Space charge sheath in plasma-neutral gas interaction

A space charge sheath is found to be formed whenever a high-velocity magnetized plasma stream penetrates a gas cloud. The sheath is always located at the head of the plasma stream, and its thickness is very small compared to the length of the plasma stream. Soon after the sheath is formed it quickly slows down to the Alfvén critical velocity. The plasma behind the sheath continues to move at higher velocity until the whole plasma stream is retarded to the critical velocity. In the interaction at gas density 10¹⁹ m^–3 the sheaths are observed to be accompanied by a single loop of current with current density of 10⁵ Å m^–2. Maximum potential in the sheath ranges between 50 and 200 V.Presently available models for the sheath may explain the initiation of the sheath formation. Physical processes like heating of the electrons and ionization of the gas cloud which come into play at a later stage of the interaction are not included in these models. These processes considerably alter the potential structure in the sheath region. A schematic model of the observed sheath is presented here.Experiments reveal a threshold value of the magnetic field for plasma retardation to occur. This seems to correspond to the threshold condition for excitation of the modified two-stream instability which can lead to the electron heating. The observed current are found sufficient to account for the plasma retardation at a gas density of 10¹⁷ m^–3.     

Observations of the OH radical in comets at 18 cm wavelength

We present recent observations of the OH radical at λ 18 cm with the Nançay radio telescope in comets Meier (1978 XXI), Bradfield (1979 X), Meier (1980q), P/Encke (1980), and Bradfield (1980t). The analysis of the OH radio line shape is a powerful tool to study the kinematics of the coma. The expansion velocity of the OH molecules is found to be ≈1.5 km sec^?1 at r_h = 1 AU, and decreases with increasing heliocentric distance. The line profile is generally asymmetric, which demonstrates the Greenstein effect on the fluorescent excitation mechanism and/or anisotropic outgassing of the nucleus. In several cases, especially for comet Meier (1978 XXI), an asymmetry is also found in the east-west brightness distribution of the OH line, showing again the Greenstein effect and/or anisotropic outgassing. An excitation model by uv pumping and fluorescence of the OH radical, which agrees with the observations at least in the first order, and the application of Haser's model lead to the production rate of the parent molecule of OH. There is a close correlation between this gas production rate and the visual brightness of the comet. Our estimates of gas production rates are smaller than or equal to those obtained from uv measurements, but both radio and uv estimates depend heavily on the parameters used in Haser's models.     

The role of electrostatic instabilities in the critical ionization velocity mechanism

The role of electrostatic instabilities in the critical ionization velocity mechanism is investigated. The analysis is based on the theory developed by Sherman, which interprets Alfvén's critical velocity in terms of a circular process. This process involves the acceleration of electrons by a two-stream instability modified by the presence of a magnetic field. A general expression for the energy and momentum of ions and electrons associated with an electrostatic mode is derived in terms of the plasma dielectric constant. This is used in the case of the modified two-stream instability to determine the distribution of energy between ions and electrons. An extrapolation from the linear phase then gives an estimate of the energy delivered to the electrons which is compared to that required to ionize the neutral gas.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978.     

The role of critical velocity ionization phenomena in the atmosphere of Io

The Alfvén's critical ionization velocity (CIV) have been observed in a number of laboratory and space experiments. In the Io-torus system, relative velocity of the plasma species in the torus with respect to the neutral species in the Io's atmosphere and neutral cloud exceeds the critical velocity required for CIV. Townsand condition is satisfied up to 6r _io , in the neutral cloud when Io passes through the torus. In this paper it is shown that during the passage of Io through the plasma torus, apart from critical velocity and Townsand condition, a number of other requirements are also satisfied. Therefore, it is concluded that, the CIV mechanism must play an important role in ionizing the neutral cloud and enriching the plasma torus.     

Experimental investigation of the critical ionization velocity in gas mixtures

The critical ionization velocity which is of cosmogonic and astrophysical interest has hitherto mainly been investigated for pure gases. Since in space we always have gas mixtures, it is of interest also to study gas mixtures. The present report, which is a summary of a more detailed report (Axnäs, 1976), summarizes the results of systematic experiments on the critical ionization velocity as a function of the mixing ratio for binary gas mixtures of H₂, He, N₂, O₂, Ne and Ar. The apparatus used is a coaxial plasma gun with an azimuthal magnetic field. The discharge parameters are chosen so that the plasma is weakly ionized. In some of the mixtures it is found that one of the components tends to dominate in the sense that only a small amount (regarding volume) of that component is needed for the discharge to adopt a limiting velocity close to that for the pure component. Thus in a mixture between a heavy and a light component having nearly equal ionization potentials, the heavy component dominates. Also, if there is a considerable difference in ionization potential between the components, the component with the lowest ionization potential tends to dominate.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978.     

Single particle approximation for the inner coma of comet Halley

Parameters of the plasma in the inner coma of comet Halley are derived from the magnetic field measurements by using single particle approximation. Both the plasma velocity and the temperature obtained by using this approach are self-consistent and happen to be in good agreement within situ measurements whereas the neutral gas production rate happens to be 2–3 times higher than the conventionally cited value 6.9 × 10²⁹ s^–1.     

Helical waves in type-1 comet tails

Oscillations of type-1 comet tails with plasma compressibility taken into account are studied. A comet tail is treated as a plasma cylinder separated by a tangential discontinuity surface from the solar wind. The dispersion equation obtained in the linear approximation is solved numerically with typical plasma parameters. A sufficient condition for instability of the cylindrical tangential discontinuity in the compressible fluid is obtained. The phase velocity of helical waves is shown to be approximately coincident with Alfvén speed in the tail in the reference system moving with the bulk velocity of the plasma outflow in the tail. The instability growth rate is calculated.This theory is shown to be in good agreement with observations in the tails of Comets Kohoutek, Morehouse and Arend-Roland. Hence we conclude that helical waves observed in type-1 comet tails are produced due to the Kelvin-Helmholtz instability, and the model under consideration is justified. If so, one may estimate comet tail magnetic field from the pressure balance at the tangential discontinuity; it turns out to be of the order of the interplanetary magnetic field.     

The formation of comets in wind-driven shells around protostars

It is shown that the dense, turbulent, decelerating shells produced by protostellar flows around young stars are a probable site for rapid grain growth by coalescing collisions. The growth of grains occurs in a thin dust layer at the leading edge of the gas shell until a critical grain size on the order of 1−10 μm is reached. Grains larger than this decouple from the turbulence and eventually reach sizes of ≈100 μm. These large grains form a thin dust shell with low-velocity dispersion, in which ultimately local gravitational instability takes place. This causes the accumulation of comet-sized aggregations of dust, assuming that the dust velocity dispersion is on the order of 10⁻² m sec⁻¹. It is proposed that the mechanism could lead to a high space density of comets in molecular clouds. The efficient formation of “giant” grains, and even comet nuclei, in the regions around young stars has important implications both for cometary astronomy and for understanding the dynamical and chemical evolution of molecular clouds and the interstellar medium.     

A probability of encounter with interstellar comets and the likelihood of their existence

A theory of the probability of encounter of the Sun with an interstellar comet at a distance comparable to the Earth-Sun distance is formulated, and a general expression is derived establishing the relationship among the influx rate of interstellar comets, the perihelion distance, the space density of the comets, the Maxwellian distribution of comet velocities in the interstellar cloud, and the cloud's systematic velocity relative to the Sun. The fact that no comet with a strongly hyperbolic orbit has so far been observed is used to determine an upper limit of 6 × 10^?4 solar masses per cubic parsec (4 × 10^?26 gcm^?3) for the space density of interstellar comets. The theoretical distribution of semimajor axes of interstellar comets is derived to show that a strong hyperbolic excess must be present in the orbits of a majority of interstellar comets regardless of the dynamical characteristics of the comet cloud, except when the cloud is moving along with the Sun and the distribution of individual velocities has a very low dispersion. This case, however, implies a possibility of capture by the Sun and thus becomes a problem of an Oort-type cloud.     

Plasma clouds in Saturn's rings

The expansion and ionization of vapor produced by impacts of meteorites on Saturn's rings is described. There is an “impact plasma” produced in the initial collision, and a “secondary plasma” produced by subsequent ionization of the neutral gas ejecta. The dynamics of these plasma clouds, their size, density, and life time are calculated. It is suggested that large clouds, produced by meter-sized meteorites (or a collection of such clouds produced, e.g., by the impact of a swarm of meteorites) may lead to the formation of spokes by the mechanism discussed in Goertz, C. K., and Morfill, G. E. (Icarus53, 219–229, 1983).     

Energization of positive ions in the cometary foreshock region

The energization of positive ions in front of a cometary bow shock is investigated. Ions produced by ionization of the cometary neutrals interact with the solar wind protons to produce, among other waves, large amplitude oscillations of the ambient magnetic field. Such oscillations are convected towards the comet at the unperturbed solar wind speed far from the shock and at a lower speed closer to the shock (due to the solar wind mass loading) ; hence, they can energize the suprathermal ions by Fermi acceleration. The spatial extension of the acceleration region is of the order of 10⁶ km and the resulting ion energy spectrum is harder than in the Earth's bow shock case. The energization of cometary ions produces an additional deceleration of the solar wind, such that the cometary bow shock of Halley-type comet may be regarded as a “cosmic ray shock”.     

The interaction of the solar wind with a comet

The flow of plasma on the sunward side of a comet is investigated by means of an axialsymmetric model based on hydrodynamics modified by source terms. The model assumes a given curvature of the isobaric surfaces, which corresponds to paraboloids around the nucleus of the comet. The flow on the axis can be represented by a solution of a system of seven ordinary differential equations (respectively five in case of pure photo-ionization). The flow pattern always contains a widely detached bow shock and a contact discontinuity separating a cavity with purely cometary plasma from the transition region containing also solar wind ions. The model is applied to the special case where the cometary gas is ionized by the solar UV radiation only. Numerical solutions are integrated for five levels of production of neutral gas by the comet and for seven typical situations in the undisturbed solar wind. The results imply standoff distances of the stagnation point from the nucleus of the order of 10 000 km or more, and distances of the bow shock of the order of 10⁶–10⁷ km.     

Acceleration of the solar wind by whistler waves from coronal holes

We investigate the possibility of an additional acceleration of the high speed solar wind by whistler waves propagating outward from a coronal hole. We consider a stationary, spherically symmetric model and assume a radial wind flow as well as a radial magnetic field. The energy equation consists of (a) energy transfer of the electron beam which excites the whistler waves, and (b) energy transfer of the whistler waves described by conservation of wave action density. The momentum conservation equation includes the momentum transfer of two gases (a thermal gas and an electron beam). The variation of the temperature is described by a polytropic law. The variation of solar wind velocity with the radial distance is calculated for different values of energy density of the whistler waves. It is shown that the acceleration of high speed solar wind in the coronal hole due to the whistler waves is very important. We have calculated that the solar wind velocity at the earth's orbit is about equal to 670 km/sec (for wave energy density about 10^?4 erg cm^?3 at 1.1R⊙). It is in approximate agreement with the observed values.     

DE-like event in the plasma tail of comet C/2006 M4 (SWAN)

We investigate a DE-like event of October 27, 2006, in the plasma tail of comet C/2006 M4 (SWAN). A set of phenomena in the comet??s tail after the brightness outburst on October 24.04, 2006, is described. The typical dimensions of the tail structure details that developed as a result of plasma instabilities are found. The velocity and acceleration of the disconnected tail fragment and the beginning time of the event are determined. The likely cause of the DE-like event is the comet??s encounter with a high-speed stream of solar wind.     

The plasma condensation region in the coma of Halley's Comet

The cometary images taken on 1986 January 8.590 and 8.638 UT (R-0.9 AU, ~ 1.29 AU) at Gurushikhar, Mt. Abu, India (24 °39 N, 72 °43 E alt: 1700 m) show distinct condensation region in the tail direction. The size of the condensation region is 4 × 10³ km. The condensation region showed up strongly in the blue emission, implying the abundance of CO⁺. It was inferred to be moving with a velocity of 37 ± 3 km/s relative to the comet at a distance of 2.3 × 10⁵ km from the nucleus in the tailward direction.The analysis show that the condensation was a result of rapid ionization mechanism, with a time scale of \~10³ to 10⁴ sec. The most probable mechanism for producing the ionization region was found to be the discharge of cross tail electric current passing through the neutral sheet in the near nucleus region followed by an outburst observed in IR wavelengths at 8.1 UT. It was accelerated by J × B drift at a rate of ~24 cm/sec² to the position observed by us.This feature, most probably is the precursor of the first dramatic Disconnection Event (DE) observed in Halley's Comet at Jan.10.375 UT. This supports the conjecture that the tail features originate in the coma with a velocity of ~20–40 km/s.     

The influence of the plasma inhomogeneity on the critical velocity phenomenon

The phenomenon of a critical ionization velocity is discussed under the aspect of experimentally measured inhomogeneities. The difficulties inherent in homogeneous plasma models on the phenomenon are shown. A simple sheath model is presented which can be compared with the local plasma parameters measured in a rotating plasma device. The origin of the observed turbulent heating is attributed to a modified two-stream instability occurring in the sheath under discussion.     

Comet Tempel-Tuttle and the Leonid meteors

The distribution of dust surrounding periodic comet Tempel-Tuttle has been mapped by analyzing the associated Leonid meteor shower data over the 902–1969 interval. The majority of dust ejected from the parent comet evolves to a position lagging the comet and outside the comet's orbit. The outgassing and dust ejection required to explain the parent comet's deviation from pure gravitational motion would preferentially place dust in a position leading the comet and inside the comet's orbit. Hence it appears that radiation pressure and planetary perturbations, rather than ejection processes, control the dynamic evolution of the Leonid particles. Significant Leonid meteor showers are possible roughly 2500 days before or after the parent comet reaches perihelion but only if the comet passes closer than 0.025 AU inside or 0.010 AU outside the Earth's orbit. Although the conditions in 1998–1999 are optimum for a significant Leonid meteor shower, the event is not certain because the dust particle distribution near the comet is far from uniform. As a by-product of this study, the orbit of comet Tempel-Tuttle has been redetermined for the 1366–1966 observed interval.     

Swift ultraviolet photometry of the Deep Impact encounter with Comet 9P/Tempel 1

We report time-resolved imaging UV photometry of Comet 9P/Tempel 1 during the interval 2005 June 29-2005 July 21, including intensive coverage of the collision with the Deep Impact probe and its immediate aftermath. The nuclear flux of the comet begins to rise within minutes of the collision, and peaks about 3 h after impact. There is no evidence for a prompt flash at the time of impact. The comet exhibits a significant re-brightening about 40 h after the initial outburst, consistent with the rotation period of the comet, with evidence for further periodic re-brightenings on subsequent rotations. Modelling of the brightness profile of the coma as a function of time suggests two distinct velocity systems in the ejecta, at de-projected expansion speeds of 190 and 550 m/s, which we suggest are due to dust and gas, respectively. There is a distinct asymmetry in the slower-moving (dust) component as a function of position angle on the sky. This is confirmed by direct imaging analysis, which reveals an expanding plume of material concentrated in the impact hemisphere. The projected expansion velocity of the leading edge of this plume, measured directly from the imaging data, is 190 m/s, consistent with the velocity of the dust component determined from the photometric analysis. From our data we determine that a total of (1.4±0.2)×¹⁰³² water molecules were ejected in the impact, together with a total scattering area of dust at 300 nm of 190±20 km².     

On the CN (0, 0) spectrum of Comet Mrkos 1957d

We have carried out an analysis of the (0, 0) vibrational band of the CN molecule in Comet Mrkos 1957d, including the effect of collisions. We found that the sum of the squares of the residuals can be reduced by a factor of ten, if collisions account for 46±3% of the population of the lower level. A rotational temperature can be assigned to the cometary gas. The best value found was 410±40 K. The best fit for the constantR ₁ was (1.07±0.10)×10^–4. The velocity of the comet was left as a free parameter. We found for it a value of 34.38±0.10 km s^–1. This result is in disagreement with the nuclear orbital velocity of 34.74 km s^–1. The discrepancy can be explained, if the CN molecules are ejected from the cometary nucleus preferentially in the sunward direction, with a mean velocity that corresponds to the above temperature.     

Comet plasma densities deduced from refraction of occulted radio sources

Eighty MHz observations of the occultation of the radio source Culgoora-1 0300 + 16 by the plasma tail of Comet Kohoutek (1973f) were made in February/March 1974 with the Culgoora radioheliograph. No detectable source broadening or change in flux density was observed, but the results showed a 2' arc anomaly in the observed position. This is greater than can be attributed to ionospheric refraction or experimental error. We suggest that it arose from refraction in the plasma tail of the comet. Similar observations of the occulation of the radio source Culgoora-1 2313-14 by the plasma tail of Comet West (1975n) were made at Culgoora in February 1976. These results were inconclusive but did suggest that the cometary plasma may have had some influence on the observed source position. The results are used to derive, from simple models, the distribution of electron density in comet tails. Peak electron densities of approximately 2 to 5 × 10⁴ cm^?3 and density gradients of ～0.05 cm^?3 km^?1 are indicated.