L. A. Manning

Radio Communication by Scattering from Meteoric Ionization

By a consideration of the amplitude and duration of echoes forward-scattered from individual meteor ionization trails, and of the probability of detecting randomly oriented trails over an oblique radio propagation path, an estimate of the contribution of meteoric ionization to extended range hf and vhf radio transmission has been obtained. It has been concluded that meteoric ionization alone would give a virtually continuous signal for a transmission path of about 1,000 km at frequencies near 15 mc. For the very high frequencies, scattering from meteor trails has been found to be at least an important contributing factor to the propagation of a signal over an oblique path. A precise evaluation of the role of this process must await a better determination of the number of trails as a function of their ionization density.

Meteoric Echo Study of Upper Atmosphere Winds

A method is developed for measuring the velocity of winds in the 90- to 110-kilometer height region of the upper atmosphere. From the Doppler frequency shift imparted to a continuous-wave reflection from a meteoric ionization column, a measure of the wind drift of the trail is found. Statistical analysis enables average wind velocities to be measured to within perhaps 20 per cent, and direction to 20°, in a period of one or two hours. Observations made during the early morning hours in the summer of 1949 show typical average wind velocities to be 125 kilometers per hour, with motions from south-southwest and north the most common. On some occasions, evidence of a nonuniform wind structure is found.

Radio Doppler Investigation of Meteoric Heights and Velocities

A continuous‐wave Doppler method of determining meteoric velocities and heights was tested during the 1948 Perseid meteor shower. The accuracy of the technique was found to compare very favorably with optical and other radio methods. Doppler measurements have the advantage of relatively great sensitivity.

The strength of meteoric echoes from dense columns

Abstract Calculations of the strength of the echoes from dense meteoric ionization columns have been made in the past by assuming that the trails behave like perfectly reflecting cylinders of that radius for which the refractive index is zero. It is shown that this procedure neglects the defocusing effect of refraction at greater radii. A general formula is obtained for the effect of refraction on echo strength. The maximum power returned from a large dense Gaussian distribution of ionization is found to be only 70 per cent of that assumed previously.

The Role of Meteors in Extended-Range VHF Propagation

It is possible to transmit a continuous signal from point to point by taking advantage of specular reflections from ionization trails formed in the E-region of the ionosphere by meteors of all sizes. A continuous signal is provided whenever such reflections are present in sufficient numbers so that they overlap in time. The factors controlling the degree of overlap are: (1) radio frequency, (2)power (overall circuit sensitivity), and (3) length of path. For best overlap, the individual meteor reflections should be as strong as possible, as numerous as possible, and as long-enduring as possible. If all other parameters are kept the same, lowering the radio frequency increases individual echo strength and duration; raising the power and circuit sensitivity increases not only echo strength and echo duration, but also number of echoes detectable above the noise; and finally, increasing the path length also increases individual-echo duration.

Oblique echoes from over-dense meteor trails

Abstract Ray paths are computed for waves refracted by meteor trails having a Gaussian radial distribution of ionization density. From the spreading of initially parallel rays as they pass through the trail, a measure of reflected signal intensity vs. scattering angle is obtained; the results are presented in polar scattering diagrams, valid in the limit of large trail size. Equivalence theorems are derived relating both intensity and scattering angle for rays incident upon the trail at an arbitrary angle to the intensity and scattering angle for rays incident in the plane normal to the trail. Curves are presented showing the dependence of echo duration on forward-scatter-angle with trail orientation as parameter; it is found that the sec 2 φ law developed for under-dense trails applies to over-dense trails only if the plane of propagation contains the trail axis. If not, the effective secant exponent may be as small as 0·3. The theory is compared with McKinley and McNamara s duration measurements. It is found that although the general agreement is satisfactory, the details of their experimental results depend on the way that winds change the trail orientation. The ray theory is also compared with Keitel s wave theory solution. Unfortunately, he could not get wave solutions for dense trails of age greater than 0·357 per cent of the minimum echo duration. Even so, the ray solutions agree with Keitel s results for scatter angles up to 155°, thus including all angles available to ground-based stations.

Air motions at meteoric heights

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

The length of ionized meteor trails

By studying the rate of coincident detection of meteors at two widely separated locations, as a function of the differential time of first detection, it is possible to deduce the distribution in length of the meteoric ionization columns. A precise determination would require, in addition, that the velocity of the individual meteors used in the test be known. However, in the absence of such data, an approximate distribution of trail lengths whose mean value is probably representative of actual conditions, can still be deduced. For equipment generating about a kilowatt of continuous wave output, and capable of detecting meteors at a total rate of 450 meteors per hour, the mean trail length was found to be 25 or 30 km. Calculations based upon this value of trail length, and upon the use of a corrected rate of meteoric arrival, show that meteors up to the sixth magnitude were detected during the test. The method described provides a measure of trail length dependent upon the ability of the trail to reflect a signal at normal incidence. The accuracy of the determination of frequency of occurrence of trails of various lengths is independent of the rate of detection, but is proportional to the square root of the period of observation.

Meteoric radio echoes

Study of meteoric effects by radio methods has resuited in gain to three fields-astronomy, upper atmosphere physics, and radio propagation. In each of these fields progress is surveyed, and some of the more important unsolved problems are pointed out.

Radio Propagation Following World War II

Since the early 1940s the development of new propagation modes has greatly increased mans ability to communicate. Some of the outstanding advances are briefly reviewed.