Ionospheric research from space vehicles

Summary

Spaceflight observations made in recent years are most consistent with the following general conclusions:

• (1) The principal ionizing agents for the D region under quiet solar conditions at an epoch corresponding to the middle of the solar cycle are cosmic radiation for the region below 70 km and Lyman α radiation at altitudes between 70 and 85 km. The negative ion abundance at all D region altitudes has been underestimated theoretically.

• (2) X-radiation (2–8 A) and energetic protons, which are respectively responsible for Sudden Ionospheric Disturbances and Polar Cap Absorption Events, lead to a D region electron density enhancement of about two orders of magnitude.

• (3) The principal ions produced in the lower ionosphere are N₂ ⁺, O₂ ² and O⁺. The ions which exist in greatest abundance are NO⁺, O₂ ⁺ and O⁺. These ionization characteristics esult from the combined influence of solar ultraviolet and X-radiation in the E region with ultraviolet radiation becoming the dominant source in the F region.

• (4) The average E region electron density decreases by about two orders of magnitude at night. Some types of sporadic-E ionization have a depth of less than one kilometer and a horizontal dimension greater than 70 km. Other types have a smaller horizontal dimensions. There is a first order increase of the electron density in a sporadic E-layer.

• (5) The formation of the F2 peak is best explained by the combined altitude dependence of the electron production rate, electron loss by an attachment like process and diffusion.

• (6) The constant electron-ion scale height which is observed for altitude intervals of a few hundred kilometers above the F2 peak is evidence that the upper ionosphere is isothermal and in diffusive equilibrium.

• (7) The upper ionosphere is characterized by the presence of three ionic constituents (O⁺, He⁺, H⁺) each predominating in a different altitude region. The thickness of the helium ion layer has a large diurnal variability. The control which these ionic constituents exercise over the electron density distribution is such that the electron density is generally higher at midday below 1000 km, shows a small diurnal variation near 1000 km, and is generally higher at altitudes between 1000 and 2000 km at night.

• (8) In an undisturbed mid-latitude ionosphere, it is generally accepted that temperature equilibrium can be expected at night. Significant departures from equilibrium can be expected at sunrise and during disturbed conditions. Toward midday under quiet mid-latitude conditions significant departures from equilibrium would be expected in the lower F region, but small differences between T _e, T _i and T _g are observed below 150 and above 450 km.

• (9) Except for the sunrise anomaly the diurnal maximum and minimum values of electron temperature at mid-latitudes approximate the diurnal extremes of the neutral gas temperature in the upper ionosphere. An observed latitude dependence of electron temperature suggest that corpuscular radiation is an important heat source at high latitudes.