Jody K. Wilson

Discovery of the distant lunar sodium tail and its enhancement following the Leonid Meteor Shower of 1998

Night-time measurements using a bare CCD all-sky imaging system have detected the presence of an extensive region of neutral sodium emission (589.1 nm) in the direction of the anti-solar/lunar points. The emission was observed to occur during the nights of 21–22 August and 18–20 November, 1998 UT, centered on the new Moon period. The Moon is the most likely source of the neutral sodium, making this the first detection of the lunar sodium tail out to a distance of hundreds of lunar radii. The greater brightness of the emission feature on 19 November is attributed to the Leonid meteor shower which peaked on 17 November, 1998, less than two days before new Moon.

Molecular Origin of Io's Fast Sodium

Neutral sodium emissions encircling Jupiter exhibit an intricate and variable structure that is well matched by a simple loss process from Ios atmosphere. These observations imply that fast neutral sodium is created locally in the Io plasma torus, both near Io and as much as 8 hours downstream. Sodium-bearing molecules may be present in Ios upper atmosphere, where they are ionized by the plasma torus and swept downstream. The molecular ions dissociate and dissociatively recombine on a short time scale, releasing neutral fragments into escape trajectories from Jupiter. This theory explains a diverse set of sodium observations, and it implies that molecular reactions (particularly electron impact ionization and dissociation) are important at the top of Ios atmosphere.

Modeling an enhancement of the lunar sodium tail during the Leonid Meteor Shower of 1998

A region of non-terrestrial sodium emission seen in the sky on the nights of November 18–20, 1998, has been interpreted as the Moons distant sodium tail, possibly enhanced by micrometeor impact vaporization of the lunar regolith by the Leonid meteor shower. We show that the location and morphology of the spot can be explained by standard steady-state models of the Moons sodium atmosphere. Moreover, using a new time-dependent simulation of the lunar atmosphere, we find that the Na escape rate from the Moon increased to 2 or 3 times its normal level during the most intense period of the 1998 Leonid meteor shower on November 16th and 17th.

A Digital High-Definition Imaging System for Spectral Studies of Extended Planetary Atmospheres. I. Initial Results in White Light Showing Features on the Hemisphere of Mercury Unimaged by Mariner 10

We present an instrumentation plan for spectral imaging of Mercurys extended atmosphere. The approach depends upon simultaneous short-exposure images in white light and sodium, with the former used to select the frames for postintegration of the sodium images. The effects of atmospheric seeing are thus minimized by the combination of high-speed exposures and subsequent selective integration. The instrumentation to be used is a long-slit imaging echelle spectrometer equipped with an image slicer and an imaging photon detector. A test of the white-light component of the technique has yielded a best-to-date image of a portion of Mercurys surface not photographed during the Mariner 10 mission.

Io's sodium directional feature: Evidence for ionospheric escape

We present evidence that Ios sodium directional feature is produced by the neutralization of pickup ions over the anti-Jovian hemisphere of Io. The localized production region requires ion pickup to occur at or below Ios exobase and is consistent with an ionospheric escape mechanism. Numerical modeling reproduces the feature in at least four distinct data sets spanning 15 years. The directional feature is sensitive to local magnetic fields and seems to indicate little or no perturbation to the local Jovian magnetic field beyond that associated with the Io/torus interaction. The sensitivity of the directional feature to electric and magnetic fields near Io makes it a convenient ground-based observable phenomenon for quantifying the complex interaction of Jupiters plasma torus with Ios atmosphere.

Imaging the surface of Mercury using ground-based telescopes

Abstract We describe and compare two methods of short-exposure, high-definition ground-based imaging of the planet Mercury. Two teams have recorded images of Mercury on different dates, from different locations, and with different observational and data reduction techniques. Both groups have achieved spatial resolutions of km , and the same albedo features and contrast levels appear where the two datasets overlap (longitudes 270–360°). Dark albedo regions appear as mare and correlate well with smooth terrain radar signatures. Bright albedo features agree optically, but less well with radar data. Such confirmations of state-of-the-art optical techniques introduce a new era of ground-based exploration of Mercurys surface and its atmosphere. They offer opportunities for synergistic, cooperative observations before and during the upcoming Messenger and BepiColombo missions to Mercury.

Does the worsening galactic cosmic radiation environment observed by CRaTER preclude future manned deep space exploration

The Sun and its solar wind are currently exhibiting extremely low densities and magnetic field strengths, representing states that have never been observed during the space age. The highly abnormal solar activity between cycles 23 and 24 has caused the longest solar minimum in over 80 years and continues into the unusually small solar maximum of cycle 24. As a result of the remarkably weak solar activity, we have also observed the highest fluxes of galactic cosmic rays in the space age and relatively small solar energetic particle events. We use observations from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter to examine the implications of these highly unusual solar conditions for human space exploration. We show that while these conditions are not a show stopper for long-duration missions (e.g., to the Moon, an asteroid, or Mars), galactic cosmic ray radiation remains a significant and worsening factor that limits mission durations. While solar energetic particle events in cycle 24 present some hazard, the accumulated doses for astronauts behind 10 g/cm2 shielding are well below current dose limits. Galactic cosmic radiation presents a more significant challenge: the time to 3% risk of exposure-induced death (REID) in interplanetary space was less than 400 days for a 30 year old male and less than 300 days for a 30 year old female in the last cycle 23–24 minimum. The time to 3% REID is estimated to be ∼20% lower in the coming cycle 24–25 minimum. If the heliospheric magnetic field continues to weaken over time, as is likely, then allowable mission durations will decrease correspondingly. Thus, we estimate exposures in extreme solar minimum conditions and the corresponding effects on allowable durations.

Three tails of comet Hale-Bopp

We report on the imaging detection of the sodium tail of comet Hale-Bopp using large field-of-view, multi-wavelength observations from the McDonald Observatory in March 1997. We subtract off-band images from sodium-filter images to obtain the atomic sodium tail, and we find it to be more extended in width than either the dust or ion tails. The cross-tail integrated brightness of the sodium tail increases with distance from the nucleus for at least 8 million km. The direction of the sodium tail in our data is different from the ion tail, the dust tail, and the sodium tails imaged at different times by other observers. To account for these characteristics, there must be an extended source of sodium atoms tailward of the comet nucleus. We propose that the sodium tail in March 1997 was produced by the release of sodium atoms from grains in the dust tail.

The sources of sodium escaping from Io revealed by spectral high definition imaging.

On Jupiter’s moon Io, volcanic plumes and evaporating lava flows provide hot gases to form an atmosphere that is subsequently ionized. Some of Io’s plasma is captured by the planet’s strong magnetic field to form a co-rotating torus at Io’s distance; the remaining ions and electrons form Io’s ionosphere. The torus and ionosphere are also depleted by three time-variable processes that produce a banana-shaped cloud orbiting with Io, a giant nebula extending out to about 500 Jupiter radii, and a jet close to Io. No spatial constraints exist for the sources of the first two; they have been inferred only from modelling the patterns seen in the trace gas sodium observed far from Io. Here we report observations that reveal a spatially confined stream that ejects sodium only from the wake of the Io–torus interaction, together with a visually distinct, spherically symmetrical outflow region arising from atmospheric sputtering. The spatial extent of the ionospheric wake that feeds the stream is more than twice that observed by the Galileo spacecraft and modelled successfully. This implies considerable variability, and therefore the need for additional modelling of volcanically-driven, episodic states of the great jovian nebula.

Monitoring the moon's transient atmosphere with an all-sky imager

Abstract An indispensable tool in terrestrial aeronomy, all-sky imaging has recently been shown to be useful in studies of the Moons exosphere. Two days after the peak of the 1998 Leonid meteor shower, an extended region of neutral sodium emission was detected in the night sky using a bare-CCD imaging system. The feature was found to be a train of sodium gas originating from the Moon. Subsequent observations indicate the feature is normally visible (max. brightness ∼15–90 Rayleighs (R)) during nights near the time of new Moon. Monthly monitoring of the feature using an all-sky system can provide useful information about the time-variability of the lunar atmosphere. Several processes are believed to be responsible for the production of lunar Na and evidence is presented indicating that two of these processes were each responsible for the observed brightness enhancements of the sodium feature on two separate new Moon periods in January and March 2000.