Middle and upper atmosphere radar observations of ionospheric electric fields

Abstract

Middle and upper atmosphere (MU) radar observations of electric fields over the period September 1986 to January 1991 are reported in the form of time‐of‐day variations for several different seasonal and solar activity cases. The perpendicular north velocity has on average a smooth and distinct northward excursion from 0600‐1500 LT followed by a period of constant southward velocity until midnight and then a slow linear change to merge with the daytime northward excursion period. This excursion feature is small in summer and enhanced in winter but, seasonally averaged, has only small solar activity variation. The individual seasonal averages show afternoon/evening excursion behavior apparently tied to the time of sunset. The perpendicular east velocity is characterized by a major eastward excursion in the evening and a minor eastward excursion in the morning, with a tendency to level off or form a slight additional local eastward excursion after midnight. The evening excursion is enhanced during winter and at high solar activity, and the postmidnight excursion is enhanced during summer. The morning excursion has no comparable counterpart in existing electric field models or data averages from Arecibo or Jicamarca. We suggest that this may be a semidiurnal component that evidences enhanced propagation of the semidiurnal tide at the location of the MU radar. The maximum daily amplitudes and the time‐of‐day and solar activity trends at the Arecibo and MU radar locations have strong resemblances, but the seasonal trends in the MU data, the Arecibo data, and existing radar models disagree strongly.