Properties of the F layer plasma in the midday throat ridge and trough


  • J. A. Whalen


The tongue or ridge of the F layer plasma in the region of the dayside throat of the convection pattern is studied in order to determine its source and its relation to the daytime trough. A global array of six ionospheric sounders near 75° MLAT situated in both hemispheres provide the observations spanning 19 consecutive months at solar maximum. The ridge is often stable for many hours and magnetically conjugate; it is variable day to day but is so persistent that it is evident in the monthly medians. The ridge maximum has a seasonal dependence consistent with ionization by solar radiation rather than by particle precipitation, and a local time dependence that is consistent with the transport from elsewhere on the dayside by convection rather than with production at the observing sites. The ridge is not an isolated structure but is flanked by the two troughs of low-density plasma transported to the dayside by the dawn and dusk convection cells. Accordingly, the structure of the throat is a ridge between two valleys that is entirely a product of convection.