On explaining the behavior of the ionospheric F region

Abstract

This paper reviews the current situation in ionospheric physics regarding the large-scale behavior of the F region. Following the introduction, in which the plan of the paper is detailed, part 2 summarizes a basic physical theory of the F region and then lists the additional physical processes that are thought to influence F-region behavior. The over-all balance of ionization in the F region is also considered in relation to the flux of ionizing solar radiation. With currently available data, it is barely possible to account for the gross quantity of ionization present. In part 3, several phenomena of the F region are reviewed and possible explanations discussed. The phenomena include seasonal and annual variations of the F2 layer, the daily variations of peak electron density (N,•F2) and the height of the peak (h,,•F2), the nighttime F layer, and the equatorial F2 layer. Part 4 deals with further topics of interest, though in less detail. These include the F2 layer during storms and at high latitudes, conjugate point effects, and the existence of negative ions in the F region. In the conclusion, the preceding discussion is recapitulated by listing the observed effects that are suggested to result from each physical process, and brief comments are made on possible future progress. 1. INTRODUCTION The F region has been defined as the par[ of the ionosphere above 150-km altitude. Although physical considerations do not support the idea of a precise lower boundary, nearly all of this paper is concerned with levels above 150 km. The upper boundary may be •aken as •he level where ligh• ions (helium and hydrogen) become more numerous [han [he ions of heavier gases, abou[ 600-1000 km. The F region is of considerable importance for radio communications since it generally contains greater concentrations of free elec[rons than the other ionospheric regions. Although the F region has been studied for more than one-third of a century, •he de•ails of i•s behavior are s•ill no• well understood. This paper is intended to ½omplemen[ a more basic review IRishbeth, 1967a]. I[ surveys some of the interesting phenomena and examines the plausibility of proposed physical explanations in the light of present knowledge. For this purpose, no very detailed description of F-region morphology is needed. Accurate numericM values of F-region p•mmeters are no• de•l• with here, since •hey h•ve been extensively discussed in the recent review by Yonezawa [1966]. The present discussion does, however, rely heavily on various aspects of contemporary ionospheric theory; therefore these are reviewed early in the paper. The mos• importan• physical parameter of the F region is the electron density, or electron concentration N. By various experimental means, a vast amoun[ of information has been gathered on the variation of N with local time, aliiJude, and location. A grea[ deal of this informalion comprises measurements of 33 34 H. RISHBETH •he critical frequencies of •he F1 and F2 components of •he F layer, obtained by vertical incidence sounding at nu...

Magnetospheric convection and the high-latitudeF2ionosphere

“…As the "tongue" of ionization is convected below the trapping boundary on the night side and toward the sunlit region of the earth, the F-layer decays primarily from dissociative recombination and also to some extent from polar wind loss. The ebb and flow boundary is a magnetic latitudinal boundary equatorward of which the return flow of H 4 " ions from the plasmasphere at night is adequate to maintain the F-layer against dissociative recombination (Park, 1970;Nagy and Banks, 1972;Rishbeth, 1968). It is approximately at the same latitude as the plasmapause.…”

A review of the theories concerning the equatorial F2 region ionosphere

“…The last term on the right is the contribution from horizontal electron density gradients and is normally neglected for midlatitude diffusion and drift studies (of. Ferraro, 1945 , for the first derivation of the midlatitudo equation and Rishbeth, 1967aand Rishbeth, , 1968 for a more complete review of this subject).…”

Vertical lifting of ionization during geomagnetic storms from satellite measurements of ion composition.

“…The storm time behaviour of the ionosphere is known to be very complex and the changes observed are found to be dependent on the phase of the storm and also on the location (OBAYASHI, 1964;RISHBETH, 1968). While most of the observations pertain to the changes in the peak density NmF2 and peak height hmF2 of the F2-region there are some studies made of the bottomside and topside profiles (MATSUSHITA, 1963;SOMAYAJULU, 1963;BAUER and KRISHNAMURTY, 1968).…”