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

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 × 10 4 ions/cm³; H + was dominant at night with a concentration of 1 × 10 4 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 × 10 4 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.