RADIO ASTRONOMICAL MEASUREMENTS OF THE DIRECTION AND SPEED OF WINDS IN THE UPPER LAYERS OF THE ATMOSPHERE

At the present time there is no doubt that the general circulation of air masses in the Earth’s atmosphere is not limited to the near-surface layers, but also embraces regions tens and even hundreds of kilometers above them. Therefore, the study of winds in the upper layers of the atmosphere is one of the most important problems of modern meteorology.

At the same time, the capabilities of the various methods for studying winds at great heights, which reduce to observing the motion of various objects that more or less accidentally enter these layers—visible meteor trails, noctilucent clouds, smoke from exploding artillery shells, etc.—are highly limited; and the sporadically obtained data, acquired with their aid, relate mainly to heights not exceeding 110–120 km¹.

Therefore the appearance of new methods that supplement the existing ones and expand the range of heights accessible for study is of considerable importance.

Particularly attractive are attempts to use radio-physical methods for this purpose; these have already successfully demonstrated their fruitfulness in the study of a number of other phenomena in the upper layers of the atmosphere. In addition, the important advantage from the meteorological point of view is that they can ensure regularity and systematic observation. Attempts of this kind to determine the motion of air masses (more precisely, electron clouds) in the region of the ionosphere

by observing the reflection from it of signals sent from the Earth’s surface, were undertaken repeatedly (see, for example,², ³, ⁴). The general idea of these measurements is based on the assumption that the motion of all possible irregularities in the structure of the ionospheric layers is due mainly to the motion of air masses and correctly reflects the character of this motion.

The practical implementation of such measurements may be very diverse, in accordance with the diversity in the character of the observed inhomogeneities themselves (from large “fields” hundreds of kilometers in diameter to small “cloudlets” only a few tens of meters in size). In particular, the objects of observation may also be meteor trails, and the speed of their displacement can be measured with the aid of radio-location devices, for example, by the Doppler effect⁵).

Some results obtained in this way are presented in the table.

The development of radio astronomy has made it possible to devise a somewhat different method for determining the direction and velocity of winds in the region of the $F_2$ layer, one which apparently possesses a number of substantial advantages and is free from some as yet insurmountable difficulties in the interpretation of observational data that hinder the development of the methods mentioned above and deprive the results obtained with their aid of the necessary degree of reliability.

The difference of this method consists in the fact that, instead of terrestrial sources of radio radiation, extraterrestrial sources (radio stars) are used, i.e. the measurements are made not with reflected radiation, but with radiation passing through the ionosphere.

As is known, the radiation of radio stars is subject to strongly pronounced fluctuations⁶. At a frequency of 80 Mc, the average duration of a single fluctuation is usually about 0.5 sec, while the depth of modulation reaches 40%.

The character of the correlation of the fluctuations observed at receiving stations separated by various distances shows that these fluctuations are basically a phenomenon analogous to the twinkling of visible stars and are caused by the influence of the Earth’s atmosphere. At present it may be regarded as generally accepted that they are a consequence of diffraction by inhomogeneities of the $F$-layer of the ionosphere, the dimensions of these inhomogeneities being close to 4–5 km³, ⁴. This view is also supported by the close correlation between fluctuations in the radiation of radio stars and changes in the sporadic $F_2$ layer observed by ordinary methods.

If one starts from the idea that the structure of the layer through which the radiation from the radio star passes changes comparatively slowly, and that the fluctuations are caused chiefly by the displacement of the diffraction pattern relative to the receiver (i.e. by the drift of the sporadic $F_2$ layer), then the possibility of a simple determination of the velocity of this drift becomes evident.

Suppose that several identical receiving devices are located at small distances from one another (of the order of several kilometers). Then, if the original assumption is valid, the fluctuations perceived at stations situated along the drift line must be identical, but shifted in time; moreover, the magnitude of the time shift is connected in an elementary way with the drift velocity. According to observations (see, for example,⁹), such identity of the fluctuation curves obtained at neighboring receiving stations, and a relative shift of these curves in time, do in fact occur; moreover, the magnitude of the time shift is sufficiently large and is easily measurable.

Some results of radiophysical measurements of wind speed at various altitudes

* Rapid changes in wind direction were sometimes observed (up to a reversal of direction). In some cases, this may be due to a change in the effective height of signal reflection.

Fig. 1. Wind roses in the region of the ionospheric layer \(F_2\), obtained from measurements of fluctuations in the radiation of radio stars at a frequency of 80 MHz.

a — May–June 1951; \(60^\circ\) N lat.; b — September–October 1951; \(53^\circ\) N lat.

Fig. 2. Histograms of wind velocities in the region of the ionospheric layer \(F_2\).

a) vertical axis: number of observations; horizontal axis: velocity (km/hour).

b) vertical axis: number of observations; horizontal axis: velocity (km/hour).

a — May–June 1951; b — September–October 1951.

FROM CURRENT LITERATURE

Similar measurements7, 8, carried out in 1949–1950 at wavelengths of 3.7 and 6.7 m with the aid of two stations separated by several kilometers, showed that in the region of the sporadic layer \(F_2\) (i.e., at heights \(>300\) km) winds prevail that blow mainly westward with velocities of the order of 100 m/sec.

The authors of the paper under review9 continued and developed these measurements. In order to determine more accurately the direction and velocity of drift, they used three stations arranged in a triangle and separated from one another by a distance of 4 km. The object of observation was the radiation of the brightest radio stars, belonging to the constellations Cygnus and Cassiopeia. In processing the data, a correction was introduced for the apparent motion of the stars caused by the rotation of the Earth.

Since the fluctuations are produced by the sporadic layer \(F_2\), they are observed predominantly at night. Accordingly, the observations were made between 1800 and 0100 local time. Two series of observations were carried out. One of them (\(a\)) covered May–June 1951 (72 measurements over 13 nights), the other (\(b\)) concerned September–October 1951 (65 measurements over 18 nights). A significant difference in the conditions was also that in the first case the radio stars were not far from the northern horizon, so that the measurements referred to the region lying approximately 800 km north of the observation site (i.e., approximately at \(60^\circ\) north latitude). In the second case the radio stars were located near the zenith and the results referred to a latitude close to \(53^\circ\).

The results of the measurements are shown in Figs. 1 and 2. They undoubtedly indicate the existence of a systematic transport motion of the irregularities of the \(F_2\) layer, and this direction of drift, as the authors note, usually predominated for many hours in succession: variations of the direction during four-hour observations usually did not exceed \(25^\circ\).

In both cases the drift occurred predominantly in a westerly direction. The mean drift velocity was approximately 350 km/hr (i.e., about 100 m/sec), with observations near the horizon (series \(a\)) showing a considerably smaller velocity than observations at the zenith (series \(b\)): 250 km/hr and 430 km/hr, respectively. Certain differences were likewise observed in the direction of the drift: in series \(a\) the predominant drift direction deviated from due west by about \(15^\circ\) to the north, and in series \(b\) by \(15^\circ\) to the south.

In addition, the authors note the following circumstances: 1) In two cases a sharp (by more than \(140^\circ\)) change in the direction of drift was observed within one hour. Analogous phenomena had been observed by the authors earlier as well (in February–May 1950), in observations with two receivers separated from each other by 11 km (see also the note to the table). 2) A linear dependence was found between the rate of fluctuations and the drift velocity (the duration of individual fluctuations varies between 0.1 and 5 min—in different nights). This, in the authors’ opinion, indicates that differences in the rate of fluctuations are due not to differences in the sizes of the irregularities of the \(F_2\) layer, but to changes in the speed of their displacement.

Thus, the use of extraterrestrial sources of radio emission makes it possible to obtain, apparently, sufficiently reliable data on the motion of ionized clouds in the region of the \(F_2\) layer.

It is to be hoped that in the coming years the application of radiophysical methods will make it possible to compile a sufficiently complete and clear picture of the character of the circulation of air masses at various altitudes.

G. R.

References Cited

1. See, for example, E. O. Halbart, UFN 34, 481 (1948).
2. Ya. L. Alpert, UFN 38, 309 (1949).
3. Nature 167, No. 4251, 626 (1951).
4. Observatory 71, No. 862, 104 (1951).
5. L. A. Manning, O. G. Villard and A. M. Peterson, Proc. Inst. Rad. Eng. 38, 877 (1950).
6. M. Ryle, UFN 46, 508 (1952).
7. M. Ryle and A. Hewish, Mon. Nat. Roy. Astr. Soc. 110, 381 (1950).
8. C. G. Little and A. Maxwell, Phil. Mag. 42, 267 (1951).
9. A. Maxwell and C. G. Little, Nature 169, 746 (1952).