Cause of the midlatitude winter night increase in<i>f</i>₀<i>F</i>₂

Abstract

Electron density measurements and determinations of the electron and ion temperatures throughout the F region have been made at midlatitudes by the incoherent backscatter technique at Millstone Hill. Results of this work are presented for the hours 1800–0600 EST for the months November and December 1963 and January 1964. The early morning increase in ƒ0F2 observable at this station near 0200–0400 EST in these months is accompanied by density increases at all the other altitudes that could be investigated, i.e., up to 600 km. Hence this phenomenon cannot be accounted for by changes only in the shape of the layer. Further, the electron and ion temperatures tended to decrease as the density increased, indicating that freshly produced ionization caused, for example, by particle precipitation cannot be invoked as an explanation. It is concluded that additional ionization must have diffused from great altitudes in the exosphere. It is suggested that this downward motion may occur at this time as a result of a lowering of the exospheric electron temperature after sunset in the southern (summer) hemisphere.