GEOPItYSICAL RESEAR½'tt I.[•TI'ERS, V()I.. 17, NO. I0, PACjI•S 1717-172t), SEP'I't:•'•MBIœR19½X}

TRITON' TOPSIDE IONOSPHERE AND ESCAPE

Y. L. Yung and J. R. Lyons

Division of Geological and Planetary Sciences,California Institute of Technology

Theprincipal ion in theionosphere of is N+. Ener- the topside ionosphere implies a massive escape rate of geckelectrons of magnetosphericorigin are the primary N + from Triton. The details are worthy of further inves- .:•e of ionization, with a srnMler contribution due to tigation only if the major conceptsprove correct. Unless ?•.•to.[onization.To explain the topside plasma scale otherwise stated, all results in this article are obtained height,we postulatethat N+ ionsescape from Triton. by solving the coupled continuity equationsfor ions and •e lossrate is 3.4 x 107 cm-2 $-t or 7.9 7. 1024ions electronsin a sphericalatmosphere with transport by am- ,-i• D'msociativerecombination of N2+ producesneu- bipolardiffusion [Banks and Kockarts,1973] using the •rat exothermic fragments that can escape from Triton. numericalcode described in Allen eta!. I1981}. Charge •'•e rate is estimatedto be 8.6 x 10• N cm-• s-• or neutrality is rigorouslypreserved at eachlevel of the at- 2•.0x !02• atoms s-l. Implicationsfor the magneto- mosphere. •phereof Neptmmand Triton'sevolution are discussed. Photochemical Models Introduction The model atmosphereadopted in this study is taken The•onosphere of Triton discoveredby Voyager2 [Tyler fromStrobel et at. [1990]for exospherictemperature equal &, 1989]is remarkablein manyways. First, the max- to 95 K, as shown in Figure 1. The simplestmodel we :•:mumel•tron concentration in ingress and egressis 2.3 can think of is one with energetic electrons impacting a 104and 4.6 x 104 cm-a, respectively.These are very pure N2 atmosphereas first proposedby Atreya[1989]. •.argo numbers indeed, if we recall that , with a sitn- N2 is readily ionized, [taratmosphere iN2) but much closerto the sun, has a .peakebctron density of lessthan 3 x 10ø cm-ø [Lindal N2+ e --+ N• + 2e , (Rla) d., 19.83].The sameexperiment in Neptunereported aaelectron density that is lessthan 3 x 10a cm-a (in an followed by rapid recombination, 'H•). The secondpuzzling feature of the iono- •:ere [s the topsideplasma scale height Hp = 128:k 25- +, + . (a7) kin. Nowfor either a molecularion in photochemicalequi- (SeeTable ! for listing,numbering of reactions,and rate :•[bfiumor an atomic ion in diffusiveequilibriurn (in the coefficients.)By trial anderror we d'mcoveredthat a mo- •'•nce of largewinds), we haveH• = 2H,• where noenergeticelectron beam with E = 20 keV per electron *•.hescale height of the correspondingneutral species and energyflux F = 0.4 erg cm-• s-t cansimulate the 1Atreya,1986]. Therefore,H,• = 64 km, a valuevery essentialfeatures of the observedegress electron profile r:k• to the neutral atmospheric scale height of 60 km (for presentpurposes, further fine tuning is not neces- {½orrerpondingto 90 K) deducedfrom the observations sary). But this modelgrossly violates other observations• •beUVS experiments on the Voyager[Broadfoot etal., The thermospherictemperature of 95 K suggestsan en- 1•9:•. Hence,it is temptingto identifythe major ion as ergyinflux of 1.6 x 10-• erg cm-2 s-t [Broadfootetal., :•'•+iTy•r e!al., 1989]. We will show that this •nodel will !989],which is considerably lessthan 0.4 erg cm -2 s-•! •haYed'mastrous consequences. Third, the electrondensi- In addition, this large flux of energeticelectrons will be f,•ezdrop off rapidly below the peak in a manner consis- accompanie.dby an inducedN2c[ stateemission of about t.teatw•th the classical Chapman profile [Charnberlain and 200 R, whichshould be comparedwith the observedemis- l{unten,1987]. Finally, there is an asymmetrybetween sion of 3-5 R. Hence, this model is entirely incompatible }-ngre•(da.wn) and egress(dusk) electron profiles by a f•%o.r of 2. with the upper atmosphereenergetics of Triton. Can N + be the dominant ion in the ionosphereof Tr•- h th•sarticle we attempt to examinethe simplesthy- ton? N + is readily producedby electronimpact, ,.•th• neededto providea satisfactoryaccount of the ..;•[de ionosphereof Triton. We rely heavilyon the neu- N•+e --. N++N+2e (Rib.) •:ralmodeling work of Strobelet al. [1990].No attempt •a• beenmade to providea modelthat is self-consistent and by photo•nization, •'•h •;'heneutral species. Rather this is a preliminaryat- •:•p• [o proposea•'•d explore a new and bold!typothesis: N•+ha, • N++N+e . (R2b) But this ion has the wrong scale height • explainedin the introduction. Now the relationH r = 2H• az d'm- C*•yright1990 by the AmericanGeophysical Union. cu•:e.deax!ier holds in an equilibriumsituation, but not •e• mu•ber 90GL0!•65 in a dynamic•ituation. If the plaSmain the atrnosphexeof Triton interactswith the Neptunianmggn.et•phere, this

!7'! 7 1718 Yungand Lyons, Triton, Ionosphere

Table 1. List of reactions considered in our models. The units for rate coefficients are s-t and..cm 3 s- • for dissociativeand two-body reactions, respectively. The values for photodissociationcoefficients refer to diurnallyaveraged values at the top of the atmosphere.

Rla N• + e -, N•+ + 2e seetext (a) R!b -+ N + N+ + 2½ seetext (a) R2• N• + •v • N• + • J•. = •.4 x •o-•0 (b) R2b • N+N ++e J•=6.7 x 10-• (b) R3 N•+N • N½+N+ k•=1.0 x 10-•* (c) R4 N•+t{• • N•H++H ks=l.7 x 10-• (c) R5 N•+H • N•+H + ki =1.9 x 10-m (c) R6 N++tt• • NH++H ks =7.0 x 10-m (c) R7 N++It • N+H + k6 =1.9 x 10-m (d) as N•+e • N+N •=•.sx •o-• (c) R9 N:H++e • Ne+H %=5.0x 10-7 (d) R10 NH++e • N+H %=2.0 x 10-• (c)

Rll N+ + e • N + hr k•o • 3.8

RI2 H+ • e • II • hY kll • 3.5 x 10-!• (c)

(a) Cross-sectionsforelectron impact axe based on Aje!1o eta/. [1989]and Krishnaku- mar add Srivastava[19901. (b) Adopted[n modelB. Cross-sectionstaken from Kirby et aL [xo7•l, w• •t aL [X0$4],and Moriokaet at. [X0S•]. (c) Prasadand Huntress [1980].(d) Estimatedby analogywith similarreactions.

T (øK) Nil ++e • H+N , (R10) 40 GO 8O !oo !00020 \ ' ,, , ,, , thus leadingto a rapid lossof ionizationbelow the sphericpeak. H2 is alsodestroyed by N• in the reactions, N2++H• --+ N•tt++H N2tl++e • N•+H , (R9} resultingin a net conversionof H• to 2H. Ultimately, H+'s are also formed. We assumethat H + will to the lower atmosphereand charge transfer CHi diffuseto the exosphereand escapefrom Triton. For plicity, we do not includeH + in the model. As will discussedlater, we do not intendto conducta 0 • .t ...... • ,•:.. _f:, 6 8 10 12 14 16 investigationof the bottomsideionosphere in this I• o Number•nsity (•.3) Therefore, by introducing additional loss proc• may obtain an ionosphericprofile that can simulate Figure 1. Model atmosphereof Triton adoptedfor iono- essentialfeatures of the observedprofile. However, sphericstudies. demandsa higherionization rate to compensatefor greaterlosses. Model runs (not shown) indicate that toionizationalone is far from beingadequate {see mightlead to a lossof N+ by escapefrom the . The lossof ionizedparticles at the upper boundarywill discussion).Electron impact is invokedas an sourceof ionization. By trial and error we arrived leadto a steepeningof the plasmagradient, resulting in a scaleheight H•, < 2H,•, as previouslynoted in the study modelshown in Figure 2. At the upper boundary, fluxesare givenby nv,.• wheren denotesthe of the ionosphereof Venus[Nagy et al., 19751. tion of a speciesand v,,.•cis its escapevelocity. A• To accountfor the lossof ionsbelow the electronpeak weinvoke the presence of H• in Triton'satmosphere [Stro- lowerboundary, the mixing ratio of H•, fH,,is fixed, bel et aL, !990]. N+ reactswith H•, thoseof N+ andN• + areset to zero.The model a fairly good fit to the observedtopside ionmphere, N++H2 -. NH++H , (R6) ceptperhaps near the upper boundary, where the uncertanties in the o•erved electron densitiesm followedby, Themajor ion is N+. N3+ is lessabundant due :•m Yungand Lyons,Triton, Ionosphere 1719

lOOO this largeflux wouldbecome unnecessary (see later d•s- cussion).The hardelectrons are neededto reproducethe observedelectron peak. To test the sensitivityof the model •o input parame- ters and boundaryconditions we conducteda number of sensitivityruns in which one changewas made at a time. N2.•.-/'/ '%-%e The resultsare presentedin Figure 4. In casesA and B, the soft{0.5 keV) andhard {20 keV) electronswere, re- spectively, %witched off." In case C, the escapevelocity for N+ at the upperboundary ve•,. (N +) wasincreased by a factorof 3 to 4.5 x !04 cm s-t, resultingin an elec- tron profile with a smaller scale height than that in the standardcase. In caseD, v•,•(N +) wasreduced by the 3.5 4.0 4 5.0 samefaci, or to 0.5 x 104cm s-•, resultingin an electron tog•oNurnber Density (cm-3) profile with a much larger scale height relative to that in case C. Thus, the magnitude of vo• has a major impact Figure2. Comparisonof modelionospheric profile (e) on the slope of electron densities. with Voyageregress observations (eo•,•). The uncer- taintyin e<,t,•is -t-2.3 x 103cm -3 [Tyleret aL,1989]. Conditionsof this modelwere: fH• = ! X 10-a at the lowerboundary (189 km) and v•½ = 7 x 103, 1.5x 104,0 c•n s-t for H,., N+ andN2 + respectively at the upperboundary (974 km).

Figure 4. Sensitivity study of model electron prorite to variations in input parameters and boundary con.di- tionsin the standardmodel (see Table 2). e,•' ob- served; A: without 0.5 keV electrons; B: without 20 keV electrons;C: v,.•,.(N+) = 4.5 x 104cm s-•; D- v,,,.(N+) = 0.5 x 104 cm s-•. F•gure3. Ionizationrates by electronimpact and sunlight [n the•nodel.Rla: N3+e-• N•++2e; Rlb: N•+ Given the crudeness of the model described in titis e--, N+N + +2e; R2a: N•+hu-, N•+ +e; R2b' work, we can only point out the inadequacies,which will N:•+ hv --• N + N+ + e. The integratedenergy fluxes be remedied in a subsequentpublication: diurnal .varia- m 2 x !0 -•, 6.7x !0 -• and2.4 x 10-a era cm-%- • tion, lower ionosphere,energy source and uniqueness. for EUV solarflux (R1), 0.5 keV electronsand 20 keVelectrons (R2), respectively. Concluding Remarks c•t d'•msociative recombination. The concentrations of The interaction between Triton and the Neptunian theor.her ions N2H + and NH+ are muchless than those m•gnetosphereis primarily responsiblefor generatingand • N+ andN• +, andare therefore not shown. maintaining the ionosphereof Triton. Impact by elec- Thecontributions to ion productionrates in the model trons is the principalsource of ionization.N + is the ma- ,ze p.•:nt• in Figure 3. The principalsource of ioniza- jor ion. Its loss from the atmosphere by escape is sur- ½•,n:• el,ectronimpact by softelectrons (0.5 keV), Rla prising but the processmay be characteristicof plan.e- •