Long Term Variations of the Decimetric Radio Emission of Jupiter (and Saturn?)

Abstract

The Jovian total flux density at decimeter wavelengths is known to possess a short modulation due to the beaming of the synchrotron radiation of the relativistic electrons trapped in the dipole magnetic field of the planet. The maximum flux (later called I _max) occurs twice per rotation when the Earth is at zero magnetic declination. It has been a matter of dispute for many years to know whether I _max was variable over long periods of time i.e. months and years. After more than a decade of observations it is clear that the flux density of Jupiter is variable (Roberts and Huguenin, 1963; Gerard, 1970; Klein et al., 1972; Berge, 1974; Gerard, 1974, in preparation) at 21, 11 and 6 cm wavelength. At 21 cm we find that I _max measured with the E-vector along the magnetic equator is 40% lower in 1973 than what Roberts and Komesaroff (1965) found in 1962. A similar variation is observed by Berge. At 13 cm Klein et al. note a decrease of 20% for I _max between 1964 and 1971 Finally at 6 cm we find more than 10% decrease for I _max measured in 1968 (Whiteoak et al., 1969) and 1973. The smaller percentage decrease towards shorter wavelengths is mostly due to the increasing thermal radiation from the disk which contributes about 10, 30 and 60% of the total flux at 21, 11 and 6 cm respectively. The non-thermal radiation (the synchrotron emission) thus suffered a 40% decrease during the last decade i.e. solar cycle No. 20 and will be much weaker during the 1974 solar minimum than it was during the 1964 solar minimum. However the observations that we made at 11 cm (Gerard, 1970) between December 1967 and August 1968 indicate that I _max underwent variations as large as 30% during the maximum of the solar cycle and rose well above its 1964 value in January 1968. A correlation with solar activity as measured by the 10.7 cm solar flux density was also suggested with phase lags lying between 3 and 9 days. In 1971 Klein et al. (1972) failed to detect variations of I _max larger than 9% but the Sun was considerably less active than in 1967–1968.