Ferraro's Contribution to Ionospheric Research

Abstract

It may be appropriate to preface this collection of papers with some remarks on a paper published by Ferraro (1945). I shall try briefly to give the background to this work and at the same time indicate the part it has played in ionospheric research.The ionosphere had been explored and its layered structure established in the early days of development of radio transmission. Appleton named two principal ionized layers in the upper atmosphere, in each of which the electron density reaches a maximum value; the E layer, with a maximum denoted by NmE (x lo5 ~m -~) occurring at an altitude hmE (about 100 km) and the F2 layer with a maximum NnzF2 ( x lo6 ~m -~) at an altitude hmF2 (about 350 km).The E layer was satisfactorily explained on the basis of electron production by solar ultraviolet radiation and electron loss by recombination. By contrast, the F2 layer was found to be much less predictable. Data obtained by radio sounding revealed a number of properties which were described as anomalous.A key to the behaviour of the F2 layer is provided by the plasma parameter o i / v i ( o i being the gyrofrequency and v i the collision frequency of ions with neutral particles). In the E layer, o i / v i = 10-and the microscopic motion of ions is essentially random. In the F2 layer, because of the smaller neutral particle density, o i / v i x 300; ions and electrons are thus effectively tied to geomagnetic field lines. Under the action of gravity and of their own pressure gradient, plasma particles in the F2 layer are free (in between the comparatively rare collisions with neutral particles) to diffuse along geomagnetic field lines. The diffusion (of electrons and ions together) is described as ambipolar. Ferraro derived the equation + f N ) aN is+--3 aN 2 aZ -= q -L + , at H for the electron density N at altitude z (measured in units of scale height H ) and time t.The diffusion coefficient D turns out to be inversely proportional to the collision frequency of ions and hence to the number density of neutral particles. It follows that D increases upwards exponentially. It is assumed for simplicity that the ions and neutral particles have the same constant scale height. Of the remaining terms in the equation, q represents the rate of electron production and L the rate of loss.The application of the diffusion equation to the F2 layer was hindered for several years because the nature of the loss process was not fully understood. Ratcliffe et at. (1956) gave justification for assuming an attachment-type law L = Be-'N, where / 3 is a constant.There remained some doubt as to the most appropriate boundary condition to apply at z = 00. The most natural assumption, for a supposedly static region, was that the flux of ionization should vanish at great height. The resulting mathematical models were in fair agreement with the diurnal behaviour of hmF2 and NmF2 in middle latitudes at times of low geomagnetic activity. 309