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RECEIVED
NASA ST! FAC!,_!TY
ACCESS DEPT.
t
r%V%SA
Technical Memorandum 80287
Voyager 1 Planetary Radio Astronomy
Observations Near Jupiter
(NASA-T"1-80287)
VOYAGFR 1 PL.ANRTARY RAT)i0
N79-29101
ASTRONOMY 08SFRVATIO)NS NEAR .IIJPTTER (NASA)
16 p HC A02/
1
1F 401
CSCL 03P
'Inctas
G 3/91 29941
J. W. Warwick, J. B. Pearce, A. C. Riddle,
J. K. Alexander, M. D. Desch, M. L. Kaiser,
J. R. Thieman, T. B. Carr, S. Gulkis,
A. Boischot, C. C. Harvey and B. M.-Pederson
MAY 1979
Nat ionn' .4ercnautics and
Space !4rmin,stration
0
VOYAGER 1 PLANETARY RADIO ASTRONOMY OBSERVATIONS NEAR JUPITER
J. W. Warwick, J. 8. Pearce, A. C. Riddle
Laboratory for Applicd Plasma Studies
Science Applications, Inc.
Boulder, CU 80302
J. K. Alexander, N. D. Descn, M. L. Kaiser, J. R. Thieman
Laboratory for Extraterrestrial Physics
Goddard Space Flight Center
Greenbelt, Msryland 20'/71
T. D. Carr
Department of Physics and Astronomy
University of Florida
Gainesville, Florida 32611
S. Gulkis
Jet Propulsion Laboratory
Pasadena, CA 91103
A. boischot, C. C. Harvey, B. M.-Pederson
Observatoire de Paris
Section d'Astrophysique e.e Meudon
92190 Meudon, Fiance
Pre,
,
::nt; to appear in
,Science
1
LV
We report results from the first low frequency radio receiver to be
transported into the Jupiter magnetosphere. We obtained dramatic new
information, both because Voyager was near or in Jupiter's radio emission
sources and also because it was outside the relatively dense solar wind
plasma of the inner solar system.
Extensive -adio spectral arcs, from
above 30 MHz to about 1 MHz, occurred in patterns correlated with
planetary longitude.
A newly discovered kilometric wavelength radio
source may relate to the plasma torus near Io's orbit. JL
Zlli.si
wave
resonances near closest approach define an electron density profile along
the Voyager trajectory and form the basis for a map of the torus. Many
detailed studies are in prog-ess and are outlined briefly.
Low frequency radio emissions from Jupiter have been observed from
Earth for nearly three decades. Thesc emissions showed that Jupiter had a
strong magnetic field, that highly variable and intense waves were
omnipresent near Jupiter, and that the satellite Io interacted strongly
with Jupiter's magnetosphere. Nevertheless, the emission mechanisms, the
locations of the sources, and many detailed propertiFF of the magneto-
a
nheric plasma are still unknown. The Planetary Radio Astronomy (PRA)
experiments on the Voyager 1 and 2 spacecraft were designed to study the
low frequency radio emissions from Jupiter both at a distance and
ip situ.
the purpose of this paper is to present selected results from the Voyager
1 PHA experiment near Jupiter closest approach on 5 March 1979.
The PRA instrument (1) consists of a radio receiver that steps in
frequency from 40.5 MHz to 1.2 kHz and an orthogonal pair of 10-meter
monopole antennas connected to provide right hand (RH) and left hand (LH)
polarization. The receiver is a superheterodyne in each of two bands,
from 1.2 MHz to 40.5 MHz (HF ) and from 1.2 kHz to 1.3 MHz ( LF) , respec-
ti4^ly. In six seconds the receiver step tunes at intervals of 307.2 kHz
and 15 2 kHz through the HF and LF ranges, respectively; the corresponding
bandwidths are 2U0 kHz and 1 kHz.
Each step alternates in polarization,
'd
2
from kH to LH, or vice versa. The receiver operated in several other data
modes under Voyager computer control, but most of the results reported
here utilized the stepping mode.
In the HF bar
e
'. the most striking observation is the ubiquitous
presence of nested families of arcs on the frequency-time plot (Fig. 1).
Almost without exception, all of the observed emissions resolve into arcs
or portions of arcs. A considerable fraction of this frequency range is
covered in ground-based observations of Jupiter, but these arcs have not
been seen before. The reason undoubtedly lies in the strong comT,uni-
cations interference and ionospheric effects that typically plague almost
all radio observations made at frequencies lower than 20 or 25 MHz. Above
20 MHz, only the high frequency portion of the greatest arcs would be
visible from the ground. The sense of frequency drift at high frequencies
is a function of Jupiter longitude. The same functional relationship has
previously been observed from the ground. The greatest arcs, covering the
widest frequency range, over 30:1 at maximum, are consistently right
handed in polarization. The sense of curvature of the arcs reverses near
longitudes 20
0
and 200
0
close to south and north dipole tip.
Below 12 MHz, emission appears quasi-continuously at all Jupiter
longitudes. The curvature of the arcs whose vertices are in this range is
larger than those at higher frequency. The polarization appears to vary
with longitude, in the sense that RH waves are most common when the
northern tip of Jupiter's magnetic dipole is tilted toward the spacecraft,
and LH, when the southern tip is.
Some of the arc structure extends into the LF band, although because
our receiver functions here in a greatly expanded frequency scale, and
with much greater sensitivity, it is more difficult to recognize. Voyager
first detected Jupiter in this band in late 1977 (2).
It
iE
very striking that near closest approach the emissions in all
bands changed dramatically in character. Below 1 MHz, where the local
plasma frequency may exceed the observing frequency, the changes may be
associated with wave propagation effects. Above 10 Mliz, however, another
3
a