ASTRONOMY

B. G. KUTUZA, B. Ya. LOSOVSKII, A. E. SALOMONOVICH

RADIO EMISSION OF SATURN AT A WAVELENGTH OF 8 mm

(Presented by Academician V. A. Kotel’nikov on 26 XI 1964)

Radio-astronomical studies of Saturn are of great interest. There are grounds to suppose that this planet, like Jupiter, is a source of magnetic bremsstrahlung, which indicates the presence of a radiation belt around Saturn. At the same time, the suggestion has been made (¹) that Saturn’s dust ring prevents the formation of a radiation belt as intense as Jupiter’s (²).

The possibility of detecting radio emission from Saturn that differs from the thermal emission of its surface or atmosphere is connected with carrying out reliable measurements of the spectrum of the planet’s radio emission in the range of superhigh frequencies. However, Saturn’s remoteness and its relatively low radiometric temperature make such measurements difficult. Until now, results of measurements of Saturn’s radio emission have been published only at wavelengths of 3 cm (³) and 10 cm (¹).

Fig. 1. Results of averaging 24 records of Saturn’s transit on 22 VII 1964 and 12 records of Saturn’s transit on 21 VIII 1964. Along the abscissa axis—the angular distance in the image plane; along the ordinate axis—antenna temperature. The dashed curve is the approximating curve.

In July–August 1964 we carried out measurements of Saturn’s brightness temperature in the millimeter range. The measurements were made with the RT-22 radio telescope of the Lebedev Physical Institute using an ordinary modulation radiometer at a wavelength of 8 mm. To eliminate errors in determining the antenna parameters, recordings of Jupiter’s radio emission were also made at the same time; its brightness temperature, referred to the optically visible disk in accordance with (⁴), was taken to be 144° K. Under conditions of optical visibility, records were made of the transits of both planets in azimuth, with visual tracking in zenith distance.

Using observations of Jupiter, the electrical axis of the radio telescope was first aligned with the optical axis of the guide. When averaging a series of records, allowance was made for attenuation in the Earth’s atmosphere, as well as for a decrease in the output signal (but not for broadening of the transit curve) due to the effect of the radiometer time constant \((^5)\). The calibration of the antenna temperature and monitoring of the constancy of the gain were carried out with the aid of a gas-discharge noise generator.

The averaged transit records of Saturn were approximated by the method of least squares using transit curves obtained on the same days for Jupiter, analogously to how this was done in \((^6)\).

In all, 36 transit records of Saturn were used for the reduction, obtained on 22 VII and 21 VIII 1964. The result of averaging 24 records from 22 VII and 12 records from 21 VIII is shown in Fig. 1. The arithmetic mean values of the brightness temperature referred to Saturn’s disk (without the ring) were, respectively, 129 and \(144^\circ\) K. The root-mean-square random errors of these values were, respectively, 10 and \(20^\circ\). The weighted mean (with allowance for random errors) value of the brightness temperature of Saturn at a wavelength of 8 mm was

\[ T_{\mathrm{b}} = 132 \pm 9^\circ \mathrm{K}, \]

where the error is the root-mean-square random error of the weighted mean.

The brightness temperature obtained by us at a wavelength of 8 mm agrees well with the results of measurements of the infrared radiation of Saturn \((^7, ^8)\), according to which its brightness temperature is \(125^\circ\) K*. Comparison with the results of Drake’s measurements at a wavelength of 10 cm \((^1)\), who obtained \(T_{\mathrm{b}} = 196 \pm 44^\circ\) K, and also with the results of Rose, Bologna, and Sloanaker \((^9)\), confirms the presence of enhanced radio emission at a wavelength of 10 cm, although the excess, which may indicate the presence of a radiation belt of Saturn, is considerably smaller than that of Jupiter. As for the brightness temperature at a wavelength of 3.45 cm, reported in \((^3)\) \((106 \pm 21^\circ\ \mathrm{K})\), this value does not contradict that obtained by us. Of course, for clarifying the nature of Saturn’s radio emission, measurements of the distribution of the brightness and of the degree of polarization of the radio emission would be of great importance.

Physical Institute named after P. N. Lebedev
Academy of Sciences of the USSR

Institute of Radio Engineering and Electronics
Academy of Sciences of the USSR

Received
23 XI 1964

REFERENCES

1. F. D. Drake, Nature, 195, 893 (1962).
2. F. D. Drake, H. Hvatum, Astron. J., 64, 329 (1959).
3. J. J. Cook, L. G. Cross et al., Nature, 188, 393 (1960).
4. D. D. Thornton, W. J. Welch, Icarus, 2, No. 3, 228 (1963).
5. А. Д. Кузьмин, А. Е. Саломонович, Radioastronomical Methods for Measuring Antenna Parameters, 1964.
6. А. Е. Башаринов, Ю. Н. Ветуховская et al., Astron. Zh., 41, No. 4, 707 (1964).
7. D. H. Menzel, W. W. Coblentz, C. O. Lampland, Astrophys. J., 63, 177 (1926).
8. E. Petit, Collection “Planets and Satellites,” IL, 1963, p. 353.
9. W. K. Rose, J. M. Bologna, R. M. Sloanaker, Phys. Rev. Lett., 10, 123 (1963).
10. F. G. Low, Astron. J., 69, 143 (1964).

* According to measurements in the region of \(10\,\mu\) \((^{10})\), the brightness temperature of Saturn is \(85 \pm 2^\circ\) K.