We report the discovery with the Ulysses unified radio and plasma wave (URAP) instrument of features in the Jovian hectometer (HOM) wavelength radio emission spectrum which recur with a period about 2-4% longer than the Jovian System III rotation period. We conclude that the auroral HOM emissions are periodically blocked from "view" by regions in the torus of higher than average density and that these regions rotate more slowly than System III and persist for considerable intervals of time. We have reexamined the Voyager planetary radio astronomy (PRA) data taken during the flybys in 1979 and have found similar features in the HOM spectrum. Contemporaneous observations by Brown (1994) show an (SII) emission line enhancement in the Io plasma torus that rotates more slowly than System III by the same amount as the HOM feature. frequency portion of the radio astronomy receiver (RAR) which sweeps through 12 logarithmically spaced channels from 52 to 940 kHz every 144 s. The RAR is driven by a 72-m dipole and a 6-m monopole. Ulysses is a spinning spacecraft with the long dipole in the spin plane and the monopole on the spin axis. Electrically combining the two antennas permits URAP to measure all four Stokes' parameters (Kraus, 1986) and the direction of arrival of the observed signals. Each of the two Voyager instruments, fully described by Warwick et al. (1977), consists of two swept-frequency receiv- ers. The lower band of these pairs of receivers is of interest here. During the era when the Voyagers were in the vicinity of Jupiter, the PRA low-band receivers swept the frequency range from 1.2 to 1326.0 kHz in 70 linearly spaced steps every 6 s and measured the left- and right-hand circularly polarized power. The PRA receivers are driven by two 10-m monopoles. Primarily owing to the longer antenna system, the URAP RAR is about a factor of 10 more sensitive near 1 MHz than the PRA. The Voyager spacecraft are three-axis stabilized.
[1]
Alain Lecacheux,et al.
Direction finding study of Jovian hectometric and broadband kilometric radio emissions: evidence for their auroral origin
,
1994
.
[2]
M. Brown,et al.
The Structure and Variability of the io Plasma Torus
,
1994
.
[3]
R. Stone,et al.
Source characteristics of Jovian hectometric radio emissions
,
1993
.
[4]
M. Kaiser.
Time‐variable magnetospheric radio emissions from Jupiter
,
1993
.
[5]
P. Zarka,et al.
Source characteristics of Jovian narrow‐band kilometric radio emissions
,
1993
.
[6]
R G Stone,et al.
Ulysses Radio and Plasma Wave Observations in the Jupiter Environment
,
1992,
Science.
[7]
P. Canu,et al.
The Unified Radio and Plasma wave investigation
,
1992
.
[8]
Y. Leblanc,et al.
The Jovian hectometric radiation : an overview after the Voyager mission
,
1991
.
[9]
Is System IV independent of System III
,
1991
.
[10]
B. Sandel,et al.
Dual periodicity of the Jovian magnetosphere
,
1988
.
[11]
F. Roesler,et al.
Periodic intensity variation in [SIII] 9531A emission from the Jupiter plasma torus
,
1984
.
[12]
Lou‐Chuang Lee,et al.
A theory of the terrestrial kilometric radiation
,
1979
.
[13]
A. C. Riddle,et al.
Planetary radio astronomy experiment for Voyager missions
,
1977
.
[14]
L. Brown.
Spectral behavior of Jupiter near 1 MHz
,
1974
.
[15]
M. Desch,et al.
Dekametric and hectometric observations of Jupiter from the RAE-1 satellite
,
1974
.