Multi-Frequency Radio Observations of Jupiter at Effelsberg during the SL9 Impact

Abstract

Jupiter's radio emission was monitored before, during, and after the impacts of the multi-nucleus comet Shoemaker–Levy 9 in July 1994 at the Effelsberg 100-m radio telescope of the Max-Planck-Institut für Radioastronomie. Full polarization measurements (four Stokes parameters) were recorded in near-simultaneous mode at the three wavelengths 2.8, 6, and 11 cm. A steady increase in the flux density levels at 6 and 11 cm became apparent after the first few impacts. The absolute (relative) increase in nonthermal flux density over the 7 days of cometary bombardment, referred to the standard distance of 4.04 AU, was 1.2 ± 0.1 Jy (28%) at λ = 11 cm and 1.4 ± 0.1 Jy (43%) at λ = 6 cm. The additional radio emission is partially linearly polarized (⇒ synchrotron radiation), implying a source from energetic electrons in Jupiter's radiation belts. The total flux density at λ = 2.8 cm also increased by 2.1 ± 1.0 Jy. This is attributed to synchrotron emission, albeit at a wavelength still dominated by the thermal contribution. The brightness increase is attributed to a change in the resident electron electron population by (a) an energization of the electrons due to enhanced radial diffusion or shock acceleration, and (b) a broadening of the undisturbed electron velocity distribution due to pitch-angle scattering.