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This chapter summarizes our current understanding of the ionosphere of Saturn. We give an overview of Saturn ionospheric science from the Voyager era to the present, with a focus on the wealth of new data and discoveries enabled by Cassini, including a massive increase in the number of electron density altitude profiles. We discuss recent ground-based detection of the effect of "ring rain" on Saturn's ionosphere, and present possible model interpretations of the observations. Finally, we outline current model-data discrepancies and indicate how future observations can help in advancing our understanding of the various controlling physical and chemical processes., Comment: 34 pages, 17 figures, Chapter 8 of a forthcoming multi-volume work entitled "Saturn in the 21st Century"

A series of near-equatorial radio occultations of Cassini by Saturn occurred in 2005 and again in 2009-2010. Comparison of the temperature-pressure profiles obtained from the two sets of occultations shows evidence of a descending pattern in the stratosphere that is similar to those associated with equatorial oscillations in Earth's middle atmosphere. This is the first time that this descent has been observed in another planetary atmosphere. If absorption of upwardly propagating waves drives the descent, the implied absorbed flux is 0.05 square meters per square second at least as large if not greater than on Earth.

We present results from the two radio occultations of the Cassini spacecraft by Titan in 2006, which probed mid-southern latitudes. Three of the ingress and egress soundings occurred within a narrow latitude range, 31.34 deg S near the surface, and the fourth at 52.8 deg S. Temperature - altitude profiles for all four occultation soundings are presented, and compared with the results of the Voyager 1 radio occultation (Lindal et al., 1983), the HASI instrument on the Huygens descent probe (Fulchignoni et al., 2005), and Cassini CIRS results (Flasar et al., 2005; Achterberg et al., 2008b). Sources of error in the retrieved temperature - altitude profiles are also discussed, and a major contribution is from spacecraft velocity errors in the reconstructed ephemeris. These can be reduced by using CIRS data at 300 km to make along-track adjustments of the spacecraft timing. The occultation soundings indicate that the temperatures just above the surface at 31-34 deg S are about 93 K, while that at 53 deg S is about 1 K colder. At the tropopause, the temperatures at the lower latitudes are all about 70 K, while the 53 deg S profile is again 1 K colder. The temperature lapse rate in the lowest 2 km for the two ingress (dawn) profiles at 31 and 33 deg S lie along a dry adiabat except within approximately 200m of the surface, where a small stable inversion occurs. This could be explained by turbulent mixing with low viscosity near the surface. The egress profile near 34 deg S shows a more complex structure in the lowest 2 km, while the egress profile at 53 deg S is more stable.

Recent ground-based and Cassini CIRS thermal-infrared data have characterized the spatial and temporal characteristics of an equatorial oscillation in the middle atmosphere of Saturn above the 100-mbar level. The CIRS data [I] indicated a pattern of warm and cold anomalies near the equator, stacked vertically in alternating fashion. The ground-based observations s2, although not having the altitude range or vertical resolution of the CIRS observations, covered several years and indicated an oscillation cycle of approx.15 years, roughly half of Saturn's year. In Earth's middle atmosphere, both the quasi-biennial (approx.26 months) and semi-annual equatorial oscillations have been extensively observed and studied (see e.g., [3]), These exhibit a pattern of alternating warmer and cooler zonal-mean temperatures with altitude, relative to those at subtropical latitudes. Consistent with the thermal wind equation, this is also associated with an alternating pattern of westerly and easterly zonal winds. Moreover, the pattern of winds and temperatures descends with time. Momentum deposition by damped vertically propagating waves is thought to play a key role m forcing both types of oscillation, and it can plausibly account for the descent. Here we report the direct observation of this descent in Saturn's equatorial atmosphere from Cassini radio occultation soundings in 2005 and 2009. The retrieved temperatures are consistent with a descent of 0.7 x the pressure scale height. The descent rate is related to the magnitude of the wave forcing, radiative damping, and induced meridional circulations. We discuss possible implications.

Ground-based and Cassini CIRS thermal-infrared data have characterized the spatial and temporal characteristics of an equatorial oscillation in Saturn's middle atmosphere above the 100-mbar level. The CIRS data indicate a vertical pattern of alternating warm and cold anomalies at the equator. From the thermal wind equation this implies a concomitant reversal of zonal winds with attitude, relative to the cloud-top winds, with peak-to-peak amplitude approximately 200 meters per second. The ground-based observations do not having the altitude range or vertical resolution of the CIRS observations, but they cover several years and indicate an oscillation cycle of 1 years, roughly half of Saturn's year. Equatorial oscillations in Earth's middle atmosphere have primarily exhibited either quasi-biennial or semi-annual "periodicities," and both types have been extensively observed and modeled. They exhibit a vertical pattern of alternating warmer and cooler zonal-mean temperatures and zonal winds analogous to that described above for Saturn. Moreover, the pattern of winds and temperatures descends with time. Momentum deposition by damped vertically propagating easterly and westerly waves is thought to play a key role in forcing both types of oscillation, and it can plausibly account for the descent. Here we report the direct observation of this descent in Saturn's equatorial atmosphere from Cassini radio occultation soundings in 2005 and 2009. The retrieved temperatures are consistent with a descent of 0.6 x the pressure scale height over this time period. The descent rate is related to the magnitude of the wave forcing, radiative damping, and induced meridional circulations. A simple calculation implies that vertical wave fluxes of zonal momentum approximately 0.05 square meters per square second could account for the observed vertical descent on Saturn, which is comparable to the magnitude of the wave fluxes associated with the terrestrial quasi-biennial oscillation.

The radiative response of Titan's atmosphere varies by several orders of magnitude with altitude. This presents an interesting situation in which most of the stratosphere---at least that part above 100km is characterized by radiative relaxation times that are short compared to the length of a Titan season, and the troposphere and tropopause region by times that are larger than seasonal timescales. Consistent with this, Cassini CIRS spectra indicate stratospheric temperatures at 100-170 kin that are 20-30 K cooler at high northern latitudes in winter than those at equatorial and southern latitudes. Given the expectation that the situation will largely reverse in southern winter, the observed large meridional contrast is likely indicative of the expected seasonal variation at polar latitudes in both hemispheres. CIRS spectra do not as easily yield temperatures below 100 km in the lower stratosphere and tropopause region, because of the contribution of heterogeneously distributed aerosols and condensates to the infrared opacity. However, Cassini radio occultations probe both the stratosphere and troposphere, and below 80 km they show the thermal contrast with latitude to be muted, e.g, approx.5 K near the tropopause at 40-50 km and approx.3 K just above the surface. This is consistent with the large radiative relaxation times at these altitudes and with efficient meridional heat transport. What is curious is the manner in which temperatures in the north winter polar atmosphere make the transition between the troposphere and lower stratosphere, where seasonal variations are relatively small, and higher altitudes, where they are large. Temperatures at all latitudes sounded by the radio occultations exhibit similar behavior in the lower stratosphere, increasing with altitude. Between 80 and 100 kin, however, the temperatures at high northern latitudes exhibit a sudden drop with increasing altitude, producing the meridional contrast in the upper stratosphere described above. While the radiative relaxation time associated with infrared gaseous coolants decreases with altitude in the stratosphere, it is not likely to account for the abrupt transition observed. Possible mechanisms for this transition are discussed, including the presence of an optically thick. cloud at thermal-infrared wavelengths.

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The electron density distribution in the ionosphere of nonmagnetic (or weakly magnetized) planet depends not only on the solar ultraviolet intensity, but also on the nature of the SW interaction with this planet. Two scenarios previously have been developed based on the observations of the bow shock crossings and on the electron density distribution within the ionosphere. According to one of them Mars has an intrinsic magnetosphere produced by a dipole magnetic field and the Martian ionosphere is protected from the SW flow except during "overpressure conditions, when the planetary magnetic field can not balance the SW dynamic pressure. In the second scenario the Martian intrinsic magnetic dipole field is so weak that Mars has mainly an induced magnetosphere and a Venus-like SW/ionosphere interaction. Today the possible existence of a sufficiently strong global magnetic field that participates in the SW/Mars interaction can no longer be supported. The results obtained by the Mars-Global-Surveyor (MGS) space-craft show the existence of highly variable, but also very localized magnetic fields of crustal origin at Mars as high as 400-1500 nT. The absence of the large-scale global magnetic field at Mars makes it similar to Venus, except for possible effects of the magnetic anomalies associated with the remnant crustal magnetization. However the previous results on the Martian ionosphere obtained mainly by the radio occultation methods show that there appears to be a permanent existence of a global horizontal magnetic field in the Martian ionosphere. Moreover the global induced magnetic field in the Venus ionosphere is not typical at the solar zenith angles explored by the radio occultation methods. Additional information is contained in the original extended abstract.

The Galileo orbiter has provided radio occultation measurements of the electron density profiles of the plasma surrounding lo and Europa. There have been six occultations of Io, providing twelve electron density profiles at various locations relative to the ram direction of the impinging particles of the Jupiter magnetosphere on Io, and eight profiles on Europa. The two satellites were found to have very different plasma environments, with Io having a proper ionosphere produced on top of an endogenous SO2 atmosphere by magnetospheric particle precipitation and solar EUV, while Europa has a tenuous plasma environment produced by the same mechanisms from an oxygen atmosphere itself also produced by sputtering of water ice from its surface by impinging magnetospheric particles. In both cases the observed electron density profiles are highly asymmetrical, with a compressed profile on the ram side, and an extended one on the wake side. The presence of several measurements for each satellite at different ram-to-wake directions provided data for estimating an approximate distribution of ionization from the ram direction to the wake direction.

The ongoing Galileo mission has provided many new insights into the Jovian system. Among them are new discoveries from the Radio Science investigations , including multiple measurements of the Jovian ionosphere, the ionospheres and plasma environments of Io, Europa, Ganymede, and Callisto, and the internal structure of the Galilean satellites. The Cassini spacecraft, which will be placed in orbit about Saturn in 2004, will conduct Radio Science investigations of many aspects of the Saturnian system with a radio instrument of unprecedented stability and versatility. It will use radio links at three wavelengths : S-band(13 cm), X-band (3.5 cm), and Ka-band (1 cm) to probe the atmospheres and ionospheres of Saturn and Titan and Saturn's rings by means of radio occultations, and to measure the masses and gravity fields of Saturn, Titan, and selected icy satellites by precision tracking. In addition, the stability of the radio instrument will be utilized to conduct a search for gravitational waves during solar oppositions, and to precisely measure general relativistic effects during solar conjunctions during the interplanetary cruise prior to arrival at Saturn.

The Galileo orbiter has provided radio occultation measurements of the electron density profiles of the plasma surrounding Io and Europa. There have been six occultations of Io, providing twelve electron density profiles at various locations relative to the ram direction of the impinging particles of the Jupiter magnetosphere on Io, and eight profiles on Europa. The two satellites were found to have very different plasma environments, with Io having a proper ionosphere produced on top of an endogenous SO2 atmosphere by magnetospheric particle precipitation and solar EUV, while Europa has a tenuous plasma environment produced by the same mechanisms from an oxygen atmosphere itself also produced by sputtering of water ice from its surface by impinging magnetospheric particles. In both cases the observed electron density profiles are highly asymmetrical, with a compressed profile on the ram side, and an extended one on the wake side. The presence of several measurements for each satellite at different ram-to-wake directions provided data for estimating an approximate distribution of ionization from the ram direction to the wake direction, which were compared with the results of MHD simulations.

The availability of a large body of radio occultation data for the Mars and Venus ionospheres makes possible comparative studies of their structures and temporal behavior. On Venus, the nightside ionosphere is produced by the transport of ionization from the dayside, which operates at solar maximum when the ionopause is high, and impact ionization by energetic electrons, which operates at all times. On Mars, transport from the dayside would seem to be inhibited for the same reason as for Venus at solar minimum, although sufficient flux of energetic electrons is present to generate nightside ionization. The Venus ionosphere at solar minimum, and that of Mars at all times, are not in a state of diffusive equilibrium above the chemical equilibrium regions.

Results are presented of observations from the changes in the electron density structure of the dayside ionosphere of Venus that were brought about by changing solar activity. The ionopause height is generally low for values of the solar zenith angle below about 50 deg regardless of the phase in the solar cycle. At solar maximum, and at times of intermediate solar activity, the ionopause height for solar zenith angles greater than about 50 deg is highly variable, ranging from a minimum of about 200 km to a maximum of more than 1000 km. At times of solar minimum the great majority of all ionopause heights for all solar zenith angles are uniformly low, lying between 200 and 300 km. It is argued that the compressed nature of the Venus atmosphere at solar minimum is produced by permeation of the ionosphere by the solar wind magnetic field, which occurs when the solar wind dynamic pressure exceeds the ionospheric plasma pressure.

The behavior of the Venus nightside ionosphere at solar maximum and solar minimum is discussed based on Pioneer Venus radio occultation measurements. Although some solar maximum measurements are similar to those observed at minimum, which have an average peak density of about 7000/cu cm, others show much higher peak densities, reaching values of about 40,000/cu cm. These elevated peak densities also occur at higher altitudes. The integrated electron column densities for these measurements indicate the presence of substantial ionization above the main peak. The magnitudes of both the peak density and the integrated content above the peak are anticorrelated with the solar wind dynamic pressure, indicating that these enhancements during solar maximum are due to transterminator transport of O(+) ions from the dayside. The resulting ionization peak can be many times the concentration produced by energetic electron fluxes impacting the neutral atmosphere on the nightside.

Remote sensing radio occultation measurements are used here to study magnetization of the ionospheres of Venus and Mars. For Venus, the measurements yield results on frequency of occurrence of magnetization during solar maximum that are similar to those obtained from Pioneer Venus in situ magnetic field measurements. During solar minimum, magnetization of the Venus ionosphere is more pervasive than at solar maximum. Magnetization extends to higher solar zenith angles and appears stronger than at solar maximum. These results confirm that during solar minimum the high solar wind dynamic pressure state is more prevalent at Venus because the ionospheric plasma pressure is weaker than at solar maximum. Comparison of a large number of electron density profiles of Mars with those of Venus shows an absence of the ledge and disturbed topside plasma observed in the Venus profiles. These results do not constitute evidence against magnetization of the ionosphere of Mars.