William H. Smyth

Theoretical overview and modeling of the sodium and potassium atmospheres of the moon

A general theoretical overview for the sources, sinks, gas-surface interactions, and transport dynamics of sodium and potassium in the exospheric atmsophere of Mercury is given. Information for these four factors, which control the spatial distribution of these two alkali-group gases about the planet, is incorporated in numerical models. The spatial nature and relative importance of the initial source atom atmosphere and the ambient (ballistic hopping) atom atmosphere are then examined and are shown to be controlled and coupled to a great extent by the extremely large and variable solar radiation acceleration experienced by sodium and potassium as they resonantly scatter solar photons. The lateral (antisunward) transport rate of thermally accommodated sodium and potassium ambient atoms is shown to be driven by the solar radiation acceleration and, over a significant portion of Mercurys orbit about the Sun, is sufficiently rapid to be competitive with the short photoionization lifetimes for these atoms when they are located on the summit surface near or within about 30 deg of the terminator. The lateral transport rate is characterized by a migration time determined by model calculations for an ensemble of atoms initially starting at a point source on the surface (i.e., a numerical spacetime dependent Greens function). Four animations for the spacetime evolution of the sodium (or potassium) atmosphere produced by a point source on the surface are presented on a videotape format. For extended surface sources for sodium and potassium, the local column density is determined by competition between the photoionization lifetimes and the lateral transport times of atoms originating from different surface source locations. Sodium surface source fluxes (referenced to Mercury at perihelion) that are required on the sunlit hemisphere to reproduce the typically observed several megarayleighs of D2 emission-line brightness and the inferred column densities of 1-2 x 10(exp 11) atoms per sq cm range from approximately 2-5 x 10(exp 7) atoms/sq cm/sec. The sodium model is applied to study observational data that document an anticorrelation in the average sodium column density and solar radiation acceleration. Lateral transport driven by the solar radiation acceleration is shown to produce this behavior for combinations of different sources and surface accomodation coefficients. The best fit model fits to the observational data require a significant degree of thermal accommodation of the ambient sodium atoms to the surface and a source rate that decreases as an inverse power of 1.5 to 2 in heliocentric distance.

Monte Carlo particle-trajectory models for neutral cometary gases. I - Models and equations. II - The spatial morphology of the Lyman-alpha coma

The mathematical derivations of various methods employed in the Monte Carlo particle-trajectory model (MCPTM) are presented, and the application of the MCPTM to the calculation of the photochemical heating of the inner coma through the partial thermalization of cometary hydrogen atoms produced by the photodissociation of water is discussed. This model is then used to explain the observed morphology of the spatially extended Ly-alpha comas of comets. The rocket and Skylab images of the Ly-alpha coma of Comet Kohoutek are examined. 90 references.

Large-aperture [O I] 6300 Å photometry of comet Hale-Bopp: Implications for the photochemistry of OH

Large-aperture photometric observations of comet Hale-Bopp (C/1995 O1) in the forbidden red line of neutral oxygen ([O I] 6300 with the 150 mm dual-etalon spectrometer that comprises the Ae ) Fabry-Pec rot

Sunlit Io atmospheric [O I] 6300 Å emission and the plasma torus

A large database of sunlit Io [O I] 6300 A emission, acquired over the period 1990–1999, with extensive coverage of Io orbital phase angle ϕ and System III longitude λIII, exhibits significant long-term and short-term variations in [O I] 6300 A emission intensities. The long-term average intensity shows a clear dependence on λIII, which establishes conclusively that the emission is produced by the interaction between Ios atmosphere and the plasma torus. Two prominent average intensity maxima, 70° to 90° wide, are centered at λIII ≈ 130° and λIII ≈ 295°. A comparison of data from October 1998 with a three-dimensional plasma torus model, based upon electron impact excitation of atomic oxygen, suggests a basis for study of the torus interaction with Ios atmosphere. The observed short-term, erratic [O I] 6300 A intensity variations fluctuate ∼20–50% on a timescale of tens of minutes with less frequent fluctuations of a factor of ∼2. The most likely candidate to produce these fluctuations is a time-variable energy flux of field-aligned nonthermal electrons identified recently in Galileo plasma science data. If true, the short-term [O I] intensity fluctuations may be related to variable field-aligned currents driven by inward and outward torus plasma transport and/or transient high-latitude, field-aligned potential drops. A correlation between the intensity and emission line width indicates molecular dissociation may contribute significantly to the [O I] 6300 A emission. The nonthermal electron energy flux may produce O(1D) by electron impact dissociation of SO2 and SO, with the excess energy going into excitation of O and its kinetic energy. The [O I] 6300 A emission database establishes Io as a valuable probe of the torus, responding to local conditions at Ios position.

Emission profiles of neutral oxygen and sulfur in Io's exospheric corona

Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) observations of Io acquired in 1997 [Roesler et al., 1999] provided the first simultaneous spatially resolved measurements of emission from neutral sulfur and oxygen, the dominant atomic species in Ios exospheric corona. Previous measurements of Ios corona relied primarily on sunlight resonantly scattered from sodium, a trace element in Ios atmosphere, and required measurement during mutual satellite eclipses to obtain the necessary spatial resolution. We present here spatial profiles of Ios extended emissions derived from observations spanning the time period from October 1997 to February 2000. The STIS Far Ultraviolet Multi-Anode Microchannel Array (FUV-MAMA) detector permits measurement of the emissions with a spatial resolution of ∼0.05 Io radii out to distances of ∼20 Io radii. Useful measurements are limited to ∼10 Io radii owing to the low signal-to-noise ratio of the extended emission features. The coronal emission profiles vary considerably in slope and intensity and are generally brighter for Io west (duskside) of Jupiter. Emission profiles obtained near western elongation are relatively symmetric about Io; profiles obtained in other orbital positions display varying degrees of asymmetry, with enhanced emissions and generally steeper slopes in the downstream direction relative to the plasma flow. The downstream-upstream profile asymmetry is thought to be caused by higher electron densities in Ios plasma wake. While the coverage of the data is limited in both Jovian System III coordinates and geocentric phase, the intensities of emission from regions both near Io and in the extended corona vary with System III longitude in a near-simultaneous fashion, suggesting local torus electron density as the probable source of this modulation. The observed ratio of oxygen to sulfur emission is relatively constant in time, perhaps reflecting the stoichiometric ratio of the SO 2 source molecules. Eclipse and posteclipse observations on February 25, 2000, show a dramatic increase in profile emission brightness and slope, suggesting a dynamic response by a sublimation-supported component of Ios SO 2 atmosphere and associated atomic species.

The sodium zenocorona

A recent narrow-band-filtered CCD image by Mendillo et al. (1990) has shown that a sodium corona, produced near Io, extends at least 400 Jupiter radii in the planets equatorial plane. Isophotes indicate that the polar to equatorial extents are in ∼1 to 3 proportions. The image can be reproduced by a model which includes both a high- and an intermediate-speed distribution, with source rates of 2.2 and 1.1 × 1026 atoms s−1, respectively. The high−speed distribution was ejected from Io with a velocity tangential to the satellite orbit of 57 km s−1 (∼74 km s−1 relative to Jupiter) plus an isotropic Maxwellian velocity distribution of ∼25 km s−1. This distribution likely corresponds to a charge exchange source of plasma torus sodium ions which are neutralized in the near-Io atmosphere and are ejected relative to Jupiter with a corotational velocity (74 km s−1) plus a thermal ion (25 km s−1) Maxwellian distribution. The intermediate speed distribution was ejected from Io with a tangential speed near 20 km s−1 (37 km s−1 relative to Jupiter) plus an isotropic Maxwellian velocity distribution of ∼12 km s−1. This distribution corresponds to the same nonthermal sodium atoms earlier identified near Io in the sodium directional features (Pilcher et al., 1984). Modeled Doppler profiles of the sodium D lines show that moderate resolution spectroscopy would prove to be highly diagnostic of both the velocity distribution of the source near Io and the resulting periodic time variations expected. On the scale of a few astronomical units, the sodium corona displays a cometary comalike appearance because of the acceleration of sodium atoms induced by solar radiation pressure. Finally, at distances of roughly 1000 planetary radii the effects of the ∼400-hour photoionization lifetime of sodium atoms should begin to become apparent in observed radial profiles.

Hubble Space Telescope Ultraviolet Imaging and High-Resolution Spectroscopy of Water Photodissociation Products in Comet Hyakutake (C/1996 B2)

Comet Hyakutake (C/1996 B2) provided a target of opportunity for performing a systematic study of water photodissociation products in which we obtained data from three instruments on the Hubble Space Telescope (HST). The HST Goddard High Resolution Spectrograph (GHRS) was used to measure the line profile of hydrogen Lyα (H Lyα) at six locations around the coma of the comet, ranging from the nucleus to a displacement of 100,000 km, and covering different directions compared with the comet-sun line. GHRS yielded line profiles with a spectral resolution (FWHM ~4 km s^(-1)) that was a factor of 2-3 better than any previous H Lyα or Hα ground-based measurements. The Wide Field Planetary Camera 2 (WFPC2) and the Woods filter were used to obtain H Lyα images of the inner coma. The faint object spectrograph (FOS) was used to determine the OH production rate and monitor its variation throughout the HST observing sequence. The GHRS H Lyα line profiles show the behavior of a line profile that is optically thick in the core for positions near the nucleus (<5000 km) and gradually becoming more optically thin at larger displacements and lower column abundances. A composite H Lyα image constructed from four separate WFPC2 exposures is consistent with the relative fluxes seen in GHRS observations and clearly shows the dayside enhancement of a solar illuminated optically thick coma. These data were analyzed self-consistently to test our understanding of the detailed physics and chemistry of the expanding coma and our ability to obtain accurate water production rates from remote observations of gaseous hydrogen (H) and hydroxyl (OH), the major water dissociation products. Our hybrid kinetic/hydrodynamic model of the coma combined with a spherical radiative transfer calculation is able to account for (1) the velocity distribution of H atoms, (2) the spatial distribution of the H Lyα emission in the inner coma, and (3) the absolute intensities of H and OH emissions, giving a water production rate of (2.6 ± 0.4) × 10^(29) s^(-1) on 1996 April 4.

An explanation for the east–west asymmetry of the Io plasma torus

One of the most interesting and unexplained spatial structures in the Io plasma torus located near Ios orbit about Jupiter is the east–west (or alternatively dawn-dusk) asymmetry in the planetocentric distance of the so called plasma “ribbon,” the brightest and most prominently observed radial feature of the torus. The average radial position of the ribbon on the sky plane in both its ground-based measured S+ optical emission (6716 A, 6731 A) and its Voyager measured S++ ultraviolet emission (685 A) is observed to be located closer to the planet and well within Ios orbit when it is west of Jupiter at the dusk (or receding) ansa and farther from the planet and very near Ios orbit when it is east of Jupiter at the dawn (or approaching) ansa. In addition, the ribbon is also observed to move about this average position as a function of its ansa System III longitude. It is shown that the location of this asymmetrical radial structure for the S+ ribbon arises naturally in the presence of an east–west electric field from a space and time dependent plasma source that is highly concentrated at Ios instantaneous orbital location (and hence initially located at a constant distance from the planet) and a plasma transport rate that increases radially outward. Model calculations reproduce both the observed average east–west asymmetry and the System III longitude dependence of the S+ ribbon location in the plasma torus. In the absence of an east–west electric field, however, the model-calculated density peak for the S+ ribbon is located essentially symmetrically about Jupiter just inside Ios orbit and does not exhibit the observed east–west asymmetry. Since the S+ ribbon during its slow outward transport will undergo electron impact ionization, the radial location of the S++ ribbon can be expected to be created naturally with the same System III longitude dependence as for the S+ ribbon and at a position radially just beyond that of the S+ ribbon, as has been observed.

The OH distribution in cometary atmospheres : a collisional Monte Carlo model for heavy species

The study presents an extension of the cometary atmosphere Monte Carlo particle trajectory model formalism which makes it both physically correct for heavy species and yet computationally reasonable. The derivation accounts for the collision path and scattering redirection of a heavy radical traveling through a fluid coma with a given radial distribution in outflow speed and temperature. The revised model verifies that the earlier fast-H atom approximations used in earlier work are valid, and it is applied to a case where the heavy radical formalism is necessary: the OH distribution. It is found that a steeper variation of water production rate with heliocentric distance is required for a water coma which is consistent with the velocity-resolved observations of Comet P/Halley.

An initial look at the Iogenic SO2 + source during the Galileo fly by of Io

Galileo in its December 7, 1995, encounter with Io flew downstream of Io and through the magnetospheric wake of the satellite with a closest approach altitude of ∼900 km. Magnetospheric instruments were therefore able to sample the Iogenic plasma source both outside as well as deeply within the Lagrange sphere of Io (∼5.81 satellite radii) where the gravity of Io dominates and where the plasma pickup processes are expected to be highly peaked about the satellite. The presence of both long-lived atomic ions (H+, O+, O++, S+, S++) and short-lived molecular ions (SO+, SO2+) was detected along the spacecraft trajectory. We have undertaken preliminary calculations for the density profile of SO2+. These calculations are compared with the SO2+ density profiles deduced from magnetic field fluctuations with periods of ∼2–3 s measured by the Galileo magnetometer and interpreted as ion cyclotron waves produced by fresh SO2+ Iogenic pickup ions created near Io. By matching the absolute SO2+ model density with the minimum ion density determined by Huddleston et al. [1997] in their analysis of the ion cyclotron waves, an SO2 source rate of ∼4 × 1027 molecules s−1 (425 kg s−1) at Ios exobase and a corresponding SO2+ source rate of 2.8 × 1026 ions s−1 (30 kg s−1) in the magnetosphere are determined. Most of the SO2 that undergoes interactions in the plasma torus is, however, rapidly dissociated primarily by electron impact, producing O, S, SO, and O2. These species subsequently undergo ionization and charge exchange reactions in the plasma torus, producing much larger mass and energy pickup plasma loading rates, including an SO+ source rate estimated to be somewhat smaller than the SO2+ source rate. Since the lifetime of SO2 is highly variable with Ios position in the plasma torus, it follows that the spatial profile for the amplitude of these magnetic fluctuations will also be highly space and time variable and will depend upon both Io System III longitude and Io geocentric phase angle.