Walter Michael Harris

Time-Resolved Observations of Jupiter's Far-Ultraviolet Aurora

Simultaneous imaging and spectroscopic observations of Jupiters far-ultraviolet aurora covering half a jovian rotation were made on 31 May 1994. The Hubble Space Telescope Wide Field Planetary Camera 2 images revealed dramatic and rapidly changing auroral features, including discrete longitudinal structures along the auroral ovals, with variable contrast; a poleward offset in a north oval sector, showing equatorward motion near dusk; emissions polewards of the ovals, apparently co-rotating; and a bright event developing near the dawn limb. Viewing geometry effects explain the rotational intensity modulation observed by the International Ultraviolet Explorer, without intrinsic longitudinal asymmetries.

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

Comparison of IUE and HST diagnostics of the Jovian Aurorae

Analysis of international ultraviolet explorer (IUE) and Voyager ultraviolet spectrometer (UVS) spectra of the Jovian auroral emission indicates that the Jovian auroral brightness is modulated in longitude (brighter near 180° in the north and 20° in the south) and that there is a color ratio asymmetry associated with this brightening. The purpose of this study is to investigate the origin of this apparent asymmetry. To that end, we use a series of six typical images of the north auroral region taken in the H 2 Lyman bands with the faint object camera (FOC) aboard the Hubble space telescope (HST) and which cover a full Jovian rotation. Although the images do not display any strong brightening near 180°, once we have simulated the signal IUE would see through its aperture, we find the characteristic longitudinal modulation. We attribute most of this modulation to a combination of viewing geometry effects near the east and west ansae of the auroral oval (already taken into account in previous studies) and of the spatial degradation of the source by the IUE instrument function (never considered so far), and we suggest qualitatively that these effects may also affect the color ratio asymmetry. Nevertheless, part of the asymmetry seems to be clue to an intrinsic modulation associated with a bright feature crossing the polar cap along the 160° meridian (transpolar emission) and present in most of the images. We then use a series of FOC images taken during an atypically strong auroral event, and we show that the same effects can again account for the anomalous brightness variations observed simultaneously with IUE.

Measurements of [C I] Emission from Comet Hale-Bopp

We present quantitative measurements of cometary [C I] 9850 A emission obtained during observations of comet Hale-Bopp (C/1995 O1) in 1997 March and April. The observations were carried out using a high-resolution (λ/Δλ ≈ 40,000) Fabry-Perot/CCD spectrometer at the McMath-Pierce Solar telescope on Kitt Peak. This forbidden line, the carbon analog of [O I] 6300 A, is emitted in the radiative decay of C(1D) atoms. In the absence of other sources and sinks, [C I] 9850 A emission can be used as a direct tracer of CO photodissociation in comets. However, in Hale-Bopps large, dense coma, other processes, such as collisional excitation of ground-state C(3P), dissociative recombination of CO+, and collisional dissociation of CO and CO2 may produce significant amounts of C(1D). The long C(1D) radiative lifetime (~4000 s) makes collisional deexcitation (quenching) the primary loss mechanism in the inner coma. Thus, a detailed, self-consistent global model of collisional and photochemical interactions is necessary to fully account for [C I] 9850 A emission in comet Hale-Bopp.

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.

Oxygen/hydrogen chemistry in the inner comae of active comets

We have constructed a concentric-shell, one-dimensional kinetic model that examines the chemistry of hydrogen and oxygen species in detail. We have studied the effects of the reactions of the reactive OH, O( 3 P), and O( 1 D) species with themselves and with the abundant stable molecules in the inner coma of moderately and highly active comets. We find that the reactions (1) Oð 1 DÞþH2O! 2OH and (2) Oð 3 PÞþOH! O2þ Hp lay important roles in the inner comae of active comets. Inclusion of reaction (2) predicts the formation of significant amounts of molecular oxygen. As the densities of O2 may be as high as 1% those of water in some cases, the possibility of detection exists. We suggest the possibility that the ion O þ may contribute to some previously unassigned features in the optical ion-tail spectra of comets. We also consider the role that reactions of the reactive species might play in the destruction of CO, NH3, and organic molecules in the inner coma of the active comet. We find that destruction offormaldehyde, for example, by reaction with OH has a small but essentially negligible effect on the predicted production rate offormaldehyde. Finally, we examine the significance of the reaction of OH with CO in the dense inner coma. Subject headings: astrochemistry — comets: general

UV PHOTOPOLARIMETRIC IMAGING OF C/1995 O1 (HALE-BOPP) WITH THE WIDE FIELD IMAGING SURVEY POLARIMETER (WISP)

We report on the reduction and analysis of UVpolarimetric images of CI (λ1657 Å) and dust continuum (2696 Å emissions from C/1995 O1 (Hale-Bopp) taken using the Wide Field Imaging Survey Polarimeter (WISP) sounding rocket on 8 April, 1997. These observations represent the first imaging polarimetry of comets in the UV, and were performed in consort with ground based measurements of gas and dust polarization and distribution. The continuum results show 9% polarization across the image field with a polarization phase angle close to the 129° prediction. Comparison with ground based data implies minimal color dependence for Hale-Bopp in either the degree of polarization and in the position angle. The carbon polarimetry implies that most production occurs in the dense inner coma, and that it leaves that area in thermodynamic equilibrium. Its radial profile further constrains the carbon outflow speed to be sufficient to travel ≥5 × 106 km without photoionization.

The Gas Production Rate and Coma Structure of Comet C/1995 O1 (Hale-Bopp)

The University of Wisconsin–Madison and NASA–Goddard conducted acomprehensive multi-wavelength observing campaign of coma emissionsfrom comet Hale–Bopp, including OH 3080 Å, [O I] 6300 Å H2O+ 6158 Å, H Balmer-α 6563 Å, NH2 6330 Å, [C I] 9850 ÅCN 3879 Å, C2 5141 Å, C3 4062 Å,C I 1657 Å, and the UV and optical continua. In thiswork, we concentrate on the results of the H2O daughter studies.Our wide-field OH 3080 Å measured flux agrees with other, similarobservations and the expected value calculated from published waterproduction rates using standard H2O and OH photochemistry.However, the total [O I] 6300 Å flux determined spectroscopically overa similar field-of-view was a factor of 3-4 higher than expected.Narrow-band [O I] images show this excess came from beyond theH2O scale length, suggesting either a previously unknown source of[O I] or an error in the standard OH + ν→ O(1D) + H branching ratio. The Hale–Bopp OH and[O I] distributions, both of which were imaged tocometocentric distances >1 × 106 km, were more spatiallyextended than those of comet Halley (after correcting for brightnessdifferences), suggesting a higher bulk outflow velocity. Evidence ofthe driving mechanism for this outflow is found in the Hα lineprofile, which was narrower than in comet Halley (though likelybecause of opacity effects, not as narrow as predicted by Monte-Carlomodels). This is consistent with greater collisional coupling betweenthe suprathermal H photodissociation products and Hale–Bopps densecoma. Presumably because of mass loading of the solar wind by ionsand ions by the neutrals, the measured acceleration of H2O+ downthe ion tail was much smaller than in comet Halley. Tailwardextensions in the azimuthal distributions of OH 3080 Å,[O I], and [C I] , as well as a Doppler asymmetry in the[O I] line profile, suggest ion-neutral coupling. While thetailward extension in the OH can be explained by increased neutralacceleration, the [O I] 6300 Å and [C I] 9850 Å emissions show 13%and >200% excesses in this direction (respectively), suggesting anon-negligible contribution from dissociative recombination of CO+and/or electron collisional excitation. Thus, models including theeffects of photo- and collisional chemistry are necessary for the fullinterpretation of these data.

Studies of H Ly-alpha emissions in the solar neighborhood with the techniques of polarimetry and spatial heterodyne spectroscopy (SHS)

Ultraviolet astronomy is an important tool for the study of the interplanetary medium and diffuse, angularly extended emissions in planetary/comet atmospheres and their near space environments. We describe a new technique for high étendue observations of emission lines at R > 105 with an all-reflective spatial heterodyne spectrometer and polarmetric sampling of these lines with an ultraviolet optimized waveplate-Brewster mirror combination. The resulting system is themo-mechanically stable, has light collecting power substantially greater than HST for extended emissions, despite having a volume of several x 10-3 m3. This makes the SHS polarimeter ideal for spacecraft applications. We describe the SHS and polarmetric optical techniques and provide a discussion of its planned development for studies of interplanetary hydrogen.

Wide Field Imaging and the Velocity Structure in the Coma of Hale–Bopp

The comae of very active comets have a substantiallymore complex coma than their weaker cousins.The primary cause of this is photolytic heating and collisionsthat occur over an ever-larger volume of the coma asQH2O increases. Collisionswith the photochemical daughters ofwater in this regionmodify the radial distributions and outflowvelocity of each species, excite and quench metastableemissions, and introduce velocity gradients from photolyticheating. Comet Hale–Bopp was the first comet forwhich the collisional coma was both spatially resolvableand comparable in extent to the scale lengths ofmajor coma species. In the case of this object,the classical assumptions that make it possible toinvert radial emission line profiles, brightnesses, andlineshapes to production rate and velocity eitherdo not hold or require adjustment to work.Here we describe how a large collision zone modifies thecoma, how it affects the classical methods for obtainingproduction rate and velocity, and discuss how wide fieldimaging may be combined with modified versions ofsimple models to address the complications and extract somestructural information.