Ronald J. Oliversen

Three-Dimensional Fabry-Perot Imaging Spectroscopy of the Crab Nebula, Cassiopeia A, and Nova GK Persei

A three-dimensional model of the [0 III] A5007 line-emitting gas in the Crab Nebula has been developed from imaging spectroscopy taken with the Goddard Fabry-Perot Imager mounted on the McGrawHill 1.3m telescope of Michigan-Dartmouth-MIT Observatory. Several interesting morphological features revealed in three-dimensional isophotal displays are discussed.

First results of the MAVEN magnetic field investigation

Two Mars Atmosphere and Volatile EvolutioN magnetic field sensors sample the ambient magnetic field at the outer edge of each solar array. We characterized relatively minor spacecraft-generated magnetic fields using in-flight subsystem tests and spacecraft maneuvers. Dynamic spacecraft fields associated with the power subsystem (≤1 nT) are compensated for using spacecraft engineering telemetry to identify active solar array circuits and monitor their electrical current production. Static spacecraft magnetic fields are monitored using spacecraft roll maneuvers. Accuracy of measurement of the environmental magnetic field is demonstrated by comparison with field directions deduced from the symmetry properties of the electron distribution function measured by the Solar Wind Electron Analyzer. We map the bow shock, magnetic pileup boundary, the V × B convection electric field and ubiquitous proton cyclotron, and 1 Hz waves in the ion foreshock region.

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.

Lyman‐α imaging of the SO2 distribution on Io

Imaging spectroscopy of Io in the ultraviolet (1160–1720 A) was carried out with the Space Telescope Imaging Spectrograph on HST on three dates in October 1997 and August 1998. Among the initial results was the observation of concentrated regions of Hi Lyman-α flux near the poles of Io that exhibited a morphology and temporal variability different from those of the atomic oxygen and sulfur emission regions seen near the equatorial limbs. We examine the suggestion that the primary source of Lyman-α emission is surface reflected solar radiation that penetrates the thin polar atmosphere, but is strongly absorbed by the thicker SO2 atmosphere near Ios equator. Spectral and spatial analyses lead to derived SO2 column densities that are in good agreement with those derived from earlier HST observations of Ios albedo in the 2000–2300 A wavelength range. The Lyman-α images clearly illustrate features of Ios atmosphere that have been deduced from previous observations and theoretical modeling: a non-uniformity with respect to the sub-solar point dominated by a freezing out of the SO2 near the poles and variation with both longitude and time due to the variability of the sources of the atmospheric gas. Lyman-α imaging is demonstrated to be an extremely powerful and direct way to globally map the dynamic atmosphere of Io.

Fabry-Perot images of NGC 1275 and its puzzling high-velocity system

We report the first images obtained with the Goddard Fabry-Perot imager, a very sensitive and tunable imaging system designed to achieve the high levels of performance required in the optical studies of faint emission-line extragalactic objects. A velocity sequence of calibrated narrow-band CCD images (FWHM ∼7 A) has been obtained to cover the 3000 km s −1 velocity space between the redshifted Hα emission lines of NGC 1275 (the central dominant galaxy of the Perseus cluster), its extended associated system of low-velocity (LV) filaments and the high velocity (HV) system of knots, projected on the same line of sight in the sky

Spatial distribution of O(1D) from Comet Halley

Abstract Images of Comet Halley in [OI]6300-A emission, obtained using the Wisconsin 150-mm Fabry-Perot spectrometer in the imaging mode, have been combined with spectra taken in the scanning mode to deduce the distribution of cometary O( 1 D) within an approximately 10-arc minute field of view centered on the comet head. The 10 km sec −1 bandpass of the system allowed the distribution to be measured with little contamination from airglow [OI]6300 and cometary NH 2 lines in the nearby spectrum. The results were modeled to provide photodestruction lifetimes for cometary H 2 O and OH, the predominant parents of O( 1 D). Our results are compared with other experimental and theoretical results.

Ultraviolet and Optical Spectroscopy of the R Aquarii Symmetrical Jet

The first ultraviolet spectrum of the southwest (SW) component of the symmetrical jet in the R Aquarii binary system has been obtained in the range 1200-2000 A with the IUE. These results are compared to more encompassing spectra of the central H II region taken at the same time and also similar spectra of the northeast (NE) jet component obtained six months earlier. Moreover, optical spectra of both the NE and SW jet components in the range 3400-9800 A were obtained within about 6 months and about 1 month, respectively, of the ultraviolet spectra. These highly complementary observations argue that excitation of the symmetrical jet may be due to shock excitation as the jet components overtake and impact the previously ionized material associated with the expanding inner nebulosity. The problems with this shock model as well as problems with competing photoionization models are discussed. It is suggested that the jet components were ejected less than 90 years ago. 28 refs.

The Io sulfur torus in 1981

Abstract A Fabry-Perot/CCD spectrometer was used to obtain images of the Io torus in emission lines of [S II] λ6716, λ6731 and [S III] λ9531 in 1981 February and March on the 2.1-m telescope at KPNO. The [S II] and [S III] images showed a large variation in brightness and radial extent. There is an indication that [S II] and [S III] emissions in the warm torus are correlated. The [S II] and [S III] emissions in the warm torus also have similar scale heights along the magnetic field lines of ∼ 0.6 R J –0.72 R J . The east-west asymmetry in the [S II] images taken at similar magnetic longitudes, but 2.5 Jovian rotations apart, supports the theory of convective motions suggested by others. In addition to the images, simultaneous measurements of the [S II] λ6731 line profile were made on one night using a Fabry-Perot scanning spectrometer on the 4-m telescope at KPNO. The [S II] spectral scans implied ion temperatures perpendicular to the magnetic field of (52 ± 10) × 10 3 K at 5.2 R J to 5.6 R J from Jupiter and a temperature greater than 3 × 10 5 K at 6 R J from Jupiter.

Production, Outflow, Velocity, and Radial Distribution of H2O and OH in the Coma of Comet C/1995 O1 (Hale-Bopp) from Wide-field Imaging of OH

Observations of OH are a useful proxy of the water production rate (Q) and outflow velocity (V) in comets. From wide-field images taken on 1997 March 28 and April 8 that capture the entire scale length of the OH coma of comet C/1995 O1 (Hale-Bopp), we obtain QOH from the model-independent method of aperture summation and Q from the OH photochemical branching ratio, BROH. Using an adaptive ring summation algorithm, we extract the radial brightness distribution of OH 0-0 band emission out to cometocentric distances of up to 106 km, both as azimuthal averages and in quadrants covering different position angles relative to the comet-Sun line. These profiles are fitted using both fixed and variable velocity two-component spherical expansion models to estimate VOH with increasing distance from the nucleus. The OH coma of Hale-Bopp was more spatially extended than those of previous comets, and this extension is best matched by a variable acceleration of H2O and OH that acted across the entire coma, but was strongest within 1-2 × 104 km from the nucleus. Our models indicate that VOH at the edge of our detectable field of view (106 km) was ~2-3 times greater in Hale-Bopp than for a 1P/Halley class comet at 1 AU, which is consistent with the results of more sophisticated gas-kinetic models, extrapolation from previous observations of OH in comets with Q > 1029 s-1, and direct radio measurements of the outer coma Hale-Bopp OH velocity. The likely source of this acceleration is thermalization of the excess energy of dissociation of H2O and OH over an extended collisional coma. When the coma is broken down by quadrants in position angle, we find an azimuthal asymmetry in the radial distribution that is characterized by an increase in the spatial extent of OH in the region between the orbit-trailing and anti-Sunward directions. Model fits specific to this area and comparison with radio OH measurements suggest greater acceleration here, with VOH ~ 1.5 times greater at a 106 km cometocentric distance than elsewhere in the coma. We discuss several mechanisms that may have acted within the coma to produce the observed effect.