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Triton Topside Ionosphere and Nitrogen Escape

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Triton Topside Ionosphere and Nitrogen Escape GEOPItYSICAL RESEAR½'tt I.[•TI'ERS, V()I.. 17, NO. I0, PACjI•S 1717-172t), SEP'I't:•'•MBIœR19½X} TRITON' TOPSIDE IONOSPHERE AND NITROGEN ESCAPE Y. L. Yung and J. R. Lyons Division of Geological and Planetary Sciences,California Institute of Technology Theprincipal ion in theionosphere of Triton 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 Titan, 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•atmosphere). 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 ionization belowthe 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 assume that H + will to the lower atmosphereand charge transfer io CHi diffuseto the exosphereand escapefrom Triton. For plicity, we do not includeH + in the model. As will discussedlater, we do not intend to 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 being adequate{see mightlead to a lossof N+ by escapefrom the exosphere. 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.
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