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Current Review of the Jupiter, Saturn, and Uranus Ionospheres

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Current Review of the Jupiter, Saturn, and Uranus Ionospheres Adv. Space Res. Vol. 7, No. 12, pp. (12)119—(12)134, 1987 0273—1177/87 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright © 1987 COSPAR CURRENT REVIEW OF THE JUPITER, SATURN, AND URANUS IONOSPHERES J. H. Waite, Jr* andT. E. Cravens** *NASA Marshall Space Flight Center, Huntsville, AL 35812, U.S.A. **Space Physics Research Laboratory, University ofMichigan, Ann Arbor, MI 48109, U.S.A. ABSTRACT The ionospheres of the major planets Jupiter, Saturn, and Uranus are reviewed in light of Pioneer and Voyager observations. Some refinements to pre—Voyager theoretical models are required to explain the results, most notably the addition of significant particle ioniza- tion from “electroglow” and auroral processes and the need for additional chemical loss of protons via charge exchange reactions with water. Water from the Saturn rings has been identified as a major modifier of the Saturn ionosphere and water influx from satellites and/Or meteorites may also be important at Jupiter and Uranus as well, as evidenced by the observed ionospheric structure and the identification of cold stratospheric carbon monoxide at Jupiter. IN~rRODUCTI0N Many excellent reviews have been written on the development of photochemical models for the study of the ionospheres of the major planets /1—5/. Therefore, no attempt will be made here to reproduce a historical summary of the development of the important photochemical schemes for modeling the outer planet lonospheres. ~irthermore, there will be little dis- cussion of the neutral atmospheres of these planets. The neutral atmosphere models will be taken from the Voyager UVS solar and stellar occultation analysis at Jupiter by Festou et al. /6/, at Saturn by Festou and Atreya /7/ and Smith et al. /8/, and at Uranus by Broadfoot et al. /9/. The purpose of this paper is to review the observational data from the Pioneer and Voyager satellites of the outer planet ionospheres and discuss the required modifica- tions to the pre—Voyager ionospheric models referenced above. We will begin with a brief overview of the pre—Voyager ionosphere chemistry, then discuss the Pioneer and Voyager measurements, followed by a post—Voyager analysis of the present stats of knowledge of the Jupiter, Saturn, and Uranus ionospheres. Finally, a summary will be presented of out- standing problems to be solved in order to make further progress. Ion theinistry Overview H 2 is the major neutral gas in the Jupiter, Saturn, and Uranus atmospheres. Therefore, it follows that the major ion formed in the upper atmosphere due to photoionization or particle impact is H2+. Once formed, H2+ reacts quickly with H2 forming H3+, breaking an H2 bond in the process. Earlier models used a rapid recombination rate for H3+ /10/ which resulted in a small contribution of H3+ to the ionosphere. The major ionospheric ion was H~,formed chiefly as a result of dissociative ionization of H2 by photons or particles. The only loss of H+ in the topside ionosphere was the slow process of radiative recombination which led to the dominance of H+ in the ionosphere and to a long lifetime for the ionosphere itself s, several planetary rotations). Recent results, however, suggest that the vibrational distribution of both H3+ and H2 may be extremely important in determining ionospheric structure. McElroy /2/ first suggested that H2 excited to V > 4 levels could possibly charge exchange with H+ and provide an additional loss for protons in the ionosphere. Subsequent models by Atreya et al. /11/, McConnell et al. /12/, and Waite et al. /13/ used this reaction to help explain observed ionospheric profiles at Jupiter. However, these studies simply assumed a vibrational temperature pro- file for H2. More recent calculations of the vibrational distribution of H2 in the upper atmosphere /14/ do indeed suggest an elevated and highly non—Boltzmann distribution of H2 in the Jupiter upper atmosphere which can have substantial effects on the ionospheric profile. Furthermore, very recent results by Smith and Adams /15/ and Michels and Hobbe /16/ indicate that H3+ recombines very slowly in its ground vibrational state and does not rapidly recom- bine unless V > 3. Although the reaction of H2~with H2 to form H3+ generally results in vibrationally excited H3+ /3/, competing processes such as vibration—translation (V—F) (12)119 JASR 7:12—I (12)120 J. H. Waite, Jr and T. E. Cravens dc—excitation by ~2 collisions and spontaneous radiative transitions are efficient in reducing E 3+ to its ground state level. This increases the concentration of H3+ in the ionosphere due to slowed recombination. Topside ions such as H3~,H2+, and H+ can be rapidly lost by reaction with methane, CM4, forming the terminal ions CH~and C2H5+ and higher order hydrocarbon ions. Therefore, the strength of eddy mixing in the atmosphere can also be important in determining ionospheric structure. Results by Waite et al. /17/ showed that fo~a sold isothermal thermosphere, values of the eddy diffusion coefficient from 310. toLarge10 valuescm ofresultedthe eddyindiffusionan ionosphereco- composedefficientof(10~gf withcm2 ~1)peakresulteddensitiesinoflarge>10~abundancesc1n of cH~in the region of maximum ioniza- tion. The enhanced chemical loss of H+ and H 3 cm3. High exospheric2+temperaturesto methane leadsmoderatedto a decreasethis effectin theby movingpeak electronthe solardensityextremetoultravioletI0 ionization region away from the methane layer. Furthermore, there is increasing evidence /18,19/ that the influx of H 2O in the outer planet ionospheres from rings, satellites, and meteorites may significantly affect ionospheric structure at Jupiter, Saturn, and Uranus. A schematic of the standard outer planet ionospheric chemistry is shown in Figure 1; additional H20 chemistry can be found in Figure 15. ~ +CH,~ _ I +hz’(A~912A) +H +2H2 H2 +H / +H2 CH5~ +hv(X~911A) +CH4 H ~e /1 HeH~ +H2 +hv(A~5O4A) He ~e +H2 CH4~ +CH4 +CH4 CH3~ +CH4 C2H5 +H2 CH2~ +H2 CH~ Fig. 1. Schematic depicting the important ion chemistry processes in the ionospheres of Jupiter and Saturn (from /34/). Sources of Ionization Prior to the Voyager mission, solar extreme ultraviolet (EU’.’) radiation was considered to be the major source of ionization for the outer planet ionospheres. However, the Voyager Ultra- violet Spectrometer (UVS) observations indicated that particle precipitation may play an important role in determining the ionospheric structure even at low— or mid—latitudes. The Voyager TJVS observed significant Lyman and Werner band emissions from 142 over the whole dayside disk of Jupiter, Saturn, and Uranus. The phenomenon has been termed the “electro— glow” by Broadfoot et al. /9/. Shemanslcy /20/ suggests that soft electrons at high alti- tudes are responsible for the observed phenomenon at Jupiter. However, more recent ~bserva— tions of the limb of the atmosphere at Saturn /21/ and Uranus /19/ suggest that the source of the “electroglow” emissions is deep in the atmosphere just above the homopause. This may indicate separate “electroglow” mechanisms operative at the different planets and strongly suggests further analysis and observations are needed. The inferred column2 s 4~ntegrated, Saturn 0.13 energyerg cm ~epo~itions , and Uranusrates for0.10theergthreecu2planets5~i• Theare:UVS Jupiterspectra0.32alsoergindicatedcm that each planet C C C C Jupiter, Saturn and Uranus lonospheres (12)121 has a characteristic electron energy that is associated with the electroglow process: Jupiter 50 eV, Saturn 30 eV, and Uranus 15 eV electrons. The particle “electroglow” ionization sources are included in all the modeling profiles in this study by simply introducing a flux of the appropriate energy electrons, distributed in altitude corresponding to the photoelectron production profile, and with a column integrated energy flux as suggested by the Voyager UVS measurements. A sample ionospheric profile for Jupiter is shown in Figure 2. Both H 3+ and H2 vibrational chemistry effects as well as particle ionization from the “electroglow” process (described above) are included in the profile. The vibrational temperature profile is a scaled—down particle precipitation case taken from Cravens /14/ which reaches an exospheric value of 2200 K. The topside ionosphere is dominated by 14+ with layers of 3H3+liesandathydrocarbonan altitudeionsnear just625 kmbelowabovethetheionospheric1 bar pressurepeak. referenceThe peak densitylevel. ofThe 6.6ionospherex 10~cmis very extended due to the hot exospheric temperature. Using this model ionosphere as a point of departure, let us now take a look at the measurements and see how they agree. 3500 \ — ELECTRONS 3000 \ —-—Hi \ H 3~ 2500 E 2000 w E 1500 1000 500 . ._~...“.~.—-———— — 5 i06 10~ DENSITY (cm~3) i0 Fig. 2. Representative ionospheric profile for Jupiter using a high exospheric 1 temperature of 100 K and a moderate eddy diffusion coefficient of 3 x 10 cm The model also includes the new H 3+ and H2 vibrational chemistry, and the “electro— glow” particle ionization sources. PIONEER AND VOYAGER IONOSPHERIC OBSERVATIONS Measurement Technique The structure of the ionospheres and tropospheres of the outer planets has been successfully measured by the technique of radio occultation employed on both the Pioneer and Voyager spacecraft. Information on the gaseous envelope is obtained from measurements of Doppler frequency shift, group delay, intensity, and polarization of the radio signal when the spacecraft swings behind the planetary body and undergoes occultation as viewed from the Earth /22—24/. The Pioneer measurements were carried out using a single frequency (2.293 GHz or S—band at 13 cm) radio link. The Voyager d’.~al—frequency technique is particularly important for the outer planet ionospheres where multi—mode propagation of the beam is caused by sharp ionospheric layers /25/. Furthermore, the signal—to—noise ratio for Voyager exceeds the Pioneer S—band values by 10 dB at S—band frequencies and 23 dB at X—band fre- quencies /25/. Due to geometric considerations, all ionospheric profiles are obtained near the dawn or dusk terminators.
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