Radio astronomy has been expanding into outer space so fast in recent years that it is pleasant to find our own solar system at last receiving the attention it deserves. In this session we are concerned with everything within the system except the sun and our own planet. I start with a question, to which I shall return later: Where does the sun end? In another session you will hear of the experiments on the far-out parts of the solar corona; here we are concerned with interplanetary space as well as with the planets themselves, and what lies within this region may or may not be considered part of the solar corona.

In recent years the way in which the moon reflects radio waves has been the subject of much study. Observations by Yaplee [1] at 2860 Mc/s, Trexler [2] at 198 Mc/s, Evans [3] at 120 Mc/s, and others show that radio waves are reflected principally by a small region at the center of the moon's visible disk which has a radius of the order of one-third of the lunar radius.

This note describes some of the initial results derived from observations of radar echoes from the moon obtained at 10-cm wavelength at the Royal Radar Establishment, Malvern. The radar, which has been described elsewhere [1], has a transmitter of 2 megawatts peak power with a pulse length of 5 microseconds and a pulse recurrence frequency of 260 per second. The receiver is of conventional design and has a bandwidth of 500 kc/s and a noise factor of 7.5. The aerial used is the 45-foot diameter radio telescope shown in Fig. 1. The telescope is controlled from a mechanical computer that converts the lunar coordinates into azimuth and elevation, which are then fed into a servo drive.

In February 1957 the U.S. Naval Research Laboratory was successful in obtaining radar echoes from the moon by using a short-pulse radar [1]. Although the signal-to-noise ratio was not as large as desired, the leading edge of the echo was sufficiently well defined to make possible an accurate measurement of the radar distance to the moon. It was hoped that geodetic information could be extracted from such distance measurements. Hence in October and November, 1957, the radar distance to the moon was measured with a radar mounted in the 50-foot steerable parabola at NRL.

In recent years many sets of experimental data have been obtained on radar reflections from the moon. After analyzing the results, workers tried to determine the scattering properties of the moon at radar wavelengths, and they concluded that rough-scattering laws could be ascribed to the surface. The available evidence no longer appears to support this view.

Observations of a lunar occultation of a radio source may provide information concerning both the distribution of radio “brightness” across the source and its accurate position. For sources of which these results are already fairly well known, observations at long wavelengths may be used to derive the density of the lunar atmosphere [1]. During recent years two such occultations have been observed at Cambridge: one, the occultation of IC 443, the large-diameter radio source in the constellation of Gemini, from which the density of the lunar atmosphere was estimated to be less than 10–12 of that of the density of the terrestrial atmosphere [2] and [3]; and two, the occultation of the Crab nebula on 1956 January 24 [4].

Mayer, McCullough, and Sloanaker [1] and Drake and Ewen [2] have measured centimeter wavelength radiation from several of the planets; the former at 3.15 and 9.4 cm, the latter at a wide bandwidth centered near 3.75 cm. The first measurements were the observations of Venus by Mayer et al., made near inferior conjunction in late spring 1956. These were obtained with a sufficiently favorable signal-to-noise ratio to permit direct recognition of Venus in individual diurnal-rate scans, and a brightness temperature of 560 ± 73°K (mean error) was deduced for the planet at inferior conjunction, i.e. for the dark side of Venus.

Observations at Yale of planetary nonthermal noise were first made in 1957 from March to June at 21.1 Mc/s by a single close-spaced phase-switching interferometer with Yagi elements. To be accepted as possibly planetary, noise events had to be unidentifiable as terrestrial; they had to fit the interferometer pattern; and they had to occur when the planet was in the Yagi beams. Thirteen events satisfied these criteria for Jupiter (Fig. 1a), giving the rotation histogram Fig. 2a when plotted according to the System III period of Carr et al. [1]. The pattern is quite similar to that obtained for the immediately preceding months by the Florida group (Fig. 2b).

Observations were made at Cambridge between 1957 March 12 and May 13 in an attempt to detect radio emission from the comet at 3.7- and 7.9-m wavelengths by observing radio stars through its tails, to derive information on the electron density in the manner used by Hewish [1] in investigating the outermost layers of the solar corona.

Les recherches récentes [1, 2, 3] ont permis d'établir l'existence des émissions planétaires dans le domaine des ondes radio, dont les sources sont plus ou moins localisées sur les surfaces planétaires. En relation avec ces observations on peut aussi étudier les ionosphères planétaires [5]. Dans ces études on jouit d'un certain avantage de pouvoir se baser sur nos connaissances relativement avancées de l'ionosphère terrestre et nous en profiterons également dans ce rapport.

The largest steerable radio telescope in the United States (Fig. 1) has recently been completed at the U.S. Naval Research Laboratory's Maryland Point Observatory in Charles County, Maryland. The new instrument, an 84-foot parabolic antenna, or “dish,” was designed by the D. S. Kennedy Co. of Cohasset, Massachusetts. Originally ordered in June 1955, the instrument has been three years in design, construction, and erection. Carried on a polar or equatorial mount, the 84-foot aluminum reflector can be aimed at any point in the sky and can track any celestial object from horizon to horizon. It is the largest antenna with this type of mounting in the world.

The new types of low-noise radio-frequency amplifiers now under development hold great promise for improving and extending radio astronomical observations. A solid-state maser, designed and built at Columbia University, has been used with the U.S. Naval Research Laboratory's 50-foot reflector since 1958 April. The maser is the 3-level type [1], with ruby [2] as the paramagnetic medium. Initially, an rms radiometer output fluctuation of about 0.1 °K was realized with a 5s output time constant. Subsequent improvements of the maser and of the associated circuitry have made it possible to observe with an rms output fluctuation of 0.04 °K for a 5s output time constant. These figures represent improvement factors of about 5 and 13 respectively over the radiometer without the maser. These significant improvements in sensitivity have made possible continuum observations which could not previously have been made with the NRL apparatus except by unwieldy averaging techniques.

The preceding papers have demonstrated the remarkable development of the radio-astronomical investigations of the solar system during the last few years. The subject of meteors has been intentionally excluded, since this is now far too extensive to be included in a symposium of this kind. The radio echo, or radar, techniques have now been extended to reach the moon, and a further extension to the nearer planets can be anticipated soon. In the meantime, the study of the thermal and nonthermal radio emissions from the planets is developing rapidly, and important new results have been presented here. It will be convenient first of all to consider the implications of the new work on the radio echo investigations of the moon.

Si les émissions radioélectriques du soleil sont dépourvues de la richesse d'informations apportée par les spectres optiques, elles se révèlent, par contre, particulièrement bien adaptées à l'étude d'une très grande variété de perturbations solaires, dont bien souvent les observations optiques n'avaient même pas permis de soupçonner l'existence, et ceci pour plusieurs raisons:

1. Ces émissions, souvent transitoires, intéressent particulièrement la couronne, ou la haute chromosphère, où les observations optiques sont discontinues et difficiles en raison de l'éblouissement par le disque photosphérique qui n'existe pas en radio.

2. Certaines ne sont pas d'origine thermique: le plasma coronal possède diverses possibilités de rayonnement (oscillations de plasma, rayonnement synchrotron ou Čerenkov) plus aisément ou seulement perceptible dans le domaine radioélectrique.

Les sources solaires d'émissions radioélectriques sont multiples et se distinguent essentiellement par leur stabilité plus ou moins grande et la nature de leur rayonnement; pour le moment on ne possède pas beaucoup d'indications sur leurs dimensions, encore moins sur leur structure.

The outer part of the sun consists of two main regions: (1) the convection zone, which extends from a depth of 105 km below the visible surface to 100 or 200 km below the surface, and in which the temperature decreases from about 106 °K near the lower limit to 6000 °K near the surface; and (2) the corona, which starts about 5000 km above the surface and extends very far, up to many solar radii, and in which the temperature is 106 °K in the low parts.

During the past few years various chromospheric models based on optical and ratio data have appeared in the literature. They exhibit an overwhelming tendency to recognize the chromosphere as a heterogeneous medium departing markedly from spherical symmetry. Beyond this general trend, however, their similarity all but vanishes. Perhaps the reason for this dissimilarity lies in the complexity of nonspherically symmetric models. The simpler ones, based on the assumption of spherical symmetry, however, show an equal diversity. It seems, therefore, that the diversity must be attributed to a lack of basic understanding of chromospheric phenomena.

The work of Ginzburg, Shklovskiĭ, and Martyn (1946) showed that the upper layers of the solar atmosphere (chromosphere and corona) are responsible for the sun's thermal emission in the radio-emission range. This was confirmed by a number of observations.

An attempt has been made to measure the one-dimensional brightness distribution of the sun at 88-cm wavelength. Strip scans of the sun made with a 4′.8 fan beam have been superimposed and the lower envelope drawn, after the manner of Christiansen [1].

Two main mechanisms have been proposed to explain the origin of the slowly varying component [1] of the decimeter-wavelength radiation from the sun. Both theories [2, 3, 4] are based on the facts that (a) this component of the sun's radiation is relatively steady in output for periods of days; (b) it originates from active regions on the sun; and (c) its magnitude is closely related to the sunspot area in the region.

Beginning in summer 1957, W. N. Christiansen and D. S. Mathewson [1] have regularly obtained two-dimensional radioheliograms at λ = 21 cm with a resolving power of 3 minutes of arc. The authors have already noticed the close connection between the regions of radio emission on the one side and the fields of spots and faculae on the other side. Considering, however, that the radio emission at the wavelength involved emerges from heights of 20,000 to 50,000, or even up to 100,000 km above the photosphere, i.e. from the inner corona, it seems to be more suitable to compare the radioheliograms with the optical emission of the corona than with the photospheric and chromospheric phenomena. Yet, as the coronal observations can be carried out at the solar limb only, it is difficult to get optical pictures of the corona in front of the sun's disk. Such a picture has to be built up from the daily limb-observations covering the period from 7 days before to 7 days after the date in question; e.g. the coronal intensities shown along the central meridian are measured 7 days before at the E-limb or 7 days later at the W-limb. Since the corona may change greatly within a few days—especially during the present high solar activity—the reliability of an optical corona-picture diminishes from the limb to the central meridian. In addition, a further uncertainty has to be considered in constructing coronal maps, in so far as no station possesses complete coronal observations; therefore, observations of different stations have to be used, which are very difficult to reduce to each other. The main difficulty of such a reduction arises from the fact that the single stations carry out their observations at different distances from the sun's limb ranging from 20,000 to 45,000 km. The coronagrams discussed in the following are based on the intensities of the green coronal line 5303 Å as published in the Quarterly Bulletin on Solar Activity [2].