Two of the main components of the atmosphere of Io, neutral oxygen and sulfur, were detected with the IUE. Four observations yield brightnesses that are similar, regardless of whether the upstream or the downstream sides of the torus plasma flow around Io is observed. A simple model requires the emissions to be produced by the interaction of O and S columns in the exospheric range with 2 eV electrons. Cooling of the 5 eV torus electrons is required prior to their interaction with the atmosphere of Io. Inconsistencies in the characteristics of the spectra that cannot be accounted for in this model require further analysis with improved atomic data. The Io plasma torus was monitored with the IUE. The long-term stability of the warm torus is established. The observed brightnesses were analyzed using a model of the torus, and variations of less than 30 percent in the composition are observed, the quantitative results being model dependent

Ballester, G. E.

Bertaux, J.-L.

Feldman, P. D.

Festou, M. C.

Moos, H. W.

Skinner, T. E.

Strobel, D. F.

English

NASA Technical Reports Server

?RECEDING PAGE BLA/_P ;- NOT FILMED
N89- 1069 1
CONTRIBUTED PAPER
IO: IUE OBSERVATIONS OF ITS ATMOSPHERE AND THE PLASMA TORUS
C.E. Ballcster, l's It.W. Moos, I's ILl) Feldman, l's D.F. Strobel, l's
T E Skinner, 2,s J -1, Bellaux, 3,s and M.C.Vestou l's
1The Johns lh)pkins University, 2 I,ASP, University ()f Colorado
aSetvice d'Adron,,nie du CNRS, tObservah,re de Besan(;on
5(;uest ()bserver xxilll the IUE Satellite
A 13STRAGT
Two of t.fie main (omponents _)["I.he at mosl)here of lo,
neutral oxygen and sulfur, weie detected for the fhst time
in 1986 with the IUE. Four .bservat_ons have vlc.hled
I)rightm'sses that arc similar, regardh'ss of whether the
upstream or the downstream sides of the l.orus l)la_ma
flow alotmd h, is observed. A simple model requires lhe
emlssi,ms to be produced by the i.teractiou of 0 and S
columns in the exospherlc range with _2eV electrons
('oolir_g of the 5eV t.orus elect_ons would be _equh'ed
prior 1.o their interaction with the atnlosphere ,)f h,. Sex"
eral inconsistencies in the cfiaraclellst:cs of lhe speclra
that cannot I)c a(c,mnted for in thls model rcquile fur-
thor analysis _xlth Improved al.i,llnc data. The ],) [)lasiila
t,ot us has been monitored with the. IUE since March 1979.
Flus study has established tile long-term stabdlty of tile
warm i.,n'us The (,I)s('rved brightncsses have be('n ana
lvzed using a model of the torus, and varial.ions ,,f h'ss
l,haII _30% in the (ompositi.n are observed, the quaul.i-
t_ve results being model dependent.
Kevw,wds: lo atmosphere, h) l.ol us, ,Jupiter
I. INTI{ODUCTION
The products of the active volcanoes discovered by
l-oyager / on h, form a torus of plasma at, the orbit
of this Jovian satellite. Although the charaterlstics of
the plasma torus arc relatively well known, the processes
through which it is replenished by 1o are not well under-
stood. Most of the w)lcanic ejecta is not energetic enough
to escape lo and thus condenses on tim surface. However,
direct surface sputtering hy the torus ions has been found
to be an inefficient supply mechanism. As a consequence,
an atmosphere has been l)ostulated as an inl.c_medlatc
agent to replenish the torus, but many (tuesli(,ns remain
unanswered. What IS the nature ,)f the atm,,sphere '_
Which are the interaction mechanisms l)etweerl the 1,,
torus plasnla and its surface and atmosphere '7 \\:hat is
the variability and stabihtv of the system':
Until recently, very few direct observaltons of h/s at
mosphereexisted. In 1986, two of the main atmospheric
c-nstituents were detected for the first tram with the Jn-
ternathmal Ultraviolet l:Txplorer (IUE). 'Phe results ob-
tained st) far for a total <)f four smh IUE observations
ale presented in Section 2.
The h, plasma Lotus has been observed with the IUE
_in(e March I,(179, when l/oyagcr 1 made ;n-s_lu measure-
ments of Ihe l:)[us. A systematic sludy of the pr()perties
and long-term behaviour of the torus has since been un-
(h'iwav and the most current results (,f this study are
presented in Se('t.ton 3
2. TIlE A'I'M()SI'tIERE O1" IO
Pwneer I0 discovered an ionosl)fiere()n I% and the
Voyager I lll.lS measured an SO2 surface, density _< 0.032
('m atnl (p _10 -7 bars) resulting from either std)limati,m
of S()2 frost or a volcanic plume. At least 20% of the
%IIF[*D('c (if I() is covered by S02 frost of volcanic origin.
The atm,,sphere could t.hus be dominated by subhmated
SO2 and be colhsionally thick or tlml_ or it could even be
episodic, driven by volcanoes. Many models of the atmo
•q)[icrc exlsl, and some law." a relatiw,ly thick atmosp]letc
that L_lll IlldllIL_llll both Jill iOllosphere alld all exosphere
that could be composed mainly of atormc oxygen and
sulfur, tim l)hotocfiemical products of $02. Others adv(>-
cate a thin aimosphere due t,o the ralher limited amount
()f SO2 surface frost In any case, neutral oxygen and
sulfllr are expected 1.o be two of the main atmospheric
c,)nstituents.
The first detection of emissions of oxygen and sul-
fur on the atmosphere of [o was made with the IUE in
1986 (Ref. 1). Four short-wavelength (1150 1950_),
low-dispersion spectra (Fig 1) have been obtained as lo
orl)ited east and west of,Jupiter (Fig. 2). Excellentl)oint -
ing accuracy was maintained throughout the 14-hour ,)l)-
servations of this moving target, allowing for a positive
detecl.ion (,f the emissions. The emissions were found to
originate from a region inside _4 Io radii from the sur-
fa ( e
The observed oxyger_ and sulfur multiplets are: a
blend of el A 1304 and SI A 1299, Oil ,_ I356, SI A 1d29,
A Decade of UV Astronomy wtth IUE, Prec. Celebratory Symposmm, GSFC, Greenbelt, USA, 12-15 April 1988, ESA SP-281, Vol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001320 2020-03-20T05:58:04+00:00Z
80 G E BALLESTER ET AL
2
//I IO WITI! TORUS 31 JUL 1987 (W) I
1 5 [_ t I Sl&Ol OI l SI St Sl _q-I
_" o5
4.
I 0
i
o -05 U I I I I I I I I
2I [_-f-I , , , , , , , ,
1 5 I II IO WITtl TORUS 1 AUG 1987 (E) I
fi I- II smotoq st st sl sl|-I
0.5
-05
45
3 5 L _1 TORUS SGP'I'EMBER 1982 1
,_, 25
,j 15
05
-o 5 W - - ,- - ""T- -"T----5 _ -7"- -',_-T_-_l
1150 1250 1350 1450 1550 1650 1750 1850 1950
Wavelength (A)
Figure 1. IUE spectra oflo and torus (top) and (cen-
ter) from the 1987 observations, together with a torus
spectrum (bottom). The emissions originating from Io
are marked in the top and center spectra, while those
originating from the torus are marked in the bottom spec-
trunl.
VIEWING GEOMETRY
FROM JOVIAN ROTATIONAL NORTH POLE
4 - - - _ Torus
/ / 1o ""
/'/f
_// \\\
tit _\ _
18 Jul 86 \ _ _ _ - !_ i'O "_/ " 12 Oct 86
1 Aug 67 "- / 31 Ju187
¥
IUE
Figure2. Viewing geometry depicted from the north
pole of Jupiter for the 1o observahons. The trajectory
of lo as it orbited east and west of Jupiter throughout
the observations is marked with a solid line. The torus,
indicated by the dashed lines, is flowing past Io.
SI A 1479, SI A 1814, and SI] A 1900, 1914. The measured
brightnesses of the four spectra are very similar within
the estimated errors (Fig. 3). Some models predict dif-
ferent atmospheric properties for regions corresponding
to the upstream (west) or downstream (east) sides of the
tl0
T_
160
140
120
100
{30
60
40
20
Io BrighLnesses (Disk of r:SR,o )
I I I I I I I
a JUL86 (E)
m OCT86 (w)
1!1 , JUL 87 (W)
_,AUG 87 (E)
0 .........................
-20 I L I I I I I__
1150 1250 1350 1450 1550 1650 1750 1850 1950
Wavelength (A)
Figure 3. Measured brightnesses h,r the four h, ob-
servations plotted as a function of wavelength, derived
using an emisison disk of a radius of 5 1o radii.
torus plasma flow (at 57 km/sec) around 1o, and t,, the
day or night sides. Our observations have viewed both
the upstream and the downstream on the day side. The
similarity in the spectra obtained should therefore con-
strain any atnlospherJc model.
Various spin-forbidden transitions were observed, sug-
gesting electron impact excitation. In fact, the emissions
were expected to be produced by the interaction of 5 eV
torus electrons with the exosphere. To study this pos-
sibility, the emiss)ons were modeled assuming electron-
impact as the only excitation process. For sulfur, only
the theoretical data of Ho and tlenry (Ref. 2) is avail-
able, and it includes only three of the observed transi-
tions. From these data, a ratio of the observed emissions
was predicted as a function of electron temperature. The
measured ratios agree with an electron temperature of
_2eV (F'ig. 4). The Ol A 1304 electron-impact data
has been recently measured in the laboratory by Doering
(Ref. 3) to have a resonance near threshold which agrees
with that predicted I)y Rountree and Henry (Ref. 4). The
theoretical data of Rountree (Ref. 5) for the O1] A1356
emission includes another resonance near threshohl and
was used to model the ratio of these two oxygen emissions
as a functiml of electron temperature The measured ra-
tio also are compatible with an electron temperature of
_2eV (Fig. 5).
The derived column densities, 9.4 x 10 14cm-2 and
2.,1 × 10 -14 cm -2 for oxygen and sulfur_ respectively, are
in the exospheric range. Therefore, in this model, an un
known mechanism is required to cool the torus electrons
from 5eV to 2eV previous to their interaction wtth the
exosphere. One further inconsistency lu fhis model ,s
that resonance scattering of the solar 0 and S line_ xv_s
ignored in the analysis because the g-factors aie smaller
than the excitathm rate due to 5eV" electr, ms. Im_ f,,r
ennssions produced by 2eV" electrons the exc,lal,,,n ,a_c
he.comes comparable t,:, the g-factors f,r s<m_e of the mul-
tHJets.
IO'SATMOSPHERE&TORUS 81
o
¢0
m
15
0 5
SI Et_ss_on ]_,H_e Ratios
I 1 I I T I 1
l':mph Icnlly Adjusted A JUI, f_,l; (J.:)
IIn nnd llemy (19f15) El OCI' JIG (W)
• JUI, 87 (W)
AUG a7 (l.:)
o
o 8
I ,lR9
/ fiii,[
...... ,gr,i
I I I__
1 2 B 4 5 6 7
To (_v)
Figure ,t Lira,' rallos ,,f lhc gulfur emisqon_ m,,Itl
ph'Ls S/ A 1429, SI A I,'17!),and SI A 1299 cal( ,dated /'t,',m
the al,mnc data ,,f ll- and Henry(ltef. 2) Themcasmed
vaHles are ltldle_ted
o
al
)-I
Ol /304 L0 1356 lClmssmn k,u_e li:at.t0
| I 1 I I I I
,5 JUI. 8(1 (1,')
eJ0CTa; (w)
¢ ,JIll, 1_7 (W)
¢, ^u(; 87 (I.:)
LJ__I___ [ I. I____
0 1 2 3 4 5 6 7
're (_v)
AIIho.gh a satisfactory exl)hmatio,_ of the IUE ob-
serval tons of the atmost)here of Io cannot be supt)hed yet,
these ol)servatmns should prove useful in the future as
mqnoved atomi( data I)ecomes avadable and more obser-
rattans are performed. The IUE is valuable for studying
h,:s main atTnost)heric COllStlf,!lelltS alid sholdd thus pro-
v,h. ms@H into the nature of [o's atmosphere and H.s
inleracl.l,m with lhe torus Future observations should
als,, mea,ure any temporal variat.i,ms and l)rovide a basis
fi,r _,hseivaltons by Gahleo and lIHT.
3. TIlE I0 P1,ASMA TORUS
The i)las,na l,(,rus is comi)osed i)rimarily of ions ,,f
s,,If,,r and ,,xvgen. Its cornpositHm is determined mainly
by the ele(:troll density (n_) and ternperature (Tel , wHh
average wdues measured by Voya.qer 1 of _2000cm 3
and _5e\;, respectively. The torus has a scale height of
about one .Iovmn radius, and is centered at the orbit of
Io wil h ,ts ce,Hr,f,)gal axis lilted 7 ° away fv'om the .Jovian
r,,I.aliomd axis As the plasma (;orot_,tes u'tlh thc.l,,_,an
Tnagnetlc [iehl, it wobbles with respec( t,(> a gi\ell hm' ,;f
s,gl_l t.hr,,ugh.ut a _10 hour ,h.,vmn r,,tati,,n -\ t,,r,,,
anna al 5.9 ,Jovian radu, where lhe column sampled vs
lllaXillllll)l_ \VlrlS viewed though(rot llloqt Of the e.xl)_,qtlT{'s
(lqg. 6) The inl.egralion time is typically b,,t,_,',', c, 1,,
I'1 h,,urs and 1he speclra ,,l)fained ICplt'Sclll ],_,i/gll,,,.hnal
average_. (Grolmd-based observations of the t,,rus are
mu(h shorter and sample the torus longitudinally)
'-,i'-,:'_'_, 0_: ":7'";:;Y/l:' _ ;;'': ...... <';'"°"-";,}2t_F{_',_'
Figure .5. Ol A lg0'l t,o O1) A 1356 line ratio olH.a,ned
from the theoretical data of Rountree and Henry (l/.ef 4)
and Ilountree (l{ef 5) ]:L>l,h data include a ,es(mance
,lear Ihteshohl, and cascade effects have been added to
Ihe O1 k 1304 data. The mesured values are indicated. Figure6 A "snal)-shot"ofthe.h,piter-h>-Torussys-
tem. The SWI' large aperture is sho_n.
Furthermore, there are some lnconsisl.encies in the
characteristics of the spectra that do not agree with the
theoretical atomic data, namely the presence of the bright
SI A 1'179 emission multiplet eoml)ared to l.he almost
undetectable SI k 1429 emission multiplet. A rocket-
spectrum of the torus (l{ef. 6) shows these lwo sulfl, r
emissions in agreement with the theoretical predictions.
Therefore, olher excitation processes should be signifi-
cant on 1o. We are presently studying other possibdities
such as electron-impact on SO2 and cascade effects aml
re¢oml>ination of S+, as suggested by the work of ,Judge
(llef. 7). Again: the alomic data is very lilmted for tllese
excitation processes
The observed torus emission features are SII A 1256,
Sill} A 1729, and S1V] A1406 (Fig. 1), whereas oxygen
ions can,,ot be detected with the IUE. The measured
l)rightnesses vary because of tim ddTerent viewing geome-
fries, bul the ratio of the brightnesses does not vary as
much. In order to compe_re the observations, a model
()f tim l.,)rus has to be employed. The original model is
described in detail in Skinner (It.el. 8) and Moos el al.
tiler. 9), in this work some n,,)del parameters have been
Ul)-dal.cd aim (.he data set has I)een extended.
82 G E BALLESTER ET AL
The model predicts an average brightness for each of
the emission features by estimating tile wdume emission
rate integrated over all the portions of the torus falling
in the field-of-view lhoughout the observation. This vol-
ume emission rate is a function of the electron tempera-
ture, electron density and ion density and the following
representative, longitudinally-averaged spatial profiles for
these quantities are adopted'
- the electron temperature and density profiles in the
torus centrifugal plane (Fig. 7) derived from the 1:01/-
a.qer I observations (Ref. 10);
- the electron density at a distance z from the centrifu
gal plane approximated with ascalinglaw (Ref. 11) and
using the revised scale height of _1 Jovian radius (Ref.
t2);
- ion densities assumed to be constant fractious of lhe
eleclron density and to follow the same spatial profile,
since the plasma is assumed to be homogeneously mixed:
and
- the electron temperature dependence in the volume
emission rate is included in the thermally averaged colli-
sion strength (Fig. 7) of the S II 1256 A transition (Ref.
13), hut no temperature dependent atomic data is avail-
able for the SIX:} A1406 emission Since the atomlc data
for the Sill} A 1729 emission is rather uncertain, lhe theo-
retically calculated, temperature-dependent values ,,[ lh,
and Henry (Ref 14) have been employed.
The ratm of the measured to model-predicted bright-
hess and the requirement of local plasma neulrality are
used to predict the characteristics of the torus plasma,
namely the electron concentration relative to the V0y-
(zfer 1 case, R, and the S +, S +_, and S +a rmxing ratios
or fractional concentrations, M II_ _'i III, and M IX:. The
results, presented in Figures 8 and 9, are: R = 1.07 ±
9%, M II = 0.10 .± 30%, M IH 0.22 5- 17%, and M lV
= 0.021 ± 35%, plus a mixing ratio of 0.39 inferred g>r
O +. (The large variability of Mtv is partly due to the
uncertainty in the low-level sir 3 ;_ 1406 signal.)
I ,| -1--
>,
? / / _
_a
"O 12-
o
.o
_q{ _ 0
'D
'_ 08
,d
7_
C<
)
1 I I I I I l 1
• I\
/ # \ /) \
x/ d' _, I
, I 1
I I I I__
19"79 1980 1981 1982 1983 1984 1985 1986 1987 1988
Datc
F,gure 8. Modeled electron concentrat,o,1. I(, Iela-
tlve to the Voyager 1 case versus date
o
v
015
0 10
0 05
O3O
0 25
0 20
0 04
0 O3
0 02
00I
I I I 1 I I
" )\ M. .
/ \ /*" t 1"_ ,) - . -,/I /{
• ¥
Mlu t\
M IV / '_ /[
.* \ / I
I I I I I k I ....
1979 1980 1981 1982 1983 1984 1985 1980 1987 1988
DaLe
Vigure 9. Modeled S +, S +2, and S *a ,ni_,,_v _,t,,,,
or fraet.i,mal concentrallons, M 1], M [I[, and IXI 1\ . veisu_
date.
lO s
tot
.%
[--,
lO__
.a
I I I I I
/
zo!
Illll
-<o_ 10 a
to
e_
q)
121
o_tot
M
t0'! ) i i ) i lO! i i ) _ i
3456789 345678
10t , , , , ,
SII X1256
Rate
,0'LI
N
\
M
._ 10U
345678
Centrifugal Plane Distance (RI)
Figure 7. Model profiles as a function of radial dis-
tance from Jupiter of the density and temperature of the
torus electrons (left and center) on the torus centrifugal
plane, derived from the Voyayer 1 observations (Ref.10).
Also plotted (right) is the profile of the thermally aver-
aged collision strength of the S II 1256 _ multiplet.
IO'SATMOSPHERE&TORUS 83
The resulis indicate that. despHe _h,,_t-t.e_m _,_im
tlons _,f le_,._ than _30%, lhe 1,,lus has lcm_uned '4aSh'
_]n<e (he l'o_laqcr l en(ounIer in 197!) lll),mgh mid l!)87
$4.,$42, S _;_,and()_wouldcalrvre_pc_tlw'lx. 10%,,1-1%,
6%, ;uld 3iY_ of the <.ha-ge. These rvsult'_ bring _,ln lliE
cstlnml.es of the I_)_ll_ plasma chalgc distIibuii,,n _h)._c
1,, Ill(' cslimates derived fr, ml the 17_-.q_lu (t'LS) pl,_sm_,
mca.',uremenls and the ulltaviolel spc<lta (Ul'5)(,f I'oq-
aqer' / as im'i('wed by I/agcn,d (_tef 1.5) b;_vr) (h,,*_gh
the rm,dcl carlm)l distmg.ulsh relatively '.mall. simuliane-
,,us (hange_ ])) l lw eh'< i i,)n i('Ill[)('ratlll(' aIl(] densiiy and
its quani)lal.i_<" le_ulls are thus m,)dvl d<'p<')Mcn(, lhe
ql)alilatlv," )','sult ,)f lhc hmg-ter))l s(abihlv <)f lhe I(,TUS
_.. qmic firm
II _ imp_,rtan( io continue these lIH '2 ,>bscrvail<)l)s as
l.lwv c<>nMtl uic a unlquc, hmg l.clm study ()f 1<)i u._ pl,_ma
dc)lsily and will be an invaluable link bc(wcen (he lll-s_{l_
l))casur('mm)_s ,,f l-O!laqCr / and _]_r>s(' ,>f H,S'T. ,'_.S'TII(),
and Gahle(,.
\¥e gralefully a<'knowledge the asslsIan('e ()f the IUI'J
<)bscrvalory siaff m the ac(luis_i.ion and reduction of ihc
_aivlhlv (lai.a. ']'lli_ r('sear(h wa_ _upp,)t(<'d bx .,N \5_ \
under gran( N5(;-539'1 i<, lhc,J<)hns lh,pk_u_ l;)_i,,,),._I\
4 It 1';t"f']R I_;N C I';S
I. Balleste), (1 E., M,,<>% 11 \V.. l"('Idman, P.D., Si)'_,b_'l.
1).ft., Summers, M.I';., Berlaux, .J. L , Skinner, T.I".,
Fest{>u, M C., and l,leske, ,} 1,. 1987a, l)ete{'l,l_>n of
Ne'.iral Oxygen and Sulfur F, missmns Neat Io Using
llJl_, Ap. J Letters, , 2__a, L33.
2. 11(i, Y.I(. and llel_rv, ll.,J \¥ 1985, (),,_ illni,*r .%ir,'n_ihq
and('o]lision StrengIhsf<>) Neu(ralbl)]fu). Iy .1. 2q))
,124
3. D(>ering, ,] 1987, pr)vate o)mmlmicall,m.
'I It<mniree, S.P and llcm_, h)..l.W Iq72. I';Ic, (,,,)) Ill)l>:,(
t_xciIai(>n (')_,_ Se(.tions fin al,,nnc Oxygen.
al'-,%'tS", f'h_s. Hey. A, 6, 2lOB.
h I/_)ul_l)ec. S P. 1977, Eh'ctr_,n-hnpact Excitaion Cross
8ecll,ms for alomlc ()xygen: ap-3s 58° and ap-3s aS°,
,1. Ph_js. H'Atom. MoIec. Phyla., 10, 2719.
(L l)u)),m_e. S.T , Pvldn)an, P.D., and Weaver, tI.A. 1983,
I{()ck('t I)eiecti()n (>f Ultraviolet Emission from Neu-
()al ()xyge)l and Sulfur m the lo 'Focus, Ap. ,I Loi-
ters,, 267, 1,125.
7 Judge, I'.G 1987, The gxc)lali,m c,fSl Emission l,incs
In (:h*<)m<>_l)lleres ,)f Late-Type (',ianl Stars, Mo,.
Nol. H ,Islr. So,'.. ac(epl.ed.
bl Skinner, T E 198-1, l)h.D thesis, Johns ltopkins Uni-
_, _.'T M ( V.
9 M,,o% II W . Skinner, T g., Durrance, S T., Feldman,
ILl).. l:('st<>u. M.C.. and Bertaux, J.-L. 1985. l,r)ng-
lerm Sial)ihty _)f the h) High-Temperalure Plasma
T,)rus, .lp. ,1. 294. 369
I() Start.h, \V.1I. and Shemanskv, 1) E. 1983, Es( ape and
h,nizali<)n <)f Atomic Oxygen from 1o. Ap. J., 271,
865
II. l/age))al, tp,S))llivm_,J.D,a))dSiscoe,G L 1980, Spa-
(ml Dlsiril>ullon of Plasma in the h) Torus, (,'eoph?is
Hcs. l, ellcr.s, . 7:'11.
12 Ilagcnal, F., McNuti. t{.L Jr , Belcher, .1.\'\: , Bridge,
11.S., and Sullivan, ,I 1). 1985, ltev_sed Ion Temper-
atures for Voyager Pl,_sma Measurements in the h)
Plasma Torus, J. 6'eoph_IS Rea., 90, 1755
13. Ilo. Y.K. and tlenry, R..I.W. 1983, Oscillator Strengths
and Collision Strengths for $11. Ap J., 267,88_3
l,l. lh,,Y.l( andlh'n)y R..l.W. 198-l.(;olhsi,,)_S(,cl)glh,
f,)r AII99 and A172.9,,fSII1, .,Ill. ,1. 28'1.-1'_
15. Bagenal, F. 1987, paper presented at (he h_lc)nai_,,nal
.Jupiter \Vatch \\7_wk_h,>p _)n 7'm_ I ar_al, l_ l'l,, ,,,,_,,
c_a _r_ thc ,lovza_ _@.#em, Flagsfaff, ,.\_lz-m)


Io: IUE observations of its atmosphere and the plasma torus

Io: IUE observations of its atmosphere and the plasma torus

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