Solitary electrostatic structures involving density depletions have been observed in the upper ionosphere by the Freja satellite [Dovner et al., 1994]. If these are interpreted as ion sound solitons, the difficulty arises that the standard Korteweg‐de Vries description predicts structures with enhanced rather than depleted density. Here we show that the presence of non‐thermal electrons may change the nature of ion sound solitary structures and allow the existence of structures very like those observed.

The electronic properties of the ferrous ion in [Fe(H20),](YH,)2(S0,), are investigated using far-infrared (FIR) Fourier transform spectra and FIREPR measurements at low temperatures in the energy range from 8 to 100cm-I. Several absorption lines can be identified to originate from electronic excitations of the ferrous ion.