Io's Sodium Cloud Research Articles

Solar control of sodium escape from Io

AbstractWe report ground‐based spectroscopic observations of the Io sodium cloud before and after eclipse by Jupiter, in a search for possible effects of sunlight on the total atomic sodium content in Io's exosphere. These observations test the importance of the Sun in maintaining the supply of atoms from the thicker atmosphere, which lies close to the surface, to the thinner, more extended, sodium cloud. We performed ground‐based spectroscopy of the sodium doublet at 589 nm with a high‐resolution échelle spectrograph on the Italian National Telescope Galileo on La Palma Island. We find that the total atomic sodium content immediately after eclipse falls by a factor of ~4 during eclipse and recovers to preeclipse values on a timescale of ~5 h. We evaluate potential causes of the precipitous drop including condensation of the atmosphere, collapse of the ionosphere, and changes in atmospheric chemistry. We conclude that most plausible cause of the drop in atomic sodium production is the interruption of photodissociation of sodium‐bearing molecules during eclipse. We discuss further implications of this change in atmospheric chemistry.

Io, the closest Galileo's Medicean Moon: Changes in its Sodium Cloud Caused by Jupiter Eclipse

AbstractWe report results of a study of true temporal variations in Io's sodium cloud before and after eclipse by Jupiter. The eclipse geometry is important because there is a hypothesis that the atmosphere partially condenses when the satellite enters the Jupiter's shadow, preventing sodium from being released to the cloud in the hours immediately after the reappearance. The challenge lies in disentangling true variations in sodium content from the changing strength of resonant scattering due Io's changing Doppler shift in the solar sodium absorption line. We undertook some observing runs at Telescopio Nazionale Galileo (TNG) at La Palma Canary Island with the high resolution spectrograph SARG in order to observe Io entering into Jupiter's shadow and coming out from it. The particular configuration chosen for the observations allowed us to observe Io far enough from Jupiter and to disentangle line-of-sight effects looking perpendicularly at the sodium cloud. We will present results which took advantage of a very careful reduction strategy. We remove the dependence from γ-factor, which is the fraction of solar light available for resonant scattering, in order to remove the dependence on the radial velocity of Io with respect to the Sun.This work has been supported by NSF's Planetary Astronomy Program, INAF/TNG and the Department of Astronomy and Cisas of University of Padova, through a contract by the Italian Space Agency ASI.

The Io Sodium Cloud: Comparison between Observations and Numerical Models

We have refined a method to derive from Io's sodium cloud spectra precise estimates of the production rate and of the ejection velocity distribution at the exobase. This method is based on a detailed comparison between the observed spectra and synthetic spectra numerically simulated with a model derived from the Smyth and Combi (1988,Astrophys. J. Suppl.66, 397–411) formalism. The innovative aspect of our approach consists in building up polienergetic spectra as linear combination of normalized single velocity spectra. In this way we simulate the dispersion of the sodium atom ejection velocities at the exobase and derive for each velocity component a production rate. Smyth and Combi (1988,Astrophys. J.328, 888–918) proposed a similar approach to study the velocity distribution of sodium atoms at the exobase. However, they did not work with spectra but with bidimensional images of the Io's cloud. Moreover they did not try to derive from the model–observation comparison an estimate of the production rate. We have applied the polienergetic model spectra approach to analyze a sequence of 59 observations of the sodium cloud spanning a period of 4 years from February 1990 to April 1994. The echelle spectra have been reduced to separate the fast sodium components and then compared to polienergetic synthetic spectra. We report the behavior of the production rate in the timespan covered by our observations and the analysis of the velocity components at the exobase. We find a relationship between both the production rate and the velocity distribution and λIII.

Observations of Io's sodium cloud and torus

The first results from a new observing programme studying the Io plasma torus and neutral clouds are presented. Images of both Io's sodium cloud and its SII torus were obtained on 11 and 12 Dec 1989 using a focal reducer CCD camera and a tunable Fabry-Perot filter. Masks were used allowing the study of both ansae of the torus simultaneously. The peak brightness in [SII] at 6716Å was 390(±50) Rayleigh at an L-shell of 5.4 (±0.1) R J. The torus was inclined by 7.8(±0.6)° to the Jovian equatorial plane. The line ratio of the forbidden red doublet lines (6716Å/6731Å) was 0.95(±0.15). These results show that the torus conditions in Dec 1989 were significantly different from those at Voyager fly-by but may be similar to conditions observed in Mar 1981 from the ground.

Io's Big Sodium Cloud Looms Even Bigger

Io's Big Sodium Cloud Looms Even Bigger

SODIUM IN IO'S EXTENDED ATMOSPHERE

This dissertation combines several new observations of the Io sodium cloud to create a consistent picture of the extended Io atmosphere and its interaction with the Jovian plasma torus. I used the LPL echelle spectrograph to obtain three types of high-resolution spectra of the extended sodium cloud at the sodium D lines (5890 A, 5896 Â). The first class of observations made use of the mutual satellite eclipses of 1985 to probe the density profile of the atmosphere in the range 1.4 to 10 Io radii, a previously unstudied region. The second type of observation examined the sodium emission in the immediate vicinity of Io, allowing an accurate measurement of the velocity structure around Io. The final method employed a high-sensitivity detector to study faint jets of high-speed sodium farther out in the extended cloud. The synthesis of these three data sets results in a better understanding of how sodium is distributed about Io as a function of position and velocity. The extended atmosphere of Io is composed of many kinematically distinct components. The distribution in space is linked to their characteristic velocities, with low-energy sodium confined near Io and faster atoms (10 to 100 km sec1) prevalent beyond ~ 25 Io radii. The sodium density falls rapidly near Io and more slowly outside 5.6 Io radii, the effective limit of the gravity of Io. The data indicate that the atmosphere is collisionally thick near the surface but becomes thin by an altitude of ~ 700 km. The upper limit of the exobase location is derived from reliable sodium density measurements made during the satellite eclipses. The lower limit is indirectly inferred from the velocity distribution of sodium near Io and the nature of high-speed jets far from Io. The high-speed jets reveal a new type of close interaction between the corotating plasma and the atmosphere of Io. The morphology and brightness of the jets require a two-reaction process in which atmospheric sodium is ionized, accelerated to high speeds, and then charge-exchanges with other sodium atoms. These processes must occur near the atmospheric exobase, indicating that the atmosphere of Io is not completely protected from the plasma flow.

A general model for Io's neutral gas clouds. II - Application to the sodium cloud

A previously developed neutral cloud model is applied to the neutral sodium cloud produced by Io. Past models for the cloud are reviewed and the motivation for developing better models is discussed. The interactions of sodium in the plasma torus are discussed. It is shown how the new model can be used to provide a consistent physical explanation for the basic physical characteristics of the spatially extended Io sodium cloud and for the east-west intensity asymmetry observed very near Io. This leads to definite conclusions regarding the flux distribution and density of sodium in Io's exosphere, the level of the collisional exobase in the atmosphere, the nature of the interaction of the corotating plasma torus and the satellite atmosphere/corona, and the presence of an east-west electric field in the planetary system.

Correlating east‐west asymmetries in the Jovian magnetosphere and the Io sodium cloud

Three east‐west intensity asymmetries in the Jupiter system that would appear to an Earth observer in optical S+, extreme ultraviolet S++ and optical Na emissions are reviewed. The three asymmetries imply that some combination of the plasma torus number densities and electron temperature is, on the long‐term average, larger west of Jupiter than east of Jupiter. A small east‐to‐west electric field, previously proposed to explain the ion intensity asymmetries, is also shown to provide a natural explanation for the sodium intensity asymmetry. A long‐term average east‐to‐west electric field of ∼3 mV/m and a corresponding west‐to‐east offset of ∼0.16 Jupiter radii for the plasma torus at Io's orbital distance is implied. On a shorter time scale, the east‐west electric field appears to be time variable and may be readily monitored from Earth by observations of the sodium cloud.

An explanation of the east–west asymmetry of Io's sodium cloud

Since the discovery of a cloud of neutral sodium atoms accompanying the jovian satellite, Io, in its orbit1, asymmetries of part of the cloud (region B)2 have often been noted3–6. Brightness variations with orbital longitude3,4 and asymmetries found by comparing images taken at orbital longitudes differing by 180° (ref. 5) have been explained as effects of solar radiation pressure7. Given that the vast majority of neutrals precede Io in its orbit, and that sputtering of Io is the principal source mechanism, Matson et al.8 postulated a hemispherical source centred 60° towards the sub-Jupiter point from the centre of the leading hemisphere to explain the general cloud shape. However, soft collisions with plasma ions may be important and call for a reassessment9. I show here that, given reasonable assumptions, the east–west brightness asymmetry can be explained by temporal changes in the number of emitting atoms in the cloud caused by atmospheric shielding of the surface.

Io's sodium directional features - Evidence for a magnetospheric-wind-driven gas escape mechanism

Elongated features in Io's sodium cloud, directed away from Jupiter and inclined both to the north and to the south of the satellite's orbital plane, have been observed. The north/south directions of the features are correlated with Io's magnetic longitude, suggesting a formation mechanism involving the oscillating plasma torus. It is shown by means of a model analysis that the features can result from a source of high-velocity (about 20 km/s) sodium combined with the oscillating neutral sodium sink provided by the plasma. The phase relationship between the features' directions and Io's magnetic longitude can be understood if escaping sodium is initially directed at near right angles to Io's orbital motion. The directionality of the features requires that the sodium flux from equatorial regions be higher than that from the poles. The initial directions and speeds of sodium atoms escaping Io to form the directional features can be understood in terms of a magnetospheric-wind-driven escape mechanism. The one sequence of directional feature observations that has been analyzed in detail implies a high-speed sodium source rate of about 10 to the 26th atoms/s.

Io's Sodium Cloud

The first two-dimensional images of the source region of Io's neutral sodium cloud have been acquired by ground-based observation. Observed asymmetries in its spatial brightness distribution provide new evidence that the cloud is supplied by sodium that is ejected nonisotropically from Io or its atmosphere. Complementary, high-time-resolution, calibrated image sequences that give the first comprehensive picture of the variations of the fainter regions of the cloud extending more than 10(5) kilometers from Io were also obtained. These data demonstrate that the cloud exhibits a persistent systematic behavior coupled with Io's orbital position, a distinct "east-west orbital asymmetry," a variety of spatial morphologies, and true temporal changes. The geometric stability of the sodium source is also indicated. Isolation of the cloud's temporal changes constitutes an important milestone toward its utilization as a long-term probe of Io and the inner Jovian magnetosphere.

Io's sodium cloud - Explanation of the east-west asymmetries. II

view Abstract Citations (18) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Io's sodium cloud - Explanation of the east-west asymmetries. II Smyth, W. H. Abstract A three- dimensional model for Io's sodium cloud incorporating the full effects of solar radiation acceleration associated with solar resonance scattering of atoms in the D-1 and D-2 lines is developed and applied to interpret two different but related east-west asymmetries exhibited in the cloud, using data obtained from ground-based telescopes. The physical changes produced in the cloud by the solar radiation acceleration as a function of satellite phase angle are documented, and how these changes explain the asymmetries is clarified. The model results suggest an east-west phase lag asymmetry of 10-15 degrees; results for the observed east-west intensity asymmetry of Bergstrahl (1977) are also presented. The east-west shape and intensity asymmetries are found to follow directly from the lack of circular symmetry about Jupiter of the forces acting on the cloud atoms. Publication: The Astrophysical Journal Pub Date: January 1983 DOI: 10.1086/160644 Bibcode: 1983ApJ...264..708S Keywords: Io; Molecular Clouds; Radiation Effects; Sodium; Solar Radiation; Asymmetry; D Lines; Data Acquisition; Jupiter (Planet); Light Scattering; Monatomic Molecules; Particle Acceleration; Resonance Scattering; Telescopes; Three Dimensional Models; JUPITER; SATELLITES; IO; SODIUM; CLOUDS; ASYMMETRY; DIAGRAMS; SHAPE; MATHEMATICAL MODELS; INTERACTIONS; SOLAR RADIATION; PRESSURE; RESONANCE; SCATTERING; INTENSITY; OBSERVATIONS; HYPOTHESES; OSCILLATIONS; DATA; PLASMA TORUS; CALCULATIONS; EMISSIONS; VELOCITY; PHASE ANGLES; COMPARISONS; ANALYSIS; Lunar and Planetary Exploration; Jupiter full text sources ADS |

Sodium remote from Io

We find that faint sodium emission originating in the middle Jupiter magnetosphere has two distinct kinematical components. The “normal” signature of atoms on bound orbits with large apojoves seems always to be present, and we suggest these atoms are an extension of the bright, near-Io sodium cloud. The “fast” signature, with speeds up to at least 100 km sec −1 , is seen only occasionally, and we suggest it is due to an interaction of the near-Io sodium cloud with the corotating, heavy-ion plasma. Both elastic and charge-exchange collisions seem consistent with the observed kinematical and temporal signatures. Elastic collisions seem marginally more capable of producing the high observed sodium atom speeds. We predict observable occurences of the fast component in the hours following passage of the Io sodium cloud through the plasma centrifugal symmetry surface if Io is at a favorable orbital longitude. Between 10 and 20 R J we find an atomic sodium density ∼10 −2 cm −3 . If the photoionization lifetime applies, an Io source of at least 10 26 sodium atoms sec − is required to maintain this remote sodium population.

A survey of Io's potassium cloud

Io's potassium cloud exhibits spatial and temporal variations similar to those observed for Io's sodium cloud. Spectra from five apparitions show that the potassium cloud is elongated so that it extends forward from Io's leading, inner hemisphere and makes an angle with Io's orbit of 10-30 deg, slightly less than the angle for the sodium cloud. The potassium cloud is a long-lived phenomenon which undergoes periodic fluctuations in response to solar radiation pressure and the ionizing influence of Jupiter's plasma torus. These give rise to east-west and north-south asymmetry variations similar to those observed for the sodium cloud. Evidence for temporary jets of potassium streaming from Io have also been observed. These similarities with the sodium cloud suggest that both sodium and potassium are ejected from nearly the same regions of Io by the same physical mechanism.

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.

Determination of magnetic field direction in a comet's head

The Stokes parameters of resonance radiation scattered by a Na atom with the angular momentum F aligned by directed unpolarized radiation in a magnetic field H ∼ 10 −5−10 −1 Oe are presented. An influence of the orientation of the magnetic field on these parameters are studied; the intensity ratio I(D 2) I(D 1) changes within ±5%, and the polarization degree P( D 2) within ±25%. Measurements of I(D 2) I(D 1) and P( D 2), if the geometry of scattering is known, may give information on the direction of the magnetic field in the sodium atmospheres of comets, as well as Io's sodium cloud or man-made cosmic clouds.

Io's sodium cloud - Explanation of the east-west asymmetries

A three- dimensional model for Io's sodium cloud incorporating the full effects of solar radiation acceleration associated with solar resonance scattering of atoms in the D-1 and D-2 lines is developed and applied to interpret two different but related east-west asymmetries exhibited in the cloud, using data obtained from ground-based telescopes. The physical changes produced in the cloud by the solar radiation acceleration as a function of satellite phase angle are documented, and how these changes explain the asymmetries is clarified. The model results suggest an east-west phase lag asymmetry of 10-15 degrees; results for the observed east-west intensity asymmetry of Bergstrahl (1977) are also presented. The east-west shape and intensity asymmetries are found to follow directly from the lack of circular symmetry about Jupiter of the forces acting on the cloud atoms.

Thermal electron density in the Jovian magnetosphere

Observations of the portions of Io's sodium cloud which are far from Io are presented. We demonstrate that the size of the sodium cloud is highly variable and relate this variability to fluctuations in the state of the thermal electron plasma in the magnetosphere of Jupiter. We show that estimates of the plasma density based on the assumption that the main sodium sink is electron impact ionization are consistent with those based on the intensities of the forbidden doublet of ionized sulfur which is excited by electron collisions. We discuss briefly the implications of our results for the magnetosphere of Jupiter.

Io's sodium cloud - Comparison of models and two-dimensional images

Io's sodium cloud - Comparison of models and two-dimensional images

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.