Io's Ionosphere Research Articles

The SO 2 atmosphere and ionosphere of Io: Ion chemistry, atmospheric escape, and models corresponding to the Pioneer 10 radio occultation measurements

Models of Io's ionosphere at the time of the Pioneer 10 encounter are constructed in the presence of an SO 2Na atmosphere on Io. The formation of the observed ionosphere on the downstream side requires precipitation of electrons; solar EUV alone is inadequate. Electron impact in the range 500–800 eV on an SO 2 atmosphere with a surface density of 14 × 10 10 cm −3 provides the best fit to the Pioneer 10 radio occultation entry data. The SO 2 +, the major ion produced, is converted rapidly to SO + and in turn to S + by reactions with the dissociation products of SO 2. Ion chemistry leads to the formation of S + as the dominant ion at and above the ionospheric peak. Na + would dominate the ion composition near the surface, and it provides important constraints on the amount of Na allowed in the atmosphere. The relatively narrow energy range and flux required for incident electrons suggests that a fraction of torus plasma is accelerated in the wake region and penetrates deep into the atmosphere. On the upstream side the torus plasma compresses the ionosphere. These characteristics support the possible presence of a weak magnetic field associated with Io. S + ions would escape from Io in the wake region at a rate of up to 10 26 sec −1.

Electromagnetic heating of Io

Electrical‐energy deposition within Io is a possible mechanism for heating the interior, the source of the electrical current system being the relative motion of Io with respect to Jupiter's magnetic field. The transverse‐electric mode currents circulate entirely within Io, with the frequency of Jupiter's rotation rate as observed in the Io frame. Maximum heating for Io is obtained if the electrical conductivity is 0.02 mho m−1, a value spanned by estimates of rock conductivity for hot interiors. Under present conditions, the maximum heating available from the transverse‐electric mode is some 3 orders of magnitude less than would be necessary to maintain Io's estimated heat flow. Confinement of Io's field by the external plasma would raise this estimate only moderately. The transverse‐magnetic mode is effectively blocked by the cold outer surface layers, the current being bypassed by an alternative current conduction path through the ionosphere, leaving an insignificant amount in the interior. Although the transverse‐magnetic heat source can be concentrated at local surface hot spots, the temperature rise is not significant on a time scale appropriate to localized heat sources. It is possible that significant heating was generated in the past by a magnetic field that was larger and had a higher frequency. Heating could have been significant also if Io's ionosphere had not been present.

Joule heating of Io's ionosphere by unipolar induction currents

The unexpectedly large scale height of Io's ionosphere (Kliore, A., et al., 1975, Icarus 24, 407–410) together with the relatively large molecular weight of the likely principal constituent, SO 2 (Pearl, J., et al., 1979, Nature 280, 755–758), suggest a high ionospheric temperature. Electrical induction in Io's ionosphere due to the corotating plasma bound to the Jovian magnetosphere is one possible source for attainment of such high temperatures. Accordingly, unipolar induction models were constructed to calculate ionospheric joule heating numerically. Heating rates produced by highly simplified models lie in the range 10 −9 to 10 −8 W/m 3. These heating rates are lower than those determined from uv photodissociative heating models (Kumar, S., 1980, Geophys. Res. Lett. 7, 9–12) at low levels in the ionosphere but are comparable in the upper ionosphere. The low electrical heating rate throughout most of the ionosphere is due to the power limitation imposed by the Alfvén wings which complete the electrical circuit (Neubauer, F.M., 1980, J. Geophys. Res. 85, 1171–1178). Contrary to the pre-Voyager calculations of Cloutier, P. A., et al. (1978, Astrophys. Space Sci. 55, 93–112), our numerical results show that the J × B force density due to unipolar induction currents in the ionosphere is much less than the gravitational force density when the combined mass of the neutral species is included. The binding and coupling of the ionosphere is principally due to the relatively dense (possibly localized) neutral SO 2 atmosphere. In regions where the ions and neutrals are collisionally coupled the ionosphere will not be stripped off by the J × B forces. However at a level above that (to which the ions move by diffusion only) the charged species would be removed. Thus there appears to be no need to postulate the existence of an intrinsic Ionian magnetic field as suggested by Kivelson, M. G., et al. (79, Science 205, 491–493) and Southwood, S. J., et al. (1980, J. Geophys. Res., in press) in order to retain the observed ionosphere.

An explanation for the alternating north-south asymmetry of Io's sodium cloud

The hot Jovian plasma torus discovered by Voyager 1 is responsible for the periodic intensity variations of Io's sodium cloud, which are correlated with Io's magnetic latitude. The plasma torus must be a long-lived phenomenon in spite of its apparent absence at the time of the Pioneer flybys. The hot electrons (∼10 5°K) must be concentrated ∼1 R J from the magnetic equator in order to produce the observed variations. Electron impact ionization in the hot plasma torus is strong enough to form and to maintain Io's ionosphere; the hot plasma torus may be the dominant agent forming the ionosphere. Io's bound atmosphere is dense enough that the plasma torus electrons cannot cause a noticeable variation in its Na emission intensity.

A model of the SO2 atmosphere and ionosphere of Io

The calculations of thermal structure for an SO2 atmosphere of Io lead to exospheric temperatures in 800‐1200K range. The Pioneer 10 electron density profiles can be fit with an SO2 surface density of 1.2 × 1011 cm−3 at 5:30 pm local time and exosphere temperature of 1030°K. Low energy electrons provide the major ionization source but the solar UV absorption dominates the heating of the atmosphere due to the long wavelength absorption threshold of SO2 and large absorption cross‐sections.

A cometary ionosphere model for Io

The ionosphere of Jupiter's satellite Io, discovered by the Pioneer 10 radio-occultation experiment, cannot easily be understood in terms of a model of a gravitationally bound, Earth-like ionosphere. Io's gravitational field is so weak that a gravitationally bound ionosphere would probably be blown away by the ram force of the Jovian ‘magnetospheric wind’ — i.e., the plasma corotating in the Jovian magnetosphere. We propose here a model in which the material for Io's atmosphere and ionosphere is drawn from the ionosphere of Jupiter through a Birkeland current system that is driven by the potential induced across Io by the Jovian corotation electric field. We argue that the ionization near Io is caused by a comet-like interaction between the corotating plasma and Io's atmosphere. The initial interaction employs the critical velocity phenomenon proposed many years ago by Alfven. Further ionization is produced by the impact of Jovian trapped energetic electrons, and the ionization thus created is swept out ahead of Io in its orbit. Thus, we suggest that what has been reported as a day-night ionospheric asymmetry is in fact an upstream-downstream asymmetry caused by the Jovian magnetospheric wind.

Jupiter-Io electrodynamic interaction

Strong interaction between Jupiter and its satellite Io is revealed by the control of the decametric radiation, by the distributions of energetic particles, and perhaps by the location of the boundary of Jupiter's plasmasphere near Io's magnetic flux tube. Two opposed theories of this interaction depend on different relative motions of Io and its flux tube. In one case the flux tube is frozen into Io and moves with Io, while in the plasma-sheath model Io moves freely across magnetic field lines. It is shown that the plasma-sheath model is unacceptable, and that Io must drive its flux tube through the magnetosphere. The first error in the sheath theory is in the mechanism of sheath creation by thermal and photoelectric electrons. The second error is in the neglect of electric currents driven through the external plasma by powerful space-charge fields. The third error is in the neglect of hydromagnetic effects of electric currents in Io: the magnetic perturbations, Lorentz forces and power supplied from the kinetic energy of Io. These effects show that Io's force tube is dragged along with Io. This frozenin model is discussed briefly in connection with energetic electrons, the decametric emission, Io's ionosphere and Jupiter's plasmasphere.

Io's atmosphere and ionosphere: New limits on surface pressure from plasma models

We have studied charge particle impact as a mechanism for the production of Io's ionosphere. Pioneer 10 thermal plasma measurements and magnetospheric plasma models which explain the observed spatial distribution of neutral hydrogen and sodium atoms in the vicinity of Jupiter’s satellite Io imply electron fluxes of ∼1010 cm−2 sec−1. The fluxes and the temperature (∼100 eV) of this plasma suggest that electron impact ionizaion is the dominant process in forming the ionosphere of Io. It is found that the surface number density of the neutral species required to match the observed electron density profiles is ∼109 cm−3 or less. This value is two orders of magnitude lower than previous estimates.

The atmosphere and ionosphere of Io

view Abstract Citations (45) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The atmosphere and ionosphere of Io. McElroy, M. B. ; Yung, Y. L. Abstract Models for Jupiter's innermost Galilean satellite's atmosphere, ionosphere, and sodium airglow are developed on the basis of recent observational data. The sodium emission detected by Brown (1973) is seen to require a collisional excitation process in Io's atmosphere, while the extended sodium emission measured by Trafton et al. (1974) may require scattering of the planetary radiation by an extended sodium cloud. The sodium is presumably present in bound form on Io's surface, and may be released by a sputtering mechanism proposed by Matson et al. (1974). The ionosphere detected by a radio occultation experiment on Pioneer 10 could be attributed to photoionization of atmospheric sodium, provided Io's atmosphere could sustain significant upward motions during daytime and downward motions during nighttime. The incomplete hydrogen torus observed by Judge and Carlson (1974) in the vicinity of Io appears to require a large supply of hydrogen from the satellite's atmosphere. Implications of the hydrogen torus for the energy and mass balance of Jupiter's magnetosphere are discussed. Publication: The Astrophysical Journal Pub Date: February 1975 DOI: 10.1086/153408 Bibcode: 1975ApJ...196..227M Keywords: Atmospheric Composition; Atmospheric Models; Diurnal Variations; Io; Natural Satellites; D Lines; Electron Density Profiles; Hydrogen; Jupiter Atmosphere; Line Spectra; Lyman Alpha Radiation; Planetary Ionospheres; Plasma Clouds; Sodium; Sputtering; Toruses; Lunar and Planetary Exploration full text sources ADS |

Does Io's Ionosphere Influence Jupiter's Radio Bursts?

Goldreich and Lynden-Bell's theory of Jupiter's Io-correlated decametric radiation sets a lower limit to Io's conductivity, high enough to carry the current associated with the radiated power. Dermott's analysis of conductivities of rocks and ice shows no such conductivity at Io's temperature. However, we show that if Io has even a small atmosphere, say of methane as suggested by Binder and Cruikshank, or of argon or nitrogen, it will have an ionosphere with adequate conductivity to meet the above criterion. A requirement for higher conductivity was found by Goldreich and Lynden-Bell on the basis of motion of magnetic lines past Io. This requirement appears to us unnecessary in view of experiments which prove that motion of the lines is not the source of the electromotance.