Ionospheric characteristics of a detached arc in the evening‐sector trough

Simultaneous measurements by the ISIS‐2 satellite and the Chatanika incoherent‐scatter radar have been used to identify the ionospheric characteristics of a detached arc in the evening‐sector trough region. On 26 February 1977 at 0737 UT, the ISIS‐2 Auroral Scanning Photometer detected a detached arc at 65° north geomagnetic latitude that extended from approximately 1900 MLT to 2100 MLT. The arc was about 2° wide in latitude and was separated by about 3° from the equatorward edge of the diffuse aurora. The ratio of observed emission intensity at 557.7 nm to that at 391.4 nm was low, indicating a fairly hard electron distribution. The ISIS‐2 Energetic Particle Detector observed energetic electron precipitation (E>40 keV) in the detached arc. The Chatanika radar found that the detached arc consisted of several narrow ionization enhancements, with peak E‐region electron densities of about 1 × 105 cm−3 at 95 km altitude. The major enhancements were about 20 km (0.2°) wide in latitude and were spaced about 50 km (0.5°) apart. Their widths and spacings were steady for about 40 minutes. This persistent spatial pattern suggests the presence of an energetic electron precipitation mechanism that is periodic in latitude and extended in longitude.