Cometary Pickup Ions Research Articles

Comparison of the wave energy transport at comets p/Halley and p/Giacobini-Zinner

Using magnetic field, plasma density and flow observations from spacecraft flybys of two comets, Elsässer variables are determined in order to study wave propagation directions. The inbound path of the Giotto spacecraft flyby of comet p/Halley is investigated outside the bow shock (between 10:36 and 19:11 UT on March 13, 1986, corresponding to distances from 3.3 × 10 6 to 1.2 × 10 6 km from the nucleus) and the inbound and outbound path of the ICE spacecraft flyby of comet p/Giacobini-Zinner is investigated outside of the bow wave (between 00:00 and 9:24 UT and between 12:25 and 23:30 UT on September 11, 1985, corresponding to distances from 8.5 × 10 5 to 1.2 × 10 5 km from the nucleus and from 1.0 × 10 5 to 9.3 × 10 5 km from the nucleus, respectively). The interaction of cometary pick-up ions and the solar wind is expected to generate waves which are assumed to be dominated by field-aligned propagating Alfvén waves. This predicted wave propagation combined with observed wave modes generated by the ion cyclotron resonance instability are compared with observed wave properties. Three mhd-modes are possible: right-hand polarized waves propagating toward the Sun ( RH −) and left-hand polarized waves propagating toward ( LH −) and away from the Sun ( LH +). Plasma conditions dictate that the RH − mode is the most unstable mode and satisfactory agreement is found between observed and predicted energy flow directions in regions outside the bow shock at Halley and the bow wave at Giacobini-Zinner, where local cometary ion pick-up conditions dominate.

Cometary ion pickup processes: Halley and Grigg-Skjellerup compared

Abstract After Giotto's encounters with two comets of quite different gas production rates we attempt to compare the results to see:- 1. a) to what extent theories which fitted the results at Halley could be applied to the much smaller Grigg-Skjellerup; 2. b) whether the different relationship between the various scale lengths involved created any observable effects.

Theoretical calculations of ion acceleration in the vicinity of comet Giacobini‐Zinner

Ionization of cometary neutral molecules produces ions which are picked up by the solar wind. The cometary ion pickup process for comet Giacobini‐Zinner is studied in two ways: (1) with a test particle method in which trajectories are numerically calculated for several thousand ions whose initial locations were chosen randomly with probability proportional to the neutral density and (2) with a quasi‐linear diffusion model. The cometary ion distribution function was calculated with the test particle model at several locations upstream of the bow shock and for several types of magnetic fluctuations (or waves). These waves were allowed to propagate in both directions along the magnetic field at the Alfven speed. Both pitch angle scattering and energy diffusion are evident in the derived ion distributions. The monochromatic waves result in less ion acceleration than turbulent fluctuations with about the same amplitude. The calculated ion distribution functions are in reasonable agreement with the distributions measured in the vicinity of comet Giacobini‐Zinner in 1985 by particle detectors on the ICE spacecraft when the ratio of power in sunward propagating Alfven waves to the power in antisunward propagating waves is assumed to lie between about 20% and 50%. However, the quasi‐linear diffusion model results agree best with the measured distribution functions when the power ratio is only about 3%.

Nongyrotropic distribution of pickup ions at comet P/Grigg‐Skjellerup: A possible source of wave activity

The pickup of newborn cometary ions in the solar wind excites strong wave activity. At comet P/Grigg‐Skjellerup the interplanetary magnetic field is nearly perpendicular to the solar wind velocity; thus the pickup ions should form a ring distribution. In opposition to P/Halley, comet P/Grigg‐Skjellerup is rather small, and its inhomogeneity scale is even smaller than the pickup scale. Therefore the ring will be filled incompletely, and the distribution becomes nongyrotropic. The dispersion of a nongyrotropic cometary pickup distribution in a cold proton‐electron background plasma is studied. Nongyrotropy couples the gyrotropic eigenmodes and drives an instability. Left‐hand polarized waves close to the cometary ion cyclotron frequency are excited.

Comment on “Comparison of observed and calculated implanted ion distributions outside comet Halley's bow shock” by T. I. Gombosi, M. Neugebauer, A. D. Johnstone, A. J. Coates, and D. E. Huddleston

It is argued that the formulation used by Gombosi (1991) does not distinguish between two fundamental types of resonant interactions (those between cometary pickup ions and LF electromagnetic waves). Two problems arise from using their overly simplified expressions. The wave polarization of the solar wind Alfven wave is not purely right-handed, and values from alpha sub com and alpha sub sw are not well known, nor are the 'polarizations' of the 'backward'-going waves in either case. Gombosi et al. discuss the antisunward vs the sunward propagating solar wind Alfven wave, and assume a reasonable value of 0.2 for alpha sub sw. However, alpha sub com is totally unknown. It is concluded that a more correct analysis of wave polarization can lead to significant changes in the formulation and results from Gombosi et al. In his reply Gombosi argues that his simplified wave-particle interaction model is consistent with Giotto observations and plasma kinetic simulations and it leads to a reasonable approximation of the implanted ion transport coefficients. The more complicated approach suggested by Tsurutani & Thorne (1993) requires the introduction of several ad hoc parameters and it does not lead to any change of the energy diffusion coefficient.

Pickup protons and water ions at comet Halley: Comparisons with Giotto observations

A quasi‐linear diffusion model including both pitch angle and energy diffusion, adiabatic compression and convective motion with the solar wind flow has been used to investigate the cometary ion pickup process along the Sun‐comet line at comet Halley. The total pickup ion densities and magnetic turbulence spectrum levels observed by Giotto were used to constrain the quasi‐linear model. Comparisons of the model results were made with energetic ion distributions observed by instruments onboard the Giotto spacecraft. The observed power spectrum index of magnetic turbulence γ is about 2–2.5. However, our simulation shows that when γ was 2, the calculated proton distributions were much more isotropic than the observed ones; hence we have chosen γ = 2.5 in our study. Furthermore, we assumed that only about 5% of the total low frequency wave power propagates away from the comet. The numerical solutions of the quasi‐linear diffusion equations show that the isotropization of the pickup ion distribution, particularly at the pickup velocity, is not complete even close to the bow shock (but upstream), which agrees with the observations. The calculated and observed proton distributions are somewhat more isotropic than the water group ion distributions in the region far upstream of the shock, but the opposite is true near the bow shock. We find that given the observed turbulence level, quasi‐linear theory gives pickup ion energy distributions that agree with the observed ones quite well and easily produces energetic ions with energies up to hundreds of keV.

Quasi‐linear velocity space diffusion of heavy cometary pickup ions on bispherical diffusion characteristics

We consider an ion transport equation describing the source, convection, adiabatic acceleration and quasi‐linear velocity diffusion of cometary pickup ions. The equation is solved numerically along solar wind plasma flow lines for the environment of comet Halley, to obtain distributions at positions on the Giotto spacecraft trajectory which may be compared with observations. We use realistic input models for the observed spectrum of upstream and downstream propagating wave turbulence and for the ion source and mass loading of the solar wind. The ion resonance condition is not approximated in the diffusion term and we obtain full two‐dimensional (pitch angle and velocity) numerical distributions which show pitch angle scattering about the wave scattering centers at ± VA where VA is the Alfvén wave speed in the solar wind frame. Peak ion phase‐space densities approximately follow the bispherical shell geometry. The energy distributions F(υ) agree well with Giotto observations at lower υ but in the high‐energy tail there is evidence possibly for the first‐order Fermi acceleration mechanism taking place in the comet Halley foreshock region.

Quasi‐linear evolution of ULF waves excited by cometary ion pickup

An analytical model is presented which describes the quasi‐linear evolution of both ultralow‐frequency (ULF) magnetic fluctuations and cometary ion flux in the solar wind due to continuous ion pickup. The derived solution yields both the initial exponential growth of the total wave intensity and the time‐asymptotic growth proportional to time obtained in numerical simulations using a one‐dimensional hybrid code with a constant rate of cometary ion injection (Gary et al., 1988, 1989). For pickup rates appropriate to cometary environments, the derived fluctuation intensity and pickup ion flux are in general accord with ULF wave observations and water group ion pitch angle distributions upstream of the bow waves at both comet Giacobini‐Zinner and comet Halley.

The directional dependence of magnetic fluctuations generated by cometary ion pickup

The properties of low frequency magnetic fluctuations generated by cometary ion pickup are examined by means of one‐dimensional hybrid simulations, in which newborn ions are created at a constant rate. The helicity and direction of propagation of magnetic fluctuations are investigated for various cometary ion injection angles, α, relative to the solar wind magnetic field. The parameter η represents the relative contribution of wave energy density propagating in the direction away from the comet, parallel to the beam. For small (quasi‐parallel) injection angles, α ∼ 0° and η is of order unity, while for larger (quasi‐perpendicular) angles, α ∼ 90° and η is about 0.5. At intermediate angles, α ∼ 60°, η can vary between 0 and 1, depending on the wave number. The wave properties are consistent with the instabilities (right‐hand cyclotron resonant modes at small α, left‐hand Alfven/ion cyclotron modes at large a) expected from linear theory. Consequences of these results for particle acceleration are also discussed.

Combined energy and pitch angle diffusion of pickup ions at comet Halley

It is well known that cometary pickup ions (e.g., H2O+, OH+, O+, CO+, H+) initially form a ring‐beam distribution in the solar wind reference frame, which is highly unstable to the growth of MHD waves (such as ion‐cyclotron waves). The low‐frequency magnetic fluctuations (or waves), which were observed upstream of comet Halley, cannot only pitch angle scatter the pickup ions so that the distribution becomes at least partially isotropized, but also stochastically accelerate the ions, resulting in the energetic ion populations observed in the vicinity of comet Halley. We have used numerical solutions of the quasi‐linear diffusion equation to investigate the cometary ion pickup process at comet Halley. Both pitch angle and energy diffusion are taken into account. Many quasi‐linear models of cometary pickup ions exist which involve one type of diffusion or the other but not both types at once. We find that the pitch angle scattering occurs faster than the energy diffusion, as expected. Moreover, our results demonstrate that the distribution of accelerated energetic ions is more isotropic than that of ions which have just been picked up. In fact, the ion distribution function on the initial pickup shell is quite anisotropic, even close to the comet Halley bow shock.

Cometary water-group ions in the region surrounding comet Giacobini-Zinner: Distribution functions and bulk parameter estimates

The bulk parameters (number density and thermal energy density) of cometary water-group ions in the region surrounding comet Giacobini-Zinner have been derived using data from the EPAS instrument on the ICE spacecraft. The derivation is based on the assumption that the pick-up ion distribution function is isotropic in the frame of the bulk flow, an approximation which has previously been shown to be reasonable within ∼4 × 10 5 km of the comet nucleus along the spacecraft trajectory (corresponding to ± 5 h about closest approach). Bulk parameter determinations are consequently restricted to this region, which corresponds physically to the inner part of the region of cometary ion pick-up in the solar wind flow, and the interior region of slowed, mass-loaded flow. The transition between the pick-up and mass-loaded regions occurs at the cometary shock, which was traversed at a cometocentric distance of ∼ 1 × 10 5 km along the spacecraft track (±85 min about closest approach). Examination of the ion distribution functions in this region, transformed to the bulk flow frame, indicates the occurrence of a “flattened” distribution in the vicinity of the local pick-up speed, and a steeply falling tail at speeds above, which may be approximated as an exponential in ion speed. Bulk parameters derived by integrating over these distributions have been compared with simple theoretical estimates based on ionization of the neutral coma of the comet by solar u.v. radiation, by charge exchange, and by electron impact, giving a combined ionization time-scale of ∼ 10 6 s, followed by ion pick-up and isotropization in the solar wind flow. In the inner pick-up region the derived parameters are in good agreement with the theoretical estimates, within a factor of ∼ 2, showing that no major additional sources of ionization, or major energization processes, are required to account for the results. Comparison of the thermal pressure of the water-group pick-up ions with the solar wind thermal pressure and with the magnetic pressure, shows how the pick-up ion pressure grows from small values at large distances from the comet, to become the dominant pressure component in the region just upstream from the shock. As a consequence, the magnetosonic Mach number falls from values of ∼ 5 at large distances, down to ∼ 2.5 just upstream of the shock. In the mass-loaded region downstream from the shock the observed ions most probably correspond to the relatively energetic population which is produced in the fast-flow region upstream from the shock, and which is subsequently converted downsteam. Reasonable agreement between derived and theoretically-estimated bulk parameters is obtained in the outer part of the mass-loaded region on this basis. In the inner part of the mass-loaded region, however, within ∼6 × 10 4 km of the comet (±45 min about closest approach), the derived densities fall far below theoretical predictions, indicating that the energetic ions created in the upstream region must be actively removed from these stream-tubes by some process, at present undetermined.

The directional dependence of cometary magnetic energy density in the quasi-parallel and quasi-perpendicular regimes

The direction of propagation of low frequency magnetic fluctuations generated by cometary ion pick-up is examined by means of one-dimensional electromagnetic hybrid simulations. The newborn ions are injected at a constant rate, and the helicity and direction of propagation of magnetic fluctuations are explored for cometary ion injection angles of 0° and 90° relative to the solar wind magnetic field. The parameter η represents the relative contribution of wave energy propagating in the direction away from the comet, parallel to the beam. For small (quasi-parallel) injection angles η was to found to be of order unity, while for larger (quasi-perpendicular) angles η was found to be of order 0.5.

Negative ions in the coma of comet Halley

IN March 1986, the Giotto spacecraft encountered comet Halley, approaching to within ∼600 km of the nucleus. Results from this encounter have shown that the inner coma contains a mixture of cometary neutral gas and dust, thermal ions and electrons, fast cometary pick-up ions, and decelerated solar-wind ions and electrons, as well as fast neutrals1produced by charge exchange between pick-up ions and cold neutrals. Here we report the detection of a new component of the inner coma of comet Halley: negatively charged cometary ions. These ions are observed in three broad mass peaks at 7–19, 22–65 and 85–110 AMU, with densities reaching ≳1, ∼5x10-2 and ∼4x10-2cm-3, respectively, at a distance of ∼ 2,300 km from the nucleus. The ion species thought to be present include O-, OH-, C-, CH-, CN- and heavier complex CHO molecular ions. As negative ions are easily destroyed by solar radiation at ∼1 AU (ref. 2) an efficient production mechanism, so far unidentified, is required to account for the observed densities. The detection of negative ions in the coma near 1 AU implies that in similar neutral gas and dust environments farther away from the Sun (in Jupiter's or Saturn's magnetospheres3, for example), negative ions should also be present. If the negative-ion densities are large enough, they could play an important part in physical processes such as radiative transfer or charge exchange.

Kinetic instability associated with spherical shell distribution of cometary pickup ions

The stability of ion shell distribution is an interesting and important issue in view of the fact that it is stable under perturbations propagating parallel to the ambient magnetic field, despite large amount of free energy it contains [Freund and Wu, 1988; Brinca, 1990]. It was found, however, that if the shell distribution of ions has a sufficient amount of drift speed with respect to the solar wind hydromagnetic waves, it can become unstable [Gary and Sinha, 1989; Yoon et al., 1989]. In addition, recently Wu and Yoon [1990] have found the obliquely propagating hydromagnetic instability associated with the ion shell distribution. This letter presents the unified stability analysis of the ion shell distribution, and show under what condition the drift‐induced instability of the type first discussed by Gary and Sinha [1989], and the oblique hydromagnetic instability discussed by Wu and Yoon [1990] are excited and how the two types of instabilities are related.

Stochastic acceleration of cometary pickup ions - The classic leaky box model

view Abstract Citations (16) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stochastic Acceleration of Cometary Pickup Ions: The Classic Leaky Box Model Barbosa, D. D. Abstract The acceleration of cometary pickup ions by magnetohydrodynamic waves at P/Giacobini-Zinner is examined in a model where acceleration predominantly occurs downstream of the bow shock throughout the cometosheath where intense magnetic turbulence exists. The mean free path for scattering by the magnetic fluctuations in this region is less than the characteristic dimension of the cometosheath so that pickup ions are rendered isotropic and energized by a modest amount in the process. This principal loss mechanism for the ions is spatial diffusion out of the acceleration region moderated by the self-same accelerating waves. This particular feature constrains the model in a way that the predicted ion spectrum is uniquely determined by the power spectrum of the magnetic turbulence. At both P/Giacobini-Zinner and P/Halley, the turbulence is non-Kolmogoroff with a spectral index of 2 resulting in an ion spectral behavior that is approximately an exponential in ion speed, consistent with a recent analysis of the Giacobini-Zinner data (see Richardson et al.). Publication: The Astrophysical Journal Pub Date: June 1989 DOI: 10.1086/167510 Bibcode: 1989ApJ...341..493B Keywords: Cosmic Rays; Giacobini-Zinner Comet; Ion Temperature; Magnetohydrodynamic Turbulence; Magnetohydrodynamic Waves; Particle Acceleration; Bow Waves; Interplanetary Medium; Planetary Magnetospheres; COMETS; ACCELERATION; PICKUP; IONS; MODELS; MAGNETOHYDRODYNAMICS; GIACOBINI-ZINNER; DOWNSTREAM; COMETOSHEATH; MAGNETIC EFFECTS; TURBULENCE; SCATTERING; DIAGRAMS; CALCULATIONS; ENERGY; SPECTRA; COMPARISONS; ICE MISSION; SPACECRAFT OBSERVATIONS; Astrophysics; Comets; COMETS; COSMIC RAYS: GENERAL; HYDROMAGNETICS; PARTICLE ACCELERATION full text sources ADS |

Stability of a spherical shell distribution of pickup ions

The stability of a spherical shell (complete or incomplete) is investigated. The study is motivated by the research of cometary pickup ions. The present discussion is restricted to low‐frequency electromagnetic waves propagating along the ambient magnetic field. It is found that a complete shell is stable, whereas an incomplete shell can be unstable. The numerical results are reported and discussed.

Time scales for formation and spreading of velocity shells of pickup ions in the solar wind

The pickup of newborn ions in the solar wind by means of low‐frequency, electromagnetic waves, either due to the intrinsic turbulence of the solar wind or induced turbulence due to instabilities excited by the newborn ions, is discussed. The pickup process is envisioned to occur in three stages, formation of a ring‐beam distribution, rapid pitch angle scattering of this initial distribution into a thin shell, and slower velocity diffusion that spreads out the shell. The process of shell formation and evolution are studied by means of numerical simulations, first for the situation of relatively low levels of turbulence such as would occur naturally in the solar wind and then for higher levels of turbulence characteristic of those encountered near cometary bow shocks. The results of the numerical experiments are compared with theory. The initial velocity spreading and growth of electromagnetic fluctuations due to the interaction of a small initial density of newborn ions interacting with the solar wind in the absence of significant background turbulence is shown to be well described by linear‐ and second‐order theory. When the initial density of newborn ions is sufficiently small or the level of intrinsic turbulence is large enough so that there is no appreciable growth and self‐interaction of generated waves with the newborn ions, these ions pitch angle scatter to form a thin, partial shell distribution. The time scale for the initial scattering is shown to be related to the gyroperiod in the perturbed wave field. On a longer time scale that is consistent with a simplified quasi‐linear theory, the ions in this shell diffuse in velocity space to fill it out in pitch angle, as well as cause it to spread in energy. However, the diffusion process slows considerably, long before a filled‐in velocity sphere is formed. These results are then used to estimate the time scales for shell formation and diffusion in several situations of recent observational interest, the interstellar helium data obtained by AMPTE and cometary ion pickup distributions obtained by various spacecraft at comets Giacobini‐Zinner and Halley.

A kinetic study of solar wind mass loading and cometary bow shocks

A detailed numerical study is conducted to understand the kinetic processes associated with solar wind mass loading due to pickup of cometary ions and the formation of cometary bow shocks. Because in this study we are most interested in phenomena which take place over time and spatial scales associated with ion dynamics, electrons are treated as a massless fluid. On the other hand, solar wind protons and heavy cometary ions are treated kinetically. It is found that solar wind deceleration and pickup of cometary ions take place through both the macroscopic electromagnetic fields embedded in the solar wind, as well as the microscopic fields associated with low‐frequency electromagnetic waves that are generated by the unstable velocity distribution function of the cometary ions. These electromagnetic waves can grow to very large amplitudes and their nonlinear evolution is controlled by their wave normal angle. At parallel propagation where the waves are noncompressional, parametric instabilities seem to be operative, while the oblique, compressional waves are found to steepen and form shocklets. As for the nature of cometary bow shocks, three different regimes are found, based on the shock normal angle. For shock normal angles at or near 90° (quasi‐perpendicular), a typically thin transition region (shock) is formed through steepening of fast magnetosonic pulses. This shock is a pure proton shock in that no sharp change in density or velocity of the cometary ions is observed across the shock. At intermediate shock normal angles (oblique), a much wider transition region is seen where the solar wind is gradually heated and decelerated. This region, whose width is much larger than the gyroradius of cometary ions, is associated with a high level of turbulence due to ion pickup instabilities and is unlike a typical fast magnetosonic shock. Finally, at small shock normal angles (quasi‐parallel), a narrow transition region is found which is similar to a parallel terrestrial shock. The formation of this shock is not due to wave steepening but rather is the result of scattering and heating of the solar wind protons by the large‐amplitude electromagnetic waves that are generated through the relative drift between the protons and the cometary ions. These results are compared and shown to be in qualitative agreement with the recent observations at comets.

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 H 2O + 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 α of the interplanetary field. For α ≲ 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 α ≳ 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° ≲ α ≲ 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.

Numerical simulation of the generation of turbulence from cometary ion pick‐up

Observations of magnetic field fluctuations near comet Halley have revealed a rapid development of a Kolmogoroff‐like turbulence spectrum extending from below 10−2 Hz to above 0.1 Hz. Spectra obtained far from the comet have a strong peak in power near the Doppler‐shifted ion‐cyclotron frequency of singly ionized water. Closer to the comet, the spectrum at higher frequencies is enhanced in power level over the background solar wind spectrum by approximately an order of magnitude. We solved the equations of incompressible MHD using a two‐dimensional 256×256 mode spectral method code to simulate this spectral evolution as an inertial range turbulent cascade. The initial conditions contained a constant magnetic field and a single coherent wave mode at a low wave number. The solar wind turbulence was modelled by a background noise spectrum having a Kolmogoroff spectral index. The coherent mode decayed into an inertial range spectrum with Kolmogoroff slope within a few eddy‐turnover times. Both the time scale and the increase in power level of the turbulence seen in the simulation are in accord with the Giotto observations.