Electron densities in the lower ionosphere

Abstract

For the purposes of this thesis, the lower ionosphere is defined as the ionized region beneath the E region. The E region normally refers to the altitude range from 90-140 km. where the temperature profile has again assumed a positive gradient (Figure A.1). The lower limit also approximately marks the level where diffusive separation of the atmosphere constituents begins to overcome the mixing processes which dominate below 80km. Ionization in the E region is attributed mainly to solar X-rays (10-100 Å), Lyman β (1025.7Å), C.III and the Lyman Continuum (910-800 Å). The photochemistry, and diurnal seasonal variation of the E region are reasonably well understood. The lower ionosphere has for many years been called the D region – and extends from 60-90 km. The main ionizing agents are thought to be Lyman α, X-rays from 1-8 Å, and galactic cosmic rays in the lower portions. However, only on quiet days, in temperature latitudes, is there a simple dependence of electron density (N(h)) on solar zenith angle, as predicted by the “Chapman layer” theory. The relationship between photochemical reactions due to solar activity and the changes in the electron density values N(h) are not established for the D region. This is partly due to the lack of knowledge concerning the concentration of some of the minor constituents which are thought to be involved in the photochemical reactions – and partly to the lack of general information concerning dynamical processes in the region. Both of these aspects are of fundamental importance when a theory of the major processes occurring in the D region are considered.