Effelsberg Radio Observations of Comets
Effelsberg Radio Observations of the Great Comet Crash
When it became obvious that comet P/Shoemaker-Levy 9 would impact Jupiter in July 1994 the Effelsberg 100 m radio telescope of the Max-Planck-Institut für Radioastronomie became a key link in a world-wide network of radio telescopes to monitor this unique event.
Full polarization measurements of the Jovian radio emission were performed at Effelsberg before, during and after the impacts at three different wavelengths (frequencies): 2.8 cm (10.55 GHz), 6.3 cm (4.75 GHz) and 11.1 cm (2.695 GHz). For increasing wavelength, these observations span a spectral range where the thermal radiation from Jupiter's "warm" atmosphere decreases and synchrotron radiation emitted by highly relativistic electrons trapped in the inner Jovian magnetosphere becomes more important.
The strength of this nonthermal radio component as viewed from earth is directly correlated to the position of Jupiter's magnetic pole relative to the observer. Because Jupiter's magnetic dipole axis is tilted by ~ 10° with respect to the rotational axis, the observed radio flux density shows two characteristic maxima and minima.
Whenever the magnetic pole is directed towards or away from the observer, a minimum in flux density will be seen. Correspondently, maximum flux density is recorded when the orientation of the magnetic pole is perpendicular to the line-of-sight. This natural modulation of the flux density is called the beaming curve.
A number of authors predicted a decrease in Jupiter's radio emission due to large amounts of absorbing dust brought into the magnetosphere by the comet (Dessler and Hill, 1994; de Pater, 1994; Ip, 1994). It was a big surprise when exactly the opposite happened. Over the week of impacts Jupiter's radio brightness was observed to increase at wavelengths ranging from 2.8 cm up to 90 cm! De Pater et al. (1995) give a summary of the results from the global radio observation network at different wavelengths. Individual reports of the observations can be found for wavelengths
13 cm: Klein et al. (1995)
18 cm: Bolton et al. (1995)
13, 22, 36 cm: Dulk and Leblanc (1995), Dulk et al. (1995),
Leblanc and Dulk (1995)
20 cm: Wong et al. (1996)
and a detailed description of the Effelsberg observations at 2.8, 6 and 11 cm is given in Bird et al. (1996).
Radio emission from ammonia, a suspected parent molecule in comets, was detected from comet Hale-Bopp near its perihelion passage with the 100-m Effelsberg Radio Telescope. Signals from the five lowest metastable inversion transitions of NH3 were obtained to derive a rotational temperature of 104±30 K, assumed to be representative of the kinetic temperature in the comet's inner coma (R < 5000 km). The ammonia production rate at perihelion is calculated from these observations to be 6.6±1.3 1028 s-1 (almost two tons of ammonia per second). Compared with independently determined water production rates near perihelion, the implied ammonia abundance ratio to water is in the range 1.0-1.8%.