Indian Journal of Radio & Space Physics Vol. 16, February 1987, pp. 192-206 Ionospheres of Outer Planets KKMAHAJAN Radio Science Division, National Physical Laboratory, New Delhi 110012 Received 26 December 1986 Ionospheres of Jupiter, Saturn and Uranus are reviewed on the basis of existing experimental measurements and theoretical models. The radio science experiments on Pioneer 10 and 11 and Voyager 1 and 2 indicate the presence of eq- uatorial anomaly in the ionospheres of Jupiter and Saturn. Peak electron densities inferred by interpreting electrostatic dis- charges as lightning associated radio bursts, indicate large diurnal changes in the ionospheres of Saturn and Uranus. Simple photochemical models cannot explain the altitude of peak electron density in case of Jupiter and the peak electron density, as well as its altitude in case of Saturn. Factors like ion drifts, Hz vibrational temperatures, particle precipitation, water from the rings can play major role in the ionospheres of these planets. It appears that while the ionosphere of Jupiter is controlled by particle precipitation processes, the ionosphere of Saturn is influenced by water from the rings. For U ranian ionosphere, particle precipitation as well as water from the rings, are likely to be important. 1 Introduction sphere. In the sections that follow, we shall first review Although the first successful planetary mission to the ionosphere of Jupiter, then of Saturn and at the explore the atmospheres and ionospheres of planets, end that ofU ranus. This review is an update of an ear- other than the earth, started with the Mariner- 2 flyby lier review by Strobel' as far as the experimental re- of Venus as early as 1962, yet the first information on sults are concerned. While reviewing ionosphere of the ionosphere of an outer planet was available only in Jupiter, a good deal of material has been selected 1975, with the radio occultation experiment on from recent papers by McConnell et al? and Waite Pioneer 10. Since then there have been three more et a[.3 missions, namely, Pioneer 11, Voyager 1 and Voyager 2 and all these three have provided information on the 2 Ionosphere of Jupiter ionospheres of Jupiter and Saturn through the same The earlies tin teres t in the ionosphere of Jupiter, as radio occultation technique. Some major characteris- mentioned by Strobel' was initiated following the ob- tics of the ionosphere of Uranus have also been mea- servations of radio emissions at 20 MHz from this sured by the radio science experiment on Voyager 2. planet. Gardner and Shain" tried to explain these as The Planetary Radio Astronomy experiment on Voy- possible plasma oscillations of the Jupiter's ionos- ager 2 has provided information on the peak electron phere thus requiring peak electron densities of the or- densities of the ionospheres of Saturn and Uranus. der of 5 x 106 em - 3. Rishbeth" immediately ex- Table 1 summarizes some relevant information on the plained these high densities by proposing that H + various missions, along with the flyby dates for the ions were formed by photoionization of H, produced three outer planets, the ionospheres of which we shall from total dissociation of H2. By making a compara- describe and discuss in this paper. tive study with the Fl region of the earth'sionosphere, While the major photochemical processes in the Rishbeth' obtained a value of 50 cm - 3S - 1 for H + pro- upper atmospheres and ionospheres of various pla- nets are similar, factors like neutral composition, energy input, acceleration due to gravity, magnetic Table 1- Planetary Missions to Outer Planets field, etc. play a dominant role in controlling the rela- Mission Flyby at tive importance of the various processes. Some of these factors depend upon the physical features of the Jupiter Saturn Uranus planet including its size, density, distance from the Pioneer-lO 4 Dec. 1973 sun, etc. The important physical features of Jupiter, Pioneer-l l 3 Dec. 1974 1 Sep. 1979 Saturn and Uranus are given in Table 2. The features Voyager-I 5 Mar. 1979 12 Nov. 1980 also include the length of the day lnight, as this could Voyager-2 9 July 1979 26 Aug. 1981 24 Jan. 1986 playa major role on the existence or decay of the iono- 192 MAHAJAN: IONOSPHERES OF OUTER PLANETS Table-2- Physical Features Parameter Planet Jupiter Saturn Uranus Mean distance from Sun, 7.783 14.27 28.69 108km Sidereal year, earth years 11.86 29.46 84.01 Rotation period 9 hrSO min30s lOhr14min 11hr Rotation sense Direct Direct Retrograde Inclination of axis 3°0S' 26°44' 82°S' Inclination of orbit (deg) 1.3 2.5 O.S Equatorial diameter (km) 1,42,800 1,20,000 Sl,800 Mass (earth 1) 317.9 9S.2 14.6 Density(g em - 3) 1.3 0.7 1.2 Magnetic dipole, 4.22 0.21 0.23 (gauss R3) Magnetic dipole tilt (deg) 9.6 60 JUPITER A,A)PIO ENTRY -500 K 0< Mil2 6000 (a) E B) PIO EXIT "" C) PII ENTRY ~ 400 5000 ~ 4000 ':J 300 - <! 3000 ~ 200 f- _ 2000 <I ~ 100 ...E 0: 1000 LIJ O~~ __ -L~L- __ ~~ __ ~~ -L ~ 0 3 2 10- 10- f-=> ION PRODUCTION RATE (c;:,,3s-l) f- A) VI ENTRY (b) ...J 6000 B) VI EXIT <X Fig. 1- Photo ion production rates P(X + ) in the ionosphere of Ju- C) V2 ENTRY piter [Note that the photodissociative ionization of Hz is the major 5000 O)V2 EXIT sourceofH + production (after Atreyaand Donahue").] 4000 duction rate and by using radiative recombination as 3000 the loss mechanism for H + he obtained the desired electron density. Mclilroy" pointed out that direct 2000 photoionization of H is not an important source of 1000 H +, but it is the dissociative photoionization of H2 OL.. ----3L-.-" 5 which is the major source of H + • This is so because 2 6 atomic hydrogen is about three orders of magnitude LOG ELECTRON DENSITY ( em3) smaller than molecular hydrogen in the main ionos- Fig. 2(a)-Sketch of Pioneer electron density profiles in the ionos- pheric region. Fig. 1 shows model calculations of phereofJupiter[A= PlO entry; B = PlO exit;C = PlI entry. Curve photoion production rates made by Atreya and D is a model electron density profile' obtained using model atmos- 7 phere A of Fig. 5 (after McConnell etaF).] Donahue , when dissociative photoionization of H, is Fig. 2(b )-Sketch of Voyager electron density profiles in the ion- included. osphere of Jupiter [A:Vl entry; B:Vl exit; C: V2 entry; D: V2 exit. Curve E is a model ionosphere calculated using the parameters 2.1 Radio Occultation Measurements on Jupite •. discussed in the text and with zero flux and Hz vibrational temper- The first measurement of electron concentration in atureequal to neutral temperature (after McConnell etaF).] the ionosphere of Jupiter was reported by Fjeldbo et at.8, following the radio occultation measurements on L3 peak around 900 km - all the heights being with Pioneer 10. This measurement is shown in Fig. 2(a) refe-rence to 1mbar pressure level. Many of the lower (curve A). The investigators have identified many layers seen in Fig. 2(a) are perhaps artifacts of the in- peaks and ledges, namely, Ll, 12, L3, etc. in the elec- version process caused by multi path propagation and tron density (Nel profiles. The Ll peak is seen to ap- ionospheric inhomogeneities and thus may not be pear around 1600 km, the 12 ledge around 1100 and really existing" The plasma scale heights derived 193 INDIAN J RADIO & SPACE PHYS, VOL 16, FEBRUARY 1987 from the topside electron distribution from these The exit measurements have not been reported but measurements have varied between 800 km for the Fjeldbo et al.9 have stated that L1 and L3 peaks occur exit to about 1000 km for the entrance profile, there- at 1800 and 750 km, respectively. by implying a very hot thermosphere. There was further improvement on the radio sci- The S-band (2.3 GHz) occultation experiment on ence experiment on the Voyager mission with two co- Pioneer 10 utilized an on-board crystal oscillator as a herent radio frequencies at 2.3 and 8.3 GHz. In addi- frequency reference. The harsh radiation environ- tion, the signal strengths were high and the on-board ment affected the reference frequency during the ex- oscillator had better stability and more radiation re- periment, and therefore two electron density profiles sistant capability. All the four N, profiles obtained were derived by the experimenters - one assuming a from Voyager measurements 10.11 are shown in Fig. continuous drift of the oscillator (curve A) and the 2(b). These profiles do not extend much below the other with a different oscillator drift rate (curve A). first peak, which on Pioneer terminology can be Although there are distinct changes in the profile termed as L1 peak. According to Eshleman et al." the characteristic like the scale height reducing to 375.km peak electron density for the exit profile on Voyager 2 4 and peak density to 3 x 10 em - 3, other features like is expected to be more than 2 x 105 em - 3 and the the L1 and L3 peaks and 12 ledge remain the same". height of the peak lower than 700 km. Other charac- The problem about the oscillator drift was attempt- teristics of the Voyager, as well as the Pioneer profiles, ed to.be solved on Pioneer 11where, in addition to the are given in Table 3, reproduced from the paper by on-board oscillator, there was an earth-based trans- McConnell et al.21t may be noted that with the excep- mitter with a stable reference frequency at 2.1 GHz tion of Voyager 2 entry profile, all the other Voyager which was used to transmit to the spacecraft.