Simultaneous FAST and Double Star TC1 observations of broadband electrons during a storm time substorm

<jats:p>Broadband electrons (BBEs) exhibit remarkable electron flux enhancements over a broad energy range (0.03–30 keV) near the equatorward edge of the auroral oval during geomagnetic storms. Here, we report a BBE event observed by the Fast Auroral Snapshot (FAST) satellite at 1355–1359 UT, ∼61°–66° invariant latitudes, ∼0600 magnetic local time (MLT), and ∼3800 km altitude during a storm on 25 July 2004. The Double Star (DS) TC1 satellite was located near the magnetic equator at <jats:italic>L</jats:italic> = 5.7, close to the same local time as FAST. We investigate the acceleration process of BBEs from the inner magnetosphere to near the ionosphere by comparing electron data obtained by FAST and DS TC1. We also investigate both plasma and field variations in the inner magnetosphere associated with substorm onset using DS TC1 data to examine the relationship between the BBEs and the storm time substorm. Ground geomagnetic field data show a positive H‐bay at ∼1349 UT at ∼0600 MLT, indicating that a storm time substorm started just before the appearance of the BBEs. At ∼1350 UT, a tailward ion flow was observed by DS TC1. Then, DS TC1 observed a local dipolarization and a drastic ion density enhancement at ∼1351 UT, indicating that particle heating associated with the substorm was occurring in the inner magnetosphere. From ∼1352 UT, electron fluxes were isotropically enhanced at energies above ∼0.5 keV as observed by DS TC1. On the other hand, the pitch angle distribution of BBEs at the FAST altitude showed field‐aligned lower‐energy electrons below ∼0.5 keV and isotropic higher‐energy electrons above ∼0.5 keV. From these data, it was inferred that the BBEs might consist of two energy components due to the acceleration or heating of electrons at different altitudes in association with the storm time substorm.</jats:p>