Research within the Ionosphere

Explanation of recent rocket and satellite results requires considerable modification to the theories of formation of the various subdivisions of the ionosphere which evolved from ground-based studies. It was not until quite recently that the difficult task of measuring D region (50-85 km) electron densities from rockets was consummated. Flights made during sunspot minimum in the absence of solar flare show that the average electron population of the D region is about 700 electrons cm-3. The altitude distribution obtained suggests that the principal ionizing agent for the region 70-85 km is Lyman alpha radiation. A firm conclusion requires a better knowledge based on laboratory investigation of the rate at which electrons are lost by recombination with the positive ions that are simultaneously produced. Electron density enhancements of up to two orders of magnitude (sufficient to cause radio blackouts) have been observed during special events. Rocket measurements show that (a) the sudden ionospheric disturbance, a short term radio absorption event occurring on the sunlit side of the earth, is caused by enhanced X-ray radiation emanating from the sun during a solar flare; (b) radio absorption events occurring in the auroral zone are attributable to direct and indirect (bremsstrahlung) effects of precipating energetic electrons; (c) polar cap absorption events which occur above the auroral zone can be correlated with satellite measurements of enhanced proton fluxes during certain types of solar flares. Rocket measurements are beginning to detail the day-to-night behavior of E region (85-140 kin) electron densities.