The Interaction of C/1995 O1 Hale–Bopp with the Solar Wind as recorded in CoCam Images: A Progress Report

The Isaac Newton Group’s CoCam instrument obtained wide-field (10°X 20°) CCD images of the H₂O⁺ tail of Hale-Bopp on March 12–15, 19, 20, 25, 28, 29, 31, April1, 7, 9–13, 21, 23–29 and May 4, 1997. A 6185å filter recorded the distribution of H₂O⁺; 6250å-centered continuum images were also obtained. Initial analysis has revealed the CoCam dataset to be an invaluable record of the large-scale structure of the comet's ion tail. Several images show evidence of upstream parabolic envelopes of ions reminiscent of those reported in C/1908 R1 Morehouse (Eddington A. S., Mon. Not. Roy. Astron. Soc. 70, 442–458, 1910). Studies of these features may help towards a better understanding of the tail ray phenomenon. During most of March and early April, the tail had a largely consistent quiescent appearance, with numerous rays exhibited, but no major disturbances; most probably as a result of being in the stream of fast polar solar wind. A significant kink propagated down the tail around April 9/10. Towards the end of April, as the comet entered more variable solar wind at near-equatorial heliographic latitudes, the tail's morphology became significantly more structured. Striking tail disturbances were recorded in late April and early May, when the comet was in the vicinity of the heliospheric current sheet. Analysis of the dataset continues.     

Comet Hale–Bopp: Velocity Field Of Ions From Fabry-Pérot Imaging

Fabry-Pérot interferograms of comet Hale-Bopp were obtained on several nights in March and April 1997. For this purpose we utilized the 2-channel focal reducer of the Max-Planck-Institute for Aeronomy at the 2-m telescope of the Pik Terskol Observatory. Solid Fabry-Pérot etalons of resolving power 30000 were used in both channels of the focal reducer. The main aim of this study is to measure the velocities and abundances of OH⁺ and H₂O⁺, both ions closely related to the same parent molecule, H₂O. In the blue channel interferograms we identified several individual OH⁺ rotational lines of the A³Π_i - X³Σ⁻ (0-0) transition and measured their Doppler shifts. The target emissions in the red channel were the H₂O⁺ lines of the A²A₁ − X²B₁ (10-0) band. We found that the line of sight velocities, obtained from the Doppler shifted wavelengths of emissions in the comet are higher in sunward direction than in the plasma tail and do not exceed 20 km s^–1. The corresponding values, deprojected in antisolar direction, are consistent with predictions by magnetohydrodynamical models of the solar-wind-comet interaction, when one accounts for the extremely high gas production rate of comet Hale-Bopp. This revised version was published online in July 2006 with corrections to the Cover Date.     

MHD Simulation of Comets: The Plasma Environment of Comet Hale-Bopp

MHD simulation results of the interaction of the expanding atmosphere of comet Hale-Bopp with the magnetized solar wind are presented. At the upstream boundary a supersonic and superalfvénic solar wind enters into the simulation box 25 million km upstream of the nucleus. The solar wind is continuously mass loaded with cometary ions originating from the nucleus. The effects of photoionization, recombination and ion-neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our MUSCL-type upwind numerical technique, MAUS-MHD (Multiscale Adaptive Upwind Scheme for MHD). The combination of the adaptive refinement with the MUSCL-scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. Detailed simulation results for the plasma environment of comet Hale-Bopp for slow and fast solar wind conditions are presented. We also calculate synthetic H₂O⁺, CO⁺ and soft x-ray images for observing conditions on April 11, 1997. This revised version was published online in July 2006 with corrections to the Cover Date.     

Large-Scale Structures in Comet Hale–Bopp

The plasma tails of comets clearly show the demarcation of the solar wind into distinct equatorial and polar regions (Brandt and Snow (2000), Icarus 148, 52–64).The boundary is determined by the maximum extent in latitude of the heliospheric current sheet (HCS). The observational record contains many well-observed equatorial comets, but observations of comets in the polar region are relatively rare. In addition to its size and brightness, comet Hale–Bopp had an orbital inclination of 89.4° and was well observed for months in the polar region. We document the comet's large-scale appearance throughout the apparition, including the polar region and its transition into the equatorial region. The bright dust tail hampered observations of the plasma tail, particularly near the head, but images taken with a CO⁺ filter show a very large disconnection event (DE) on May 7 and May 8, 1997. The time of disconnection is estimated at approximately May 4.0. This DE is associated with a crossing of the HCS. The model calculations of the HCS indicate that other crossings might have occurred in late April, but given the uncertainty in the calculation, the comet might have missed the HCS. Sparse observational coverage and the bright dust tail prevent further investigation of the potential earlier HCS crossings. The plasma tail shows anomalous orientations at the highest latitudes and possible explanations are discussed.     

Observations of Ions in Comets: A Contribution Towards Understanding the Comet-Solar Wind Interaction

Since many years cometary ions have been observed by the authors and their coworkers in order to study the comet-solar wind interaction. Comets with water production rates ranging from 10²⁸(46P/Wirtanen) to 6 10³⁰molecules s⁻¹(C/1995 O1 Hale-Bopp) have been observed. In this paper we briefly introduce the physics of the comet-solar wind interaction. New observations of comet C/1996 Q1 (Tabur) are presented, where for the first time H₂O⁺and CO⁺ions have been recorded exactly simultaneously with a two-channel system. They are compared with previous observations of comets C/1989 X1 (Austin), 46P (Wirtanen) and 109P (Swift-Tuttle). We use a new method of Wegmann et al. (1998), based on the MHD scaling law, to determine the water production of comet Tabur from its H₂O⁺column density map and obtain a value of 3.3 10²⁸water molecules s⁻¹. Nonstationary phenomena like tail rays and so-called tail disconnections are very briefly reviewed. A movie of plasma envelopes observed in the light of OH⁺in comet 1995 O1 (Hale-Bopp) is presented on the attached CD-ROM. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

The role of solar-wind velocity-waves in comet outburst activity

Comet outburst activity and the structure of solar wind streams were compared on the basis of Pioneer 10, 11, Vela 3 and IMP 7, 8 measurements at the heliocentric distance r ≈ 1–6 AU. It is shown that the solar wind velocity waves which are evolving into corotating shock waves beyond the Earth orbit may be responsible for comet outburst activity. The correlation between variations of comet outburst activity with heliocentric distance and the behavior of the solar wind velocity waves is established. The closeness of the characteristic times for the velocity waves and comet outburst activity (7–8 days at r = 1 AU) as well as the simultaneous growth of both the characteristic times with r are noted. The observed distribution of the comet outburst activity parameters during the 11-year cycle is also in good agreement with the phase distributions during the 11-year cycle of variations of the coronal hole areas and the rate of change of the sunspot area δS _p.     

Evolution of C/2006 M4 (SWAN) comet tails

We studied variations in the structure of plasma and dust tails of the C/2006 M4 (SWAN) comet during a long observation period (September–December 2006). We found sizes of grains ejected by the comet from the synchronic-syndynamic analysis of comet images. We calculated solar wind speed for high heliographic latitudes from calculations of the aberration angle of the comet plasma tail. Rapid changes in the calculated values of the solar wind speed are caused by its variable transversal component.     

New Molecular Species in Comet C/1995 O1 (Hale-Bopp) Observed with the Caltech Ssubmillimeter Observatory

We present millimeter-wave observations of HNCO, HC₃N, SO, NH₂CHO, H¹³CN, and H₃O⁺ in comet C/1995 O1 (Hale-Bopp)obtained in February–April, 1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first detected at the CSO in comet C/1996B2 (Hyakutake), is securely confirmed in comet Hale-Bopp via observations of three rotational transitions. The derived abundance with respect to H₂O is (4-13) × 10^-4. HC₃N, SO, and NH₂CHO are detected for the first time in a comet. The fractional abundance of HC₃N based on observations of three rotational lines is (1.9 ± 0.2) × 10^-4. Four transitions of SO are detected and the derived fractional abundance, (2-8) ×10^-3, is higher than the upper limits derived from UV observations of previous comets. Observations of NH₂CHO imply a fractional abundance of (1-8) × 10^-4. H₃O is detected for the first time from the ground. The H¹³CN (3-2)transition is also detected and the derived HCN/H¹³CN abundance ratio is 90 ± 15, consistent with the terrestrial¹²C/¹³C ratio. In addition, a number of other molecular species are detected, including HNC, OCS, HCO⁺, CO⁺, and CN(the last two are first detections in a comet at radio wavelengths). This revised version was published online in July 2006 with corrections to the Cover Date.     

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(¹ D) + 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–Bopp's 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.