The day side mid-latitude plasma trough

Above 2500 km the electrostatic probe experiment aboard the Isis 1 satellite detects a persistent dayside midlatitude plasma trough. The superposition of measurements taken over two-week intervals at noon is used to illustrate the form and location of the quiet time trough within each season. These measurement results contrast with those observations of a less prominent dayside trough at altitudes below 2500 km. Associated with the trough above 2500 km is an electron temperature maximum of about 6000 K that is sharp at midnight and broad at noon. In spring and summer a second noontime temperature maximum often appears poleward of 70-deg invariant latitude accompanied by an enhancement in the ionization. When it is assumed that the geophysical processes producing a plasmasphere act most directly upon the light ions, one factor contributing to the distinction between the plasma trough and the equatorial plasmapause is the increasing influence of O(+) on total plasma behavior at lower altitudes. Other factors are the local processes, such as dayside F region photoionization by energetic cusp particles that produce enhancements in plasma density and electron temperature.