M8: UV Observations of the Io Plasma Torus From New Horizons and Rosetta
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Transcript of M8: UV Observations of the Io Plasma Torus From New Horizons and Rosetta
M8: U
V O
bservation
s of the Io P
lasma T
orus
From
New
Horizon
s and
Rosetta
A.J. S
teffl (Sw
RI)
*, N.J. C
unningham (S
wR
I), P. D
. Feldm
an (JHU
), G. R
. Gladstone (S
wR
I),
J. Wm
. Parker (S
wR
I), K. D
. Retherford (S
wR
I), S. A
. Stern (N
AS
A H
Q)
*To w
hom correspondence should be addressed: steffl@
boulder.swri.edu
P-A
lice as an E
lectron D
etector
Though not fully appreciated until the Jupiter encounter, P
-Alice is quite
sensitive to high-energy electrons. Generally, the A
lice detector count rate is sam
pled at a rate of 1 Hz w
henever the instrument is on. A
lice count rates during the Jupiter encounter are show
n in the above plot.
At the start of the Jupiter encounter, A
lice count rates with the aperture door
closed averaged ~150 Hz. H
owever, at D
OY
56.75 (2007-02-25 18:00 UT
C)
at a distance of 67.3 RJ from
Jupiter, the count rate suddenly increased by a factor of 50. B
etween D
OY
57-60 the dark count rate often exceeded 15 kHz,
causing the instrument to safe. T
he count rate exhibited spikes when N
ew
Horizons w
as at a System
III longitude of 130° and 280°, suggesting particles in the Jovian current sheet w
ere responsible for the increase.
A com
parison with data from
PE
PS
SI (P
EP
SS
I data have been graciously provided by R
.L. M
cNutt, D
.K. H
aggerty, and the PE
PS
SI team
) showed
Alice count rates are highly correlated w
ith the energetic electron flux, as show
n in the two plots below
. Many (if not m
ost) of the Alice counts are
likely due to secondary electrons. Although no spatial or spectral inform
ation about the energetic electron distribution is available from
the Alice count
rates, it is sampled at a higher rate than either of the nom
inal particles instrum
ents, PE
PS
SI and S
WA
P.
Ab
stractD
uring the New
Horizons flyby of Jupiter in F
ebruary 2007, the Alice
UV
spectrograph obtained numerous high-quality spectra of the Io torus. T
hese spectra w
ere obtained in observations of Jupiter and the Galilean satellites, in
which the Io plasm
a torus appears as "background" emission. A
s New
Horizons
flew dow
n the Jovian magnetotail, the A
lice instrument on that spacecraft w
as not able to observe the Io torus due to solar elongation constraints. H
owever, a
nearly-identical Alice U
VS
instrument aboard the R
osetta spacecraft was able to
observe the Io plasma torus and the Jovian aurora for a total of 378 hours
between 27 F
ebruary 2007 and 08 May 2007. A
lthough there is no spatial inform
ation in the Rosetta A
lice data (since Rosetta w
as near Mars, roughly 4.2
AU
from Jupiter) these observations show
the temporal variability of the Io
torus and Jovian aurora on the timescale of days to w
eeks. We present spectra of
the Io plasma torus obtained from
both Alice instrum
ents and show a tim
e series of em
ission intensity observed by Rosetta A
lice. Com
pared to the Cassini
epoch, we find em
issions from the Io torus w
ere fainter with a relative increase
in emissions from
higher ionization states.
Th
e Alice U
V S
pectrom
etersA
lice is a light weight, low
-cost, and low-pow
er UV
Spectrom
eter. Tw
o Alice
spectrometers are presently in flight: R
-Alice aboard R
osetta and P-A
lice aboard N
ew H
orizons. Tw
o additional “Alice” spectrom
eters are being built: L
AM
P for L
RO
and UV
S for Juno.
Mass:
3 kg (R-A
lice); 4.4 kg (P-A
lice)
Pow
er: 4 W
(R-A
lice); 4.4 W (P
-Alice)
Wavelen
gth R
ange:
700-2050Å (R
-Alice); 520-1870Å
(P-A
lice)
Sp
ectral Resolu
tion: ~5Å
FW
HM
(point source); ~10Å (F
illed-slit)
Disp
ersion:
~1.8 Å/pixel
Detector:
1024x32-pixel Double D
elay Line (D
DL
)
Ph
otocathod
e: <
1180Å: K
Br
1180-1250Å: N
o photocathode
>1250Å
: CsI
Show
n below is a schem
atic and picture of P-A
lice. R-A
lice is largely similar,
except that it lacks the Solar O
ccultation Channel and pick-off m
irror.
R-A
lice Ob
servations of Ju
piter
ES
A’s
Rosetta
spacecraft flew
past
Mars
on 2007-02-25
(DO
Y
056) at
01:54:13 UT
C. T
hree days later, R-A
lice began observations of Jupiter and the Io plasm
a torus in support of the New
Horizons m
ission. Observations
continued intermittently until 2007-05-08 (D
OY
128). In total, 378 hours of integration w
ere acquired.
From
a
distance of
~4.2 A
U,
both Jupiter
and the
Io plasm
a torus
are effectively point sources for R
-Alice, so no spatial inform
ation is available. O
bserved count rates are rather low, as show
n below.
The R
-Alice Jupiter spectrum
has three components: em
ission from the Io
plasma torus, em
ission from the Jovian aurora, and sunlight reflected from
Jupiter. E
xamples of these three com
ponents are labeled in the calibrated com
posite spectrum show
n below. S
ubtraction of the background Lym
an alpha em
ission from the Jovian spectrum
is imperfect, so this region has been
set to zero.
Jovian Aurora
Reflected sunlight
Torus emissions
The 10-hour running average of the total lum
inosity of the Io torus emission
features at 834Å, 900Å
and 1410Å and the Jovian auroral em
ission feature near 1610Å
are shown below
. Given the signal-to-noise of the data, no
obvious temporal variations are present. C
ompared to the C
assini epoch, the torus em
issions are nearly a factor of two fainter, although som
e of this may
be due to calibration errors.
P-A
lice Ob
servations of th
e Io Toru
s
Iogenic neutral emissions
Although
Alice
science operations
during the New
Horizons Jupiter flyby
were
optimized
for observations
of the G
alilean satellites and the Jovian aurora, the large field of view
of the instrum
ent enabled
observations of
the Io plasma torus as w
ell.
The illustration to the left show
s the A
lice observing geometry for an Io
observation taken on 2007-02-25. We
fit the
P-A
lice spectrum
of
the Io
torus with a spectral m
odel based on the
CH
IAN
TI
emissions
database v5.2. T
he spectrum and m
odel fit are show
n below.
Spectrum extracted from
here
While there are som
e deviations, as a whole, the m
odel is a good match to the
observed torus spectrum. T
he best-fit model torus spectrum
, broken down by
ion species, is shown above. C
ompared to the C
assini epoch, the average charge state of the torus plasm
a is higher (1.67 vs. 1.55 during Cassini).
Together w
ith the lower torus lum
inosity seen by Rosetta A
lice, this may
imply a low
er neutral source rate. Mixing ratios (ion density/electron density)
for the two epochs are show
n in the table below.
The
imaging
capability of
Alice com
bined with tim
e-tagging
of photons
allows
the creation
of spatial-
spectra maps.
Show
n to the left are two
preliminary
spectral m
aps show
ing the noon ansa of the torus at 680Å
(S III) and
833Å (O
II & O
III).
Con
clusion
s•
The Io plasm
a torus has been observed by Alice spectrom
eters on New
H
orizons and Rosetta
•W
e find no significant evidence for a weeks-to-m
onths long change in the com
position of the torus plasma.
•T
he overall luminosity of the Io torus m
ay be up to a factor of two low
er during the N
ew H
orizons epoch, as compared to the C
assini epoch.
•S
pectral modeling show
s that the average charge state of the plasma has
increased.
•T
entatively suggests the neutral source rate was low
during the New
H
orizons flyby
•S
tay tuned!