Search

A new high power radar at Malvern is being used to investigate the ionosphere by the incoherent scatter technique.

SINCE October 1944 we have carried out investigations of the short duration scatter echoes observable in the neighbourhood of the E region of the ionosphere at frequencies well above the critical frequencies for either the normal or abnormal E layers. The general occurrence of such echoes was first reported by Appleton, Naismith and Ingram1 in their observations during the Polar Year 1932–33. Schafer and Goodall2, who worked in collaboration with Skellett3 in an investigation of meteors as a source of abnormal E region ionization, had also noted them as a specific feature of the Leonid shower of 1931. An adequate reference to subsequent research cannot be given in a brief communication; but the work has for the most part led to the view that the transient ionospheric echoes are caused by meteors (see, for example. Appleton4). Our experiments have led us to confirm that the majority of scatter echoes must be of meteoric origin and are due to reflexions from meteor trains or streaks : these columns of ionized gas, caused by meteoric impact with the molecules of the upper atmosphere, present their maximum echoing areas when viewed at right angles to their length.

Meteor Astronomy By A. C. B. Lovell. (International Series of Monographs on Physics.) Pp. xiv + 463 + 3 plates. (Oxford: Clarendon Press; London: Oxford University Press, 1954.) 60s. net.

THE study of the physics of shock waves of sufficiently high intensity to produce appreciable gaseous excitation and ionization has in recent years become of increasing interest in relation to the gas dynamics of astrophysical phenomena and to the properties of gases at very high temperatures.

A SHOCK tube consists essentially of two sections, one containing a gas at high pressure and separated from the test gas in the other section by means of a diaphragm. A shock wave is produced in the test gas when the diaphragm is suddenly broken. As the initial pressure ratio is increased, the shock front attains a limiting velocity and the corresponding limiting shock Mach number is U/a1, where a1, a2 are the speeds of sound and γ1, γ2, the specific heat ratios (Cp/Cv) of the gases in the test and high-pressure sections respectively. Thus if both sections contained the same monatomic gas for which γ = Cp/Cv = 1.67 the limiting shock Mach number would be approximately 4. Greater shock Mach numbers could be achieved if the speed of sound in the high-pressure section were increased, for example, by using a light gas and by raising its temperature. Thus hydrogen is a suitable driver gas, and it could also be brought to a high temperature by burning oxygen in it. An alternative method of producing a high shock velocity is to introduce an intermediate section into the shock tube1. When the first diaphragm is broken, a shock wave is produced in the intermediate section, and the resulting column of hot gas flowing at high speed in this section then acts as the final driver to initiate a faster shock wave in the test section. In a previous communication2 we described the radio measurement of the velocity of shock waves by means of the Doppler effect. The value of this technique, which provides detailed measurement as the shock wave travels along the tube, is illustrated by its application to the study of methods for producing hypersonic flow.

IT is important both for the understanding of the nature of the localized radio sources and the determination of interstellar radio absorption to extend the measurement of the spectra of the sources to as low radio-frequencies as possible. Ionospheric transmission sets a limit, but by choosing frequencies not too close to the critical frequency, and restricting observations to days of low ionospheric disturbance, reasonably accurate measurements may be obtained. In this way, we have recently made measurements at a frequency of 22.6 Mc./s. (λ = 13.3.m.) by means of an interferometer consisting of two aerials spaced in the east–west direction at 20 wave-lengths apart, thus producing lobes in the aerial pattern at intervals of about 3°. Each aerial consists of an array of six full-wave dipoles giving a vertically directed beam of width ± 22° and ± 9° in the north–south and east–west directions respectively. The recording apparatus is a simplified form of the type described by Machin, Ryle and Vonberg1, whereby the receiver is continually switched to a standard noise diode source which is automatically adjusted by a servo-mechanism to equalize the received radiation.

In their description of the solar radio emissions from the great sunspot of February 1946, Appleton and Hey1 pointed out that a number of sudden enhancements of intensity, observed at about 5 metres wave-length, occurred simultaneously with increases in D-layer ionization as indicated by fade-outs on long-distance radio-communication circuits. This increased ionization, which is believed to be due to enhanced solar ultra-violet radiations, strongly absorbs the radio communication waves (of the order of 20 metres wave-length) but does not produce any marked absorption of the solar radio emissions at wave-lengths of 5 metres or less.

THE discovery of electromagnetic 'noise' radiations at radio frequencies was first made by K. G. Jansky1 who, in a series of measurements at about 20 Mc. s. ( 15 m.) was able to establish the close connexion between the direction of greatest intensity and the centre of the galaxy. G. Reber2 later made measurements at 160 Mc./s. ( 2 m.), and he was able, by virtue of a narrower beam, to detect a number of subsidiary peaks. He found the intensity of radiation at this higher frequency was very much less ; he also suggested a probable explanation of the radiations, namely, the interactions in encounters between positive ions and free electrons released from interstellar gases by the photoelectric action of stellar radiations. The theoretical treatment for radiation arising from such encounters was first given by H. A. Kramers3 to account for continuous radiation in X-ray spectra. The cosmic noise radiation is not observable at frequencies lower than a few megacycles per second, owing to the screening action of the ionosphere.

IN two recent communications to these columns1,2 we have reported the existence of powerful radio emissions in the 5-metre wave-length band from sunspot areas. Since such radio waves must travel through regions of considerable ionization in escaping from the sun, it occurred to us that the magneto-ionic theory of radio wave propagation3, which has proved so useful in elucidating phenomena in the terrestrial ionosphere, would be applicable in the case of the corresponding solar envelope. According to, this theory, characteristic polarizations are imposed on radio waves in their transmission through an ionized medium under the influence of a magnetic field, due either to differential absorption of the oppositely polarized magneto-ionic components or to the suppression of one component by electron limitation. Such effects are most pronounced if the radio wave frequency in question is either of the same order as, or less than, the electronic gyro-frequency determined by the imposed magnetic field. There is also the possibility that the noise itself has a magneto-gyric (electronic) origin.

Radio Astronomy By Dr. J. L. Pawsey and Dr. R. N. Bracewell. (International Monographs on Radio.) Pp. X + 361 + 23 plates. (Oxford: Clarendon Press; London: Oxford University Press, 1955.) 55s. net.

IN 1948, Hey, Parsons and Phillips1 reported that at 4.1-metres wave-length bursts of radio emission, usually of a few minutes duration, occurred more often in association with flares on the eastern half of the sun's disk than on the western half. Subsequent observations have confirmed this conclusion. In a recent analysis of radio emissions at 1.5-metres wavelength, Dodson, Hedeman and Owren2 could find no evidence of this asymmetry. The following reasons may explain why they failed to observe the effect.

Radio Studies of the Universe By Dr. R. D. Davies and Dr. H. P. Palmer. Pp. viii + 200 + 14 plates. (London: Routledge and Kegan Paul, Ltd., 1959.) 25s. net.