Characteristics of a stable arc based on FAST and MIRACLE observations

A stable evening sector are is studied using observations from the FAST sat ellite at 1250 km altitude and the MIRACLE ground-based network, which cont ains all-sky cameras, coherent radars (STARE), and magnetometers. Both FAST and STARE observe a northward electric held region of about 200 km width a nd a held magnitude of about 50 mV/m southward of the are, which is a typic al signature for an evening-sector are. The field-aligned current determine d from FAST electron and magnetometer data are in rather good agreement wit hin the area. Outside the arcs, the electron data misses the current carrie rs of the downward FAC probably because it is mainly carried by electrons o f smaller energy than the instrument threshold. Studying the westward propa gation speed of small undulations associated with the are using the all-sky cameras gives a velocity of about 2 km/s. This speed is higher than the ba ckground ionospheric plasma speed (about 1 km/s), but it agrees rather well with the Idea originally proposed by Davis that the undulations reflect an E x B motion in the acceleration region. The ground magnetograms indicate that the main current flows slightly south of the are. Computing the ionosp heric conductivity from FAST electron data and using the ground magnetogram s to estimate the current yields an ionospheric electric field pattern, in rather good agreement with FAST results.