A magnetohydrodynamical model of the inner coma of comet Halley

Cravens, T. E.

doi:10.1029/ja094ia11p15025

Cited by 63 publications

(65 citation statements)

References 35 publications

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“…Since n g » n p , the former has an approximate r _ 2 dependence, and the latter is proportional to n p *. Thus, if the flow time does not exceed the chemical time, one expects n p to exhibit an r _ 1 dependence [14]. This is approximately verified above the initial fast n p increase region (see Figure 9 and quoted references) up to the vicinity of the DC boundary, where additional processes come into play.…”

Section: N I Electric Charge Budgetmentioning

confidence: 60%

“…Other authors use a separate electron fluid, but keep the ions inside the neutral fluid [58, 59,60,69,83]. In a few studies, both ions and electrons are treated as separate fluids [14,51,57]. In fact, the lumpings of all ions in a single fluid and of all electrons in a single fluid raise difficulties as we will comment below [51].…”

Section: Electrons and Ionsmentioning

confidence: 99%

“…It seems that MHDEs can indeed account satisfactorily for most observed properties of the cometary plasma (e.g., [14,69,83,84]). Here, we will discuss only the innermost region of the coma, the so-called "diamagnetic cavity" (DC), where, as revealed by the Giotto mission magnetometer, the magnetic field is negligibly small (less than 0.1 nT).…”

Section: Electrons and Ionsmentioning

confidence: 99%

“…An inviscid equation of the form (3) is used in [69] for electrons; an intermediate equation (NSE, but with the viscous dissipation term omitted) is used for electrons in [57,58,59,60] and for ions and electrons in [14,51]. Using, e.g., equation (12) of [20], we can write this energy equation: 2dr r…”

Section: Energy Equationmentioning

confidence: 99%

“…When there is no separate ion fluid, V p = V g . Otherwise, the inviscid equation (2) is used for V p [51] or the exact NSE ( [14], but notice that the Cartesian form of the viscous term is written there in lieu of the spherical form). In spherical geometry, one has: in which j labels electrons or ions, Mj n is the photoproduction source, M; e is the recombination sink, and |0j is the shear viscosity.…”

Section: Momentum Equationmentioning

confidence: 99%

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Hydrodynamic Models of the Collisional Coma

Crifo

1991

International Astronomical Union Colloquium

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ABSTRACT. The hydrodynamic approach to the modeling of the coUisional regions of the cometary atmospheres is reviewed. A concise account of the existing Monte Carlo simulations is also incorporated. Emphasis is placed on works stimulated by the Halley flyby missions. The basic physical processes are described, based on the analogy with the physics of underexpanded jets. The general structure of the governing equations is presented and their legacy discussed. A detailed intercomparison is made of the application of these methods to the modeling of the comas. The reliability of the conclusions brought forward by existing models is discussed. . IntroductionIn view of the extreme rarity of past and foreseeable in situ cometary missions, remote sensing of comets has been and will continue to be the essential tool of cometary investigation. Therefore, most properties (and, in particular, the statistical properties) of the solid nucleus of all comets have been and will continue to be inferred from observations of the cometary atmosphere-the coma. With the refinements of the observational techniques, a detailed and reliable understanding of the formation of the coma has emerged as a key objective of cometary physics.The application of gas-dynamic methods to the formation of the coma seems to date only back to the year 1965. Fluid equations were first used in order to account for the acceleration of cometary dust and for the formation of many minor molecules. It is not a simple problem to delineate the precise extent of the collision-dominated part of a coma, i.e., that region where hydrostatic or hydrodynamic methods provide satisfactory results (see Sections 3.3 to 3.5). Primitive considerations, based on evaluations of the ratio of the local collision mean free path (m.f.p.) to the cometocentric distance, suggest that a waterdominated coma has a very small collision zone beyond an heliocentric distance = 3 AU, while that zone covers the whole water survival zone within = 0.5 AU (see, e.g., [13,17]). The essential point, however, is that the distribution of matter in the coma, the dynamical properties of all constituents, and the chemical composition of the gas phase are 937

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Section: N I Electric Charge Budgetmentioning

confidence: 60%

Section: Electrons and Ionsmentioning

confidence: 99%

Section: Electrons and Ionsmentioning

confidence: 99%

Section: Energy Equationmentioning

confidence: 99%

Section: Momentum Equationmentioning

confidence: 99%

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Hydrodynamic Models of the Collisional Coma

Crifo

1991

International Astronomical Union Colloquium

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Four‐fluid MHD simulations of the plasma and neutral gas environment of comet 67P/Churyumov‐Gerasimenko near perihelion

et al. 2016

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The neutral and plasma environment is critical in understanding the interaction of the solar wind and comet 67P/Churyumov‐Gerasimenko (CG), the target of the European Space Agency's Rosetta mission. To serve this need and support the Rosetta mission, we have developed a 3‐D four‐fluid model, which is based on BATS‐R‐US (Block‐Adaptive Tree Solarwind Roe‐type Upwind Scheme) within SWMF (Space Weather Modeling Framework) that solves the governing multifluid MHD equations and the Euler equations for the neutral gas fluid. These equations describe the behavior and interactions of the cometary heavy ions, the solar wind protons, the electrons, and the neutrals. This model incorporates different mass loading processes, including photoionization and electron impact ionization, charge exchange, dissociative ion‐electron recombination, and collisional interactions between different fluids. We simulated the plasma and neutral gas environment near perihelion in three different cases: an idealized comet with a spherical body and uniform neutral gas outflow, an idealized comet with a spherical body and illumination‐driven neutral gas outflow, and comet CG with a realistic shape model and illumination‐driven neutral gas outflow. We compared the results of the three cases and showed that the simulations with illumination‐driven neutral gas outflow have magnetic reconnection, a magnetic pileup region and nucleus directed plasma flow inside the nightside reconnection region, which have not been reported in the literature.

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Pluto's interaction with the solar wind

et al. 2016

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This study provides the first observations of Plutogenic ions and their unique interaction with the solar wind. We find ~20% solar wind slowing that maps to a point only ~4.5 RP upstream of Pluto and a bow shock most likely produced by comet‐like mass loading. The Pluto obstacle is a region of dense heavy ions bounded by a “Plutopause” where the solar wind is largely excluded and which extends back >100 RP into a heavy ion tail. The upstream standoff distance is at only ~2.5 RP. The heavy ion tail contains considerable structure, may still be partially threaded by the interplanetary magnetic field (IMF), and is surrounded by a light ion sheath. The heavy ions (presumably CH4+) have average speed, density, and temperature of ~90 km s−1, ~0.009 cm−3, and ~7 × 105 K, with significant variability, slightly increasing speed/temperature with distance, and are N‐S asymmetric. Density and temperature are roughly anticorrelated yielding a pressure ~2 × 10−2 pPa, roughly in balance with the interstellar pickup ions at ~33 AU. We set an upper bound of <30 nT surface field at Pluto and argue that the obstacle is largely produced by atmospheric thermal pressure like Venus and Mars; we also show that the loss rate down the tail (~5 × 1023 s−1) is only ~1% of the expected total CH4 loss rate from Pluto. Finally, we observe a burst of heavy ions upstream from the bow shock as they are becoming picked up and tentatively identify an IMF outward sector at the time of the NH flyby.

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A magnetohydrodynamical model of the inner coma of comet Halley

Cited by 63 publications

References 35 publications

Hydrodynamic Models of the Collisional Coma

Hydrodynamic Models of the Collisional Coma

Four‐fluid MHD simulations of the plasma and neutral gas environment of comet 67P/Churyumov‐Gerasimenko near perihelion

Pluto's interaction with the solar wind

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