signature and rapidly varying field-aligned fluxes at or below the isotropic spectral peak. The isotropic precipitation had a flux discontinuity inside the arc indicating the arc was present on a boundary between two different magnetospheric plasmas. Dispersive and nondispersive bursts of field-aligned electrons were measured throughout the arc, appearing over broad energy ranges or as monoenergetic beams. Dispersive bursts gave variable source distances <8000 km. Plateauing of some of the most intense bursts suggests that waves stabilized these electrons.

During the lower altitude arc crossing, the field-aligned component formed a separate inverted-V energy-time signature whose peak energy was half the isotropic peak energy. This structure suggests that two separate potential drops may have formed along the same flux tube with cold plasma present between them. High time resolution measurements of electron distributions, together with measurements of the current system plasma flows, and plasma waves, provide a detailed picture of the narrow arc. This complete set of measurements illustrates the complex structure present in narrow discrete arcs and demonstrates the advantages of high resolution measurements by multiple spacecraft. ¿ American Geophysical Union 1990