Movements of ionspheric irregularities and gravity waves

Abstract

The nature and movements of ionospheric irregularities at E- and F-region heights are considered. Experimental evidence from a mid-latitude station is discussed which suggests that the night-time irregularities which produce the spread- F, spread- E 8 or sporadic- E-cloud features on ionograms represent a class of irregularities which are frontal disturbances. These have quasi-sinusoidal cross sections at F-region heights. Wavelengths range from 25 to 350 km and periods range from 5 to 60 min. Their characteristics indicate that they may be produced by internal gravity waves propagating through the ionospheric regions. These night-time irregularities are somewhat similar to daytime travelling ionospheric disturbances which have been in the past, associated, by certain workers, with internal gravity waves. Another class of irregularities, with dimensions of the order of a kilometre, appear to be responsible for the fading patterns from which ionospheric drifts are calculated. When the movements of the two classes of irregularities are compared a certain amount of agreement is evident. It is suggested that, because of this association between drift velocities and the movements of the larger-scale irregularities, the drift measurements are, at times, influenced by the presence of internal gravity waves.