Synchrotron evidence for Amalthea's influence on Jupiter's electron radiation belt

Abstract

High‐resolution radio images of Jupiter at wavelength λ = 20 cm obtained with the Very Large Array (VLA) in June 1994 (a few weeks before comet Shoemaker‐Levy 9 collided with the planet) are compared with detailed model calculations. All major features of the radio emission can be explained or simulated through model calculations. In particular, we infer how the pitch angle distribution of high‐energy electrons varies with L value. The electron pitch angle distribution seems to undergo a dramatic change at Amalthea's orbit: A fraction of the electron population is redistributed in pitch angle (isotropized) there, so that fewer electrons mirror near the magnetic equator and more electrons mirror off the equator at L ≲ 2.65 than beyond. The isotropic component leads to the high‐latitude emission regions, while the decreased number of equatorially mirroring electrons results in a “shoulder” or flattening in the radio intensity pattern at L ∼ 2.5, as is observed. Perhaps Amalthea's motion through Jupiter's magnetic field induces Alfvén or whistler wings or electrostatic high‐frequency waves which lead to the “observed” pitch angle scattering. Jupiter's ring absorbs 80%–100% of electrons with small pitch angles that diffuse through the region it occupies. The observed effect of this absorption is that the high‐latitude emission peaks remain distinct from the equatorial maximum. Ring absorption causes nearly all electrons at L ≲ 2 to be narrowly confined to the magnetic equator, a distribution which accounts for the east‐west asymmetry, which is very prominent at certain central meridian longitudes. The azimuthal variation (east‐west asymmetry) over a Jovian rotation is completely determined by the magnetic field configuration, as was suspected by many researchers in the past but never modeled succesfully before. We infer, however, that Connerney's O6 magnetic field model from 1992–1993 is slightly oversimplified, since the radiation characteristics cannot be completely matched at all Jovian longitudes: Deviations appear in particular at longitudes λcml ∼ 140°–180° and λcml ∼ 300°–340° (corresponding to λIII ∼ 30°–90° and λIII ∼ 210°–270° in Jovicentric coordinates).