The Atmosphere and Magnetosphere of Jupiter

Radio emission from Jupiter was first detected in 1955 at a frequency of 22.2 MHz. This emission was sporadic in character, and confined to frequencies less than 40 MHz. It is commonly referred to as decametric radiation. In subsequent years the planet's thermal emission was detected at short centimeter wavelengths, and its synchrotron radiation at wavelengths between $\sim$ 2 cm and a few meters. The latter radiation is emitted by high energy electrons in a Jovian Van Allen belt. At present, this radiation has been observed and imaged regularly at frequencies between 300 and 8000 MHz. Jupiter's thermal emission becomes dominant at higher frequencies, and separation of the nonthermal component, although clearly present, becomes increasingly more difficult at frequencies above 8 GHz ( $\lambda < 3.6$ cm). Jovian emission at frequencies below 40 MHz (decametric range) does have a synchrotron radiation component, we believe, but this is overwhelmed by sporadic bursts of radio emission closer to the planet (Jovian DAM emission) which can reach intensities up to 10⁵ Jy. Due to strong background interference, very few, if any, observations have been made at frequencies below 300 MHz ( $45< \nu <300$), though some are planned with the VLA and WSRT arrays for the near future. With the SKA, however, the planet's spectrum can be measured from 40 MHz up to 20 GHz, simultaneously; this gives interesting prospects to determine not only its spectrum at one particular time, but also to monitor any time variations in its spectrum. Such time variability has been measured at the higher frequencies (3 - 90 cm), for instance, following the impact of comet Shoemaker-Levy 9. If the spectrum changes regularly this would contain a lot of information on the physical processes going on in Jupiter's magnetosphere, such as the interaction between electrons and dust (e.g. de Pater & Goertz 1990).

Good brightness temperature measurements of the giant planet atmospheres at the longer wavelengths ( $\nu < 1 GHz$) would provide invaluable information on the deeper layers of a planet's atmosphere. Such information can be used to provide an accurate measurement of the ammonia abundance at deep levels, and possibly of any stratification within the ammonia abundance. Although it may push atmospheric models, it is not inconceivable that information on the water abundance can be obtained for Saturn and Jupiter. Nothing really is known about this.