Pickup Ion Production Research Articles

Angular Scattering in Charge Exchange: Issues and Implications for Secondary Interstellar Hydrogen

Abstract Interstellar neutral atoms provide a remote diagnostic of plasma in the outer heliosheath and the very local interstellar medium via charge exchange collisions that convert ions into atoms and vice versa. So far, most studies of interstellar atoms assumed that daughter hydrogen atoms directly inherit the kinetic properties of parent protons. This assumption neglects angular scattering of the interacting particles. However, for low relative velocities, as expected for charge exchanges in the outer heliosheath, this scattering is significant. In this study, we present how the parameters of daughter populations depend on the relative velocity and temperatures of parent populations. For this purpose, we numerically compute collision terms with and without this scattering. We find that the secondary population of interstellar hydrogen atoms, for the parent populations with the relative bulk velocity of 20 km s−1 and equal temperatures of 7500 K, has ∼2 km s−1 higher bulk velocity if the scattering is taken into account. Additionally, temperatures are higher by ∼2400 K and ∼1200 K in parallel and perpendicular directions to the relative motion of parent populations, respectively. Moreover, a significant departure of secondary atoms from the Maxwell–Boltzmann distribution is expected for high relative velocities of parent populations. This process affects the distribution and density of interstellar atoms in the heliosphere and production of pickup ions. Thus, we show that angular scattering in charge exchange collisions is important to include in analyses of interstellar neutral atoms and pickup ions observed at 1 au and in the outer heliosphere.

Energetic Neutral Atoms from the Heliosheath as an Additional Population of Neutral Hydrogen in the Inner Heliosphere

Abstract Interstellar neutral hydrogen (ISN H) gas penetrates freely the heliopause. Inside the inner heliosheath, the charge-exchange interaction of this gas with the shocked solar wind and pickup ions creates energetic neutral atoms (ENAs). ISN H is strongly depleted inside the termination shock but a fraction reaches the Earth’s orbit. In these regions of the heliosphere, ISN H is the source population for interstellar pickup ions and for the heliospheric backscatter glow. The globally distributed flux (GDF) of ENAs created in the inner heliosheath has been sampled directly by the Interstellar Boundary Explorer. Based on these measurements, we calculate the density of the GDF ENA population at the Earth’s orbit. We find that this number density is between 10−4 and 10−3 cm−3, i.e., comparable in magnitude to the number density of ISN H in the downwind portion of the Earth’s orbit. Half of this atom population has energies less than ∼80 eV. This GDF population of neutral hydrogen is likely to provide a significant contribution to the intensity of heliospheric glow in the downwind hemisphere, may be the source of the inner source of hydrogen pickup ions, and may be responsible for the excess of production of pickup ions found in the analysis of magnetic wave events induced by the proton pickup process in the downwind region at 1 au from the Sun.

Evidence for Neutrals‐Foreshock Electrons Impact at Mars

AbstractBackstreaming electrons emanating from the bow shock of Mars reported from the Mars Atmosphere and Volatile EvolutioN/Solar Wind Electron Analyzer observations show a flux fall off with the distance from the shock. This feature is not observed at the terrestrial foreshock. The flux decay is observed only for electron energy E ≥ 29 eV. A reported recent study indicates that Mars foreshock electrons are produced at the shock in a mirror reflection of a portion of the solar wind electrons. In this context, and given that the electrons are sufficiently energetic to not be affected by the interplanetary magnetic field fluctuations, the observed flux decrease appears problematic. We investigate the possibility that the flux fall off with distance results from the impact of backstreaming electrons with Mars exospheric neutral hydrogen. We demonstrate that the flux fall off is consistent with the electron‐atomic hydrogen impact cross section for a large range of energy. A better agreement is obtained for energy where the impact cross section is the highest. One important consequence is that foreshock electrons can play an important role in the production of pickup ions at Mars far exosphere.

ANALYTIC MODEL OF THEIBEXRIBBON WITH NEUTRAL SOLAR WIND BASED ION PICKUP BEYOND THE HELIOPAUSE

Energetic neutral atom (ENA) full sky maps obtained with the Interstellar Boundary Explorer revealed a surprising narrow band of increased intensity, the Ribbon, which has been attributed to the ordering effect of the interstellar magnetic field immediately outside the heliosphere. Among models to explain the enhanced ENA Ribbon intensity, Heerikhuisen et al. base theirs on neutral solar wind origin. It reflects the Ribbon angular and energy distribution correctly, but experiences inherent challenges from the long-term stability of the pickup ion (PUI) ring in velocity space, required for the observed ENA fluxes, and from time variations observed after less than one year due to the long integration length. We provide a simplified analytic model of the neutral solar wind, PUI production beyond the heliopause, and subsequent ENA production. We include convection of the PUIs with the interstellar flow toward the heliopause perpendicular to the interstellar magnetic field, thus far not in any model, extinction of the outward propagating neutral solar wind, and of the PUIs and ENAs on their way inward. Based on hybrid simulations of PUI driven instabilities, with injection rates from this model, scattering and isotropization of the PUIs is noticeably weaker than previously thought, yet too fast for ENA production. Assuming a narrow PUI velocity ring, we find a strong concentration of the ENA origin just outside the heliopause and Ribbon intensities comparable with the observations. Conversely, an isotropic PUI distribution produces ENA fluxes factor of ten too low, thus reemphasizing the need of very slow scattering.

Charge‐exchange X‐ray emission at hot/cool gas interfaces

AbstractCharge‐transfer (CT) X‐ray emission may occur at interfaces between a partially neutral gas and gas possessing high ions, provided there is a relative motion between those two phases. The CTX surface brightness from distant objects must be taken into account if it is not far below other “classical” emission sources, especially the thermal emission from the hot phase. I discuss those conditions and potential spectroscopic or photometric diagnostics. I also mention potential indirect effects of the CT reactions by means of pickup ion production, acceleration and subsequent modification of interface and plasma properties (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Saturn's magnetosphere interaction with Titan for T9 encounter: 3D hybrid modeling and comparison with CAPS observations

Global dynamics of ionized and neutral gases in the environment of Titan plays an important role in the interaction of Saturn's magnetosphere with Titan. Several hybrid simulations of this problem have already been done (Brecht et al., 2000; Kallio et al., 2004; Modolo et al., 2007a; Simon et al., 2007a, 2007b; Modolo and Chanteur, 2008). Observational data from CAPS for the T9 encounter (Sittler et al., 2009) indicates an absence of O+ heavy ions in the upstream that change the models of interaction which were discussed in current publications (Kallio et al., 2004; Modolo et al., 2007a; Simon et al., 2007a, 2007b; Ma et al., 2007; Szego et al., 2007). Further analysis of the CAPS data shows very low density or even an absence of H+ ions in upstream. In this paper we discuss two models of the interaction of Saturn's magnetosphere with Titan: (A) high density of H+ ions in the upstream flow (0.1cm−3), and (B) low density of H+ ions in the upstream flow (0.02cm−3). The hybrid model employs a fluid description for electrons and neutrals, whereas a particle approach is used for ions. We also take into account charge-exchange and photoionization processes and solve self-consistently for electric and magnetic fields. The model atmosphere includes exospheric H+, H2+, N2+ and CH4+ pickup ion production as well as an immobile background ionosphere and a shell distribution for active ionospheric ions (Mi=28amu). The hybrid model allows us to account for the realistic anisotropic ion velocity distribution that cannot be done in fluid simulations with isotropic temperatures. Our simulation shows an asymmetry of the ion density distribution and the magnetic field, including the formation of Alfvén wing-like structures.The results of the ion dynamics in Titan's environment are compared with Cassini T9 encounter data (CAPS).

Density of neutral interstellar hydrogen at the termination shock from Ulysses pickup ion observations

Aims. By reevaluating a 13-month stretch of Ulysses SWICS H pickup ion measurements near 5 AU close to the ecliptic right after the previous solar minimum, this paper presents a determination of the neutral interstellar H density at the solar wind termination shock and implications for the density and ionization degree of hydrogen in the local interstellar cloud. Methods. The density of neutral interstellar hydrogen at the termination shock was determined from the local pickup ion production rate as obtained close to the cut-off in the distribution function at aphelion of Ulysses. As shown in an analytical treatment for the upwind axis and through kinetic modeling of the pickup ion production rate at the observer location, with variations in the ionization rate, radiation pressure, and the modeling of the particle behavior, this analysis turns out to be very robust against uncertainties in these parameters and the modeling. Results. Analysis using current heliospheric parameters yields the H density at the termination shock equal to 0.087 ± 0.022 cm −3 , including observational and modeling uncertainties.

Nature of the iogenic plasma source in Jupiter's magnetosphere: II. Near-Io distribution

Three-dimensional calculations are presented for the distribution of the iogenic plasma source that is created near Io (i.e., within ∼24 satellite radii about Io) by O and S gases located in the volume above Io's exobase (i.e., corona and escaping extended neutral clouds, designated as the “Outer Region”) and are complementary to a preceding paper for calculations on a circumplanetary scale. The instantaneous pickup ion production rates for both electron impact and charge exchange have significant radial, north–south, and orbital-plane asymmetries beginning just inside and/or beyond Io's Lagrange sphere (5.81 Io radii) and, within the Lagrange sphere, are distributed nearly symmetrically about Io and are highly peaked about Io's exobase. The spatial natures of the corresponding pickup ion density, mass loading rates, and the pickup ion conductivity, current, and magnetic field are investigated. Spatially integrated rates are calculated for the corona volume and compared to larger Outer Region circumplanetary volumes and are also compared to estimates drawn from the scientific literature (but not modeled here) of the spatially integrated rates for pickup processes in the strongly perturbed “Inner Region” below the exobase. Within the corona volume, the spatially-integrated net-mass loading rate and total (electron impact and charge exchange) mass loading rate are only a factor of ∼3 and ∼5, respectively, smaller than those estimated for the Inner Region, whereas for the whole plasma torus, the Outer Region rates are larger or comparable to those estimated for the Inner Region. The total pickup ion gyration power supplied to the whole plasma torus is estimated to be significantly less than the power radiated by the plasma torus, indicating that an additional power source, likely a circumplanetary distribution of nonthermal electrons, is present.

On radial evolution of stochastic acceleration in a dissipative solar wind turbulence with spatially distributed sources

The spatial behavior of stochastic acceleration of particles in the expanding solar wind plasma is considered. For this, a standard model is used representing a radially evolving, driven solar wind turbulence. On the basis of a turbulence correlation timescale approach, an analytical model is developed describing the radial dependence of the correlation length of the turbulence driven by stream interactions and the production of pickup ions. This analytical model, which might also be beneficial for other interesting problems such as solar modulation of cosmic rays, is used to consider the radial variation of Fermi‐II acceleration of pickup ions in a dissipative, fully driven solar wind turbulence on their way to the heliospheric termination shock. This might be of particular interest for the injection of pickup ions into the process of diffusive shock acceleration and their subsequent conversion to anomalous cosmic rays.

Expected heliospheric attributes of Jovian pickup ions from the extended neutral gas disk

The MIMI CHEMS Instrument on the Cassini Orbiter detected Jovian pickup ions almost an AU upstream of Jupiter during the 2001 flyby. The clue to their planetary origin is the presence of singly ionized sulfur ions in quantities exceeding those expected from the interstellar gas entering the heliosphere (Nature 415 (2002) 994). Earlier modeling of the extended Jovian neutral gas disk suggested how the combination of the orbiting, localized Jovian source and interplanetary ionization processes should combine to produce a distinctive reservoir for heliospheric pickup ion production, different from its interstellar gas counterpart. Here the expected characteristics of pickup ions from the Jovian source are considered using a simplified model. The results provide an idea of the signatures in physical and phase space that reflect both the initial velocities and directionalities of the parent neutral population. Long-term measurements can easily test for these attributes given sufficient spatial and ion energy coverage.

Simulation of the Saturnian magnetospheric interaction with Titan

Flowing plasma interactions with small bodies in the solar system are frequently complicated by the significant size of the ion gyroradii relative to the scale size of the body and the interaction region. This situation often applies to both the flowing ion populations and the atmospheric pickup ions added to the flow in the vicinity of the body. Titan represents an important example of this type of system and is scheduled to be probed in some detail during the Cassini mission tour of Saturn. Initial results from global hybrid (particle ions, fluid electrons) numerical simulations of Saturn's magnetospheric plasma encountering Titan's ionosphere reveal the complexity of the interaction introduced by the ion kinetics. The scale of the interaction region is dominated by the heavy ion gyroradii of the ambient and pickup ions rather than the size of Titan and is found to depend on the mass loading of the magnetospheric flow encountering Titan. Comparison with data from the Voyager 1 flyby suggests that ion kinetics as well as the relative orientations of external plasma ram and pickup ion production regions affect what was observed.

Voyager observations of the magnetic field, interstellar pickup ions and solar wind in the distant heliosphere

Voyagers 1 and 2 are now observing the latitudinal structure of the heliospheric magnetic field in the distant heliosphere (the region between ≃ 30 AU and the termination shock). Voyager 2 is observing the influence of the interstellar medium on the solar wind. The pressure of the interstellar pickup protons, measured by their contribution to pressure balanced structures, is greater than or equal to the magnetic pressure and much greater than the thermal pressures of the solar wind protons and electrons in the distant heliosphere. The solar wind speed is observed to decrease and the proton temperature increase with increasing distance from the sun. This may result from the production of pickup ions by the charge exchange process with the interstellar neutrals. The introduction of the pickup ions into the dynamics of the magnetized solar wind plasma appears to be an important new process which must be considered in future theoretical studies of the termination shock and boundary with the local interstellar medium.

The production of different pick-up ion species in the heliosphere and their convection towards the three-dimensional termination shock

The production of different pick-up ion species in the heliosphere due to local ionization of corresponding neutral gas species entering from the local interstellar medium is investigated. Starting from the usual values for the cosmic elemental abundances and taking into account the most probable conditions for a fractional ionization of the local interstellar medium, we consider first the effect of an element-selective interface transmission in order to achieve values for the inflow of neutral elements into the heliosphere. It is then shown that the element-specific heliospheric distributions of neutral gases in connection with the relevant local ionization rates lead to element-specific upwind-downwind asymmetries in the resulting pick-up ion production rates. Starting from those we calculate spatial heliospheric patterns of pick-up ion fluxes for the different species assuming that these species are essentially convected with the solar wind bulk flow towards the outer heliospheric regions. As we are going to show here, the effects of specific ion drifts and diffusive motions superimposed on the solar wind bulk motion in all cases can be considered as negligible, unless pick-up ion energies at their convection to larger heliospheric distances are substantially increased by factors of the order of 100 or more. We present radial and longitudinal profiles of the different pick-up ion fluxes which, upon their arrival at the termination shock, may serve as measure of production rates for corresponding anomalous cosmic ray species.