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!