M. L. Marconi

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.

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.

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.

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.

Analysis of hydrogen H-alpha observations of the coma of Comet P/Halley

Ground-based Fabry-Perot observations of the hydrogen coma of comet P/Halley in the Hα (6563 A) line were acquired on 10 nights between 1985 mid-December and 1986 mid-January. These observations are analyzed using a Monte Carlo Particle Trajectory Model. Model calculations were undertaken for both the line profile and the two-dimensional Hα brightness on the sky plane. Model calculations accurately reproduced both the measured line profile and the integrated brightness obtained through the observing aperture. The H 2 O production rates determined in this manner were compared with revised values derived from [O I] 6300 A observations of comet Halley, also taken with the same instrument