The short wavelength spectrography on the IUE satellite was used to obtain spectra of the plasma torus near the orbit of Io about Jupiter. Three exposures of about 8 hours each taken in March and May 1979 show emission features due to SII, SIII, and OIII. The absence of features at other wavelengths permits upper limits to be other species in the torus

Atreya, S. K.

Clarke, J. T.

Lane, A. L.

Moos, H. W.

English

NASA Technical Reports Server

OBSERVATIONS OF THE Io PLASMA TORUS
H. W. Moos and J. T. Clarke
Physics Department, The Johns Hopkins University, Baltimore, MD 21218
\
S. K. Atreya
Department of Atmospheric & Oceanic Science
University of Michigan, Ann Arbor, MI 48109
A. L. Lane
Jet Propulsion Laboratory, California Institute of Technology
Pasadena, California 91103
ABSTRACT
The short wavelength spectrograph on the IUE satellite has been used to
obtain spectra of the plasma torus near the orbit of Io about Jupiter. Three
exposures of about 8 hours each taken in March and May 1979 show emission
features due to SII, Sill, and 0111. The absence of features at other wave-
lengths permits upper limits to be set on other species in the torus.
INTRODUCTION
It is now known that ionic species are trapped in the orbital path of Io.
The basic picture is that atoms ejected from the satellite are ionized and
thus trapped on Jovian magnetic field lines. The field lines move with the
i0 hour planetary rotational period, spreading the ions into a toroidal-like
cloud along the orbital path. The only identified ionic emissions are those
of sulfur and oxygen. However, a number of species have strong lines in the
spectral region covered by the short wavelength spectrograph. For this
reason, three observations of the Io torus were made near the times of the
Voyager i and Voyager 2 flybys (5 March and i0 July 1979).
OBSERVATION DETAILS
Large aperture, low dispersion spectra of the torus were taken with
the SWP camera on i March 1979, 3 March 1979 and 19 May 1979 (SWP 4448, 4463,
5302). The aperture, aligned approximately perpendicular to the orbital
plane, was pointed between 3 and 6 Rj. The exposure times were 520, 445 and
440 minutes respectively.
Due to the long exposure times, the spectra contained in addition to
readout noise, noise spikes due to fast particle hits, radioactive spots or
blemishes on the camera face and geometry correcting reseaux marks. Running
a median filter perpendicular to the dispersion direction before adding the
line by line spectra was found to significantly reduce these noise spikes.
The photometric error in SWP data during this time period has been corrected
using the standard algorithim (IUE Newsletter #8). The three spectra were
then summed to produce Fig. i.
49
https://ntrs.nasa.gov/search.jsp?R=19810017364 2020-03-21T12:27:58+00:00Z
To determine the baseline and noise signal, the averagesand standard
deviations were computed for the 1278 - 1376 _, 1426 - 1651 _, and 1776 -
1951 _ wavelength regions. The baseline varied (3121, 883 and 5030 FN
respectively). The standard deviation was approximately constant with an
average value of 3163 FN.
DISCUSSION
Fig. 1 shows the sum of the three observations. The features at _256 9
and 1198 X are identified as SII% 1257 and SIII% 1199. The 1729 _ feature
has not been identified. There is a persistent blemish which produces a hit
like feature near this wavelength in lengthy SWP exposures; however when this
is removed either by hand or by a median filter perpendicular to the
dispersion, the feature remains in the torus spectra. In addition, all three
photowrite images show a distinct image of the entrance aperture indicating
that the 1729 A feature is real. The feature at 1664 _ is identified as
OIII% 1664. However, the peak is only 10% above the 3_ noise level.
Therefore, although statistically significant the identification must be
considered marginal. Although the apparent peak at 1397 _ is close to the
expected SIV% 1406, it may be due to a shift in the background level near
this point. In any case, a statistically significant signal does not exist
at 1406 _.
Table 1 lists a series of ionic species, in order of decreasing cosmic
abundance, with strong emission lines in this spectral region. The measured
brightness of the three identified species are listed. For the other
species the upper limit to the brightness was determined from three times the
standard deviation and the spectrograph sensitivity.
To estimate the number densities, a simple homogenous torus model
similar to that used by Broadfoot et al (ref. i) was used. (In this case the
torus was centered on the orbit rather than the magnetic equator.) Although
it is likely that the emission structure was quite complex, these IUE spectr_
were averaged over most of a rotational period; thus, the simple model may be
appropriate_ The number densities were computed using an electron density of
2 x 103 cm-j (ref. 2) an electron temperature or i0 eV and an average path
length of 7.3 Rj. Also shown in Table 1 are the number densities reported
by the Voyager 1 UVS group. These values are based on short wavelength
transitions observed by the Voyager 1 extreme ultraviolet spectrograph and a
model similar to ours (ref. i). The most serious discrepancy is in the case
of OIII where the IUE value is one sixth the Voyager value. The uncertain-
ties in the number densities depend directly on those of the excitation
coefficients used. A more detailed discussion of the data analysis and the
sources of the excitation rates will be published at a later date.
We thank the IUE Observatory staff for their aid in the acquisition and
reduction of the data. This work has been supported by a grant from NASA.
50
REFERENCES
i. Broadfoot, A. L. et al 1979 Science 204, 979.
2. Bridge, H. S. et al 1979 Science 204, 987.
51
Tableol
OBSERVED IONIC SPECIES AND UPPER LIMITS
IUE Voyager I
Species X Brightness number number
(_) (Rayleighs) density density _
cm - (cm-3)
0 III * 1664 12 * 140 850
C II 1335 6.7 4.2
C III 1909 8.5 8.6
N III 1750 9.4 50
N IV 1485 i0 18
Si II 1264 6.7 2.0
Si III 1892 8.5 7.9
Si IV 1395 7.9 I i.i
S II * 1257 43 * 140
S III * 1199 30 - 60 * 160 - 320 95
S IV i406 8.2 49 55
AI II 1671 ii 0.22
A1 III 1856 8.4 0.72
Ca IX 1839 8.4 47
P II 1308 6.4 12
PIII 1344 6.7 17
t
Reference 1
* Observed: The brightness value is a measured one rather than an upper limit.
52
Io Plosmo Torus
(Sum of 3 Exposures)
,,o--,,I,o,,Lo,_1,o,,_o,,Lo,,Lo.J,o,,& ,_o ._Lo,,Lo.,Lo,.Lo,oko,_1,o,,Lo,ol,o
I_V(L(NO|H IN RNGSTROflS
Figure 1


Observations of the Io plasma torus

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server