Density distribution of solar wind protons and “loaded” ions in the shock layer ahead of a cometary ionosphere

We propose a simple method that allows the density fields of solar wind protons and heavy ions of cometary origin (“loaded” ions) in the solar wind-cometary ionosphere interaction region to be separated from the general density field calculated within the framework of a single-fluid model. The method is based on the assumption that the velocities of both components are identical. We analyze the density fields in the solar wind obtained in this way before and after the passage of the bow shock ahead of the cometary ionosphere and make a comparison with the distributions measured with various instruments onboard the Giotto spacecraft when it flew past Comet Halley and calculated on the basis of more complex multi-fluid models.     

The February 1986 solar activity: A comparison of GIOTTO,VEGA-1, and IMP-8 solar wind measurements with MHD simulations

Large disturbances in the interplanetary medium were observed by several spacecraft during a period of enhanced solar activity in early February 1986. The locations of six solar flares and the spacecraft considered here encompassed more than 100° of heliolongitude. These flares during the minimum of cycle 21 set the stage for an extensive multi-spacecraft comparison performed with a two-dimensional, magnetohydrodynamic (MHD) numerical experiment. The plasma instruments on the European Space Agency (ESA)'s GIOTTO spacecraft, on its way to encounter Comet Halley in March 1986, made measurements of the solar wind for up to 8 hours per day during February. We compare solar wind measurements from the Johnstone Plasma Analyzer (JPA) experiment on GIOTTO with the MHD simulation of the interplanetary medium throughout these events. Using plasma data obtained by the IMP-8 satellite in addition, it appears that an extended period of high solar wind speed is required as well as the simulated flares to represent the interplanetary medium in this case. We also compare the plasma and magnetometer data from VEGA-1 with the MHD simulation. This comparison tends to support an interpretation that the major solar wind changes at both GIOTTO and VEGA-1 on 8 February, 1986 were due to a shock from a W05° solar flare on 6 February, 1986 (06:25 UT). The numerical experiment is considered, qualitatively, to resemble the observations at the former spacecraft, but it has less success at the latter one.     

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”.     

Ionization mechanisms of cometary comas

A new approach is considered to the problem of ionization of the inner comas of comets connected with two phenomena: meteor-like process due to cometary molecules — interplanetary meteoroids impacts and explosion-type process due to high-velocity collisions between cometary dust grains and interplanetary meteoroids. It is found that the efficiency of explosive ionization exceeds the efficiency of meteor ionization approximately 100 times. The explosive ionization may be possible mechanism for anomaly ionization of the inner comae of dusty comets like Halley 1986 III with the dust to gas production rate ratio more than 0.1.     

A cometary aurora

It is proposed that the cometary analog of a terrestrial aurora was responsible for the enhanced fluxes of suprathermal (keV) electrons and associated plasma waves observed in the cometosheath of Comet Halley during its VEGA 2 encounter. The non-detection of such suprathermal electron fluxes during the GIOTTO encounter is ascribed to the much quieter solar wind conditions at that time.     

Alfvén wave plasma turbulence during solar wind-comet interaction

The large differences in drift velocities between the solar wind protons and the picked-up ions of cometary origin cause the Alfvén waves (among others) to become unstable and generate turbulence. A self-consistent treatment of such instabilities has to take into account that these cometary ions affect the solar wind plasma in a decisive way. With the help of a previously developed formalism one finds the correct Alfvén instability criterion, which is here nondispersive, in contrast to recent calculations where the cometary ions are treated as a low-density, high-speed, and non-neutral beam through an otherwise undisturbed solar wind. The true bulk speed of the combined solar wind plus cometary ion plasma clearly shows the mass-loading and deceleration of the solar wind near the cometary nucleus, indicating a bow shock. The instability criterion is also used to determine the region upstream where the Alfvén waves can be unstable, based upon recent observations near comet Halley.     

Solar wind interactions with Comet 19P/Borrelly

The Plasma Experiment for Planetary Exploration (PEPE) made detailed observations of the plasma environment of Comet 19P/Borrelly during the Deep Space 1 (DS1) flyby on September 22, 2001. Several distinct regions and boundaries have been identified on both inbound and outbound trajectories, including an upstream region of decelerated solar wind plasma and cometary ion pickup, the cometary bow shock, a sheath of heated and mixed solar wind and cometary ions, and a collisional inner coma dominated by cometary ions. All of these features were significantly offset to the north of the nucleus-Sun line, suggesting that the coma itself produces this offset, possibly because of well-collimated large dayside jets directed 8°-10° northward from the nucleus as observed by the DS1 MICAS camera. The maximum observed ion density was 1640 ion/cm³ at a distance of 2650 km from the nucleus while the flow speed dropped from 360 km/s in the solar wind to 8 km/s at closest approach. Preliminary analysis of PEPE mass spectra suggest that the ratio of CO⁺/H₂O⁺ is lower than that observed with Giotto at 1P/Halley.     

Solar-wind flow past a cometary ionosphere

An axisymmetric gasdynamical model of an interaction between the solar wind and comet atmospheres is formulated. Photoionization of the cometary neutral particles as well as their resonance charge-exchange are taken into account to determine the flow in the distured region containing three discontinuity surfaces (two shocks and the contact discontinuity).It is shown that these two processes are significant for many comets. The problem so formulated is solved numerically. A dependence of the obtained solutions on comets gas production, charge-exchange cross-section, photoionization time scale and other parameters is studied. The numerical solutions are in good agreement with the experimental data obtained during the spacecraft missions to the comet Halley in March 1986.     

The HELIOS spacecraft zodiacal light photometers used for comet observations and views of the comet west bow shock

The HELIOS A and B zodiacal light photometers can be used to view comets as they pass the spacecraft. Because the HELIOS spacecraft orbit the Sun on their own, and are generally far from Earth, the spacecraft allow us to view comets from a different perspective than normally available. Comet West (1976VI) passed through perihelion on February 25, 1976. The comet crossed the HELIOS A and B spacecraft zodiacal light photometer fields of view, allowing them to record the brightness, polarization and color of the comet. Data from the U, B and V photometers showed a distinct blueing followed by a slight reddening corresponding to the ion and dust tails, respectively, entering the field of view of each photometer sector. The extent of the tail of Comet West was far greater seen from the HELIOS spacecraft than seen from Earth, even taking into account their generally closer viewing perspective. As Comet West traveled away from the Sun, it was observed in the zodiacal light photometer fields of view at a solar distance of more than 1.4 AU. The zodiacal light photometers also viewed Comet Meier (1978XXI). Comet Meier is far more compact than Comet West, extremely blue and unlike Comet West showed no significant dust tail. The interplanetary medium is observed to a level of the variations in the brightness of the electron-scattering component near Comet West. A brightness bump present in the data before the comet reached some photometer positions can be shown to approximately form a parabolic shape sunward and ahead of the orbital motion of the Comet West nucleus. We presume that this bump is evidence of the position of the cometary atmosphere or an enhancement of the ambient interplanetary medium ahead of the comet motion. The brightness bump in terms of density generally corresponds to a density enhancement of the ambient medium by a few times in the vicinity of the comet. When compared with Comet Halley and couched in terms of the shock stand-off distance, the distance of this brightness increase from the nucleus implies a neutral gas production rate of approximately 2.5 times that of Halley. This is in agreement with the neutral gas production rate measured from Comet West using more direct techniques.Now at Scientific Applications Inc., La Jolla, California, U.S.A.     

The solar wind interaction with comet P/Grigg-Skjellerup

The Giotto spacecraft is scheduled to intercept comet P/Grigg-Skjellerup on July 10, 1992. The observed outgassing rate of this comet is over an order of magnitude smaller than comet Giacobini-Zinner and over two orders of magnitude smaller than that of comet Halley. Consequently, the new data obtained during the upcoming encounter will strengthen our understanding of how the solar wind interaction with comets depends upon the neutral gas production rate. In this brief note, we make predictions of the location of the flow transition regions — i.e., the bow shock and the ionopause, and discuss the expected level of wave turbulence.     

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.     

Simulation of ICE-Giacobini-Zinner and VEGA/GIOTTO-Halley encounters

The formation of a weak shock at the interaction of the solar wind with the cometary neutral gas is confirmed in a non-stationary gasdynamic model. No shocks will be detectable along the ICE path and if the VEGA/GIOTTO spacecraft enter the cometary atmosphere. A shock can only be expected at an exit from the Halley atmosphere.     

First evidence of IMF control of Jovian magnetospheric boundary locations: Cassini and Galileo magnetic field measurements compared

The Cassini spacecraft, en route to Saturn, passed close to Jupiter while the Galileo spacecraft was completing its 28th and 29th orbits of Jupiter, thus offering a unique opportunity for direct study of the solar wind-Jovian interaction. Here evidence is given of response of the Jovian magnetopause and bow shock positions to changes of the north-south component of the solar wind magnetic field, a phenomenon long known to occur in equivalent circumstances at Earth. The period analyzed starts with the passage over Cassini of an interplanetary shock far upstream of Jupiter. The shock's arrival at Galileo on the dusk-flank of the magnetosphere caused Galileo to exit into the solar wind. Using inter-spacecraft timing based on the time delay established from the shock arrival at each spacecraft, we point out that Galileo's position with respect to the Jovian bow shock appears to correlate with changes in the disturbed north-south reversing field seen behind the shock. We specifically rule out the alternative of changes in the shape of the bow shock with rotations of the interplanetary magnetic field as the cause.     

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.     

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.     

Plasma environment of Jupiter family comets

As any comet nears the Sun, gas sublimes from the nucleus taking dust with it. Jupiter family comets are no exception. The neutral gas becomes ionized, and the interaction of a comet with the solar wind starts with ion pickup. This key process is also important in other solar system contexts wherever neutral particles become ionized and injected into a flowing plasma such as at Mars, Venus, Io, Titan and interstellar neutrals in the solar wind. At comets, ion pickup removes momentum and energy from the solar wind and puts it into cometary particles, which are then thermalised via plasma waves. Here we review what comets have shown us about how this process operates, and briefly look at how this can be applied in other contexts. We review the processes of pitch angle and energy scattering of the pickup ions, and the boundaries and regions in the comet-solar wind interaction. We use in-situ measurements from the four comets visited to date by spacecraft carrying plasma instrumentation: 21P/Giacobini-Zinner, 1P/Halley, 26P/Grigg-Skjellerup and 19P/Borrelly, to illustrate the process in action. While, of these, comet Halley is not a Jupiter class comet, it has told us the most about cometary plasma environments. The other comets, which are from the Jupiter family, give an interesting comparison as they have lower gas production rates and less-developed interactions. We examine the prospects for Rosetta at comet Churyumov-Gerasimenko, another Jupiter family comet where a wide range of gas production rates will be studied.     

Interaction of interplanetary shocks with the bow shock

Fast forward interplanetary (IP) shocks have been identified as a source of large geomagnetic disturbances. However, the shocks can evolve in the solar wind, they are modified by interaction with the bow shock and during their propagation through the magnetosheath. A few previous papers refer the inclination and deceleration of the IP shock front in this region. Our contribution continues this effort and presents the study of an IP shock interaction with the bow shock. Since the bow shock is a reversed fast shock, the interaction of the IP shock and bow shock is a problem of interaction of two fast MHD shocks.

We compare profiles of magnetic field and plasma parameters observed by several spacecraft in the solar wind and magnetosheath with the profiles of the same parameters resulting from the MHD numerical model. The MHD model suggests that the interaction of an IP shock with the bow shock results in an inward bow shock displacement that is followed by its outward motion. Such motion will result in an indentation propagating along the bow shock surface. This scenario is confirmed by multipoint observations. Moreover, the model confirms also previous suggestions on the IP shock deceleration in the magnetosheath.     

Disconnection Events in Comets

Cometary and solar wind data are compared with the purposeof identifying the solar wind conditions which are associated with comet plasma tail disconnection events (DEs),i.e., when the plasma tail appears disconnected from the cometaryhead. The cometary data are fromThe International Halley Watch Atlas ofLarge-Scale Phenomena (Brandt et al.,1992a). A systematic visual analysis of the atlas images(Voelzke and Matsuura, 1998)revealed, among other morphological structures, 47 DEs alongthe plasma tail of comet P/Halley. This work compares the current competitive theories, based on the triggeringmechanisms, in order to explain the cyclic phenomena of DEs, i.e., the ion production effects, the pressure effects and themagnetic reconnection effects are analysed. The distribution of the DEs in time or heliocentricdistance presents abimodal character possibly associated with the cometary passage through the magnetic sector boundaries in the interplanetary medium.The 47 DEs documented in 47 different images allowed the estimation of 19 onsets of DEs, i.e., the time when the cometsupposedly crossed a frontier between magnetic sectors of the solar wind. The solar wind data are taken from in situ measurements of IMP-8 (King, 1982), which is used to construct the actual variation of solarwind speed, density and dynamic pressureduring the analysed interval. These in situ measurements arereferenced to the comet by standard co-rotationmethods. The preliminary results ofthis research reveal that the DEs onsetsof comet P/Halley are correlated with pressureeffects only in 23% of the analysed cases,therefore these effects should not be the principaltriggering mechanism of DEs.     

Resonant interactions between cometary ions and low frequency electromagnetic waves

We explore the conditions for resonance between cometary pick-up ions and parallel propagating electromagnetic waves. A model ring—beam distribution for the pick-up H₂O⁺ ions is adopted which allows a direct comparison of the source of free energy for growth from either the beam or the gyrating ring in the limit near marginal stability. Under average solar wind conditions in the inner solar system, the gyrating ring provides the dominant contribution to wave growth. The presence of a field-aligned beam is only important to allow resonance with R-mode waves which occur in two distinct frequency bands either well above or below the pick-up ion gyrofrequency. The most unstable mode is the low frequency R-mode or fast MHD wave, though higher frequency whistlers or low frequency L-mode waves may also be excited by the same source of free energy. The nature of the unstable waves is strongly influenced by the inclination of the interplanetary field. For 3° the rate of the low frequency R-mode growth is dramatically reduced and resonant L-mode waves should experience net ion beam damping. Conversely for 75°, the ion beam velocity will be insufficient to allow resonant R-mode instability; L-mode waves should therefore predominate. The low frequency fast MHD mode should experience the most rapid amplification for intermediate inclination; 30° 75°. In the frame of the solar wind such waves must propagate along the field in the direction upstream towards the Sun with a phase speed lower than the beaming velocity of the pick-up ions. The waves are consequently blown back away from the Sun and would thus be detected with a left-hand polarization by an observer in the cometary frame. We consider this the most likely mechanism to account for the interior MHD waves observed by satellites over an extended spatial region surrounding comets Giacobini-Zinner and Halley.