Quantitative Analysis of H2OComa Images Using a Multiscale MHD Model with Detailed Ion Chemistry

The observation of ions created by ionization of cometary gas, either by ground-based observations or byin situmeasurements can give us useful information about the gas production and composition of comets. However, due to the interaction of ions with the magnetized solar wind and their high chemical reactivity, it is not possible to relate measured ion densities (or column densities) directly to the parent gas densities. In order to quantitatively analyze measured ion abundances in cometary comae it is necessary to understand their dynamics and chemistry. We have developed a detailed ion–chemical network of cometary atmospheres. We include production of ions by photo- and electron impact-ionization of a background neutral atmosphere, charge exchange of solar wind ions with cometary atoms/molecules, reactions between ions and molecules, and dissociative recombination of molecular ions with thermal electrons. By combining the ion–chemical network with the three-dimensional plasma flow as computed by a new fully three-dimensional MHD model of cometary plasma environments (Gombosiet al.1996) we are able to compute the density of the major cometary ions everywhere in the coma. The input parameters for our model are the solar wind conditions (density, speed, temperature, magnetic field) and the composition and production rate of the gas. We applied our model to Comet P/Halley in early March 1986, for which the input parameters are reasonably well known. We compare the resulting column density of H₂O⁺with ground-based observations of H₂O⁺from DiSantiet al.(1990). The results of our model are in good agreement with both the spatial distribution and the absolute abundance of H₂O⁺and with their variations with the changing overall water production rate between two days. The results are encouraging that it will be possible to obtain production rates of neutral cometary constituents from observations of their ion products.     

Atomic oxygen in the comae of comets

We report on the detection of atomic oxygen lines in the spectra of 8 comets. These forbidden lines are a result of the photodissociation of the parent oxygen-bearing species directly into an excited state. We used high resolution spectra obtained at the McDonald Observatory 2.7 m telescope to resolve the cometary oxygen lines from the telluric oxygen lines and from other cometary emissions. We find that the relative intensities of the two red lines (6300.304 and 6363.776 Å) are consistent with theory. The green line (5577.339 Å) has an intensity which is about 10% of the sum of the intensities of the two red lines. We show that collisional quenching may be important in the inner coma. If we assume the relative excitation rates of potential parents which have appeared in the literature, then H₂O would be the parent of the cometary green oxygen line. However, those rates have been questioned. We measured the width of the three oxygen lines and find that the green line is wider than either of the two red lines. The finding of a wider line could imply a different parent for the green and red lines. However, the constancy of the green to red line flux ratio suggests the parent is the same for these lines but that the exciting photons have different energies.     

Time-dependent behaviour of a stable auroral red arc excited by an electric field

We examine the electric field hypothesis as a possible explanation of a stable auroral red arc. An electric field perpendicular to the geomagnetic field in the ionosphere heats the ambient F-region electrons and ions. Given large enough electric fields, the electrons can be heated sufficiently to excite the OI (¹D) term of atomic oxygen by electron impact, giving rise to the λ6300 emission characteristic of the red arc. The electron and ion heating rates are determined by the relative drift between the plasma and neutral gas.     

Changes in the Martian atmosphere induced by auroral electron precipitation

Typical auroral events in the Martian atmosphere, such as discrete and diffuse auroral emissions detected by UV spectrometers onboard ESA Mars Express and NASA MAVEN, are investigated. Auroral electron kinetic energy distribution functions and energy spectra of the upward and downward electron fluxes are obtained by electron transport calculations using the kinetic Monte Carlo model. These characteristics of auroral electron fluxes make it possible to calculate both the precipitation-induced changes in the atmosphere and the observed manifestations of auroral events on Mars. In particular, intensities of discrete and diffuse auroral emissions in the UV and visible wavelength ranges (Soret et al., 2016; Bisikalo et al., 2017; Gérard et al., 2017). For these conditions of auroral events, the analysis is carried out, and the contribution of the fluxes of precipitating electrons to the heating and ionization of the Martian atmosphere is estimated. Numerical calculations show that in the case of discrete auroral events the effect of the residual crustal magnetic field leads to a significant increase in the upward fluxes of electrons, which causes a decrease in the rates of heating and ionization of the atmospheric gas in comparison with the calculations without taking into account the residual magnetic field. It is shown that all the above-mentioned impact factors of auroral electron precipitation processes should be taken into account both in the photochemical models of the Martian atmosphere and in the interpretation of observations of the chemical composition and its variations using the ACS instrument onboard ExoMars.     

UVIS observations of the FUV OI and CO 4P Venus dayglow during the Cassini flyby

We analyze FUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus. We use a least-squares fit method to determine the brightness of the OI emissions at 130.4 and OI 135.6 nm, and of the bands of the CO fourth positive system which are dominated by fluorescence scattering. We compare the brightness observed along the UVIS foot track of the two OI multiplets with that deduced from a model of the excitation of these emissions by photoelectron impact on O atoms and resonance scattering of the solar 130.4 nm emission. The large optical thickness 130.4 nm emission is accounted for using a radiative transfer model. The airglow intensities are calculated along the foot track and found to agree with the observed 130.4 nm brightness within ∼10%. The modeled OI 135.6 nm brightness is also well reproduced by the model. The oxygen density profile of the VTS3 model is found to be consistent with the observations. We find that self-absorption of the (0, v″) bands of the fourth positive emission of CO is important and we derive a CO vertical column of about 6.4 × 10¹⁵ cm⁻² in close agreement with the value provided by the VTS3 empirical atmospheric model.     

Dust tail of the active distant Comet C/2003 WT42 (LINEAR) studied with photometric and spectroscopic observations

We present the study of dust environment of dynamically new Comet C/2003 WT42 (LINEAR) based on spectroscopic and photometric observations. The comet was observed before and after the perihelion passage at heliocentric distances from 5.2 to 9.5 AU. Although the comet moved beyond the zone where water ice sublimation could be significant, its bright coma and extended dust tail evidenced the high level of physical activity. Afρ values exceeded 3000 cm likely reaching its maximum before the perihelion passage. At the same time, the spectrum of the comet did not reveal molecular emission features above the reflected continuum. Reddening of the continuum derived from the cometary spectrum is nonlinear along the dispersion with the steeper slop in the blue region. The pair of the blue and red continuum images was analyzed to estimate a color of the comet. The mean normalized reflectivity gradient derived from the innermost part of the cometary coma equals to 8% per 1000 Å that is typical for Oort cloud objects. However, the color map shows that the reddening of the cometary dust varies over the coma increasing to 15% per 1000 Å along the tail axis. The photometric images were fitted with a Monte Carlo model to construct the theoretical brightness distribution of the cometary coma and tail and to investigate the development of the cometary activity along the orbit. As the dust particles of distant comets are expected to be icy, we propose here the model, which describes the tail formation taking into account sublimation of grains along their orbits. The chemical composition and structure of these particles are assumed to correspond with Greenberg’s interstellar dust model of comet dust. All images were fitted with the close values of the model parameters. According to the results of the modeling, the physical activity of the comet is mainly determined by two active areas with outflows into the wide cones. The obliquity of the rotation axis of the nucleus equals to 20° relative to the comet’s orbital plane. The grains occupying the coma and tail are rather large amounting to 1 mm in size, with the exponential size distribution of a^−4.5. The outflow velocities of the dust particles vary from a few centimeters to tens of meters per second depending on their sizes. Our observations and the model findings evidence that the activity of the nucleus decreased sharply to a low-level phase at the end of April–beginning of May 2007. About 190 days later, in the first half of November 2007 the nucleus stopped any activity, however, the remnant tail did not disappear for more than 1.5 years at least.     

Negative ions in comets

The primary negative ion sources in comets are shown to be: for the inner coma—both polar photodissociation of HCN, electron attachment of OH and collisions with alkalis; in the vicinity of the nucleus—plasma, excavated during interplanetary dust impacts on the nucleus; for both the contaminated solar wind region and sporadic discharges in the non-homogeneous inner coma plasma—dissociative electron attachment and charge inversion during keV positive ion scattering by cometary dust are also significant sources. Negative ion abundance for Halley's Comet has been estimated to be from 10^–6 to 10^–10 of electron densities. However, this ratio may be more due to the formation of clusters A^–(H₂O)_n. Some possible cometary plasma effects, caused by negative ions, have also been discussed.     

Vibrational and rotational cooling of electrons by water vapor

The cooling of electrons by vibrational and rotational excitation of water molecules plays an important role in the thermal balance of electrons in cometary ionospheres. The energy loss function for rotational excitation and de-excitation of H₂O by electron impact is calculated theoretically. The rotational cooling rate is calculated using this loss function for a wide range of electron and neutral temperatures. The vibrational cooling rate is calculated using measured values of electron impact vibrational excitation cross sections. Analytical formulae are provided for some of the cooling rates. The interaction of ions with H₂O molecules is also discussed and a formula is suggested for the momentum transfer collision frequency.     

Suprathermal electron fluxes on the nightside of Mars: ASPERA-3 observations

Recently aurora-type UV emissions were discovered on the nightside of Mars [Bertaux, J.-L., Leblanc, F., Witasse, O., et al., 2005. Discovery of an aurora on Mars. Nature 439, doi:10.1038/nature03603]. It was suggested that these emissions are produced by suprathermal electrons with energies of tens of eV, rather than by the electrons with spectra peaked above 100 eV [Leblanc, F., Witasse, O., Winningham J., et al., 2006. Origin of the martian aurora observed by spectroscopy for investigation of characteristics of the atmosphere of Mars (SPICAM) onboard Mars Express. J. Geophys. Res. 111, A09313, doi:10.1029/2006JA011763]. In this paper we present observations of fluxes of suprathermal electrons (E_e≈30-100 eV) on the Martian nightside by the ASPERA-3 experiment onboard the Mars Express spacecraft. Narrow spikes of suprathermal electrons are often observed in energy-time spectrograms of electron fluxes at altitudes between 250 and 600 km. These spikes are spatially organized and form narrow strips in regions with strong upward or downward crustal magnetic field. The values of electron fluxes in such events generally could explain the observed auroral UV emissions although a question of their origin (transport from the dayside or local precipitation) remains open.     

Monte Carlo modeling of Io’s [OI] 6300 Å and [SII] 6716 Å auroral emission in eclipse

We present a Monte Carlo (MC) model of [OI] 6300 Å and [SII] 6716 Å emission from Io entering eclipse. The simulation accounts for the 3-D distribution of SO₂, O, SO, S, and O₂ in Io’s atmosphere, several volcanic plumes, and the magnetic field around Io. Thermal electrons from the jovian plasma torus are input along the simulation domain boundaries and move along the magnetic field lines distorted by Io, occasionally participating in collisions with neutrals. We find that the atmospheric asymmetry resulting from varying degrees of atmospheric collapse across Io (due to eclipse ingress) and the presence of volcanoes contributes significantly to the unique morphology of the [OI] 6300 Å emission. The [OI] radiation lifetime of ∼134 s limits the emission to regions that have a sufficiently low neutral density so that intermolecular collisions are rare. We find that at low altitudes (typically <40 km) and in volcanic plumes (Pele, Prometheus, etc.) the number density is large enough (>4 × 10⁹ cm⁻³) to collisionally quench nearly all (>95%) of the excited oxygen for reasonable quenching efficiencies. Upstream (relative to the plasma flow), Io’s perturbation of the jovian magnetic field mirrors electrons with high pitch angles, while downstream collisions can trap the electrons. This magnetic field perturbation is one of the main physical mechanisms that results in the upstream/downstream brightness asymmetry in [OI] emission seen in the observation by Trauger et al. (Trauger, J.T., Stapelfeldt, K.R., Ballester, G.E., Clarke, J.I., 1997. HST observations of [OI] emissions from Io in eclipse. AAS-DPS Abstract (1997DPS29.1802T)). There are two other main causes for the observed brightness asymmetry. First, the observation’s viewing geometry of the wake spot crosses the dayside atmosphere and therefore the wake’s observational field of view includes higher oxygen column density than the upstream side. Second, the phased entry into eclipse results in less atmospheric collapse and thus higher collisional quenching on the upstream side relative to the wake. We compute a location (both in altitude and latitude) for the intense wake emission feature that agrees reasonably well with this observation. Furthermore, the peak intensity of the simulated wake feature is less than that observed by a factor of ∼3, most likely because our model does not include direct dissociation-excitation of SO₂ and SO. We find that the latitudinal location of the emission feature depends not so much on the tilt of the magnetic field as on the relative north/south flux tube depletion that occurs due to Io’s changing magnetic latitude in the plasma torus. From 1-D simulations, we also find that the intensity of [SII] 6716 and 6731 Å emission is much weaker than that of [OI] even if the [SII] excitation cross section is 10³ times larger than excitation to [OI]. This is because the density of S⁺ is much less than that of O and because the Einstein-A coefficient of the [SII] emission is a factor of ∼10 smaller than that of [OI].     

Multi-fluid model of comet 1P/Halley

A new three-dimensional magnetohydrodynamic model of the coma of a comet has been developed and applied to simulations of a Halley-class coma using the solar-wind conditions of the Giotto flyby of Halley in 1986. The code developed for high-performance parallel processing computers, combines the high spatial resolution of smaller than 1 km grid spacing near the nucleus, with a large computational domain that enables structures nearly 10 million km down the comet tail to be modeled. Ions, neutrals, and electrons are considered as separate interacting fluids. Significant physical processes treated by the model include both photo and electron impact ionization of neutrals, recombination of ions, charge exchange between solar-wind ions and cometary neutrals, and frictional interactions between the three fluids considered in the model. A variety of plasma structures and physical parameters that are the output of this model are compared with relevant Giotto data from the 1986 Halley flyby.     

Spatially Resolved Far Ultraviolet Spectroscopy of the Jovian Aurora

Spatially resolved spectra in four 50-Å FUV spectral windows were obtained across the jovian aurora with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Nearly simultaneous ultraviolet imaging makes it possible to correlate the intensity variations along the STIS slit with those observed in the images and to characterize the global auroral context prevailing at the time of the observations. Spectra at ∼1-Å resolution taken in pairs included an unabsorbed window and a spectral region affected by hydrocarbon absorption. Both sets of spectra correspond to an aurora with a main oval brightness of about 130 kilorayleighs of H₂ emission. The far ultraviolet color ratios I(1550-1620 Å)/I(1230-1300 Å) are 2.3 and 5.9 for the noon and morning sectors of the main oval, respectively. We use an interactive model coupling the energy degradation of incoming energetic electrons, auroral temperature and composition, and synthetic H₂ spectra to fit the intensity distribution of the H₂ lines. It is found that the model best fitting globally the spectra has a soft energy component in addition to a 10 erg cm⁻² s⁻¹ flux of 80 keV electrons. It provides an effective H₂ temperature of 540 K. The relative intensity of temperature-sensitive H₂ lines indicates differences between the auroral main oval and polar cap emissions. The amount of methane absorption across the polar region is shown to vary in a way consistent with temperature. For the second spectral pair, the polar cap shows a higher attenuation by CH₄, indicating a harder precipitation along high-latitude magnetic field lines.     

EUV spectroscopy of the Venus dayglow with UVIS on Cassini

We analyze EUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus on 24 June 1999, a period of high solar activity level. Emissions from OI, OII, NI, CI and CII and CO have been identified and their disc average intensity has been determined. They are generally somewhat brighter than those determined from the observations made with the HUT spectrograph at a lower activity level, We present the brightness distribution along the foot track of the UVIS slit of the OII 83.4 nm, OI 98.9 nm, Lyman-ß + OI 102.5 nm and NI 120.0 nm multiplets, and the CO C-X and B-X Hopfield-Birge bands. We make a detailed comparison of the intensities of the 834 nm, 989 nm, 120.0 nm multiplets and CO B-X band measured along the slit foot track on the disc with those predicted by an airglow model previously used to analyze Venus and Mars ultraviolet spectra. This model includes the treatment of multiple scattering for the optically thick OI, OII and NI multiplets. It is found that the observed intensity of the OII emission at 83.4 nm is higher than predicted by the model. An increase of the O⁺ ion density relative to the densities usually measured by Pioneer Venus brings the observations and the modeled values into better agreement. The calculated intensity variation of the CO B-X emission along the track of the UVIS slit is in fair agreement with the observations. The intensity of the OI 98.9 nm emission is well predicted by the model if resonance scattering of solar radiation by O atoms is included as a source. The calculated brightness of the NI 120 nm multiplet is larger than observed by a factor of ∼2-3 if photons from all sources encounter multiple scattering. The discrepancy reduces to 30-80% if the photon electron impact and photodissociation of N₂ sources of N(⁴S) atoms are considered as optically thin. Overall, we find that the O, N₂ and CO densities from the empirical VTS3 model provide satisfactory agreement between the calculated and the observed EUV airglow emissions.     

Diffuse auroral precipitation in the jovian upper atmosphere and magnetospheric electron flux variability

The deposition of energetic electrons in Jupiter's upper atmosphere provides a means, via auroral observations, of monitoring electron and plasma wave activity within the magnetosphere. Not only does particle precipitation indicate a potential change in atmospheric chemistry, it allows for the study of episodic, pronounced flux enhancements in the energetic electron population. A study has been made of the effects of such electron injections into the jovian magnetosphere and of their ability to provide the source population for variations in diffuse auroral emissions. To identify the source region of precipitating auroral electrons, we have investigated the pitch-angle distributions of high-resolution Galileo Energetic Particle Detector (EPD) data that indicate strong flux levels near the loss cone. The equatorial source region of precipitating electrons has been determined from the locations of Galileo's in situ measurements by tracing magnetic field lines using the KK97 model. The primary source region for Jupiter's diffuse aurora appears to lie in the magnetic equator at 15-40 RJ, with the predominant contribution to precipitation flux (tens of ergs cm⁻² s⁻¹ sr⁻¹) stemming from <30 RJ. Variability of flux for energetic electrons in this region is also important to the irradiation of surfaces and atmospheres for the Galilean moons: Europa, Ganymede, and Callisto. The average diffuse auroral precipitation flux has been shown to vary by as much as a factor of six at a given radial location. This variability appears to be associated with electron injection events that have been identified in high-resolution Galileo EPD data. These electron flux enhancements are also associated with increased whistler-mode wave activity and magnetic field perturbations, as detected by the Galileo Plasma Wave Subsystem (PWS) and Magnetometer (MAG), respectively. Resonant interactions with the whistler-mode waves cause electron pitch-angle scattering and lead to pitch-angle isotropization and precipitation.     

Production,in a stationary state,of solid material in a coma of cometary nuclei under the action of solar radiation

A stationary state of production of solid material in the coma of a comet is sought by assuming a production and dynamics of solid grains liberated during the vaporization of cometary nuclei under the action of solar radiation.     

Dust in cometary comae: Present understanding of the structure and composition of dust particles

In situ probing of a very few cometary comae has shown that dust particles present a low albedo and a low density, and that they consist of both rocky material and refractory organics. Remote observations of solar light scattered by cometary dust provide information on the properties of dust particles in the coma of a larger set of comets. The observations of the linear polarization in the coma indicate that the dust particles are irregular, with a size greater (on the average) than about 1 μm. Besides, they suggest, through numerical and experimental simulations, that both compact grains and fluffy aggregates (with a power law of the size distribution in the −2.6 to −3 range), and both rather transparent silicates and absorbing organics are present in the coma. Recent analysis of the cometary dust samples collected by the Stardust mission provide a unique ground truth and confirm, for comet 81P/Wild 2, the results from remote sensing observations. Future space missions to comets should, in the next decade, lead to a more precise characterization of the structure and composition of cometary dust particles.     

The effects of solar and ionospheric parameters on seasonal variation of 6300 Å line emission at Calcutta

The purpose of this paper is to find correlation between OI 6300 Å line intensity with solar and ionospheric parameters. A critical study have been made and the following important results are obtained:

Rosetta-Alice observations of exospheric hydrogen and oxygen on Mars

The European Space Agency’s Rosetta spacecraft, en route to a 2014 encounter with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on 25 February 2007, closest approach being at 01:54 UT. The Alice instrument on board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for in situ cometary spectroscopy in the 750-2000 Å spectral band, was used to study the daytime Mars upper atmosphere including emissions from exospheric hydrogen and oxygen. Offset pointing, obtained five hours before closest approach, enabled us to detect and map the H i Lyman-α and Lyman-β emissions from exospheric hydrogen out beyond 30,000 km from the planet’s center. These data are fit with a Chamberlain exospheric model from which we derive the hydrogen density at the 200 km exobase and the H escape flux. The results are comparable to those found from the Ultraviolet Spectrometer experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen emission at 1304 Å is detected at altitudes of 400-1000 km above the limb during limb scans shortly after closest approach. However, the derived oxygen scale height is not consistent with recent models of oxygen escape based on the production of suprathermal oxygen atoms by the dissociative recombination of .     

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.     

Intensity variation of atomic oxygen red line in the day airglow with solar activity

Rates of production of O(¹ D) atoms in the upper atmosphere by photodissociation of O₂, dissociative recombination of O₂ ⁺, NO⁺ and electron impact excitation of O(³ P) have been calculated for low, medium and high levels of solar activity. Variations with solar activity, of neutral and ionic composition, electron and neutral temperatures of the upper atmosphere and solar extreme ultraviolet fluxes incident on it have been taken into consideration.Emission rates ofOi red line (6300Å) have been computed taking into account the deactivation both by molecular oxygen and nitrogen. It has been shown that the integrated intensity from low to high activity period varies by approximately an order of magnitude in agreement with the results of experimental observations.