Diurnal and seasonal variation of atmospheric ion composition; Correlation with solar zenith angle

Abstract

A global measurement of the diurnal and seasonal variations of the primary atmospheric ions in the earth's topside ionosphere between 280 and 2700 km has been made using the ion mass spectrometer on Explorer 32. Evidence of direct correlation between solar zenith angle and the low and midlatitude distributions of thermal O+, N+, He+, and H+ was obtained as the satellite orbit plane traversed one complete diurnal cycle between June 11 and October 5, 1966. The northern midlatitude ionosphere at 1600 km, near the O+‐H+ transition level, exhibited strong diurnal variation, O+ rising sharply at dawn to become the dominant daytime ion with a concentration of 1 × 104 ions/cm³; H+ was dominant at night with a concentration of 1 × 104 ions/cm³, a factor of 5 higher than its daytime value. The diurnal variations of N+ and He+ in this region resembled those of O+ and H+, respectively, at reduced concentrations. Measurements at 1200 km in the geomagnetic latitude intervals 45° to 55° (summer hemisphere) and −45° to −55° (winter hemisphere) showed a strong seasonal change in the O+ and N+ diurnal distributions, but little change in H+ and He+. In summer the O+ concentration increased to 2 × 104 ions/cm³ by 0600 LT; this daytime level was maintained until dusk at 1900 LT. The winter O+, however, was maintained at this daytime level only between 1100 and 1600 LT. The N+ distributions paralleled those of O+, the O+/N+ ratio being about 10. It is found, at least for midlatitudes, that the observed diurnal and seasonal variation of O+ scale height between 1000 and 1500 km is directly related to the variation of solar zenith angle. Less pronounced diurnal variations are observed at the midlatitude F maximum and in the low‐latitude protonosphere.

Temporal variations of thermospheric hydrogen derived from in situ measurements

“…[ Hanson and Ortenburger, 1961], using n(H + ) measurements by the Explorer 32 Bennett RF ion mass ,pectrometer (described in Brinton et al, 1969). Simultaneous measurements of n by two cylindrical density gage measurements, as reported by Newton [1970].…”

Diurnal Behavior of the Equatorial He+ Trough at an Altitude of 1100 km.

“…The cooling at night of the plasma contained along the flux tube also emphasizes the decrease in the He' density due to the reduction of the scale height. Because the field line, which passes the 1100 km altitude at the equator, is short enough so that the diffusive equilibrium state is established in a short time for this field line (BRINTON et al, 1969), the diurnal distribution of the He' density at the equator closely follow that of the solar zenith angle, as evidenced in Fig. 5.…”

“…4 for He+. During solar minimum, the plasma at 1000 km is mainly composed of H+ (BRINTON et al, 1969) so that the density of electron there is almost equal to that of H+. Thus, H+ during such periods behaves similarly to He'.…”

“…On the other hand, the profiles of He', NmF2 and Ne at the equator are very similar to each other. This variation at the equator can be interpreted in terms of the solar zenith angle (BRINTON et al, 1969).…”

“…The He' density in the topside ionosphere shows complex latitudinal, diurnal or seasonal variations due to many competing processes (BRINTON et al, 1969;TAYLOR et al, 1968). The equatorial trough is one of the most interesting characteristics of He' distribution.…”