X-Ray Emission from Comet McNaught-Hartley (C/1999 T1)

Comet McNaught-Hartley was observed in five 1-h exposures on January 8-14 2001 using the advanced CCD imaging spectrometer on board the Chandra X-ray Observatory. The X-ray image of the comet does not show a crescent-like shape. The brightest region is offset from the nucleus between the sunward and comet velocity directions. The comet mean X-ray luminosity is equal to 7.8×10¹⁵ erg s⁻¹ for photon energy E>150 eV and aperture ρ=1.5×10⁵ km where the comet X-ray brightness exceeds 20% of the peak value. Gas production rate was 10²⁹ s⁻¹ during the observations, and the efficiency of X-ray excitation was equal to 4×10⁻¹⁴ erg AU^3/2. Day-to-day variations in X-rays reached a factor of 5. The strongest short-term variation was by a factor of 1.75 for 1600 s. This variation may be explained by a decline in the solar-wind flux by the same factor in ≈800 s. The comet and Earth were seeing different faces of the Sun, and time delay in the solar-wind events on the Earth and the comet was long, equal to 6 days. The best correlation between the comet X-ray luminosity and the solar-wind proton density is for the time delay of 5.5 days and may be explained by the higher velocity of heavy ions.Careful background subtraction made it possible to extract the comet spectrum from 150 to 1000 eV. No signal was detected at E>1000 eV, and a 3σ upper limit to any emission with E>1000 eV is 0.3% of the photon emission at 150-1000 eV. The best χ²-fit model to the spectrum consists of nine narrow emission features. The emission energies and intensities are in good agreement with a charge exchange spectrum calculated by us for the slow solar wind. Using this spectrum, we identify the observed emissions as (Ne⁷⁺+Mg⁷⁺+Mg⁸⁺) at 195 eV, (Mg⁸⁺+Mg⁹⁺+Si⁸⁺) at 250 eV, C⁵⁺ at 370 and 460 eV, O⁶⁺ at 560 eV, O⁷⁺ at 650, 780, and 840 eV, and Ne⁸⁺ at 940 eV. X-ray spectroscopy of comets may be used to diagnose the solar-wind composition and its interaction with comets.     

SOHO/SWAN observations of comets with small perihelia: C/2002 V1 (NEAT), C/2002 X5 (Kudo–Fujikawa), 2006 P1 (McNaught) and 96P/Machholz 1

SWAN, the all-sky hydrogen Lyman-alpha camera on the SOHO spacecraft, designed primarily to image the interplanetary neutral hydrogen around the Sun, also observes comets continuously over large portions of their apparitions to the north and south of the ecliptic and at small solar elongation angles. Because of SOHO’s location at the L1 Lagrange point, analysis of SWAN images provides excellent temporal coverage of water production. We report here our results of observations of some interesting target comets selected from the extensive SWAN archive. These include three Oort Cloud Comets C/2002 V1 (NEAT), C/2002 X5 (Kudo–Fujikawa), C/2006 P1 (McNaught) and three apparitions of atypical short-period Comet 96P/Machholz 1. The common aspect of these four comets is their small perihelion distances, which are 0.19, 0.09, 0.17, and 0.12 AU, respectively. Their water production rates over their whole apparitions can be approximated by power laws in heliocentric distance (r in AU) as follows: 1.3 × 10²⁹ r^−2.1 s⁻¹ for C/2002 V1 (NEAT), 7.5 × 10²⁸ r^−2.0 s⁻¹ for C/2002 X5 (Kudo–Fujikawa), 5.4 × 10²⁹ r^−2.4 s⁻¹ for C/2006 (P1 McNaught) and 4.6 × 10²⁷ r^−2.1 s⁻¹ for 96P/Machholz 1. We also present daily-average water production rates for the long-period comets over long continuous time periods. We examine these results in light of our growing survey of comets that is yielding some interesting comparisons of water production rate variations with heliocentric distance and taxonomic classes.     

Helium on Mars and Venus: EUVE observations and modeling

Long-exposure spectroscopy of Mars and Venus with the Extreme Ultraviolet Explorer (EUVE) has revealed emissions of He 584 Å on both planets and He 537 Å/O⁺ 539 Å and He⁺ 304 Å on Venus. Our knowledge of the solar emission at 584 Å, eddy diffusion in Mars' upper atmosphere, electron energy distributions above Mars' ionopause, and hot oxygen densities in Mars' exosphere has been significantly improved since our analysis of the first EUVE observation of Mars [Krasnopolsky, Gladstone, 1996, Helium on Mars: EUVE and Phobos data and implications for Mars' evolution, J. Geophys. Res. 101, 15,765-15,772]. These new results and a more recent EUVE observation of Mars are the motivation for us to revisit the problem in this paper. We find that the abundance of helium in the upper atmosphere, where the main loss processes occur, is similar to that in the previous paper, though the mixing ratio in the lower and middle atmosphere is now better estimated at 10±6 ppm. Our estimate of the total loss of helium is almost unchanged at 8×¹⁰²³ s⁻¹, because a significant decrease in the loss by electron impact ionization above the ionopause is compensated by a higher loss in collisions with hot oxygen. We neglect the outgassing of helium produced by radioactive decay of U and Th because of the absence of current volcanism and a very low upper limit to the seepage of volcanic gases. The capture of solar wind α-particles is currently the only substantial source of helium on Mars, and its efficiency remains at 0.3. A similar analysis of EUV emissions from Venus results in a helium abundance in the upper atmosphere which is equal to the mean of the abundances measured previously with two optical and two mass spectrometers, and a derived helium mixing ratio in the middle and lower atmosphere of 9±6 ppm. Helium escape by ionization and sweeping out of helium ions by the solar wind above the ionopause is smaller than that calculated by Prather and McElroy [1983, Helium on Venus: implications for uranium and thorium, Science 220, 410-411] by a factor of 3. However, charge exchange of He⁺ ions with CO₂ and N₂ between the exobase and ionopause and collisions with hot oxygen ignored previously add to the total loss which appears to be at the level of 10⁶ cm⁻² s⁻¹ predicted by Prather and McElroy [1983, Science 220, 410-411]. The loss of helium is compensated by outgassing of helium produced by radioactive decay of U and Th and by the capture of the solar wind α-particles with an efficiency of 0.1. We also compare our derived α-particle capture efficiencies for Mars and Venus with observed X-ray emissions resulting from the charge exchange of solar wind heavy ions with the extended atmospheres on both planets [Dennerl et al., 2002, Discovery of X-rays from Venus with Chandra, Astron. Astrophys. 386, 319-330; Dennerl, 2002, Discovery of X-rays from Mars with Chandra, Astron. Astrophys. 394, 1119-1128]. The emissions from both disk and halo on Mars agree with our calculated values; however, we do not see a reasonable explanation for the X-ray halo emission on Venus. The ratio of the charge exchange efficiencies derived from the disk X-ray emissions of Mars and Venus is similar to the ratio of the capture efficiencies for these planets. The surprisingly bright emission of He⁺ at 304 Å observed by EUVE and Venera 11 and 12 suggests that charge exchange in the flow of the solar wind α-particles around the ionopause is much stronger than in the flow of α-particles into the ionosphere.     

C/2002 VQ94 (LINEAR) and 29P/Schwassmann-Wachmann 1 — CO and N2 rich comets

We investigated comets active at large heliocentric distances using observations obtained at the 6-m BTA telescope (SAO RAS, Russia). Long-slit and photometric modes of the focal reducer SCORPIO were used. Two of the comets, 29P/Schwassmann-Wachmann 1 (SW1) and C/2002 VQ94 (LINEAR) were observed to be emission rich. Detection of CO⁺ and N⁺₂ emissions in the comae of these comets is evidence that they were formed in the outer regions of the Solar System or in a pre-solar interstellar cloud in a low temperature environment with T?25 K. The ratio of N⁺₂/CO⁺ is equal to 0.011 and 0.027 for SW1 and LINEAR, respectively. Comet LINEAR is the most distant object in the Solar System (7.332 AU) for which CO⁺ and N⁺₂ are measured. The photometric maximum of the isolated CO⁺ coma in Comet LINEAR is shifted by 1.4 arcsec (7.44×¹⁰³ km) relative to the photometric maximum of the dust coma. This shift deviates from the sunward direction by 63 degrees.     

Ion flow and momentum transfer in the Venus plasma environment

An analysis of ion data from 390 Venus Express, VEX, orbits demonstrates that the flow of solar wind- and ionospheric ions near Venus is characterized by a marked asymmetry. The flow asymmetry of solar wind H⁺ and ionospheric O⁺ points steadily in the opposite direction to the planet’s orbital motion, and is most pronounced near the Pole and in the tail/nightside region. The flow asymmetry is consistent with aberration forcing, here defined as lateral forcing induced by the planet’s orbital motion. In addition to solar wind forcing by the radial solar wind expansion, Venus is also subject a lateral/aberration forcing induced by the planet’s orbital motion transverse to the solar wind flow.The ionospheric response to lateral solar wind forcing is analyzed from altitude profiles of the ion density, ion velocity and ion mass-flux. The close connection between decreasing solar wind H⁺ mass-flux and increasing ionospheric O⁺ mass-flux, is suggestive of a direct/local solar wind energy and momentum transfer to ionospheric plasma. The bulk O⁺ ion flow is accelerated to velocities less than 10 km/s inside the dayside/flank Ionopause, and up to 6000 km in the tail. Consequently, the bulk O⁺ outflow does not escape, but remains near Venus as a fast (km/s) O⁺ zonal wind in the Venus polar and nightside upper ionosphere. Furthermore, the total O⁺ mass-flux in the Venus induced magnetosphere, increases steadily downward to a maximum of 2 × 10⁻¹⁴ kg/(m² s) at ≈400 km altitude, suggesting a downward transport of energy and momentum. The O⁺, and total mass-flux, decay rapidly below 400 km. With no other plasma mass-flux as replacement, we argue that the reduction of ion mass-flux is caused by ion-neutral drag, a transfer of ion energy and momentum to neutrals, implying that the O⁺ plasma wind is converted to a neutral (thermosphere) wind at Venus. Incidentally, such a neutral wind would go in the same direction as the Venus atmosphere superrotation.     

Isotopic abundance in the CN coma of comets: Ten years of measurements

Over the past 10 years the isotopic ratios of carbon (¹²C/¹³C) and nitrogen (¹⁴N/¹⁵N) have been determined for a dozen comets, bright enough to allow obtaining the required measurements from the ground. The ratios were derived from high-resolution spectra of the CN coma measured in the B²∑⁺−X²∑⁺ (0, 0) emission band around 387 nm. The observed comets belong to different dynamical classes, including dynamically new as well as long- and short-period comets from the Halley- and Jupiter-family. In some cases the comets could be observed at various heliocentric distances. All values determined for the carbon and nitrogen isotopic ratios were consistent within the error margin irrespective of the type of comet or the heliocentric distance at which it was observed. Our investigations resulted in average ratios of ¹²C/¹³C=91±21 and nitrogen ¹⁴N/¹⁵N=141±29. Whilst the value for the carbon isotopic ratio is in good agreement with the solar and terrestrial value of 89, the nitrogen isotopic ratio is very different from the telluric value of 272.     

Detection Of Soft X-Ray Emission From Comet C/1995 O1 (Hale–Bopp)

We report the detection of soft X-rays from comet C/1995 O1 (Hale-Bopp) by the Low Energy Concentrator Spectrometer (LECS) on-board the X-ray satellite, BeppoSAX. The observations took place on 1996 September 10–11 approximately 1 day after a large dust outburst (Schulz et al., 1997–1999). After correcting for the comets motion, a 7σ enhancement was found centered (2.1 ± 1.3) x 10⁵ km from the position of the nucleus, in the general solar direction. The total X-ray luminosity in the 0.1–2.0 keV energy band is 5 x 10¹⁶ erg s⁻¹ which is at least a factor of ∼ 3 greater than measured by the Extreme Ultraviolet Explorer (EUVE)4 days later and suggests that the bulk of the emission measured by the LECS is related to the dust outburst. The extracted LECS spectrum is well fit by a thermal bremsstrahlung-like distribution of temperature of 0.29 ± 0.06 keV - consistent with that observed in other comets. We find no evidence for fluorescent carbon or oxygen emission and place 95% confidence limits of 1.0 x 10¹⁵ and 7.8 x 10¹⁵ erg s⁻¹ to narrow line emission at 0.28 and 0.53 keV, respectively. We calculate that if such lines are present, they constitute at most 18% of the 0.1–2.0 keV continuum luminosity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Changes in thermospheric composition inferred from twilight O+(2P) emission

Measurements of the twilight enhancement of airglow emission from O⁺(²P) near 7325 Å reveal major changes which accompany geomagnetic activity, no significant distance between evening and morning and an increase in brightness paralleling the approach to solar maximum. The principal source for O⁺(²P) is direct photoionization from O(³P) but at low solar activity there appears to be a contribution from another source in early twilight which may be local photoelectron ionization into O⁺(²P). The geomagnetic and solar effects appear to reflect changes in the O and N₂ density in the thermosphere; ground based twilight measurements of O⁺ emissions thus provide a simple means for monitoring thermospheric structure from 300 km to ～ 500 km at solar minimum and to ～600 km at solar maximum.     

Spectra Of Split Comet C/1999 S4 (Linear)

We obtained spectra of comet C/1999 S4 (LINEAR) with the UAGS spectrograph(long slit and CCD) installed on the 1-m Zeiss reflector of the SAO of the RAS(Northern Caucuses, Nizhny Arkhyz) on July 23/24, 26/27 and 27/28, 2000. OnJuly 22/23, before the splitting of the cometary nucleus, several emission lines,such as C₂, C₃, CN, NH, CH, NH₂, CO⁺, H₂O⁺ wereclearly identified in the spectra. The inspections of the CCD spectra obtainedon July 27/28, 2000 reveals only very weak emission lines superimposed on thesolar reflection spectrum. From analyzing the surface brightness profile of C₂ along the slit the velocity of separation of two secondary fragments (V = 10 km/h) and the energy of the fragment separation (E = 8.7 × 10¹⁵ erg) were estimated. A luminescence cometary continuum of 26% of the total continuum level is detected in the spectra of the comet at 5000 Å. Possible mechanisms of nucleus splitting are discussed.     

Water production and release in Comet 153P/Ikeya-Zhang (C/2002 C1): accurate rotational temperature retrievals from hot-band lines near 2.9-μm

Multiple non-resonance fluorescence lines of water (H₂O) were detected in Comet 153/P Ikeya-Zhang (2002 C1) between UT 2002 March 21.9 (R_h=0.51 AU) and April 13.9 (R_h=0.78 AU), using the Cryogenic Echelle Spectrometer (CSHELL) at the NASA Infrared Telescope Facility. Analysis of 2.9-μm water lines enabled accurate determination of rotational temperatures on three dates. The derived H₂O rotational temperatures were 138⁺⁶₋₅, 141⁺¹⁰₋₉, and 94±3 K on UT 2002 March 22.0, March 23.0, and April 13.8, respectively. Water production rates were retrieved from spectral lines measured in nineteen separate grating settings over seven observing periods. The derived heliocentric dependence of the water production rate was Q=(9.2±1.1)×10²⁸[R_h^{(−3.21±0.26)}] molecules s⁻¹. The spatial distribution of H₂O in the coma was consistent with its release directly from the nucleus (as a native source) on all dates.     

Meridional variations in stratospheric acetylene and ethane in the southern hemisphere of the saturnian atmosphere as determined from Cassini/CIRS measurements

These are the first results from nadir studies of meridional variations in the abundance of stratospheric acetylene and ethane from Cassini/CIRS data in the southern hemisphere of Saturn. High resolution, 0.5 cm⁻¹, CIRS data was used from three data sets taken in June-November 2004 and binned into 2° wide latitudinal strips to increase the signal-to-noise ratio. Tropospheric and stratospheric temperatures were initially retrieved to determine the temperature profile for each latitude bin. The stratospheric temperature at 2 mbar increased by 14 K from 9° to 68° S, including a steep 4 K rise between 60° and 68° S. The tropospheric temperatures showed significantly more meridional variation than the stratospheric ones, the locations of which are strongly correlated to that of the zonal jets. Stratospheric acetylene abundance decreases steadily from 30 to 68° S, by a factor of 1.8 at 2.0 mbar. Between 18° and 30° S the acetylene abundance increases at 2.0 mbar. Global values for acetylene have been calculated as (1.9±0.19)×¹⁰⁻⁷ at 2.0 mbar, (2.6±0.27)×¹⁰⁻⁷ at 1.6 mbar and (3.1±0.32)×¹⁰⁻⁷ at 1.4 mbar. Global values for ethane are also determined and found to be (1.6±0.25)×¹⁰⁻⁵ at 0.5 mbar and (1.4±0.19)×¹⁰⁻⁵ at 1.0 mbar. Ethane abundance in the stratosphere increases towards the south pole by a factor of 2.5 at 2.0 mbar. The increase in stratospheric ethane is especially pronounced polewards of 60° S at 2.0 mbar. The increase of stratospheric ethane towards the south pole supports the presence of a meridional wind system in the stratosphere of Saturn.     

Simultaneous mapping of H2O and H2O2 on Mars from infrared high-resolution imaging spectroscopy

New maps of martian water vapor and hydrogen peroxide have been obtained in November-December 2005, using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infra Red Telescope facility (IRTF) at Mauna Kea Observatory. The solar longitude Ls was 332° (end of southern summer). Data have been obtained at 1235-1243 cm⁻¹, with a spectral resolution of 0.016 cm⁻¹ (R=8×¹⁰⁴). The mean water vapor mixing ratio in the region [0°-55° S; 345°-45° W], at the evening limb, is 150±50 ppm (corresponding to a column density of 8.3±2.8 pr-μm). The mean water vapor abundance derived from our measurements is in global overall agreement with the TES and Mars Express results, as well as the GCM models, however its spatial distribution looks different from the GCM predictions, with evidence for an enhancement at low latitudes toward the evening side. The inferred mean H₂O₂ abundance is 15±10 ppb, which is significantly lower than the June 2003 result [Encrenaz, T., Bézard, B., Greathouse, T.K., Richter, M.J., Lacy, J.H., Atreya, S.K., Wong, A.S., Lebonnois, S., Lefèvre, F., Forget, F., 2004. Icarus 170, 424-429] and lower than expected from the photochemical models, taking in account the change in season. Its spatial distribution shows some similarities with the map predicted by the GCM but the discrepancy in the H₂O₂ abundance remains to be understood and modeled.     

Ion composition and solar wind interaction: Observations of comet C/1995 O1 (Hale-Bopp)

The extraordinary brightness of comet Hale-Bopp (C/1995 O1) offered the rare opportunity to investigate its ion tail by observations over a wide range of wavelengths. For the first time, ion emission lines could be detected in the radio domain, originating from H₃O⁺, HCO⁺ and CO⁺ ions. Observations in the extreme ultraviolet range showed O⁺ emissions. Optical imaging observations show a spectacular detail of structures in the ion tail. The combined measurements of the ion spatial distribution and dynamic provide a wealth of information which needs to be understood in terms of models of the comet-solar wind interaction and ion-neutral molecule reactions in the coma. This revised version was published online in July 2006 with corrections to the Cover Date.     

A high-resolution infrared spectral survey of Comet C/1999 H1 Lee

We obtained high-resolution (λ/Δλ∼25,000) spectra of Comet C/1999 H1 (Lee) on UT 1999 August 19.6 and 21.6 using the cross-dispersed Near InfraRed SPECtrometer (NIRSPEC) at the Keck Observatory atop Mauna Kea, HI. Here we present spectra of Comet Lee between 2.874 and 3.701 μm (3479-2702 cm⁻¹) representing the most complete high-resolution infrared survey of a comet to date in this wavelength region. Using published line lists and laboratory spectra we have identified 444 of the 545 distinct emission features present in these spectra. We have tabulated the rest frequencies, assignments, relative intensities, and signal-to-noise ratios of all detected emissions. In addition to gaining insights into the chemistry of Comet Lee, this survey provides a valuable tool for planning future high-resolution infrared observations of comets and other astronomical targets, and for retrospective comparison to existing high-resolution infrared datasets.     

Spectroscopic Observations of Comet C/1996 B2 (Hyakutake) with the Caltech Submillimeter Observatory

The apparition of Comet C/1996 B2 (Hyakutake) offered an unexpected and rare opportunity to probe the inner atmosphere of a comet with high spatial resolution and to investigate with unprecedented sensitivity its chemical composition. We present observations of over 30 submillimeter transitions of HCN, H¹³CN, HNC, HNCO, CO, CH₃OH, and H₂CO in Comet Hyakutake carried out between 1996 March 18 and April 9 at the Caltech Submillimeter Observatory. Detections of the H¹³CN (4–3) and HNCO (16_0,16–15_0,15) transitions represent the first observations of these species in a comet. In addition, several other transitions, including HCN (8–7), CO (4–3), and CO (6–5) are detected for the first time in a comet as is the hyperfine structure of the HCN (4–3) line. The observed intensities of the HCN (4–3) hyperfine components indicate a line center optical depth of 0.9 ± 0.2 on March 22.5 UT. The HCN/HNC abundance ratio in Comet Hyakutake at a heliocentric distance of 1 AU is similar to that measured in the Orion extended ridge— a warm, quiescent molecular cloud. The HCN/H¹³CN abundance ratio implied by our observations is 34 ± 12, similar to that measured in giant molecular clouds in the galactic disk but significantly lower than the Solar System¹²C/¹³C ratio. The low HCN/H¹³CN abundance ratio may be in part due to contamination by an SO₂line blended with the H¹³CN (4–3) line. In addition, chemical models suggest that the HCN/H¹³CN ratio can be affected by fractionation during the collapse phase of the protosolar nebula; hence a low HCN/H¹³CN ratio observed in a comet is not inconsistent with the solar system¹²C/¹³C isotopic ratio. The abundance of HNCO relative to water derived from our observations is (7 ± 3) × 10⁻⁴. The HCN/HNCO abundance ratio is similar to that measured in the core of Sagittarius B2 molecular cloud. Although a photo-dissociative channel of HNCO leads to CO, the CO produced by HNCO is a negligible component of cometary atmospheres. Production rates of HCN, CO, H₂CO, and CH₃OH are presented. Inferred molecular abundances relative to water are typical of those measured in comets at 1 AU from the Sun. The exception is CO, for which we derive a large relative abundance of 30%. The evolution of the HCN production rate between March 20 and March 30 suggests that the increased activity of the comet was the cause of the fragmentation of the nucleus. The time evolution of the H₂CO emission suggests production of this species from dust grains.     

Flowing afterglow studies of temperature dependencies for electron dissociative recombination of HCNH, CH3CNH and CH3CH2CNH and their symmetrical proton-bound dimers

A study has been made using a variable temperature flowing afterglow Langmuir probe technique (VT-FALP) to determine the equilibrium temperature dependencies of the dissociative electron-ion recombination of the protonated cyanide ions (RCNH⁺, where R=H, CH₃ and C₂H₅) and their symmetrical proton-bound dimers (RCNH⁺NCR). The power law temperature dependencies of the recombination coefficients, α_e, over the temperature range 180 to 600 K for the protonated ions are α_e(T)(cm³ s⁻¹)=3.5±0.5×10⁻⁷ (300/T)^1.38 for HCNH⁺, α_e(T)=3.4±0.5×10⁻⁷ (300/T)^1.03 for CH₃CNH⁺, and α_e(T)=4.6±0.7×10⁻⁷ (300/T)^0.81 for CH₃CH₂CNH⁺. The equivalent values for the proton-bound dimers are α_e(T)(cm³ s⁻¹)=2.4±0.4×10⁻⁶(300/T)^0.5 for (HCN)₂H⁺ to α_e(T)=2.8±0.4×10⁻⁶(300/T)^0.5 for (CH₃CN)₂H⁺, and α_e(T)=2.3±0.3×10⁻⁶(300/T)^0.5 for (CH₃CH₂CN)₂H⁺. The relevance of these data to molecular synthesis in the interstellar medium and the Titan ionosphere are discussed.     

Photometric investigations of distant comets C/2002 VQ94 (LINEAR) and 29P/Schwassmann-Wachmann-1

We present an analysis of the results of photometric investigations of two distant comets, C/2002 VQ94 (LINEAR) and 29P/Schwassmann-Wachmann-1, obtained with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The comets under study demonstrate sufficient activity out of the zone of water ice sublimation (at heliocentric distances longer than 5 AU). In the spectra of the investigated comets, we found the CO⁺ and N₂⁺ emission. The presence of this emission may say that the comets were formed in the outer parts of the Solar System, in a protoplanetary cloud at a temperature ≤25 K. We found that the photometric maximum of the ionosphere (in the CO⁺ filter) of the comet C/2002 VQ94 (LINEAR) is shifted relative to the photometric center of the dust coma by 1.4″ (7.44 × 10³ km) in the direction deflected by 63° from the direction to the Sun. Using special filters to process the images, we picked out active structures (jets) in the dust coma of the 29P/Schwassmann-Wachmann-1 comet.     

On the production of positive molecular ions in cometary comas

Positively charged molecular ions, such as H₂O⁺, which have been observed in cometary. comas, may be efficiently produced by the evaporation of positively charged clathrate grains of radii in the range 10^–6–10^–5 cm. Such grains may be expelled from nuclei of comets, along with gaseous molecules. Grain charging occurs via interaction with solar ultraviolet photons and/or solar wind protons. Observational data on the total quantities as well as the distributions of H₂O and H₂O⁺ in cometary comas are shown to be in accord with detailed model calculations.On leave from: Tata Institute of Fundamental Research, Bombay, India.     

Charge coupled device (CCD) spectroscopy of comets: Tuttle,Stephan-Oterma,Brooks 2, and Bowell

Spectra of the four comets, Tuttle, Stephan-Oterma, Brooks 2, and Bowell, were taken with a prototype space telescope charge coupled device (CCD) camera using a 500 × 500 Texas Instruments chip. The spectra extended from 5600 to 10,400 Å at a resolution of $\text{～25}\text{A}\text{?}$. The spatial coverage along the slit was 180^?; its resolution was defined by the seeing ($\text{2–3}{}^{\mathrm{?}}$). Both absolute flux scales and spectral albedos were determined with the data reduction procedure which included flat fielding and sky subtraction. Comet Tuttle displayed extensive emissions by NH₂, the red system of CN, and the C₂ Swan bands as well as emissions by the forbidden oxygen lines [OI] ¹D at 6300 and 6364 Å, and the ionic species H₂O⁺. A feature at 6851 Å has been tentatively identified as the 3-0 band of CS⁺. Notable is the absence of the C₂ Phillips bands whose transitions are optimally placed in our spectrum. The much dustier comet, Stephan-Oterma showed emissions by CN, NH₂, and [OI] while only [OI] could be discerned in the noisier Brooks 2 spectrum. The fresh comet Bowell exhibited an unusually extended coma with an albedo times cross section two orders of magnitude larger than the other comets, a very flat albedo spectrum, and no emission features. For Tuttle and Stephan-Oterma, CN and NH₂ column densities using a number of bands were calculated. The CN band intensity ratios show good agreement with theoretical fluorescence models. The spatial profiles for CN and NH₂ were compared to two step Haser model decay calculations. The scale lengths most consistent with the data were compared with values previously reported and with values expected for various photodissociation reactions. Production rates were calculated for CN and NH₂. These should be less model dependent because of the simultaneous collection of spectral and spatial information. The production rate ratios of the parents of CN and NH₂ to the parent of OH are several orders of magnitude smaller than the solar abundance ratios of C/O and N/O.