Air motions at meteoric heights

Abstract

IN previously reported studies, methods have been described for finding ionospheric winds using the Doppler frequency shift imparted to meteoric echoes by drift of the ionization trails. Both average and root mean square values of mind velocities have been measured by these techniques (MANNIKG, 1950, 1953, 1954). In the original studies the writer and his colleagues found the average horizontal drift velocity of the whole meteoric region to be of the order of 30 m/set. An average vertical drift velocitjy of 1.5 m/see in the downward direction was found with a standard deviation of 2.4 m/see. The r.m.s. value of the instantaneous values of the horizontal wind components was found to be 50 m/set, while the r.m.s_ value of the vertical wind components was found to be of the order of 12 m/set, but with an error of observation that would not exclude a value of zero. It was suggested that a horizontally stratified wind structure would serve to explain the results. Further application of the preceding method was made by ELFORD and ROBERTSON (1953) who found 12 and 24 hr components in mean velocities. GREENHCIW has also applied the method and obtained similar results; he has reported in detail on the changes in mean wind properties with time and with height (GREENHOW, 1954). In the present study the results will be given of a new method of analysing t,he detailed wind structure at meteoric heights. The results are based on t’he fading, diversity and aspect properties of meteoric echoes. It is assumed that when a meteor enters the SO-110 km height region, it produces an initially straight column of ionization perhaps 25 km long (MANNIXG et al., 1953). This straight trail is acted on by horizontal winds whose direction and magnitude vary with height. As the trail partakes of t#he motion of the surrounding air. it is distorted into some sinuous shape determined by the vert,ical wind profile. It is assumed that the deviation from the mean of the north-sout,h and east-west components of the wind profile may be described by Gaussian “noise” functions. There is then some spectrum function describing the relative amplitude of the Fourier components of the wind velocity-height profile. This spectrum function is descriptive of the shears and turbulence. The strengths of radio signals reflected from the distorted trails may be computed by integral methods involving summation of the elementary scattering contributions from each differential length of trail. The results may be shown to be very nearly the same for practical trail dimensions as from a simple analysis based on “glints”. It is assumed that whenever the trail is tangent at a point to an ellipsoid of revolution having the transmitter and receiver as foci, a reflection is obtained. The strength of the reflected power at this “glint” is