Latitudinal observations of the thermal balance in the nighttime protonosphere

Abstract

Alouette-2 electrostatic probe measurements of the nighttime electron temperature (T_e ) and concentration (N_e ) in the height range 2500 ± 500 km are presented on global basis (-60 to +60 geomagnetic latitude). The major feature of the protonosphere at this altitude is a broad minimum in T_e at the low latitudes with values around 1800° K. The electron temperature increases sharply to over 3000° K at the middle latitudes. The electron concentration exhibits a pair of maxima at about 30° north and south with values ranging from 4 × 10³/cm³ in fall to 8 × 10³/cm³ in local summer. No definitive seasonal dependence is apparent in T_e . Comparison of the present Alouette 2 measurements with nearly simultaneous measurements on Explorer 22 during summer 1966 reveal positive temperature gradient of T_e along field lines. Typical gradients at 1000 km are 0.6° /km on the 35° field line and 1.2° /km on the 50° field line. These gradients are consistent with the thermal energy stored in the nocturnal protonosphere, and therefore no external souce of heat is required to explain the observations.