On Io's excitation and the origin of Jupiter's decametric radiation.

Details On Io's excitation and the origin of Jupiter's decametric radiation. On Io's excitation and the origin of Jupiter's decametric radiation.

Oct 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...290..660l&link_type=abstract

Astronomy and Astrophysics 290, 660-673 (1994)

Astronomy and Astrophysics

Astrophysics

20

Planets And Satellites: Jupiter, Io, Radio Continuum: Solar System

Scientific paper

A new method of deriving the source locations in three dimensions of Jupiter's decametric radio emission is presented. We use the observed elliptical polarization state and the derived axial ratios T of 33 Io-related Jupiter events to determine the angle of emission θ at the source, using the theoretical result that cos θ=T. Knowledge of this angle of emission, and that the radiation is emitted from very near L=6 at the gyrofrequency level, allows us to specify the source location in three dimensions with the aid of a model of Jupiter's magnetic field. This source location is then compared with Io's position, i.e. the location of the instantaneous Io flux tube (IFT). The derived source longitudes λ_active_ coincide with the longitudes of the IFT λ_Io_ only for the Io-A and Io-C RH sources. For Io-D and Io-B (LH and RH) sources λ_active_ is downstream of Io, by as much as 80deg for Io-B LH. However, λ_active_ is found to be about 30deg upstream for the Io-C LH source. With respect to the observer, λ_active_ remains at about the same position from the start to the end of each event. This is despite the fact that the magnetic field rotates considerably and Io revolves significantly. During about half of the 12.95 hr cycle of Jupiter's rotation with respect to Io, when the northern end of the magnetic dipole is in Io's direction, both LH and RH emissions are activated. The excitation by Io is continuous for all this period, with a maximum when the dipole is toward Io. Near the extremities of the range, LH radiation tends to be predominant, while near the center, RH radiation is predominant. The first situation corresponds to times when Io is south of or near the center of the Io torus, and the second to when Io is near the northern edge of the torus. A model of Io-related radiation is proposed where disturbances triggered by Io propagate upward and downward towards the northern and southern hemipheres, reaching the Jovian polar regions where the DAM radiation is generated. To explain the fact that Io-controlled excitation is not confined to a narrow region close to Io's IFT suggests that direct acceleration of electrons by Io is not the sole source of excitation. An extended source of excitation downstream of Io may be produced by multi-bounces of Alven waves. However, the occurrence of emission at locations that are as much as 30deg upstream of Io is not understood.

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