NASA Technical Reports Server (NTRS) 20110007237: Electron Density Dropout Near Enceladus in the Context of Water-Vapor and Water-Ice

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GEOPHYSICAL RESEARCH LL°ITFRS, VOL 36. € 10203s dc^i:ifl.€f?>i?if^ (tt.,03'IO$, 2009
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Article
Electron density dropout near Enceladus in the context of rater-vapor
and water-ice
W M, Farrell,' W S. Kurth,^ l). A. Gurnett,2 R. T Johnson M. L. Kaiser,'
1.-t:. Wa.hlund,`t and J. H, Waits Jr.`
€:e iv ci 22 ?eeer , t 211(}k: rn sad 26 AU:rch 20W: acctiptcd 1 Apri! -1009: publishe i 20 \t -^y 2009.
[, j 0n U 'March 2008, the Cassini spacecraft trtade a close W Given this extraordinary rind, the C asstnt science
encounter with the Saturn wn rnoon Enceladus, passim operations loan planned a set of close spacecraft passages
within 52 km. of the moon- The spacecraft mtjectory was by the riioon to obtain further details of its structure and
?
inter)ri€^naliv-oriented. its a sout herly di _-c3ion to create a physical processes_ One such encounter occurred on 12
close alignment with the intense warter-doll-itlate'd plumes March 2008, Ei(Tyre ' shores the tra;ectory of the spacecraft
emitted from the south polar region. During the passa3e, the in all eladtis fratne cif reference where ~x is oriented i71
Cassinj radio and Plasma Wa N_ System ( IIWS) detected the dire non of the moon's orbital. velocity, -Iv is oriented
tkvo distinct radio sionatttres: I) Impulses associzued 1uith towards Satin, and -r, is oriented out of the rn€ oiVs orbital
2)
small water-ice dust grain impacts and an upper hybrid^ plane. `I.°he water p€unt le is ill.€tstrated in Figure 1 as th e
(t,d l resonance ernission that both intensified and displayed. purple-shaded region approximately indicating the location
a sharp frequency decrease is the near-viciraitx- of the moon. of highest water density-. The spacecraft moved primarily
The frequenc:y decrease of the UII emission is associated southward at x-14 km/sec, passim; tear arid. aligned with the
in
with art unexpectedly sharp decrease electron density water geysers. Given that Enccladus' radios is —`O km, the
frorti ^-gCt elan to below 20 elicrri` that rlcettrs Girl a time entire encounter (defined herein as = 5 rnadii about the moons
scale of a minute near the closest encounter with the moon. lasted for a little over4 ininutes, and the encounter with peak
In this work, we consider a. number of scenarios to explain geyser activify> itself transpired on a titnescale of mmutes.
this sharp electron dropout, but s€trmise that electron [tj The Cassini Ton and Neutral Mss Spectrometer
absorption by ice grains is the most likely process. (I S) (b itcz E1 aL, 20p4 . found (on a previous encounter)
Citation: Farrell, W. M., W. S. Kurth, D. A. Gurnctt, R.. E. that the geysers consisted of ernissions similar in nature to
jolmsk)!I, 4t. L Kaiser, J.-E. 41%; hlund, and J. H. suite Jr. r;2+y09 , cot, etard pla rues a dominance of water molecules but
Electron deitsity dropout iii ar F? icehidas in the conicxl c ^°crier- also the presence of methane. carbon monoxide, carbon
vapour and water-icc, Cre.,pr'yl ° , 1te Le a.. 36, 1,110203, dioxide, and bath simple and complex organics [Harte> ei
doi: i (l. i Ct29 20088( I.G3 108. rat., 20061. During this i2 March 2008 en ountct.. the
IN"NIS-tneasured water concentration was found to lie
<l04%cm-' before closest approach (C A) at 19:06:11. SC ET,
L Introduction
outside the plume, However, concentrations increased
['] A nia.jor discovery of the Cassini niissioti was the sharply thereafter peakingy Dear 10 ictr between
s
observation ofa substantial water geyser at the outh pole of 19:0 :3[).....19:0":00 SCE 1`, Dut:ng this period. the space-
Saturn's moon .Enceladus [T orcv et apt., 2006: Waiw c,i of„ craft was traveling aligned with the plume but also migrat-
2006, Hanv n ei ad. 2006'j. The em itted water 1) is believed ing axially-inward toward the gar center- intersecting its
to be the source of the large toroidal cloud of hydroxyl largest concentrations at appro-drntat€ly 300 krn from the
radicals with a peak density near 4 R, previously observed southerly°-directed surface sourco. Passim„ southerly at larg-
by HST [.Shera aat kv et aL 19131, 2) creates an emended er radial distances, INIMS cowinued to detect plinne emis_
neutral and pl.asrna tones that drives processes in the inner sions at teve. s % 3 x 1€1''crn a4s late at 600 seconds after
magnelosphere [flichardson and Juror, 2004; Jurac and CA.
of,
Richardson, 2005, 20071, and 3) is the source ice As we d€rnonstrate herein, the prescnc€; of large
comprising the E-ring [Ila'fe et aL. 1983: Ilrr° in, ° ; et aa," , concentrations of water char-es Vic very nature of the
2€3081. `I`he observation of both water and internal energetic plasma-neutral interactions. Specifically, prior to CA, the
activity at the moon places Enceladus on a growing list of plasnia can by considered col.lisionless. in contrast, daring
astrobi€>lkVical targets-ofinterest that includes Tf tan, Mars, the close erneciunter, the plasma e ectrons have many colli-
Europa, comets, and asteroids. sions with water vapor and ice such that its effect oti the
p,asn,a inflow cannot be immediately neglected. The effect
Spaic I.xploration Di°tic :, NASA 61oddard Space Fli!,^i (eraser.
(3reenbeit. Nlary land, USA. is to create a stead: depletion of electrons with inward
13, Yraruaicnt of Flt x sic, -sags As_;mc „ t .s a;o efts f , foe^ (;i y radial distance th°<rt is rerniniscent of the exponential elec-
tron decrease; at the bottom-side of an ionosphere - but now
Departtmeal of Mzlvc —,f science and Ullivcrjty cal.
defined by wwaalteerr gas and ice chetnistry.
v`rginia, l;la .rl ttcsvel6 ,\-i.> a;tit+, ,'S:4..
S :s di l= ji1 ii_utL c>t 5pcac I'1 VO".", U=p. ala, sv,' dcaa_
Spacc :S sync -a d 1.-,2gineeing, Sou lnk t Rc_^c vk::h sot€:a.fc, safe 2. Observations
inscfraio, Tcxas. USA.
t{: i Fr gurc 2 sbows a Cassan: Radio? and Plasma `%.Vav
I?si,^ paper i5 :r€si ;Ut jcc`. to i S. t:opyright. ,cience iRPWS) radio spectrograrn [Gurnefr et cal., 20041
Vublished in 2009 by the _ inlericura G,,o>pt ysical t,:`nion.
L 10203t cif
1,10293 FAR.tt.F.1. L E AL: ELECTRON LOSS NEAR F C'f;.t ADDS L10203
Mar& 12: Day 0,72, 2 8 M's r 12. Day ay 2, 20,08
.'
^, 2 .^.-
aa` ; =7rY't..3.^ ikfe
-4 -S 2 2 A 6 _8.1. -4 <2 u 2 $
Figure 1. The trajectory of the Cassini spacecraft past F riceladu.s; with (lel.I) sho in,g a `top' view looking down frotn
above the mt-ital plane- and (right) showing m ridionai view iooking opposite the direction of the orbital velocity vecior.
The electron concentration drops below 70 cm bemreen the red markers.
encompassing a 30 minute period centered about the F:nce- ('labeled f% in the spectrogram) that displays a clear and
iadus encounter. Closest approach to the mood (52 km distinct frequency downshift centered about CA., Unfortu-
above the surface:) is indicated in Figure 2, The measure- nately, the intentse broadband dust impact signatures obscure
ments are from the wideband wavetbrrn system with a 3--dB the upper hybrid emission between 19:06 to 19:07:311 SCE'
bandwidth of ---''5 kffz. The upper hybrid ennission (f h) and when the impact rate is highest, making it difficult to track
clectron cyclotron frequency (f,.-, -- derived frorn rnaan tom- the band throughout the entire: interval, However, a period
eter results) are presented on f=igure 2 for reference. of ernission frequency dow-n-shift is observed between
[') In this presentation, there are two clear and distinct. 19.05-19:06 SCET and up-shift to near pre-encounter
signals: 1) A broadband signal (bandwidth of —80 kli'z.-) that levels between '19:0-:30 ,- 19:08 SCET.
commences near 18:55 SCET and extends to at least Erl Regarding (1), water-ice dust grains are incident can
N 20 SC:FT that con-silt 01' impulsive Spike-like events the spacecraft and antnna, creating impact ionizations that
resulting from micron-sized dust «rain impacts on the are detected as a bipolar voltage pulse can time scales of a
spacecraft system [[rrtrnett et oil., 1943, Kw-rh er al., fete milliseconds [Ci,.rnett fir al- 1983j, When Fourier-
2106; Ming of a1., 20061 and 2) and upper hybrid emission transfonned into a frequency-versus-tinge spectroprarn they
Orti4€ 61 Eno-6adus Ryby
March 12, flay 072. 2W8
10
Tx1^'
s
z 9 12
iv iv
w
v^1a^
2.1C,4
E'er'
WIG
4..2'_ 3A F--24 473S
v^.°s .32.`:B r33_€Pi 433.7 87 115,' w1fi.7^ 1i, 3
maf WIG c .23 6,7 53.9 75,25 -72,Yt _?147
LT 2.26 2.26 2 5 2.2^ 44,E 1:4.""'^ 14.28
Figure 2, Cassini. RPWS wideband wa,, eforrn spectrograms showing both the broadband nripulsive dint grain impacts
and. upper hybrid emission (i € }, The electron cyclotron frego ncy is ako indicated for reference. Closest approach to the
moon is indicated by CA.
1-10203 FARR ALL E'f At-.: LLECTRON LOSS 'NEAR f. NCf::L, DUS L1©2U3
C;^mni R "S
NM 072 (Mw ^12^ !&M,30 -M Igl0=
V
1
dN6fdr 30eVe
z
4
3
i
^ 3
a' s i 't D'
to 50 ig 90 19 Za`r
fix" T
Figure 3. The electron density (in orn I and in 3) as derived from the upper hybrid resonance emission.
form a broadband impulsive event extending to the iimll of the electron density decrease vvith radial distance from
receiver bandwidth. A clearly-presented derivation of the Enceladus, dn, dr, is on the order of ­ 80 ei,'rrs', This large
dust charge transfer transform. is provided in the appendix of negative gradient can be considered a primary marker that
14,4,ng er at. [2006j. During the Enceladus encounter, dust defines the values of the source and loss terms in the
grant impact rates progressively increased from M56 electron continuity e gtm6on. By the Lime Cassini exits the
SCET, peaking on the order of 1100/sec bet,veen 19:04 plume, the electron density has recovered to approximately
'>9:1€I SCET. Impact rates progressively decreased thereaf- its pre-encounter- level.
ter. During peak activity near Cry, the bandwidths were
observed to increase: to latme° extents. The impact rate 3. Interpretation
appears to have an abrupt decrease near 19:14 SCEJ but
still some level of impact detection persists intermittently to [iii The electron cominuity equation can be written as
beyond 1€3:20 scET. t rrE:-' -= V - (n,.v._) = S L where vQ, is the electron flow
3] Regarding (2), an upper hybrid emission is typi- speed past the moon, S -pre electron source; (primarily
ca.11y detected throughout the entire roller niagrietosphere photo-ionization) arid. L arc clectron toss processes that
[Garnett et ul., 2005, Persoon et car'., 2005. 2006.1. It allows include electron recombination. electron dissociative attach-
a &rivation of the electron plasma frequency (since IP A. — ment and a new t€;nn defining dust absorption.
(f„r fz)'') and thus electron density at a planet-Centered Assuming a time-stationary situation (constant gas
radial distance of 2-'0 Saturn radii, NVell aw'av fronni the emission and steady spatial gradient) then On i4't -- 0. One
moon, the upper hybrid ernission (i.e., the electron. density) can write an approximate l-D electron continuity equation
varies in a progressive fashion, With Variations on scale along the Cassin$ trajectory past the moon as:
sizes €tf a fraction of a Saturn radii [see Persoon tit al_ 7006.
Figure 5.1. However, figure 2 indicates that very near 0 ( v.,' !0s v..lin, !0s,il, r1 t b„j t`#S
Enceladus, there is as significant reduction in f,,, indicating
the presence of a clear and d.isdnct -bite-out' in electron
density that was not anticipated occurring on spatial where the left band side describes the divergence of the
scale~ of a few Enceladus radii_ In fact, due to pick-up electron flux upon approach to the moon, and the right hand
processes (7crkrtr et ak, 2-006, Pontius and Hill, 20061 the side accounts for sources and losses with a water photo-
plasma flow velocity near Enceladus has beer) measure: ionization source described by v 14120, e1CCU011 recombina-
to slow substantially from. —216 krn-_ec to <10 kin 'sec tion lasses with water ions t of assumed comparable density
[WahIEind et jai., 20C>5, 200 ]. The plasma densities would as electrons) described a., —k,n , electrc» dissociative
thus be expected to increase by, a tiictor of 2,-_3 in the near- attachment Losses described by k<i nfj, €:) n,, and electron
vicinity of the moon 'In orde=r to conserve tdrix, Clearly, such fosses via dust absorption defined as r?o. v, ne [Jack-.wn es gal.,
an increase is not observed for electTons and an unexpected, 2€ 08 i.
sharp decrease was observed instead. [i:a] We will now apply this equation during the period
[i9] Fitg,urc 3 shows the derived electronden city from the before CA between 19:05- 19:06 SCET, during the R.PF'vS-
flri, emission w ich displasy, the clear and € i t inct dropout in ibs ery ed vers steep electron density ,gradient of ;bra N r;S —
electron density. Note that beov een 1 andd 19:06 SCET, k11 cl`r) .The electron dera4a -v, rig, Was —8 x it^'im at
3 of 5
1.10203 I=ARRE LL P 't' L: t"LE.('"l'RON LOSS NEAR L-NC:F:::t.sADUS 1-10203
the start of the drop-out period near 19:05 S6 `E=T and <2 x [i,¢] Herein then lies our dilemma: if we consider the
10 `iii at the end £f the period near 19:66 SCT:;T 'the hand usraal processes associated with water molecules - photo-
is difficult to identify betvveen 19:06--19:07:30 SUET). We ionization, recombination, and &ssociation - along the
note that during this period, n[tzo was not at. its peak Cassini transit, we find that none of these molecular source
levels, but rangLs from 0.2-1.0 x ill's`=rn and loss processes at — a few I Ws of el/m-s can account for
f14] Regarding the flux divergence during this time_. both the intense electron flux divergence at —10" elrnt' s. These
electron density and velocity are undergoing, changes upon water inolecules and ion processes alone cannot account- for
approach to the moon. Specifically, due to pickup,imass the severe loss of electrons observed before CA.
loading effects, the plasma flow speed was previously- [m€ liov, ,e -er. as indicated in Figure 2, there is clearly the
observed ttoo progressively slow ffrroomm corot.ational values of presence of water-ice (dust) in the environment and this also
26 kmrsee. at 14 R, to below 10 ktr/sec at 9 RL, j 7bkar et can aster the plasma character. In equation (1 } ,the ettect of
sal., 2006; f ,,azhlund et a1-, 2005; I'c*n ues and Mill, 2006; this dust. absorption is quantified via the v, n1- term,
J,f'ala€rand of cal., 20061, Close to the Y-storm, the flows, should where q,, = -7,, n is the inverse of the dusvelectron collision
slovt substantially (in theory, approaching zero), The previ- paean free path [Jacl yon et sal„ 2€)08]. The quantity n ot is the
ously-observed plasma flow rtreasurements suggest a veloc- dust density and ej is the dustv,'electron collision cross-
ity- gradient that decreases with approach to the moon as section, 7-, a£ f, where a,ff is an `effective capture radius'
Ov i0s 0.007/5_ If there were no ether sources or losses, about the dust grain.
the electron density gradient should became positive to keep P1 We now consider the constraints oil dust particulate
the flux constant in the progressively slower flow. However, absorption of electrons that are required to be consistent
we instead. observ=e a strongly decreasin electron density with the divergence of the flux (left hand aide of equation
s
with inward distance; and conclude there must be a very (1)) in es ence reversirny the argument to examim- self-
substantial lass mechanism. consistency. If we assume that the observed t.ux divergence
11-] Assuming that the decrease; in fluty speed described is created solely and completely by dust absorption, and
above continues into the plume, the flow speed would then define electron flux as g == n v we find that dg/ds — t 0° el.%
become v,-7 kriV, near 4.3 lt,, lasing n, -- S x 1 O'/ ', the in''-s. = — f")s g. For n1- W 8 x 10T-na3 and vE = kIn sec, vve
divergence of the flue; 0(n, v "j,Os — --1(1" el.;7mt -s. The loss find that rl,j = g ` crgids n.: 10 `' rn. Given that fist is also
processes described on the right hand side of the continuity equal to f-,j nE„ we now want to get an independent
equation (t) mast then account for this rate on the left hand estimates of dust density, n,,, to determine if the self=
side of'the equation. t ow.,istent dust cross section is in any way realistic for this
[ 361 The fifst tcnrt oil the right-hand side of equation (1) v'a'lue of q,.
represents the photoionization sources of electrons. The [-,n] As indicated in Figure 2, during- the C:assint Encela-
photoiotiization rate of [1 0- af, it Saturn is —4 x 10 `'- dus f?ybv. dust (water-icy) was directly- detected via impact
see [Rieharchi:on of cat., 1998 and for €;assini 1Ni;1S-mca- ionization in ali extended region about the body (=30 R,)
sured water levels at i 0' An , we obtain a si7 , sotarce symmetric about CA, Eased on counting RPWS wideband
wren, S, of-.40 el/na'-s. vvaveforni b=polar impact signatures, it was found impacts
[.i%.[ Electron dissociative recombination (111,O t- e 1-1 on the approximate order of 1000 per second were detected
0714) is considered the dominant recombination process between 19:04-19:10 SCET. So mans impacts occurred
[Richard-an, 19921 and the process that appears to lit-nit during this period. that the automated counter run in the
the molecular ion life time near the orbit of Enceladus R.PWS ground data processing system saturated. The inci-
[.Sif ler• et a1., 20081. The recombination rate constant, kr, dent dust flux (4. second) being sensed by R.P S is F = n ; v
was calculated using the integral k, (2e/r))"' f ^7411 u A. > t 000,'second. For a spacecraft speed of 14 kin" see and
t(u) du- where u is the energy in 0", and ,.74u) the cross- effective spacecraft collecting area of —0.4 m'' in-th et ai—
section of,'tlaa?<t' cal- [.1983,9 varyittg aappro^:irriately as -.: a ^'' 2006; i% ano- et- ul., 20()61, this count rate translates to a dust
We assume a Tvl xwellian distribution of electrons with density exceedin iii > 0.2/Y , .
u4 . n,; ep(
thermal e tiergy u,. t? a} ? uz,.t, such i. Given the constraint on qo. and an independent.
that l u f(a) da = 1. We find that k, — 7 x 10 `` in.:>i impact-based estimate ot` ne, we find an effective cross-
assuming a 5 e`v' electron temperature. In a quasi-neutral section for the electron absorbers to be a,, ^ q,In', - 5 x
situation, nFt2£,:rte ri,,, and the losses frorn electron dlsSO- 10 `' in' which corresponds to a predicted el ect.ve radius
ciative recombination, -k,.n , are —45 ell!m. -s (as we of less than about 1 millimeter -- consistent with electron
discuss below, the plasma is most likely neutral to order absorption by small particles. Thus, the. observed changes in.
of magnitude). As such, recombination appears t€ju"t offset electron flux can be considered consistent with the pres-
photo-ionization sources during this period. ence of large concentrations (> 0.2/m:) of small absorbers
E _ [ fosses via electron dissociative attach.meint can also (<1 Trim). We mote that these are bounding values and there
he derived. Near 6.5 eta, an electron can dissociate water Wray be larger concentrations of ,ubstantially smaller sub-
predoininatety into OH and lf- (e ^ 11LO = 01-1 - 1.14 The inicron sized Grains that would go undetected via impact
crass section for this process has a peak near 5 x 10 m' ionization with the RPWS antenna syrstem [Kurth et call.
and a width of Ali w 1 eV Utaxka,,va and Vl anon, 2002; 20066. 'The fornialism also applies to water clusters (groups
l et€)ry cat cal., 2006-). The rata: constant is thus ka (2o"in)ls.' of molecules) that may, he large in number and also
r-ju,?) Uj 1(utj) An — 5 x 10 1I15% with Liz. -= 6-5 eV and absorbing c lectrons ('although the process Pray be more
ti.,; = 5 eV. The lass rate for this process is thus —k.,i fait;-t, na. similar to dissociative attachment at the cluster level),
°^^ ell /1111"-'S7 ii ret<Frit3v'eiy SiT1af[ L'a1Ur.' as well, [-7t! to the above argumem, the detailed electrostatic
processes are effectively 'lumped` into the absorption cross
4 of 5
1,10203 FARRi:iLL EA_ ATL PLECTRON LOSS NEAR ENC'E'LAL)US 1;1.0203
section, emu, which provides limited physical insight. We Jackson "f. I:.--., 1x'. VJ. Falwell. G. T. Delory, and J. Nithianandani 1200S),
C ,7eci cii' du t aabsorption }n dnc electIi-o : ax al metic p rcitics4 cecurrmg
now consider an independent deist-plasma electrostatic
within NIartian dust atonns, Geopi,ix. R.,:s. L€sir_ . 35, L-16201,
model by Goertz 11989E that describes the micro-physical deli: l IL 1029, 20MG1_034i2 4-
_c
interaction between the plasma and dust. The mode! indi- Ju= S. and J. D. Rjchardso (2_004) A sclf eons ^tcni model of pla n ma
cates that the electrostatic nature of the dusty-plasma is told Sci Y is ai S -zmn N"trad aloud anon pbolog)_ ?. Geo ifv.s Res" 110,
AM20, doi:,0.102912004P,010635.
quantified by a P factor defiled as (a u,,) (nj1njj whore a. is )uric, 4., and J. D. Richardson (2007 1. Neutral cloud in i_ Tactio with
the grain radius and uz is the electron plasma temperature in Saturn'a ntai?i r.rigs. Cie a,a,r, Res L— ,, 34. LOS 102— dri:10.10241
c`v' [see Goer Lz, 1989, Figure 5, 'fable, 1]. For P = t o " n)- 200 GLtJ2c)567.
Kurth, w`. S.. at al. (200((5 j, (assitii 1'iPi " S obse°vations of dusl in Saivan's F,
eV, the larger concentration of fYrains creches o enapping
Debye sheaths that leaves the inter-amain gas mediurn with Mul. P. M_ Ce al. (1£)83), McTged elcc<ron-ton .heat;, cxp-ra it°aa: Y. Ns-
an overall negative plasma potential [see Goetz, 1989, sociaive nxon binati n of 011 H;0` and I3,O . J, r',r,- . 8 Al.
figure 4;. applying the limits provided by the continuity ?1r;:. Ph­ 16, 3()99€
Pes ,on, A. M.. D. A 6unnea, W. S. K€art:n.. B. Hospodar,,ky- J. B.
G.
equation (i.e._ ;^ 10 ^ m, n^ ^- 0.2,rm' U, eV, and n R ar,Ei %M. K. Doug eny r.2(105:. Equatorial zie.Ltron dcn-
Gffo€me7 Cinu,
— 8 x 10 in ) we find that P — 10 m-eV and conclude iav in s,.xct;n's Mar i 41L,IeLo" ,L c. Ge op;? 's-_ Rcs 1 e tl_,
32. 1.23105. dtai:10.Ii129'`200%,102421M,
that the dusty-plasma should indeed be acting in a Collective
N' rsoon, A, M., E). A u€rn,_°a. w', S. K.an fi, mW J. f3. G—tOa e C006), A
way. From Goer z [ 1989, Figure 5] at P -t- 10 -" m-et', othr e viniple scale }ic!i8M modal of h eleoron de of v in S atut a ptb< n. a disk,
plasma potential drop is predicted to be o , --M/ U" /e a ,t,'t= S. R 4'. LE'ZF., 's 3, j ; 1 t f1f.;, der:: [, i - 3 .()it!3( 2'ft Ji,.
few volts negative, and should result in (at least) a factor of Pctst;tis, D. 11, jr- and Y' 4rk`. t'till 02006), F,3.iccladus: 'A signi.f mn_ plasma
2 decrease in electron density In (tact_ the RPM'S L,angmuir dsotriu: rce iiFtr %S2a( YwCr)nfiJ's_ iin ll e1l 6gp?4,e.Cta ptacre, .l Cie>c;131s :,s. Res-. 111, A69214,
S C). i ti2)
Probe detected. a clear Fai`1d distinct drop in plasma potential Porm C C_ r, al. (124)(36). C: s ini tic^,, c. vts : is active south pole of En-
of a few volts exactly coinc dent with the near-Fncetadu celadus. Scierwi% 3# a93,
electron dropout, Ri kardsn;., .l. 1). i.t 3a3"_.S. A nl[ Tnodet fbr plasma nainsport ind clierssistrc
2€ Smunn,.1, Gv€) h 1{c ., 97. i.3, 1
J. l)., and S. Jwnac. (2004), - sett=c(a isiste t modei of'plasti.a
4. Conclusions and wu rats aa,, Saturt: 1-he .ion tone Gcopi= . Res'. t a., 3I, 1_248€'3,
dpi: I ti.1029i20(14(il,C12#1959.
Rxit rdsor, .t_ D.. - L aa. 099s), € li ill s atum' - _- to ct < derc [^bsers -
L^`e considered a number of p. ossible loss ppr"ocesses tions and implications. d. Gcojjlt, - RF _. 103, 20.215.
t.o explain the near- neelidus electron drop-out including Shcman3ky, D. E., i ai. (; 99 1, €} (eetroi; cif the hyda)\ I radical in Saturn',
water-ion recombination and. water molecule dissociation, I-naf._ Lftosp1"lim,, _ `wt-rier. 36-3, 3213.
but found that these could not generate the sharp and Sitdcrt I:i C_ ct A. (2008), Ton and n.eu- t > .urc s and sinks within `satum,'
distinct gradient. However, if we consider absorption of iri:ier r 1a« act,,spltciv: Gl,s; ni ros€tits, llkmea- Space Sea., 56, 3.
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