On the diurnal and seasonal variations of H +, He +, N +, O +, and Ne at 1400-km altitude

The diurnal and seasonal variations of H ⁺, He ⁺, N ⁺, O ⁺ and Ne are analyzed at 1400-km altitude. Using longitudinally averaged observations of ISIS-2 (April 1971 to December 1972), the ion and electron densities are decomposed via spherical harmonics and Fourier series into time-independent, seasonal and diurnal terms. The time-independent terms of H ⁺ and He ⁺ show a plateauor trough-like structure at medium to low latitudes and a strong decrease towards the poles; N ⁺ and O ⁺, on the other hand, yield an almost inverse picture with a density increase at high latitudes. All constituents, except He ⁺, show at polar latitudes an enhancement during local summer conditions and a depletion during local winter conditions; He ⁺, however, exhibits a winter bulge and a density minimum during local summer. The diurnal variations are strongly latitude dependent; while the amplitudes (relative) of H ⁺, He ⁺, and Ne are rather small, the heavier ions N ⁺ and O ⁺ show a deep minimum early in the morning and a high but flat maximum during daytime.