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NASA NTRS1974

A search for posteclipse brightening of Io in 1973. II

Four eclipse reappearances of Io were observed with an area-scanning photometer during the 1973 apparition of Jupiter. The results of these observations and of the ones reported in the preceding paper are discussed in the context of recent physical models for posteclipse brightening. An evaluation of the relative merits and deficiencies of all observational techniques which have been used to search for posteclipse brightening of Io leads to the conclusion that the reality of this phenomenon remains very much in doubt.

Franz, O. G.↗
NASA NTRS1975

The atmosphere of Io from Pioneer 10 radio occultation measurements

The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S-band radio occultation measurements of its atmosphere. The day-side entry measurements revealed an ionosphere having a peak density of about 60,000 el/cu cm at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na(+) and 495 K if N2(+) is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9,000 el/cu cm at an altitude of 50 km. The topside plasma temperature is 160 K for N2(-) and 131 K for Na(+). If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10 to 1 trillion per cu cm, corresponding to a surface pressure of less than 10 to 1 nanobars.

Kliore, A. J.↗
NASA NTRS1975

Characteristics of Jovian trapped electrons and protons for the region within 20 Jupiter radii and their interaction with Io

A brief summary is given of Pioneer 10 observations of trapped particles in the inner-core region (within 20 Jupiter radii) of the Jovian magnetosphere. The three sensor systems used to study trapped radiation in this region are described. Intensity profiles are plotted as a function of magnetic-shell parameter (L) for electrons with energies of at least 3 MeV, protons with energies of at least 35 MeV, and protons with energies between 0.5 and 1.8 MeV. The effect of trapped-particle absorption by Io is clearly seen in the intensity profiles of all the observed particle species, and evidence is presented for preferential absorption of small-pitch-angle particles by Io. Conclusive evidence is given for the fact that Jupiter's trapped radiation is maintained by the inward diffusion of particles across L shells.

Simpson, J. A.↗
NASA NTRS1975

Photometric properties of the surface of Io and their influence on line formation in the atmosphere

A quantitative theory is given for line formation in an atmosphere above a surface with backscattering properties. Sufficiently high spatial and spectral resolution spectra of resonance lines in Io region A can yield data on the surface scattering properties as well as the number density of scattering molecules. Macroscopically homogeneous models of scattering from the surface of Io are discussed and it was concluded that multiple reflection from crystal facets is the most likely cause for the observed phase variations of the geometric albedo.

Yung, Y. L.↗
NASA NTRS1975

Electron impact ionization of Io's sodium emission cloud

The geometry of the sodium cloud associated with Io (Jupiter I) indicates that the lifetime of the neutral sodium atoms is an order of magnitude less than the photoionization lifetime. We suggest that ionization by thermal plasma electrons in the Jovian magnetosphere is the dominant Na loss process. Using plasma densities deduced from Pioneer 10 measurements, the lifetime and density distributions are calculated for Na and other species which may be present in the cloud around Io. Electron ionization of Na is found to be an order of magnitude faster than photoionization, in agreement with the lifetime deduced from Na cloud observations.

Carlson, R. W.↗
NASA NTRS1975

High-resolution spectra of Io's sodium emission

The shape of Io's sodium D2 emission profile varies markedly with Io's orbital phase, and its width suggests that the sodium cloud may be expanding with velocities up to 18 km/sec. This offers an explanation for at least part of the orbital phase asymmetry observed for the D-line emission.

Trafton, L.↗
NASA NTRS1976

Proton aurora on Io

The Jovian satellite Io should exhibit substantial auroral activity due to interactions of the satellite's atmosphere with Jovian magnetospheric particles. Plasma sheaths that may form around the satellite will cause substantial proton aurorae on the Jupiter-facing hemisphere and electron aurorae on the outward-facing hemisphere. A sample calculation indicates that Doppler-shifted radiation from high-velocity hydrogen atoms (proton aurora) will be observable from a spacecraft in the vicinity of Jupiter. Emission near Lyman-alpha may be present at a level as much as 100 times that of the solar background reflected from Io's surface.

Pilcher, C. B.↗
NASA NTRS1976

Photometric properties of the surface of Io and their influence on line formation in the atmosphere

The paper presents a quantitative theory of line formation in an atmosphere above a surface with backscattering properties. Sufficiently high spatial and spectral resolution spectra of resonance lines in Io region A can yield data on the surface scattering properties as well as the number density of scattering molecules. Macroscopically homogeneous models of scattering from the surface of Io are discussed, and it is concluded that multiple reflection from crystal facets is the most likely cause for the observed geometric albedo and phase variation.

Yung, Y. L.↗
NASA NTRS1976

Proton aurora on Io

The Jovian satellite Io should exhibit substantial auroral activity due to interactions of the satellite's atmosphere with Jovian magnetospheric particles. Plasma sheaths that may form around the satellite will cause substantial proton aurora on the Jupiter-facing hemisphere and electron aurora on the outward-facing hemisphere. A sample calculation indicates that Doppler-shifted radiation from high-velocity hydrogen atoms (proton aurora) will be observable from a spacecraft in the vicinity of Jupiter. Emission near Lyman-alpha may be present at a level as much as 100 times that of the solar background reflected from Io's surface.

Pilcher, C. B.↗
NASA NTRS1977

Sodium D-line emission from Io - A second year of synoptic observation from Table Mountain Observatory

Measurements of sodium D-line emission from Io are reported which were made during synoptic patrols of the 1975 and 1974 apparitions. Corrected equivalent widths of the D lines are plotted against the satelliite's orbital phase reckoned from superior heliocentric conjunction, and quantitative results of selected patrol observations are provided. The results indicate that: (1) resonant scattering is most likely the dominant emission mechanism; (2) the mechanism which injects neutral sodium into the space immediately around Io underwent no major secular variation in the period between the two apparitions; (3) a less pronounced asymmetry between the peak emission rates at 90-deg and 270-deg orbital phases is present in the selected data; and (4) the measured D2/D1 intensity ratios are consistent with a value of 1.6 + or - 0.3, which corresponds to a value of the order of unity for the maximum optical depth possible for resonant scattering the D1 line center. It is suggested that an unexplained scatter of about 20% in the general emission levels is due to a variation in the ionization rate.

Bergstralh, J. T.↗
NASA NTRS1976

The atmosphere of Io

Observations of Io suggest that it may have an atmosphere in which sodium vapor, ammonia, and nitrogen are important constituents. Several atmospheric models consisting of these gases are tested as a function of total content against the Pioneer 10 observations and for stability against escape. The results suggest that the atmosphere is very tenuous and that the interpretation of the ionosphere detected by Pioneer 10 by a static model may be inconsistent with the sodium-cloud observations. It is postulated that ionization may also be escaping and that sodium may be comparable in content in the atmosphere with some molecular constituent such as NH3 or N2. Sodium and this molecular component then dominate the atmosphere. It is also suggested that particle precipitation contributes to heating of the atmosphere and to the production of ionization; the difference between day- and nighttime ionospheres and possible trailing and leading side effects may relate to the nature of the particle energy distributions. These distributions may result from the peculiar interaction of Io with the Jovian magnetosphere.

Gross, S. H.↗
NASA NTRS1977

The sodium and hydrogen gas clouds of Io

Models are developed to describe the spatial distribution of gases emitted by Io and are applied to recent observations which indicate extensive gas clouds of hydrogen and sodium in orbit around Jupiter. Hydrogen and sodium atoms are emitted from Io with velocities in the range from 2 to 3 km/sec, with fluxes of about 10 billion and 100 million per sq cm/sec for hydrogen and sodium, respectively. Hydrogen atoms may be formed by photodecomposition of gases such as NH3 or H2S released from the satellite surface and may escape thermally from an exosphere whose temperature is about 500 K. Sodium may be ejected from the surface by energetic particles or by ultraviolet radiation, and it appears that a nonthermal mechanism drawing energy from Jupiter's magnetic field is required in order to account for its release to space.

Smyth, W. H.↗
NASA NTRS1978

An emission mechanism for the Io-independent Jovian decameter radiation

A theory of the Io-independent decameter radiation is developed. The radiation results from excitation of the electromagnetic loss-cone instability by keV electrons, stably trapped near L = 6. The radiation is excited in Band 3 of the extraordinary mode. When the effects of refraction are estimated, it is shown that above 10 MHz radiation is beamed into the equatorial plane in a wide, but thin, conical sheet (Psi approximately equals 80 degrees). When the instability analysis is coupled with one of the octupole models of the Jovian magnetic field, the maximum convective growth of the instability occurs in the directions of the non-Io A, B, and C sources. The shape of the peak radio flux frequency spectrum is found to be a consequence of the loss cone shape of the electron distribution function.

Goldstein, M. L.↗
NASA NTRS1977

On determining magnetospheric diffusion coefficients from the observed effects of Jupiter's satellite Io

A method is derived for determining the radial diffusion coefficient from observed satellite effects of the inner Jovian satellites on the energetic particle fluxes. The method is based on data from L values which are significantly removed from the actual sweeping region. With regard to the large losses to the protons at Io's L shell, it is suggested that in addition to satellite sweepup, the losses may be associated with an enhanced precipitation due to resonant interaction with ion cyclotron waves near Io's orbit. It is noted that such additional loss mechanisms may also apply to electrons, and that such losses may significantly affect the estimated diffusion coefficient.

Thomsen, M. F.↗
NASA NTRS1978

On the modulation of the Jovian decametric radiation by Io. I - Acceleration of charged particles

A steady-state analysis of the current circuit between Io and the Jovian ionosphere is performed, assuming that the current is carried by electrons accelerated through potential double layers in the Io flux tube. The circuit analysis indicates that electrons may be accelerated up to energies of several hundred keV. Several problems associated with the formation of double layers are also discussed. The parallel potential drops decouple the flux tube from the satellite's orbital motion.

Smith, R. A.↗
NASA NTRS1978

Reflectance spectra for sodium and potassium doped ammonia frosts - Implications for Io's surface

This paper reports measurements of the reflection spectra of sodium- and potassium-doped ammonia frosts as a function of alkali metal concentration for the wavelength range 0.35-2.5 microns. The purpose of the measurements was to determine whether or not the reflection spectra for such a solid was compatible with the spectra albedo of Io. The data show that, with a sufficiently large alkali metal concentration, the reflection spectra of the doped ammonia frosts do not display the characteristic ammonia features at 2.0 and 2.25 micron. The high reflectance of the more concentrated samples and the character of the observed reflection spectrum make it difficult to rule out sodium-doped ammonia frost as a surface constituent on Io on the basis of existing data.

Rosen, M. D. A.↗
NASA NTRS1978

Io may have a bright dawn terminator

A test of the thermoluminescence properties of Io is proposed which can be performed with the unique optical observations programmed for Voyager 1's close flyby of that Jovian satellite. It is suggested that if Io's surface is thermoluminescent with a characteristic post-eclipse-brightening time of the order of 10 min, there should be a characteristic bright rim on the sunlit side of the satellite's dawn terminator. It is noted that the spatial resolution of six-color pictures to be taken by Voyager 1 should be sufficiently adequate to reveal the bright dawn terminator if it exists and is bright enough. A mathematical model illustrating the bright-dawn-terminator effect is presented.

Forman, M.↗
NASA NTRS1978

Pictures of the Io sodium cloud

We obtained and processed 56 pictures of Io's sodium cloud on 15 nights during the 1976-1977 apparition of Jupiter. These show rapid changes in the appearance of the cloud, but may be consistent with a steady-state cloud whose appearance is controlled by the viewing geometry. The sodium is in a column extending forward from Io, with the axis pointing slightly inside the orbit. The size and shape of the cloud convey information on the anisotropy and velocity of emission and the lifetime of neutral sodium. We find some confirnation for Trafton's findings of north/south asymmetries related to the position of the magnetic field.

Murcray, F. J.↗