During the last five years, we at the McMath-Hulbert Observatory have been attempting to determine the characteristics of radio-frequency radiation associated with certain of the transient solar features that we normally observe on monochromatic spectroheliograms or in integrated light. We have had the privilege of using the original 200 Mc./s. records at Cornell University and the 2800 Mc./s. records at the National Research Council in Ottawa.

Noise storms on metre wave-lengths originate high in the corona over particular sunspots. Generally, the enhanced radiation comes to a maximum when these sunspots cross the central meridian. If the radiation exceeds a fixed intensity, the sunspots are called noisy (R sunspots); the others are called quiet (Q sunspots). Unfortunately until now only a few interferometer measurements enabled us to know these ‘noisy sunspots’, but the strong directivity of this emission can be used to determine these sunspots in a statistical manner. We have done this for the sunspots having an average area bigger than 100 millionths of the sun, from 1947 to the middle of 1951. We have thus studied the statistical features for 350 sunspots, 160 of which are noisy and 190 quiet.

The detailed physical processes involved in centres of activity still remain obscure. Optical and radio noise observations enable us to state in increasing detail the phenomena that any comprehensive theory must explain. At times it seems that the complexity of the picture provided by these new data grows more rapidly than our ability to invent adequate theoretical models.

The so-called enhanced radiation appears at all frequencies in connexion with sunspot-regions. For λ < 25 cm. it may be interpreted satisfactorily as thermal radiation of the coronal condensations (Waldmeier, 1955) [1]. For λ > 50 cm. the mechanism responsible for the production of this emission is still unknown. The enhanced radiation from 62 to 200 Mc./s. shows a strong concentration in the radial direction and, therefore, becomes observable only when the source of radiation is close to the centre of the sun's disk. For sin θ = 0·3, i.e. at an angular distance of θ = 17° from the centre of the solar disk, the intensity has dropped to 50 % of that for θ = 0° and for sin θ = 0·5 to about 25 %. For sin θ = 0·8 the radiation intensity is certainly < 10%. This property has recently been investigated again by Machin and O'Brien (1954) [2] as well as by H. Müller[3], using the entire material of the years 1947–53 published in the Quarterly Bulletin on Solar Activity. According to Müller the limb darkening is very similar for the frequencies 62 to 200 Mc./s., the radiation intensity having decreased at sin θ = 0·29 to half of its value for sin θ = 0. A diminution to 10% may be found for 62 Mc./s. at sin θ = 0·54, for 80 Mc./s. at sin θ = 0·59, for 98 Mc./s. at sin θ = 0·62, for 175 Mc./s. at sin θ = 0·72 and for 200 Mc./s. at sin θ = 0·77. For 600 and 1200 Mc./s. the limb darkening is much weaker, the intensity decreasing to 50% at sin θ = 0·57 and 0·75 respectively.

The radio-frequency emission observed in solar bursts cannot reasonably be interpreted as thermal radiation. Its origin is to be sought for rather in terms of co-operative behaviour of systems of charged particles. In any case, we cannot avoid having to examine the physical consequences that arise from such co-operative behaviour.

Today it is taken for granted that the solar outbursts are produced by plasma oscillations of coronal matter, excited by a corpuscular radiation originating in the inner corona (Pawsey and Smerd, 1953) [1]. A certain component of this corpuscular radiation is obviously identical with that generating geomagnetic storms.

It is proposed to discuss the role of the radio-frequency emission in the whole set of the non-thermal emissions of the sun, which originate in the solar corona and the uppermost regions of the chromosphere. Of these, the radiative emissions give probably an amount of between 104 and 105 ergs/cm.2 sec., to which the radio-frequency region contributes only very little (even during an intense outburst, when the total radiative emission is much larger, not more than 10°–102 ergs/cm.2 sec.); the contribution of the lower chromosphere, however, is not yet well known. The corpuscular emissions under normal conditions seem to require ≈ 105 ergs/cm.2 sec. (but again much more in active, e.g. ‘M’, regions), and to constitute a normal feature of the outer solar corona. These emissions, we propose, are maintained by the same supply of mechanical energy which secures the thermal and radiative equilibrium of the inner corona. That is to say, some part of the flux of acoustic energy originating in the hydrogen convection zone and, according to this theory, heating the upper chromosphere and the inner corona, is believed always to reach the outer corona, where the radiative loss is smaller, and this part is then at least comparable with that dissipated in the inner corona.*

The shift of the emitted frequencies towards lower frequencies during a solar outburst is usually interpreted as due to a progressive rarefaction of the emitting gas. If one assumes that the emitted frequency is identical with the plasma frequency and furthermore that the density of the emitting plasma is similar to the density of the solar corona at the location of the radiating material, then it follows that this material is subject to an acceleration throughout the solar corona which compensates or exceeds the effect of the gravitational field of the sun.

The polarization observation of solar radio emission at 200 Mc./s. was started in December 1954 at the Tokyo Astronomical Observatory with a new type of radio polarimeter[1, 2]. With a pair of crossed dipoles at the focus of a 10 metre paraboloid of equatorial mounting, it gives almost simultaneously six components of polarization, two circular and four linear, on a time-sharing basis by using electronic switching. Fig. 1 shows the block diagram of the circuit and the combination of modulators and demodulators. 6AS6 pentodes are used as the time modulators. The duration of each gate pulse is 1/1600 sec. and one set of observations is obtained in 1/200 sec. One of the principal features of the present scheme is to use a common receiving system for all the components in order to avoid inevitable errors due to differences or changes in the gain and in the central frequencies of different receivers. A new type of square-law detector is employed in the present system [3]. It has rapid response time and excellent stability.

Observations of the radio emission from the sun carried out during recent years at the Crimean Station of the Physical Institute of the U.S.S.R. Academy of Sciences showed that the occurrence of spots appreciably increases the intensity of the solar radio emission in the range of metre wave-lengths. This increase of intensity has two components. The first (S-component) changes comparatively slowly with time. The second (P-component) consists of individual brief bursts (of the order of a second and less) of small amplitudes (10–100 % of the intensity of the quiet sun). The P-component is manifested most clearly in the emission connected with spots of small areas, when the general increase of the intensity is insignificant. Such a situation has been utilized for the study of the spectrum of individual small peaks.

Some 6 km. of high-speed (Brush) records of the solar radio radiation at 200 Mc./s. made at Cornell University by Ch. L. Seeger in 1949 and 1950 have been kindly put at our disposal. They were made with a paper speed of 0·5 cm./sec., sometimes with 2·5 cm./sec. The response time of the recording pen was 0s·02. Some records have been made simultaneously at 205 Mc./s. and at 200 Mc./s. This paper gives some results of a discussion of these records. The properties that were studied are: the clustering tendencies among the radio-pips, their band-widths, the echo-phenomena and the duration of the pips.

Some authors (Payne-Scott and Little (1952) [1]; Owren (1952) [2]) have mentioned a phenomenon in the enhanced solar radio emission which they call ‘non-selective fading’. The present paper is meant to call attention to another, rather peculiar, type of scintillation in the radio emission of the sun, which differs from the non-selective fading in some important respects. This scintillation has been observed since 1952 by the division ‘Ionosphere and Radio Astronomy’ of the Netherlands Telecommunications Service in the course of a continuous survey of solar radio radiation. It has been found at the radio frequencies 140, 200 and 545 Mc./s.

We are now in an era of remarkable progress in the study of meteors. The electronic techniques developed at Jodrell Bank by Lovell, Clegg and others, and now by Davies, have culminated in giving us the power to observe the equivalent of 8th or 9th magnitude meteors for velocities, radiants and orbits. And certainly the limit has not yet been reached. The Harvard Super-Schmidt meteor cameras represent nearly the limit of current photographic-optical techniques; they approach close to the visual limit for very slow meteors, and with great precision, to 0·1 % in velocity and radiant.

A survey of sporadic meteor orbits has been made using a spaced receiver technique. The apparatus consists of three receivers spaced on the ground by about 4 km. The received signals are brought together by radio links and displayed on a single film. In this way both diffraction pattern and the time delay between the diffraction patterns received at the different stations can be measured. From this the velocity and the direction of motion of the meteor can be calculated. The range of the echo and the local sidereal time are also recorded on the film.

Kaiser (1953) [1] has shown how it is possible, from statistical studies of radio echoes from meteor trails, to obtain values of the exponent s in the assumed equation for the number of meteors n(α) of line density α where αm is the maximum electron line density (number of electrons per cm.) produced in the trail of a meteor. If the simple theories of the meteor ionization process are correct, αm is proportional to the meteor mass m, so that from radio echo studies it should be possible to obtain a measure of the meteor mass distribution: where n(m) is the number of meteors with mass in the range from m to m + dm.

Records obtained at a declination of about + 22° during the first quarter of 1955 with the 22·2 Mc./s. Mills Cross of the Carnegie Institution of Washington occasionally exhibited an interference-like event which was apparently a function of sidereal time. A plot of the right ascensions of beginning and end of each event (duration roughly fifteen minutes) against the date of occurrence revealed a smooth change of Right Ascension corresponding, initially, to a westward motion (Fig. 1). This pre-cluded correlation of the event with passage through the pencil beam of fixed objects like the galactic cluster NGC2420 and the planetary nebula NGC2392 which otherwise would have been candidates. The retrograde motion suggested a planet, and a canvass of the solar system uncovered only Jupiter and Uranus as possibilities. Of these, Jupiter exhibited the same position and the same change of position as did the event recorded, while Uranus was well out of the pencil beam much of the time. It was therefore concluded that the source of the radio emission was associated with Jupiter.

A search of old records of cosmic noise, taken in the Radiophysics Laboratory at 18·3 Mc./s., has confirmed the discovery, by Burke and Franklin [1], of radio radiation from Jupiter, and has further shown that in 1951 the radio radiation came from a very localized region of the planet.

In the period 1955–57 a number of occultations of two radio sources 05·01 Taurus and 06·01 Gemini by the Moon takes place [1]. They will give an excellent opportunity to examine not only the exact position and the shape of these sources but possibly also some traces of the lunar atmosphere [2].

Observations were made at Cambridge on 26 April 1955 of the lunar occultation of the large-diameter radio source in the constellation of Gemini. This radio source, having r.a. 06h 13m 37s and dec. 22° 38′ (1950·0), has been identified by Baldwin and Dewhirst (1954) [1] as the galactic nebulosity IC443, which consists of a filamentary structure contained within a circular region of 24′·5 radius. Baldwin and Dewhirst also succeeded in measuring the distribution of radio ‘brightness’ across the source using an interferometric method; their measurements indicate that the diameter of the radio source is approximately the same as that of the visible nebulosity.

A theoretical study of the integral radio emission of the moon, measured at the wave-length of 3·2 cm. (Zelinskaja and Troitzky[1]; Kajdanovsky, Turusbekov and Khaikin[2]), was carried out at the Gorky radio astronomical station ‘Zimenky’ and at the Physical Institute of the Academy of Sciences of the U.S.S.R. The following expression for the average radio temperature of the entire lunar disk, as a function of the lunar phase, Ωt, was obtained (Troitzky, 1954) [3]: Here tan ξ = δ/(1 + δ) and δ = β/κ, where β is the attenuation coefficient of the thermal wave, κ the power attenuation coefficient of the radio wave. Further, Tm = 374°K. is the temperature of the subsolar point, Tn is the temperature at the lunar midnight, Θ = Tm – Tn and k0 is the reflexion coefficient of radio waves for vertical incidence (k0 ≈ 0–1). The numerical coefficients in equation (1) were obtained as a result of averaging the Fresnel reflexion coefficients over the whole disk. The degree of polarization of the total radio emission was calculated and was found to be about 4 %.