Topside ionosphere at sunrise

Abstract

The sunrise behavior of the midlatitude topside ionosphere at low sunspot numbers was examined by using Alouette I electron density and plasma scale height profiles obtained at Stanford University during May and June 1963. Under the assumption that observed variations result from solar control, the data were averaged to form height profiles of plasma scale height and electron density from 450 to 750, km altitude, which vary smoothly with solar zenith angle x. The first manifestation of sunrise is a sudden and rapid decrease in plasma scale height, beginning between •: • 105 ø and 102 ø and ending between •: • 98 ø and 96 ø. Electron densities increase rapidly between x • 98 ø and 95 ø. Our interpretation of the observations is that the sudden decrease in plasma scale heights is due to an increase in percentage concentration of O f ions at all altitudes, due primarily to a thermal expansion of the ionosphere that causes an upward movement of the topside ionization. The velocity of the thermal motion is estimated, and we attempt to resolve conflicting evidence from other observations that the topside ionosphere is controlled by either geomagnetic latitude or solar zenith angle.

Incoherent scatter observations of the ionosphere over Chatanika, Alaska

“…Any conclusions redependence on solar variations. Midlatitude ob-garding causal relationships associated with servations [Watt, 1971;Evans, 1971] suggest the observed vertical velocities should await a very pronounced upward drift of plasma im-the availability of hard data on these param- 57.3øS, 168.2øE) is well west from changes in the production rate of ions. of Chatanika, and thus conjugate sunrise and It is reasonable to look for the same qualitative sunset occur later in time than local sunrise behavior at high latitudes whenever rapid tem-and sunset throughout the year except for two perature changes take place.…”