Broadband electrons during storm-time substorm: Simultaneous FAST and Double Star observations

We report on broadband electrons (BBEs) observed simultaneously by the FAST and Double Star TC1 satellite during a storm-time substorm on 25 July 2004. The BBEs were observed at 13:55 UT by FAST at ~6.1 MLT, 66° ILAT, and an altitude of ~4000 km. The BBEs observed by FAST were electron flux enhancements over a broad energy range (50 eV-30 keV) at lower latitude side of the auroral oval (at 61-66° ILAT for this event). The pitch angle distribution of BBEs was isotropic at a higher energy range above ~0.5 keV. At a lower energy range below ~0.5 keV, field-aligned electron fluxes tend to be larger than the perpendicular fluxes. At this time, the Double Star TC1 satellite was located near the magnetic equator at L= 5.7 (65° ILAT) in nearly the same local time as that of FAST. At ~13:51 UT (4 min before the BBE observation by FAST), TC1 observed a clear dipolarization (decrease in Bx and increase in Bz). Geomagnetic field data obtained at ground-based stations showed that positive H-bay took place at this time. These facts indicate that a storm- time substorm started at 13:51 UT. At the same time, TC1 observed field-aligned upward electrons and downward ions, and intense low frequency (<10 Hz) waves. From 13:52 UT, TC1 observed drastic enhancements of electron and ion fluxes at ~0.5-30 keV, suggesting that particle acceleration was occurring in the inner magnetosphere at L= 5.7 associated with the substorm. Field aligned electron fluxes within pitch angles of ~10° at TC1 altitude can precipitate to the FAST altitude. Flux intensity of the field aligned components in the enhanced electrons at 0.5-30 keV observed by TC1 comparable to those observed by FAST. We suggest that the enhanced and field aligned higher-energy electrons at TC1 altitude became isotropic as they approach to the FAST altitudes by mirror force so that they were observed as the isotropic higher energy components of the BBEs by FAST. Note that the energy spectra of electrons observed by FAST show flux enhancements over a broad energy range not only at ~0.5-30 keV but also at ~0.05-0.5 keV. The field aligned lower energy components of BBEs could be locally accelerated in field-aligned directions near the FAST altitude.