Turbulence in the ionosphere with applications to meteor-trails, radio-star scintillation, auroral radar echoes, and other phenomena

Abstract

The irregularities in electron-density responsible for incoherent scattering of radio waves in the ionosphere are discussed on the assumption of isotropic turbulence in the neutral molecules, with allowance made for the effect of the earth's magnetic field on the associated irregularities in the density of the charges particles. The atmospheric model used is based on rocket observations, extrapolated upwards in height where necessary. Tentative formulas are deduced for the large eddies based on a non-standard application of the Richardson number. For the small eddies, the standard formulas of turbulence-theory are used. These formulas all depend on a quantity w, which is the rate of supply of turbulence-energy to the large eddies and also the rate of removal of turbulence-energy from the small eddies, measured per unit mass of atmosphere. The value of w at the meteoric level (90 km) is found to be around 25 watts/kg by comparison between the theory and meteoric observations (both visual and radio). By the same technique, a more tentative value of 1,000 watts/kg is deduced for the level responsible for scintillation of radio stars, although a lower value is probably appropriate when scintillation is weak. These values of w in the ionosphere are high compared with Brunt's value of 5×10−4 watt/kg for the troposphere. It is shown, however, that these high values of w in the ionosphere are quite possible and even reasonable. It is deduced that the time of onset of irregular fading of meteoric echoes in the VHF band is more likely to be due to roughness of the trail caused by the small eddies than to gross distortion of the trail caused by the large eddies. It follows that, after about a second, VHF radar echoes from a meteor-trail must be calculated using a theory based on incoherent scattering, thereby questioning the theory of Kaiser and Closs [37] as an explanation of long-duration meteor-echoes. It is also shown that radio-star scintillation cannot be explained in terms of turbulence at a level of 400 km, but that reasonable results can be obtained if the level is reduced to 200–300 km. Among other applications considered is the possibility of radio communication via incoherent scattering in the F region of the atmosphere. The conditions under which such communication should be sought are described in section 11.