Cometary Ions Research Articles

Deep Space 1 encounter with Comet 19P/Borrelly: Ion composition measurements by the PEPE mass spectrometer

We report in situ ion mass spectrometer measurements from the coma of Comet 19P/Borrelly, obtained during the flyby of the Deep Space 1 spacecraft on 22 September 2001. Cometary ions were detected at distances from ∼5.5 × 105 km to 2200 km from the nucleus of the comet. The predominant heavy ions observed during the seven minute interval about closest approach to Borrelly included OH+ (at 57% of the total water‐group ion density), H2O+ (at 29%), O+ (at 13%), CH3+ (at 5%), and CH2+ (at 4%). Of particular note is the small amount of H3O+ (<9%), as this was the most abundant molecular ion observed at closest approach to Comet Halley. This difference is due to the difference in water production rates between the two comets.

Fine structure of the diamagnetic cavity boundary in comet Halley

We calculated the electric current density in the diamagnetic cavity boundary layer (DCBL) using magnetic field data obtained during the Giotto mission to comet Halley. This current possesses both the component perpendicular to the local magnetic field and the parallel component. The perpendicular current is responsible for the screening of the diamagnetic cavity boundary from the field in the magnetic barrier. This current is supported by the electric field tangential to the boundary. The behavior of the parallel electric current component resembles the Alfvén wings which arise due to the interaction of the magnetized plasma flow with a conducting obstacle. However, the electric current connecting the two wings does not flow through the whole volume of the obstacle. On the contrary, the wings are connected by the perpendicular current in the DCBL. In the inner portion of the DCBL this current diverges producing the parallel current component whose direction is opposite to that in the outer portion of the DCBL. In order to support such a current system at the diamagnetic cavity boundary, the potential drop across DCBL should be as small as 0.5V. Such a small value of the potential drop agrees with the penetration depth of cometary ions from the cavity to the magnetic barrier: it is equal to the ion gyroradius and therefore is not affected by any significant electric field normal to the diamagnetic cavity boundary.

Large-scale disturbance of the solar wind by a comet

Model calculations for the interaction of the solar wind with a comet are presented that extend 30 million km into the tail. It is shown that the disturbance of the interplanetary magnetic eld (IMF) (the draping) is limited to timescales of 10 to 50 hours and length scales of 10 to 50 million km. This is supported by a theoretical argument about the acceleration of the cometary ions. The distribution of ions and protons at the end of the model tails agrees with measurements made by Ulysses far in the tail of comet Hyakutake. It is shown that the ion tail is concentrated in the current sheet between two flux lobes as long as the draping persists. The far tail, however, is flat and concentrated in a plane parallel to the IMF.

3D hybrid simulations of the interaction of the solar wind with a weak comet

The interaction of the solar wind with weak comets, leading to the formation of the cometary magnetosphere with different types of structures in the solar wind and heavy ion plasmas, is simulated using a three-dimensional hybrid code. The simulations were performed for a wide range of gas production rates (6.2×10 26 mol/ s⩽Q⩽10 28 mol/ s) and for an interplanetary magnetic field which is perpendicular to the incoming solar wind. When Q<8.2×10 26 mol/ s the cometary atmosphere forms a strong cycloid-type tail, whereas for Q⩾10 27 mol/ s the cometary atmosphere forms a cone-type tail and structuring of the coma occurs. The results of these simulations may be applied to other weak massloading sources, e.g., dusty plasmas and cometary ion dynamics in the inner coma, AMPTE releases, and nonmagnetic bodies like Phobos, Deimos or even Pluto. Furthermore, the results presented here may be important for studying the dynamics of the ionized environment near the ‘Solar Probe’ spacecraft in the future.

Sungrazing Comets: Properties of Nuclei and in Situ Detectability of Cometary Ions at 1 AU

A one-dimensional sublimation model for cometary nuclei is used to derive size limits for the nuclei of sungrazing comets and to estimate oxygen ion fluxes at 1 AU from their evaporation. Given that none of the ≈300 sungrazers detected by the SOlar and Heliospheric Observatory (SOHO) was observed after disappearing behind the sun, and that small nuclei with a radius of ≈3.5 m could be observed, it is assumed that all SOHO sungrazers were completely destroyed. For the case that sublimation alone is sufficient for destruction, the model yields an upper size limit as a function of nuclear density ϱ, albedo A, and perihelion distance q. If the density of the nuclei is that typical of porous ice (600 kg m −3), the maximum size is 63 m. These results confirm similar model calculations by Weissman (1983). An analytical expression is derived that approximates the model results well. We discuss possible modifications of our results by different disruption mechanisms. While disruption by thermal stress does not change the upper size limits significantly, they may be somewhat increased by tidal disruption (up to 100 m for a density of 600 kg m −3), dependent on the isotropy of the sublimation process and the tensile strength of the comet. Implications for the Kreutz family of sungrazers are discussed. Oxygen ions from the sublimation of sungrazing comets form a tail. Fluxes from this tail are sufficiently high to be measured at 1 AU by particle detectors on spacecraft, but the duration of a tail crossing is only about half an hour. Therefore, the probability of a spacecraft actually encountering a tail of an evaporating sungrazer is only of the order of 2% per year.

2D hybrid simulations of the solar wind interaction with a small scale comet in high Mach number flows

We investigate the structure and dynamics of the interaction between high Mach number solar wind flows and a cometary source that is “small scale” with respect to the cometary ion gyroscales, examples being Grigg‐Skjellerup or P/Wirtanen. A series of 2D hybrid simulations are used and we focus on IMF orientations parallel and perpendicular to the simulation plane, these permit or prohibit propagation of modes with a component of k parallel to B0. The perpendicular cases show the onset of magnetosonic turbulence as MA is increased; introducing a parallel B0 field component significantly changes this turbulence and the apparent shock structure seen on a ‘fly‐by'. This implies that the Alfvén‐ion cyclotron mode plays a critical role in the observed structure, and we suggest the relevant mechanism must be resonant growth of such waves in the presence of magnetosonic turbulence from either the unstable ring beam distribution of the pickup ions, or shock reflected solar wind protons.

Interception of comet Hyakutake's ion tail at a distance of 500 million kilometres

Remote sensing observations and the direct sampling of material from a few comets have established the characteristic composition of cometary gas. This gas is ionized by solar ultraviolet radiation and the solar wind to form 'pick-up' ions, ions in a low ionization state that retain the same compositional signatures as the original gas. The pick-up ions are carried outward by the solar wind, and they could in principle be detected far from the coma (Sampling of pick-up ions has also been used to study interplanetary dust, Venus' tail and the interstellar medium.) Here we report the serendipitous detection of cometary pick-up ions, most probably associated with the tail of comet Hyakutake, at a distance of 3.4 AU from the nucleus. Previous observations have provided a wealth of physical and chemical information about a small sample of comets, but this detection suggests that remote sampling of comet compositions, and the discovery of otherwise invisible comets, may be possible.

On the distribution of pickup ions as observed by the Vega spacecraft at comet Halley

Abstract Two electrostatic analysers of the PLASMAG-1 plasma instrument package detected energy/charge spectra of ions (without mass separation) aboard the Vega spacecraft when encountering comet Halley. They could observe only two relatively small sections of velocity space showing a mixed effect of decelerated solar wind particles and cometary pickup ions downstream of the bow shock. The separation of the different ion components was attempted by comparing the energy spectra with simple plasma distributions. The effect of cometary pickup ions observed by Vega-1 was different from that observed by Vega-2 at cometocentric distances >0.5 million km. These differences could be interpreted when using the bispherical shell distribution based on magnetic field vectors measured by the magnetometer.

X-ray emission from comets and planets

X-ray emission has been observed from many objects throughout the solar system including the Sun, the Moon, the Earth, Jupiter, and comets. A brief review of these observations and some of the emission mechanisms suggested to explain the observed X-rays are given in this paper. Cometary X-ray emission and Jovian X-ray emission will be emphasized. The initial discovery of X-rays from comet Hyakutake (Lisse et al., 1996) was surprising and a number of explanations were put forth, including bremsstrahlung associated with hot electron collisions with cometary neutrals or ions, scattering or fluorescence of solar X-rays from cometary neutrals or from dust particles, and charge transfer of heavy solar wind ions with neutrals. X-rays have also been observed both from Jupiter's auroral region as well as from low latitudes (Waite et al., 1997). Again, a number of emission mechanisms have been proposed, including precipitation of energetic heavy ions from the magnetosphere. Predictions of X-ray emission from Venus, Mars, and from interstellar neutrals in the heliosphere are made.

Mirror mode structures and ELF plasma waves in the Giacobini-Zinner magnetosheath

Abstract. We show evidence for mirror mode structures at comet Giacobini-Zinner. These are plasma structures with alternating high ß and low ß regions driven unstable when ß&lt;perp&gt; /ß&lt;parallel&gt; &gt; 1+ 1/&lt;perp&gt;. These structures are detected in a region just adjacent to the magnetic tail and have scale sizes of ≈ 12 H2O group ion cyclotron radii. Calculations are presented to show that mirror mode instability can occur due to the perpendicular pressure associated with H2O+ cometary pickup ions in the region of mirror mode observation. Adjacent regions (in the magnetic tail and further in the sheath) are found to be stable to the mirror mode. Plasma waves are detected in relation with the mirror mode structures. Low frequency 56 to 100 Hz waves are present in the high beta portions, and high frequency, 311 Hz to 10 kHz, waves are present in low beta regions. These may be electromagnetic lion roar waves and electrostatic festoon-shaped waves, respectively, in analogy to plasma waves detected in the Earth's magnetosheath.

Instabilities of Alfvén and magnetosonic waves in dusty cometary plasmas with an ion ring beam

The effects of charged dust grains on the instabilities of electromagnetic waves in the solar wind plasma due to ring-beam cometary ion velocity distributions are considered. The waves are assumed to propagate parallel to the solar wind magnetic field. Collisionless cold plasma theory is used to derive the dispersion relations of linear waves in the solar wind, for a delta-function ring-beam velocity distribution of cometary water-group ions. The resonant and nonresonant instabilities are considered, and it is shown that the dust has its greatest effect on the nonresonant firehose instability at very long wavelength. The dust can cause an unstable dust-free firehose mode to be stable, and vice versa. It is also shown that as the frequency approaches the cometary ion cyclotron frequency, there is an upper limit to the wave number for the unstable firehose mode. For the left-hand polarized wave, the presence of dust increases this upper wave number limit, until the firehose mode merges with the resonant mode. The right-hand polarized firehose mode also has an upper wave number limit, which is almost independent of the presence of dust. It is shown that, although there is a high frequency resonant instability of the normal whistler mode induced by the ring beam, there is no corresponding instability of the dust-induced whistler-like mode at low frequencies.

Solitons in multi-ion plasmas

In the heliosphere large fluctuations of the magnetic field are sometimes observed, especially at shocks (e.g. travelling interplanetary shocks, planetary bow waves, and shocks related to corotating interaction regions). The large fluctuations very often have an amplitude that reaches easily four or five times the magnitude of the quiet magnetic field. Since the plasma mostly contains a non-negligible fraction of heavy ions (e.g. helium ions or cometary water ions), these magnetic field fluctuations are described in terms of three-fluid equations. Here electrons, protons and heavy ions are considered as separate fluids interacting with themselves by electromagnetic forces. The three-fluid equations are treated in such a way that we are searching for one-dimensional stationary waves. Thus the equations are reduced to a fully nonlinear set of ordinary differential equations, which have been evaluated numerically with respect to solitary waves. Subsequently, examples of solitary waves in a electron- proton plasma loaded with 10 % 4He2+ ions are presented. For instance, an enrichment of helium ions is observed within the solitary waves.

Electromagnetic ion-beam-plasma instabilities

It is well known that ion beam-plasma interactions can destabilize right- and left-hand polarized electromagnetic waves. However, these instabilities have been studied for frequencies less than the ion-beam gyrofrequency, with the exception of Goldstein and Wong (1987), and Brinca and Tsurutani (1988), who studied also the region of high frequencies ( ω > Ω i , where Ω i is the ion-beam gyrofrequency), in connection with pickup cometary ions. It has been recently demonstrated that in the cold plasma theory, left- and right-hand modes can be destabilized in a rather large range of frequency and wavenumber values. It has also been shown that, in some cases, the unstable spectrum has a band structure. We study here thermal effects on the cold modes and show that, as the temperature increases, the band structure becomes a double-humped unstable spectrum. For even larger temperatures, the double-humped structure is washed out, remaining a single-humped spectrum, but with an instability range which is similar to the cold plasma case. We then apply these results to the solar wind.

Results from a gyrotropic two‐ion fluid model for the comet interaction with the solar wind

A two‐and‐a‐half‐dimensional two‐ion fluid model (with gyrotropic pressure tensor) for the interaction of cometary pickup plasma with the solar wind has been developed. Whereas previous models have, in general, assumed a one‐fluid plasma, this generalized model follows the evolution of two fluids: solar wind protons and cometary ions. The two fluids are coupled by the magnetic field and by coupling terms which are the velocity moments of the time relaxation model of Bhatnagar et al. [1954]. Furthermore, a gyrotropic pressure tensor replaces the isotropic pressure assumed in other models. Isotropization terms couple the pressures in the directions perpendicular and parallel to the magnetic field. Among the results received, the standoff distance of the shock increased significantly when the isotropization rate was reduced. The large cometary plasma pressure forces at the shock accelerated the cometary plasma along the shock. As a result, the cometary plasma drifted ahead of the solar wind along the shock flanks parallel to the magnetic field. Close to the nucleus of the comet in the far tail lobes and in the far inner tail the cometary plasma lagged behind the solar wind.

One‐sided velocity distributions at comet P/Grigg‐Skjellerup

During the hour following the bow shock of the outbound leg of Giotto's encounter with comet P/Grigg‐Skjellerup, asymmetric cometary ion distributions were observed in velocity space. Various explanations for the observed asymmetry include sudden changes in the magnetic field or magnetic mirroring. Several workers have noted that the parallel Alfvénic Mach number was close to unity during this interval, a result of the near orthogonal orientation of the interplanetary magnetic field with respect to the solar wind. Newly formed cometary ions would be implanted into the upstream diffusion gap and would therefore be more likely to diffuse through resonant interactions with downstream propagating waves. Starting with this observation, a simple finite difference representation of Boltzmann's equation has been utilized to model the velocity space distributions. The results of the simulation show that the asymmetry is a consequence of the geometry of the implanted ion distribution and the lifetime of the cometary ions in the solar wind and that no other mechanisms are needed to explain the observations.

The effect of Poisson statistics on studies of turbulence in the solar wind

Studies of Alfvén turbulence in the solar wind require the use of magnetic field and particle data. However, particle data is subject to a source of noise not present in magnetic field data, i.e. statistical noise because the bulk parameters of the plasma are calculated from a finite number of counts. The derivation of bulk parameters is unavoidably affected by Poisson statistics which add variance to the real value. The magnitude of the effects of Poisson noise on the frequency spectrum of Alfvén waves in the solar wind has been calculated by numerical simulation for the case of cometary pickup ion turbulence observed at comet Halley. The results show that Poisson noise is “white” and flattens the frequency spectrum at high frequencies, where its power spectral density exceeds the solar wind turbulence. If a power law is fitted to the spectrum, the power law index can be reduced significantly even if the flattening is not obvious in the spectrum, and the effect can be significant even when the particle count rates seem high. In Elsässer variable studies the influence of Poisson noise is seen as a tendency to equality of the two spectra at high frequencies since each contains uncorrelated noise. The simulations show that Poisson noise effects must be taken into account in the interpretation of Alfvén wave observations. A concrete result is that the difference between the spectral indices obtained separately from magnetic and particle data at comet Halley can be explained quantitatively by Poisson noise.

The turbulent state of cometary ion pickup generated fluctuations near comet P/Halley

One of the principal effects of an outgassing comet is to introduce additional mass into the solar wind flow. This mass‐loading process results in the generation of Alfvén waves via ion‐cyclotron resonance. In the case of the Halley encounter this source of “new” fluctuations was introduced into a low speed solar wind 0.89 AU from the sun. Previous in situ observations in low speed flows at this distance have described the fluctuations as turbulently evolved or “aged.” The spectral characteristics and time dependent anisotropies of these new cometary fluctuations have been effectively explained by a conservation‐of‐energy description of the cometary ion pickup process. An alternative description has been proposed whereby turbulent cascading generates the observed spectrum, implying that the fluctuations evolve significantly in the time between their generation and their observation. Analysis of the rugged invariants and, in particular, the Alfvén ratio suggests that throughout the interaction region upwind from the comet the cometary ion pickup generated fluctuations became less evolved as the bow shock was approached. Hence turbulent cascading of the fluctuation energy is unlikely to be an effective description of the observations or to modify the spectral characteristics of these fluctuations significantly in the upwind interaction region.

Interaction of the solar wind with dusty cometary plasmas

Dusty space plasmas contain negatively charged, heavy dust grains with characteristic frequencies much below those of usual plasmas. Pickup of ions of cometary origin by the solar wind is partly due to relative streaming between cometary and solar wind ions parallel to the interplanetary magnetic field, partly because of skewed velocity distributions perpendicular to it. Both pickup mechanisms excite low-frequency electromagnetic turbulence, which is altered by the presence of charged dust in cometary environments. We focus on parallel modes at frequencies below the cometary watergroup gyrofrequency. Existing firehose-like instabilities are stabilized at very long wavelengths. On the contrary, when modes would be stable in the absence of charged dust, the left-hand circularly polarized mode shows a new instability window. The scarce data available from recent cometary missions indicate that the wavelengths needed probably exceed the dimensions of the cometary structures. However, more data are needed to elucidate this.

Rosetta Orbiter Spectrometer for Ion and Neutral Analysis—ROSINA

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) will answer outstanding questions concerning the main objectives of the mission. To accomplish the very demanding objectives, ROSINA will have unprecedented capabilities, including: very wide mass range from 1 amu to >300 amu; very high mass resolution (ability to resolve CO from N 2 and 13C from 12CH); very wide dynamic range and high sensitivity; the ability to determine cometary gas and ion flow velocities and temperatures.

Electron energization by lower-hybrid waves as a possible source for x-ray emission from comets

A goal of this presentation is to demonstrate that recently observed x-ray emission from comets can be interpreted as a combination of bremsstrahlung and line k-shell radiation produced by interaction of energetic electrons with the cometary atmosphere. Electron energization inside the cometary bow-shock is due to mass-loading of the shocked solar wind flow by newly born heavy cometary ions created by photoionization of the neutral gas outflow. The new ion component in the crossed E × B fields of the host (solar wind) plasma moves in cycloidal orbits, forming an unstable ring distribution, which drives the modified two stream instability and excites lower hybrid waves. These waves have the phase velocity along the magnetic field much larger than across it. For this reason, they can be in simultaneous Landau resonance with both slow unmagnetized ions and fast, but magnetized (field aligned), electrons efficiently transferring ion energy to the electrons and leading to their energization. We have investigated this process in a quasilinear approximation. By solving the Fokker-Planck equation for the field aligned electron energy diffusion, we have been able to calculate fluxes and energy spectra of electrons accelerated by the lower hybrid waves and to determine spectrum and total luminosity of x-ray radiation. Results of theoretical analysis are compared with ROSAT and Beppo-Sax observations of x-ray emission from the different comets.