Axisymmetric dusty gas jet in the inner coma of a comet: II. The case of isolated jets

The behavior of isolated pure and dusty gas jets ejected from an active spot on the sunlit side of the nucleus surface is hydrodynamically investigated in the inner coma of an H₂O-dominated comet that is assumed to have no ambient ejection of the gas and dust from the dust-covered surface except the active spot. Steady-state solutions of the expanding jets are obtained by numerically solving the axisymmetric, time-dependent, coupled hydrodynamic equations of H₂O gas and the dust in polar coordinates (r, θ, φ). The dust particles are treated as multicomponents composed of the three radii of a = 0.01, 0.1, and 1 μm. The boundary conditions of a slip wall are applied to the dust-covered surface. Discussion is given on the no-slip-wall conditions. Compared with the previous study on the jets surrounded by ambient gas and dust ejected from a nonactive region by Y. Kitamura (1986, Icarus 66, 241–257), the jet features can be clearly discerned even at large distances from the nucleus center, and the shift of the density peaks from the central axis to the wings, which was seen in the previous study, does not occur, because the jets can freely expand in the θ direction without being decelerated by the ambient gas and dust. The gas flow in the θ direction is supersonic, and consequently it is predicted that the shock waves are formed in the interactive regions among several jets. For the isolated jets with no ambient ejection, it is to be noted that the flow of the gas and dust along the nucleus surface arises in spite of the radial ejection from the active spot, and that this flow may change the surface structure. In the dusty case, the gas temperature increases immediately from 200 to ∼275°K in the vicinity of the surface owing to strong heating by the fine dust particles with the radius as small as 0.01 μm. In addition to the fine dust, the hot dust mantle (300–400°K) on the surface may considerably heat the gas near the mantle.     

Formation of jets in Comet 19P/Borrelly by subsurface geysers

Observations of the inner coma of Comet 19P/Borrelly with the camera on the Deep Space 1 spacecraft revealed several highly collimated dust jets emanating from the nucleus. The observed jets can be produced by acceleration of evolved gas from a subsurface cavity through a narrow orifice to the surface. As long as the cavity is larger than the orifice, the pressure in the cavity will be greater than the ambient pressure in the coma and the flow from the geyser will be supersonic. The gas flow becomes collimated as the sound speed is approached and dust entrainment in the gas flow creates the observed jets. Outside the cavity, the expanding gas loses its collimated character, but the density drops rapidly decoupling the dust and gas, allowing the dust to continue in a collimated beam. The hypothesis proposed here can explain the jets seen in the inner coma of Comet 1P/Halley as well, and may be a primary mechanism for cometary activity.     

The Deep Space 1 Encounter with Comet 19p/Borrelly

NASA's Deep Space 1 (DS1) spacecraft successfully encountered comet 19P/Borrelly near perihelion and the Miniature Integrated Camera and Spectrometer (MICAS) imaging system onboard DS1 returned the first high-resolution images of a Jupiter-family comet nucleus and surrounding environment. The images span solar phase angles from 88° to 52°, providing stereoscopic coverage of the dust coma and nucleus. Numerous surface features are revealed on the 8-km long nucleus in the highest resolution images(47–58 m pixel). A smooth, broad basin containing brighter regions and mesa-likestructures is present in the central part of the nucleus that seems to be the source ofjet-like dust features seen in the coma. High ridges seen along the jagged terminator lead to rugged terrain on both ends of the nucleus containing dark patches and smaller series of parallel grooves. No evidence of impact craters with diameters larger thanabout 200-m are present, indicating a young and active surface. The nucleus is very dark with albedo variations from 0.007 to 0.035. Short-wavelength, infrared spectra from 1.3 to 2.6 μm revealed a hot, dry surface consistent with less than about10% actively sublimating. Two types of dust features are seen: broad fans and highlycollimated “jets” in the sunward hemisphere that can be traced to the surface. The source region of the main jet feature, which resolved into at least three smaller “jets” near the surface, is consistent with an area around the rotation pole that is constantly illuminated by the sun during the encounter. Within a few nuclear radii, entrained dustis rapidly accelerated and fragmented and geometrical effects caused from extended source regions are present, as evidenced in radial intensity profiles centered on the jet features that show an increase in source strength with increasing cometocentric distance. Asymmetries in the dust from dayside to nightside are pronounced and may show evidence of lateral flow transporting dust to structures observed in the nightside coma. A summary of the initial results of the Deep Space 1 Mission is provided, highlighting the new knowledge that has been gained thus far.     

Magnetic-Tower Jet Solution for Launching Astrophysical Jets

In spite of the large number of global three-dimensional (3-D) magnetohydrodynamic (MHD) simulations of accretion disks and astrophysical jets, which have been developed since 2000, the launching mechanisms of jets is somewhat controversial. Previous studies of jets have concentrated on the effect of the large-scale magnetic fields permeating accretion disks. However, the existence of such global magnetic fields is not evident in various astrophysical objects, and their origin is not well understood. Thus, we study the effect of small-scale magnetic fields confined within the accretion disk. We review our recent findings on the formation of jets in dynamo-active accretion disks by using 3-D MHD simulations. In our simulations, we found the emergence of accumulated azimuthal magnetic fields from the inner region of the disk (the so-called magnetic tower) and also the formation of a jet accelerated by the magnetic pressure of the tower. Our results indicate that the magnetic tower jet is one of the most promising mechanisms for launching jets from the magnetized accretion disk in various astrophysical objects. We will discuss the formation of cosmic jets in the context of the magnetic tower model.     

HiRISE observations of gas sublimation-driven activity in Mars’ southern polar regions: IV. Fluid dynamics models of CO2 jets

The High Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter (MRO) has been used to monitor the seasonal evolution of several regions at high southern latitudes on Mars and, in particular, the jet-like activity which may result from the process described by Kieffer (Kieffer, H.H. [2007]. J. Geophys. Res. (Planets) 112, E08005. doi:10.1029/2006JE002816) involving translucent CO₂ ice. In this work, we concentrate on attempting to model the dusty CO₂ gas jets using a computational fluid dynamics code. Models that included surface slopes of up to 20° (as an analogy to the jet activity seen in “Inca City”, 81°S, 296°E), wind (from 0 to 6 m s⁻¹), variable vent cross-section and length, particles (including a particle size distribution) and mass loading (with dust to gas ratios exceeding 1) were investigated. The structure of the resulting gas jets, the particle distribution within the jets, the deposition patterns (including their dependence on particle size), and the appearance of jets when viewed from different orientations (including from a nadir-pointing camera) have been investigated for a range of input parameters. The results provide predictions for the size-dependency of altitudes of particles within a plume and the distribution of particle sizes in the deposition fans. Where slopes are a dominant influence, larger particles are expected to be seen furthest from the vent. Where wind is dominant, smaller particles should travel to larger distances. Models producing deposition patterns consistent in length (∼80 m) and form with fans observed by HiRISE on MRO have been demonstrated. The models also suggest that downward flow of gas produced by drag effects from particles falling from the jet under gravity could provide a mechanism for the production of bright haloes which are observed to surround dark fan deposits in MOC, HiRISE and CRISM.     

Dust coma morphology in the Deep Impact images of Comet 9P/Tempel 1

We present an overview of the dust coma observations of Comet Tempel 1 that were obtained during the approach and encounter phases of the Deep Impact mission. We use these observations to set constraints on the pre-impact activity of the comet and discuss some preliminary results. The temporal and spatial changes that were observed during approach reveal three distinct jets rotating with a 1.7-day periodicity. The brightest jet produces an arcuate feature that expands outward with a projected velocity of about 12 m s⁻¹, suggesting that the ambient dust coma is dominated by millimeter-sized dust grains. As the spatial resolution improves, more jets and fans are revealed. We use stereo pairs of high-resolution images to put some crude constraints on the source locations of some of the brightest features. We also present a number of interesting coma features that were observed, including surface jets detected at the limb of the nucleus when the exposed ice patches are passing over the horizon, and features that appear to be jets emanating from unilluminated sources near the negative pole. We also provide a list of 10 outbursts of various sizes that were observed in the near-continuous monitoring during the approach phase.     

Oscillations in solar jets observed with the SOT of Hinode: viscous effects during reconnection

Transverse oscillatory motions and recurrence behavior in the chromospheric jets observed by Hinode/SOT are studied. A comparison is considered with the behavior that was noticed in coronal X-ray jets observed by Hinode/XRT. A jet like bundle observed at the limb in Ca II H line appears to show a magnetic topology that is similar to X-ray jets (i.e., the Eiffel tower shape). The appearance of such magnetic topology is usually assumed to be caused by magnetic reconnection near a null point. Transverse motions of the jet axis are recorded but no clear evidence of twist is appearing from the highly processed movie. The aim is to investigate the dynamical behavior of an incompressible magnetic X-point occurring during the magnetic reconnection in the jet formation region. The viscous effect is specially considered in the closed line-tied magnetic X-shape nulls. We perform the MHD numerical simulation in 2-D by solving the visco-resistive MHD equations with the tracing of velocity and magnetic field. A qualitative agreement with Hinode observations is found for the oscillatory and non-oscillatory behaviors of the observed solar jets in both the chromosphere and the corona. Our results suggest that the viscous effect contributes to the excitation of the magnetic reconnection by generating oscillations that we observed at least inside this Ca II H line cool solar jet bundle.     

Observations of Rotating Jets of Carbon Monoxide in Comet Hale–Bopp with the Iram Interferometer

CO was observed on March 11, 1997 in comet Hale–Bopp with theIRAM Plateau de Bure interferometer. The maps show evidence for asymmetrical patterns, due to the Existence of CO jets. Analysis of the spectra and their velocity shifts shows that there is a spiral CO jet rotating in a plane almost perpendicular to the sky plane.This is the first time that rotating jets are observed for parent molecules.We have developed a 3-D model simulating rotating spiral jets of CO gas.We present here the comparison between the observations and our model.     

Interpretation of HMC images by a combined thermal and gasdynamic model

Images of comet Halley's nucleus taken by the HMC camera during the GIOT-TO encounter in 1986 show that a major part of the total dust production is localized in a few active areas which are the sources of gas-dust jets. The global dust distribution in the inner coma is dominated by two main jets roughly directed to the sun. A combination of a 1D thermal nucleus model with an axisymmetric continuum model of the jet outflow was used to investigate the properties of the inner coma. Detailed investigations show that the characteristics of the observed jets can be reproduced by outgassing from free sublimating active areas of a few km in diameter, a dust to gas ratio of 1–2.5 and a size distribution dominated by the larger grains. It is further shown that most of the observational constraints provided by the HMC data can be met simultaneously by a model of three jets superimposed on a weak background.     

Interpretation of HMC images by a combined thermal and gasdynamic model

Images of comet Halley's nucleus taken by the HMC camera during the GIOT-TO encounter in 1986 show that a major part of the total dust production is localized in a few active areas which are the sources of gas-dust jets. The global dust distribution in the inner coma is dominated by two main jets roughly directed to the sun. A combination of a 1D thermal nucleus model with an axisymmetric continuum model of the jet outflow was used to investigate the properties of the inner coma. Detailed investigations show that the characteristics of the observed jets can be reproduced by outgassing from free sublimating active areas of a few km in diameter, a dust to gas ratio of 1–2.5 and a size distribution dominated by the larger grains. It is further shown that most of the observational constraints provided by the HMC data can be met simultaneously by a model of three jets superimposed on a weak background.     

Temporal deconvolution of the hydrogen coma: II. Pre- and post-perihelion activity of Comet Hyakutake (1996 B2)

Comet 1996 B2 (Hyakutake) displayed strong evidence for break-up, with a prominent antisunward dust spike and fragments traveling antisunward for many days after an eruptive event in late March 1996. Because of its high orbital inclination and rapid southward motion after perihelion, its post-perihelion activity was not well monitored from the ground. The SWAN all-sky Lyman-alpha camera on the SOHO spacecraft was ideally placed for long-term monitoring of the hydrogen coma of Comet Hyakutake both before and after perihelion. The SWAN images were analyzed with a new time-resolved model (TRM) that provides daily averages of the water production rate and an estimate of the hydrogen atom lifetime (dominated by charge exchange with solar wind protons) during extended periods throughout the apparition. A long-term variation of water production rate of , where r is the heliocentric distance in AU was found. The daily average values of the production rate covered the March 19 outburst and two more outbursts seen in the April before perihelion, which had progressively shorter durations at respectively smaller heliocentric distances. The long-term variation of the production rate was found to be consistent with the seasonal effect predicted by the jet rotation model of Schleicher and Woodney [2003. Analyses of dust coma morphology of Comet Hyakutake (1996 B2) near perigee: Outburst behavior, jet motion, source region locations, and the nucleus pole orientation. Icarus 162, 190-213] when added to a more steady source that is about two-thirds of the maximum of the jet source. The seasonal effect in their model found the dust jet source largely not illuminated after perihelion, coinciding with somewhat reduced overall activity and the absence of outbursts and fragmentation. The locations of the dust jets appear to be responsible for the outbursts and fragmentation before perihelion. The erratic behavior of the pre-perihelion jet sources as contrasted with the smoother variation from the rest of the surface after perihelion indicates there is a strong heterogeneity in the physical make-up of active areas on the nucleus.     

Axisymmetric dusty gas jet in the inner coma of a comet

The behavior of expanding pure and dusty gas jets is investigated in the inner coma of an H₂O-dominated comet by numerically solving the axisymmetric, time-dependent, coupled hydrodynamic equations for H₂O gas and single-sized dust (0.65 μ) in polar coordinates (r, θ, φ). The jet profile is assumed to be Gaussian on the surface of a nucleus. The viscosity of the gas is taken into account. Two-dimensional distributions of the densities, velocities in the r and θ directions, and temperatures for the gas and dust have been obtained. For the dusty jet, the axisymmetric transonic solution for the gas has been calculated time-dependently. For a narrow dusty gas jet (i.e., of breadth 10°), the gas density peaks shift from the central axis of the jet (θ = 0°) to its wings (θ ∼ 30°) with the gas flowing away from the cometary nucleus, owing to a steep density gradient in the θ direction. Dragged by this laterally expanding gas outflow, the dust particles are swept away from the central axis and are also concentrated more sharply at θ ∼ 35° than the gas particles. This lateral expansion of the jet is overwhelming only within the innermost region (r ≦ 10 km). The jet feature for the gas becomes indiscernible by the time the flow reaches the outer boundary (r = 100 km), while the corresponding dust feature remains even at the outer boundary. The radial velocities of the gas and dust are enhanced inside the jet, compared with those in the background. For a broad pure gas jet (i.e., of breadth 30°), on the other hand, the gas density peaks do not shift to the wings and the jet feature can still be seen at the outer boundary, in contrast to the narrow case.     

The bulk kinetic power of radio jets in active galactic nuclei

Based on the Königl's inhomogeneous jet model, we estimate the jet parameters, such as bulk Lorentz factor Γ, viewing angle θ and electron number density n _e from radio very long-baseline interferometry and X-ray data for a sample of active galactic nuclei (AGNs) assuming that the X-rays are from the jet rather than the intracluster gas. The bulk kinetic power of jets is then calculated using the derived jet parameters. We find a strong correlation between the total luminosity of broad emission lines and the bulk kinetic power of the jets. This result supports the scenario that the accretion process is tightly linked with the radio jets, though how the disc and jet are coupled is not revealed by present correlation analysis. Moreover, we find a significant correlation between the bulk kinetic power and radio extended luminosity. This implies that the emission from the radio lobes is closely related with the energy flux transported through jets from the central part of AGNs.     

What Drives Molecular Outflows from Young Stars?

After briefly reviewing observations of molecular outflows from young stars, we discuss current ideas as to how they might be accelerated. Broadly speaking it is thought that such outflows represented either deflected accreted gas, or ambient material that has been pushed by a poorly collimated wind or accelerated by a highly collimated jet. Observations tend to favour the latter model, with jets being the clear favourite at least for the youngest flows. Jets from young stars may accelerate ambient gas either through the development of a boundary layer, where ambient and jet material are turbulently mixed, or at the working surface of the jet, i.e. the bow shock, via the prompt entrainment mechanism. Recently, we (Downes and Ray, 1999) have investigated, through simulations, the efficiency of prompt entrainment in jets from young stars as a means of accelerating ambient molecular gas without causing dissociation. Prompt entrainment was found to be very poor at transferring momentum from the jet to its surroundings in both the case of ``heavy' (not surprizingly) but also ``equi-density' (with respect to the ambient environment) jets. Moreover the transfer efficiency decreases with increasing density as the bow shock takes on a more aerodynamic shape. Models, however, in which jets are the ultimate prime movers, do have the advantage that they can reproduce several observational features of molecular outflows. In particular a power law relationship for mass versus velocity, similar to what is observed, is predicted by the simulations and the so-called ``Hubble Law' for molecular outflows is naturally explained. Pulsing of the jet, i.e. varying its velocity, is found to have little effect on the momentum transfer efficiency at least for the dynamically young jets we have studied. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemical and physical properties of gas jets in comets: I. Monte Carlo model of an inner cometary coma

We describe a 3-dimensional, time-dependent Monte Carlo model developed to analyze the chemical and physical nature of a cometary gas coma. Our model includes the necessary physics and chemistry to recreate the conditions applicable to Comet Hale-Bopp when the comet was near 1 AU from the Sun. Two base models were designed and are described here. The first is an isotropic model that emits particles (parents of the observed gases) from the entire nucleus; the second is a jet model that ejects parent particles solely from discrete active areas on the surface of the comet nucleus, resulting in coma jets. The two models are combined to produce the final model, which is compared with observations. The physical processes incorporated in both base models include: (1) isotropic ejection of daughter molecules (the observed gases) in the parent's frame of reference, (2) solar radiation pressure, (3) solar insolation effects, (4) collisions of daughter products with other molecules in the coma, and (5) acceleration of the gas in the coma. The observed daughter molecules are produced when a parent decays, which is represented by either an exponential decay distribution (photodissociation of the parent gas) or a triangular distribution (production from a grain extended source). Application of this model to the analysis the OH, C₂ and CN gas jets observed in the coma of Comet Hale-Bopp is the focus of the accompanying paper [Lederer, S.M., Campins, H., Osip, D.J., 2008. Icarus, in press (this issue)].     

Chandra finds that X-ray jets are common in low-power radio galaxies

We present results for the first three low-power radio galaxies from the B2 bright sample to have been observed with Chandra . Two have kiloparsec-scale radio jets, and in both Chandra resolves jet X-ray emission, and detects soft X-ray core emission and an X-ray-emitting galaxy-scale atmosphere of luminosity a few ×10⁴¹ erg s⁻¹. These are the first detections of X-ray jets in low-power radio galaxies more distant than Centaurus A and M87. The cooling time of the galaxy-scale gas implies mass infall rates of the order of 1 M_⊙ yr⁻¹. The gas pressure near the jets is comparable to the minimum pressure in the jets, implying that the X-ray-emitting gas may play an important role in jet dynamics. The third B2 radio galaxy has no kiloparsec-scale radio jet, and here only soft X-ray emission from the core is detected. The ratio of X-ray to radio flux is similar for the jets and cores, and the results favour a synchrotron origin for the emission. Kiloparsec-scale radio jets are detected in the X-ray in ∼7-ks exposures with Chandra more readily than in the optical via Hubble Space Telescope snapshot surveys.     

Challenging a Paradigm: Do We Need Active and Inactive Areas to Account for Near-Nuclear Jet Activity?

We briefly describe an advanced 3D gas dynamical model developed for the simulation of theenvironment of active cometary nuclei. The model canhandle realistic nucleus shapes and alternative physical models for the gas and dust production mechanism.The inner gas coma structure is computed by solving self-consistently(a) near to the surface the Boltzman Equation(b) outside of it, Euler or Navier-Stokes equations.The dust distribution is computed from multifluid ``zero-temperature' Euler equations,extrapolated with the help of a Keplerian fountain model.The evolution of the coma during the nucleus orbital and spin motion,is computed as a succession of quasi-steady solutions. Earlier versions of the model using simple,``paedagogic' nuclei have demonstrated that the surface orographyand the surface inhomogeneity contribute similarly to structuring the near-nucleusgas and dust coma,casting a shadow on the automatic attribution of such structures to ``active areas'.The model was recently applied to comet P/Halley, for whichthe nucleus shape is available. In the companion paper of this volume,we show that most near-nucleus dust structuresobserved during the 1986 Halley flybys are reproduced, assuming that the nucleus is strictly homogeneous. Here, we investigate the effect of shape perturbations and homogeneityperturbations. We show that the near nucleus gas coma structure is robust vis-a-vissuch effects. In particular, a random distribution of active and inactive areaswould not affect considerably this structure, suggesting that such areas,even if present, could not be easily identified on images of the coma.     

Dust Morphology Of Comet Hale-Bopp (C/1995 O1): I. Pre-Perihelion Coma Structures In

In 1996 comet Hale-Bopp exhibited a porcupine-like coma with straight jets of dust emission from several active regions on the nucleus. The multi-jet coma geometry developed during the first half of 1996. While the jet orientation remained almost constant over months, the relative intensity of the jets changed with time. By using the embedded fan model of Sekanina and Boehnhardt (1997a) the jet pattern of comet Hale-Bopp in 1996 can be interpreted as boundaries of dust emission cones (fans) from four — possibly five — active regions on the nucleus (for a numerical modelling see part II of the paper by Sekanina and Boehnhardt, 1997b). This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared emission from P/Halley's dust coma during March 1986

2 to 20 micrometers photometry of the inner dust coma of comet Halley was obtained at the NASA IRTF on Mar 6.85, 12.8, 13.75, 17.7, and 24.8. Positions offset 10" were measured as well as the central brightness. The strength of the 10 micrometers emission feature was observed to vary with location in the coma. The infrared emission is in general agreement with the dust size distribution measured from the Vega and Giotto spacecraft. Mar 6.8, 17.7, and 24.8 corresponded to strong dust jet activity. The strength of the 10 micrometers silicate emission is shown to be a sensitive indicator of grain size and thus of jet activity. Dust production rate on March 13.75, 6 h before Giotto encounter, was approximately 10(7) gm s-1.     

Formation of Working Surfaces in Radiatively Cooled Laboratory Jets

Whilst observations provide many examples of collimated outflows or jets from astrophysical bodies, there remain unresolved questions relating to their formation, propagation and stability. The ability to form scaled jets in the laboratory has provided many useful insights. Experiments (Lebedev et al.: 2002, ApJ 564, 113) using conical arrays of fine metallic wires on the MAGPIE generator (1MA in 240 ns) have produced radiatively cooled collimated jets in vacuum using the redirection of convergent flows by a conical shock. Here we present results of a jet produced by this method propagating through a photo-ionized, quasi-stationary gas cloud. A working surface is observed at the head of the jet. The velocity of this working surface is lower than the velocity of a jet tip in vacuum.