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We present a recent analysis with 1996 SOHO CELIAS CTOF data, which reveals correlations of He+ pickup ion fluxes and spectra with the magnetic field strength and solar wind density. The motivation is to better understand the ubiquitous large variations in both pickup ion fluxes and their velocity distributions found in interstellar pickup ion datasets. We concentrate on time periods of that can be associated with compression regions in the solar wind. Along with enhancements of the overall pickup ion fluxes, adiabatic heating and acceleration of the pickup ions are also observed in these regions. Transport processes that lead to the observed compressions and related heating or acceleration are discussed. A shift in velocity space associated with traveling interplanetary compression regions is observed, and a simple model presented to explain this phenomenon based on the conserved magnetic adiabatic moment., Comment: 4 pages, 5 figures, Solar Wind 10 Conference Proceedings Paper

Suprathermal and pickup ion tails exist ubiquitously in slow solar wind. What is the intrinsic property of slow solar wind that causes ubiquitous particle acceleration? Observations from Ulysses have shown that statistical acceleration through transit time damping of fluctuations in field strength is the vehicle by which pickup ion tails are produced. But the source of compressive fluctuations in slow solar wind has not been established. We show that shearing of magnetic fields by small‐scale fluctuations in the solar wind speed provides a strong and continuous source of compressive fluctuations. This effect occurs naturally if slow solar wind is generated from material stored in coronal loops and released due to organized drifts of open magnetic field footpoints on the Sun. Another consequence of these organized footpoint drifts is the creation of large‐scale magnetic field structures that connect fast and slow solar wind — located for example in the outer heliosphere at the latitudinal boundaries betwee...

This paper reviews solar wind observations in the outer heliosphere, concentrating on the recent data near solar maximum. The speed and temperature tend to be lower at solar maximum, due to the lack of coronal holes. The near‐absence of a latitudinal speed gradient at solar maximum allows us to measure the speed decrease of the solar wind and find a value for the H density in the local interstellar medium (LISM) at the termination shock of 0.09 cm−3. The temperature profile is well‐matched by a model using pickup ion heating and a speed dependence of the temperature. The density profile at solar maximum is dominated by MIRs; we show one case where converging CME ejecta form a MIR.

Context. Pickup ions in the inner heliosphere mainly originate in two sources, one interstellar and one in the inner solar system. In contrast to the interstellar source that is comparatively well understood, the nature of the inner source has not been clearly identified. Former results obtained with the Solar Wind Ion Composition Spectrometer on-board the Ulysses spacecraft revealed that the composition of inner-source pickup ions is similar, but not equal, to the elemental solar-wind composition. These observations su ered from very low counting statistics of roughly one C + count per day. Aims. Because the composition of inner-source pickup ions could lead to identifying their origin, we used data from the Charge-TimeOf-Flight sensor on-board the Solar and Heliospheric Observatory. It o ers a large geometry factor that results in about 100 C + counts per day combined with an excellent mass-per-charge resolution. These features enable a precise determination of the inner-source heavy pickup ion composition at 1 AU. To address the production mechanisms of inner-source pickup ions, we set up a toy model based on the production scenario involving the passage of solar-wind ions through thin dust grains to explain the observed deviations of the inner-source PUI and the elemental solar-wind composition. Methods. An in-flight calibration of the sensor allows identification of heavy pickup ions from pulse height analysis data by their mass-per-charge. A statistical analysis was performed to derive the inner-source heavy pickup ion relative abundances of N + , O + , Ne + , Mg + , Mg 2+ , and Si + compared to C + . Results. Our results for the inner-source pickup ion composition are in good agreement with previous studies and confirm the deviations from the solar-wind composition. The large geometry factor of the Charge-Time-of-Flight sensor even allowed the abundance ratios of the two most prominent pickup ions, C + and O + , to be investigated at varying solar-wind speeds. We found that the O + /C + ratio increases systematically with higher solar-wind speeds. This observation is an unprecedented feature characterising the production of inner-source pickup ions. Comparing our observations to the toy model results, we find that both the deviation from the solar-wind composition and the solar-wind-speed dependent O + /C + ratio can be explained.

Context. He+ pickup ions are either born from the ionization of interstellar neutral helium inside our heliosphere, the so-called interstellar pickup ions, or through the interaction of solar wind ions with small dust particles, the so-called inner source of pickup ions. Until now, most observations of pickup ions were limited to reduced 1D velocity spectra, which are insufficient to study certain characteristics of the He+ velocity distribution function (VDF).Aims. It is generally assumed that rapid pitch-angle scattering of freshly created pickup ions quickly leads to a fully isotropic He+ VDF. In light of recent observations, this assumption has found to be oversimplified and needs to be reinvestigated.Methods. Using He+ pickup ion data from the PLASTIC instrument on board the STEREO A spacecraft, we reconstruct a reduced form of the He+ VDF in two dimensions. This allows us to study relative changes of the 2D He+ VDF as a function of the configuration of the heliospheric magnetic field.Results. Our observations show that the He+ VDF is highly anisotropic and even indicates that, at least for certain configurations of B , it is not fully gyrotropic. Our results further suggest, that the observed velocity and pitch angle of He+ depends strongly on the local solar magnetic field vector, B , the ecliptic longitude, λ , the solar wind speed, v sw , and the global distribution of B . Conclusions. We found two distinct signatures that systematically change as a function of the alignment of B : (1) a ring beam distribution that is most pronounced at w sw > 0.5 and likely attributed to interstellar He+ ; (2) a beam signature aligned parallel to B that is most pronounced at w sw . The strong anisotropy and the aforementioned dependencies of the He+ VDF also imply that observations of 1D velocity spectra of He+ pickup ions are potentially deceiving.

Models of the large-scale heliosphere show that the very local interstellar medium (VLISM), bounded by the heliopause and either a bow wave or bow shock, experiences significant heating by the deposition of neutral hydrogen (H) originating from the hot inner heliosheath. As hot neutrals stream into the interstellar medium, they experience charge exchange with the background cooler interstellar protons, creating a population of energetic (~ 1 keV) pickup ions. Similarly, fast neutrals created in the supersonic solar wind stream into the local interstellar medium and create a pickup ion population in the VLISM. The importance of these pickup ions is thought to manifest itself in the creation of the IBEX ribbon. Like the outer regions of the supersonic solar wind and the inner heliosheath, the VLISM is a pickup ion mediated plasma. Here we derive a model of a pickup ion mediated plasma using an approach analogous to a Chapman-Enskog expansion. We derive the anomalous heat flux and obtain a three-fluid model comprising electrons, thermal protons, and pickup ions. We investigate waves in a pickup ion mediated VLISM plasma, comparing the basic properties to those of the better known two-fluid model.

International audience; This paper presents a new approach for calculating the sputtering of an atmosphere by impacting particles when there are molecular species at the exobase. We couple a test particle Monte Carlo approach with a molecular dynamic model to describe the collisions between hot particles and cold molecules in the Martian atmosphere. It is shown that both the heating efficiency and sputtering by pickup ions are reduced from that in an atomic thermosphere. The yields are given for three different ratios of atoms to molecules at the exobase and are used to obtain a better estimate of the total loss of atmosphere due to the pickup ion sputtering. Using the pickup ion fluxes of Zhang et al. [1993], ∼120 mbar of O and ∼60 mbar of CO2 are lost. The loss of O if associated with H2O would be equivalent to ∼4 m averaged over the Mars surface.

[1] Recent spacecraft observations reveal the continuous presence of energetic tails in solar wind and pickup ion spectra in the quiet, slow low-latitude solar wind. To explain this phenomenon, we take into account growing evidence from observations, theory, and simulations that MHD turbulence in the solar wind has a dominant two-dimensional (2-D) component. Of special significance is that 2-D MHD turbulence simulations feature turbulent field-aligned electric fields due to turbulent convection of 2-D magnetic field fluctuations. Such electric fields have been observed in the low-latitude solar wind. We explore pickup ion propagation and acceleration between the Sun and Earth with a newly developed kinetic numerical model. It includes a random distribution of large-scale field-aligned electric field fluctuations consistent with observations in the low-latitude solar wind. The main result is the qualitative reproduction of observed accelerated interstellar pickup He + spectra at Earth. We find that a spectral "knee" connects the pickup ion core with the accelerated "tail." Like observations, the simulated tail is approximately a power law f(v) ν - 4 . 5 until it reaches particle speeds ∼10 times the solar wind speed, where a rollover occurs. The energetic tail particles originate closer to the Sun, where acceleration is most efficient. The knee forms because the local pickup ion source particles are abundant but weakly accelerated. Further predictions, such as large knees for pickup ion species formed from interstellar neutrals with large ionization cavities, and the absence of knees in accelerated spectra of inner source pickup ions are also discussed.

Pickup ions (PUIs) in the outer heliosphere and the local interstellar medium are created by charge exchange between protons and hydrogen (H) atoms, forming a thermodynamically dominant component. In the supersonic solar wind beyond >10 AU, in the inner heliosheath (IHS), and in the very local interstellar medium (VLISM), PUIs do not equilibrate collisionally with the background plasma. Using a collisionless form of Chapman-Enskog expansion, we derive a closed system of multi-fluid equations for a plasma comprised of thermal protons and electrons, and suprathermal PUIs. The PUIs contribute an isotropic scalar pressure to leading order, a collisionless heat flux at the next order, and a collisionless stress tensor at the second-order. The collisionless heat conduction and viscosity in the multi-fluid description results from a non-isotropic PUI distribution. A simpler one-fluid MHD-like system of equations with distinct equations of state for both the background plasma and the PUIs is derived. We investigate linear wave properties in a PUI-mediated three-fluid plasma model for parameters appropriate to the VLISM, the IHS, and the solar wind in the outer heliosphere. Five distinct wave modes are possible: Alfven waves, thermal fast and slow magnetoacoustic waves, PUI fast and slow magnetoacoustic waves, and an entropy mode. The thermal and PUI acoustic modes propagate at approximately the combined thermal magnetoacoustic speed and the PUI sound speed respectively. All wave modes experience damping by the PUIs through the collisionless PUI heat flux. The PUI-mediated plasma model yields wave properties, including Alfven waves, distinctly different from those of the standard two-fluid model.

We discuss microscale fluctuations of the hour averages of the magnetic field B observed on a scale of one day by Voyager 1 (V1) from 2011.0 to 2012.3143 (when it was within the distant heliosheath, where the average magnetic field strength (B) = 0.17 nT) and during the interval from 2012.6503 to 2013.5855 (when it was within the interstellar plasma with (B) = 0.47 nT). In both regions, the fluctuations were primarily compressive fluctuations, varying along the average B (≈T direction in RTN coordinates). In the heliosheath, the average of the daily standard deviations (SDs) of the compressive and transverse components of B were (SD{sub c}) = 0.010 nT and (SD{sub t}) ≤ 0.005 nT (which is the limit of the measurement). In the distant heliosheath (SD{sub c})/(B) = 0.06, and the distributions of SD were skewed and highly kurtotic. The interstellar magnetic field (ISMF) strength was B = 0.48 nT, but the fluctuations were below the limit of measurement: (SD{sub c}) = 0.004 nT and (abs(SD{sub t})) = 0.004 nT. The distributions of these interstellar SDs have skewness and kurtosis consistent with a Gaussian noise distribution. We also discuss the fluctuations of 48 s averages of B on amore » scale of 1 day during a 30 day interval when V1 was observing the ISMF. For the fluctuations in all three components of B, SD = 0.010 nT, which gives an upper limit on the fluctuations of the ISMF on the scales observed by V1. This SD rules out the possibility that there is significant power in electromagnetic fluctuations generated by pickup ion ring instabilities at these scales, which strongly constrains models of the IBEX ribbon.« less

We investigate the signals from neutral He atoms observed from Earth orbit in 2010 by IBEX. The full He signal observed during the 2010 observation season can be explained as a superposition of pristine neutral interstellar He gas and an additional population of neutral He that we call the Warm Breeze. The Warm Breeze is approximately two-fold slower and 2.5 times warmer than the primary interstellar He population, and its density in front of the heliosphere is ~7% that of the neutral interstellar helium. The inflow direction of the Warm Breeze differs by ~19deg from the inflow direction of interstellar gas. The Warm Breeze seems a long-term feature of the heliospheric environment. It has not been detected earlier because it is strongly ionized inside the heliosphere, which brings it below the threshold of detection via pickup ion and heliospheric backscatter glow observations, as well as by the direct sampling of GAS/Ulysses. Possible sources for the Warm Breeze include (1) the secondary population of interstellar helium, created via charge exchange and perhaps elastic scattering of neutral interstellar He atoms on interstellar He+ ions in the outer heliosheath, or (2) a gust of interstellar He originating from a hypothetic wave train in the Local Interstellar Cloud. A secondary population is expected from models, but the characteristics of the Warm Breeze do not fully conform to modeling results. If, nevertheless, this is the explanation, IBEX-Lo observations of the Warm Breeze provide key insights into the physical state of plasma in the outer heliosheath. If the second hypothesis is true, the source is likely to be located within a few thousand of AU from the Sun, which is the propagation range of possible gusts of interstellar neutral helium with the Warm Breeze characteristics against dissipation via elastic scattering in the Local Cloud., submitted to ApJS

We present new calculations of the photoionization of interstellar matter within ~5 pc of the Sun (which we refer to as the Complex of Local Interstellar Clouds or CLIC) by directly observed radiation sources including nearby hot stars and the diffuse emission of the Soft X-ray Background (SXRB). In addition, we model the important, unobserved EUV emission both from the hot gas responsible for the SXRB and from a possible evaporative boundary between the CLIC and the hot gas. We carry out radiative transfer calculations and show that these radiation sources can provide the ionization and heating of the cloud required to match a variety of observations. The ionization predicted in our models shows good agreement with pickup ion results, interstellar absorption line data towards epsilon CMa, and EUV opacity measurements of nearby white dwarf stars. Including the radiation from the conductive boundary improves agreement with data on the temperature and electron density in the cloud. The presence of dust in the cloud, or at least depleted abundances, is necessary to maintain the heating/cooling balance and reach the observed temperature. Using the column density observations as inputs, we derive the gas phase abundances of C, N, O, Mg, Si, S and Fe. Based on these inferred depletions, it appears that silicate and iron dust exists in the CLIC, while carbonaceous dust has been destroyed., 39 pages (incl. figures), 5 figures, AASTeX, accepted for publication in ApJ, some additional discussion and minor corrections added at request of the referee

We have previously calculated the spectrum of anomalous cosmic rays (ACR) by employing the supposed spectrum of interstellar pickup ions as the seed population for a Monte Carlo model of the solar wind termination shock. This pickup ion spectrum was extrapolated from measurements some distance from the shock under the assumption that adiabatic loss was the only energy change process acting prior to reaching the shock. Our results while reasonable in many respects were underabundant in He+ and O+ ions relative to H+ as determined by observation. le Roux and Ptuskin have shown that stochastic preacceleration is more effective for He+ and O+ ions than for H+ ions thereby redressing this underabundance. We have employed the results of le Roux and Ptuskin as input to our previous model and will show to what extent the relative abundances are improved thereby.

Pickup ions, created by ionization of slow moving atoms and molecules well inside the heliosphere, provide us with a new tool to probe remote regions in and beyond the heliosphere and to study injection and acceleration processes in the solar wind. Comprehensive and continuous measurements of H, He, C, N, O, Ne and other pickup ions, especially with the Solar Wind Ion Composition Spectrometer (SWICS) on both Ulysses and ACE, have given us a wealth of data that have been used to infer chemical and physical properties of the local interstellar cloud. With SWICS on Ulysses we discovered a new population of pickup ions, produced from atomic and molecular sources deep inside the heliosphere. The velocity distributions and composition of these “inner source” pickup ions are distinctly different from those of interstellar pickup ions, showing effects of strong adiabatic cooling, and a composition resembling that of the solar wind. Strong cooling indicates that the source of these pickup ions lies close to the Sun. The similarity of composition of inner source heavy ions to that of the solar wind implies that the dominant production mechanism for these pickup ions involves the absorption and re-emission of solar wind from interplanetary dust grains. While interstellar pickup ions are the seed population of the main Anomalous Cosmic Rays (ACRs), inner source pickup ions may be an important source of the rarer ACRs such as C, Mg, Si, S, and Fe. We present new results and review previous work with an emphasis on characteristics of the local interstellar cloud and properties of the inner source.

The objective of our work was to study latitudinal and time dependent effects in H+ pickup ion fluxes, potentially interesting with respect to Ulysses data analysis. Assuming models of radiation pressure and hydrogen ionisation rates based on long-term, multispacecraft observations (Bzowski, 2001a, b; and references therein), we calculate a time- and latitude-dependent distribution of interstellar neutral hydrogen in a plane perpendicular to the gas inflow direction. Based on this density model we compute expected fluxes of H+ pickup ions (H+ PUI) during various phases of recent solar cycles at various heliographic latitudes to check the amplitude of both types of variations at heliocentric distances relevant for Ulysses/SWICS observations (Gloeckler et al., 1993; Gloeckler, 1996a).

A self-consistent model is presented that gives a unified, fully theoretical description of pickup ion (PUI) and anomalous cosmic ray (ACR) proton transport in the near ecliptic regions of the upwind heliosphere. Unification is achieved through multiply reflected ion (MRI) acceleration theory which is used for the injection of pickup protons into the process of diffusive shock acceleration at the quasi-perpendicular, supercritical termination shock (TS). The results show that MRI accelerated pickup protons strongly mediate the TS while anomalous protons also significantly modify it. Although this lowers the cutoff speed of the MRI spectrum, a sufficient population of anomalous protons is formed at the TS to give a favorable comparison of modulated spectra upstream with observations at large heliocentric distances.

International audience; Kinetic simulations of collisionless shocks have provided a wealth of information on injection and acceleration of thermal ions into a diffusive acceleration process. At quasi-parallel shocks upstream diffuse ions and the induced upstream turbulence are an integral part of the collisionless shock structure. Before injected into a diffusive acceleration process thermal ions are trapped near the shock and are accelerated to higher energies. The injection and acceleration process for thermal ions at quasi-perpendicular shocks depends on the possibility of these ions to recross the shock many times. A viable mechanism for injection is cross-field diffusion of the specularly reflected ions after they have crossed the shock into the downstream region. Determination of the cross-field diffusion coefficient in strong turbulence suggests that specularly reflected ions can recross the quasi-perpendicular shock and can get further accelerated. At more oblique shocks the same injection process as at quasi-parallel shocks can work: particles gain high enough velocities during their first shock encounter so that they can escape the shock along the magnetic field in the upstream direction. Because of the form of the pickup ion distribution in velocity space there seems to be no problem for accelerating these ions at either quasi-parallel, quasi-perpendicular, or perpendicular shocks.

The interaction of interstellar pickup ions with the solar wind termination shock is reviewed and assessed. The pickup ions mass and momentum load the wind and increase its pressure, effects which decrease the strength of the shock and its distance from the Sun. The pickup hydrogen may contribute substantially to the “reflected” ion population, which should provide most of the dissipation at the supercritical quasi-perpendicular shock. A fraction of the pickup ions impinging on the shock is “injected” into the process of diffusive shock acceleration to form the anomalous cosmic ray component. An injection mechanism which accounts for the apparent absence of solar wind ions in the anomalous component is “shock surfing”, in which pickup ions which approach the shock slowly may be trapped between the upstream Lorentz force and the shock potential and accelerated in the motional electric field beyond the energy threshold for diffusive shock acceleration. However, the simplest interpretation of shock surfing would favor less massive pickup ion species, in contradiction with Voyager observations of anomalous component composition. A possible extension of the shock surfing mechanism is considered, as well as other injection mechanisms. Finally, the pressure of the anomalous component may modify the structure of the termination shock, which in turn may influence injection rates.

Fluxes of interstellar pickup helium have recently been observed to be strongly anti-correlated with solar wind speed in a manner not explained by the standard picture of local pickup and isotropization. We investigate the possibility that spatial diffusion of these ions between solar wind streams could lead to the observed substantial flux changes. However, we find that such spatial transport produces higher fluxes in fast solar wind, and would yield a dependence which is opposite to that observed.

The interstellar cloud surrounding the solar system regulates the galactic environment of the Sun, and determines the boundary conditions of the heliosphere. Both the Sun and interstellar clouds move through space, so these boundary conditions change with time. Data and theoretical models now support densities in the cloud surrounding the solar system of n(H0) = 0.22 ± 0.06 cm-3, and n(e-) ∼.0.1 cm-3, with larger values allowed for n(H0) by radiative transfer considerations. Ulysses and Extreme Ultraviolet Explorer satellite He0 data yield a cloud temperature of 6400 K. Nearby interstellar gas appears to be structured and inhomogeneous. The interstellar gas in the Local Fluff cloud complex exhibits elemental abundance patterns in which refractory elements are enhanced over the depleted abundances found in cold disk gas. Within a few parsecs of the Sun, inconclusive evidence for factors of 2–5 variation in Mg+ and Fe+ gas phase abundances is found, providing evidence for variable grain destruction. In principle, photoionization calculations for the surrounding cloud can be compared with elemental abundances found in the pickup ion and anomalous cosmic-ray populations to model cloud properties, including ionization, reference abundances, and radiation field. Observations of the hydrogen pile up at the nose of the heliosphere are consistent with a barely subsonic motion of the heliosphere with respect to the surrounding interstellar cloud. Uncertainties on the velocity vector of the cloud that surrounds the solar system indicate that it is uncertain as to whether the Sun and α Cen are or are not immersed in the same interstellar cloud.

The Solar Wind Ion Composition Spectrometer (SWICS) and the Solar Wind Ions Mass Spectrometer (SWIMS) on ACE are instruments optimized for measurements of the chemical and isotopic composition of solar and interstellar matter. SWICS determines uniquely the chemical and ionic-charge composition of the solar wind, the thermal and mean speeds of all major solar wind ions from H through Fe at all solar wind speeds above 300 kms-1 (protons) and 170 km s-1 (Fe+16), and resolves H and He isotopes of both solar and interstellar sources. SWICS will measure the distribution functions of both the interstellar cloud and dust cloud pickup ions up to energies of 100 keV e-1. SWIMS will measure the chemical, isotopic and charge state composition of the solar wind for every element between He and Ni. Each of the two instruments uses electrostatic analysis followed by a time-of-flight and, as required, an energy measurement. The observations made with SWICS and SWIMS will make valuable contributions to the ISTP objectives by providing information regarding the composition and energy distribution of matter entering the magnetosphere. In addition, SWICS and SWIMS results will have an impact on many areas of solar and heliospheric physics, in particular providing important and unique information on: (i) conditions and processes in the region of the corona where the solar wind is accelerated; (ii) the location of the source regions of the solar wind in the corona; (iii) coronal heating processes; (iv) the extent and causes of variations in the composition of the solar atmosphere; (v) plasma processes in the solar wind; (vi) the acceleration of particles in the solar wind; (vii) the physics of the pickup process of interstellar He in the solar wind; and (viii) the spatial distribution and characteristics of sources of neutral matter in the inner heliosphere.

Solar wind plasma and magnetic field measurements by Ulysses have been used to study magnetohydrodynamic turbulence in different heliospheric regions. Four intervals of six solar rotations have been analyzed. Two of them are on the ecliptic around 2 and 5 AU, respectively, one is at midlatitude near 5 AU, and the last one is at high latitude around 3 AU. Conditions on the ecliptic are those typical of high solar activity periods. The midlatitude interval is characterized by very strong gradients in the wind speed, due to an intermittent appearance of the wind coming from the polar coronal hole. In the high-latitude interval, fully inside the polar wind, the speed is steadily high. We investigated at three different scales (1, 4, and 12 hours) the level of correlation between velocity and magnetic field fluctuations, as given by the normalized cross-helicity, and the sharing of the fluctuation energy between its kinetic and magnetic component, as measured by the normalized residual energy. The observations on the ecliptic, while confirming previous findings based on Voyagers data, clearly indicate that the normalized cross-helicity is well different from zero also at distances as large as 5 AU. The midlatitude turbulence, when compared to that at low and high heliographic latitudes, appears much more evolved, with a remarkably lower normalized cross-helicity (in absolute value). This unambiguously highlights that processes at velocity gradients are an important factor in the turbulence evolution. For all the analyzed intervals the residual energy values indicate an imbalance in favor of magnetic fluctuations, in agreement with previous results. The strongest imbalance is observed for the high-latitude sample, where the turbulence is comparatively the least evolved. This is a quite unexpected result, probably related to the presence of interstellar pickup ion populations. In conclusion, our analysis indicates that (1) velocity gradients play a dominant role in driving the turbulence evolution in the solar wind and (2) pickup ion effects might be significant.

The solar system is moving through the partially ionized local interstellar cloud (LIC). The ionized matter of the LIC interacts with the expanding solar wind forming the heliospheric interface. The neutral component (interstellar atoms) penetrates through the heliospheric interface into the heliosphere, where it is measured directly "in situ" as pick-up ions and neutral atoms (and as anomalous cosmic rays) or indirectly through resonant scattering of solar Ly-alpha. When crossing the heliospheric interface, interstellar atoms interact with the plasma component through charge exchange. This interaction leads to changes of both neutral gas and plasma properties. The heliospheric interface is also the source of radio emissions which have been detected by the Voyager since 1983. In this paper, we have used a kinetic model of the flow of the interstellar atoms with updated values of velocity, temperature, and density of the circumsolar interstellar hydrogen and calculated how all quantities which are directly associated to the observations vary as a function of the interstellar proton number density. These quantities are the degree of filtration, the temperature and the velocity of the interstellar H atoms in the inner heliosphere, the distances to the bow shock (BS), heliopause, and termination shock, and the plasma frequencies in the LIC, at the BS and in the maximum compression region around the heliosphere which constitutes the "barrier" for radio waves formed in the interstellar medium. Comparing the model results with recent pickup ion data, Ly- alpha measurements, and low-frequencies radio emissions, we have searched for a number density of protons in the local interstellar cloud compatible with all observations., Comment: 14 pages, 5 figures, JGR-Space Physics, accepted, AGU preprint style

We have modeled the injection and acceleration of pickup ions at the solar wind termination shock and investigated the parameters needed to produce the observed Anomalous Cosmic Ray (ACR) fluxes. A non-linear Monte Carlo technique was employed, which in effect solves the Boltzmann equation and is not restricted to near-isotropic particle distribution functions. This technique models the injection of thermal and pickup ions, the acceleration of these ions, and the determination of the shock structure under the influence of the accelerated ions. The essential effects of injection are treated in a mostly self-consistent manner, including effects from shock obliquity, cross-field diffusion, and pitch-angle scattering. Using recent determinations of pickup ion densities, we are able to match the absolute flux of hydrogen in the ACRs by assuming that pickup ion scattering mean free paths, at the termination shock, are much less than an AU and that modestly strong cross-field diffusion occurs. Simultaneously, we match the flux ratios He$^{+}$/H$^{+}$ or O$^{+}$/H$^{+}$ to within a factor $\sim 5$. If the conditions of strong scattering apply, no pre-termination-shock injection phase is required and the injection and acceleration of pickup ions at the termination shock is totally analogous to the injection and acceleration of ions at highly oblique interplanetary shocks recently observed by the Ulysses spacecraft. The fact that ACR fluxes can be modeled with standard shock assumptions suggests that the much-discussed "injection problem" for highly oblique shocks stems from incomplete (either mathematical or computer) modeling of these shocks rather than from any actual difficulty shocks may have in injecting and accelerating thermal or quasi-thermal particles., Comment: 15 pages including 1 table and 10 figures, AASTeX emulateapj format, accepted for publication in The Astrophysical Journal

Recent observations from the Voyager 1 spacecraft show that it is sampling the local interstellar medium (LISM). This is quite surprising because no realistic, steady-state model of the solar wind (SW) interaction with the LISM gives an inner heliosheath width as narrow as ~30 AU. This includes models that assume a strong redistribution of the ion energy to the tails in the pickup ion distribution function. We show that the heliopause (HP), which separates the SW from the LISM, is not a smooth tangential discontinuity, but rather a surface subject to Rayleigh-Taylor-type instabilities which can result in LISM material penetration deep inside the SW. We also show that the HP flanks are always subject to a Kelvin-Helmholtz instability. The instabilities are considerably suppressed near the HP nose by the heliospheric magnetic field in steady-state models, but reveal themselves in the presence of solar cycle effects. We argue that Voyager 1 may be in one such instability region and is therefore observing plasma densities much higher than those in the pristine SW. These results may explain the early penetration of Voyager 1 into the LISM. They also show that there is a possibility that the spacecraft may start sampling the SW again before it finally leaves the heliosphere.

Energetic neutral atoms (ENAs) observed by the Interstellar Boundary Explorer (IBEX) provide powerful diagnostics about the origin of the progenitor ion populations and the physical mechanisms responsible for their production. In this paper, we extend the work of Desai et al. and Fuselier et al. and combine and compare ENA spectra from the first 3 yr of observations by the IBEX-Hi and -Lo ENA imagers along the lines-of-sight (LOSs) from the inner heliosphere through to the locations of Voyager 1 and 2 with results from an updated physics-based model of the three-dimensional heliosphere and its constituent ion populations. Our results show that (1) IBEX ENA fluxes and spectra above ∼0.7 keV measured along the LOSs of the Voyagers are consistent with several models in which the parent pickup ion (PUI) populations originate in the inner heliosheath, and (2) a significant fraction of lower energy ENAs between ∼0.1-0.5 keV may originate from interstellar neutral gas charge-exchanging with a non-thermalized (hot) population of PUIs in the outer heliosheath beyond the heliopause. We discuss the implications of ENAs observed by IBEX originating from distinct parent populations as well as from two distinct locations in the heliospheric interface. These results indicate that ENA spectralmore » measurements at various energies can be used to remotely probe distinct physical processes operating in vastly different regions of the distant heliosphere.« less

The focused transport theory is appropriate to describe the injection and acceleration of low-energy particles at shocks as an extension of diffusive shock acceleration (DSA). In this investigation, we aim to characterize the role of cross-shock potential (CSP) originated in the charge separation across the shock ramp on pickup ion (PUI) acceleration at various types of shocks with a focused transport model. The simulation results of energy spectrum and spatial density distribution for the cases with and without CSP added in the model are compared. With sufficient acceleration time, the focused transport acceleration finally falls into the DSA regime with the power-law spectral index equal to the solution of the DSA theory. The CSP can affect the shape of the spectrum segment at lower energies, but it does not change the spectral index of the final power-law spectrum at high energies. It is found that the CSP controls the injection efficiency which is the fraction of PUIs reaching the DSA regime. A stronger CSP jump results in a dramatically improved injection efficiency. Our simulation results also show that the injection efficiency of PUIs is mass-dependent, which is lower for species with a higher mass. In addition, the CSP is able to enhance the particle reflection upstream to produce a stronger intensity spike at the shock front. We conclude that the CSP is a non-negligible factor that affects the dynamics of PUIs at shocks.

Over the last 10 years the experimental basis for the study of the very local interstellar medium (VLISM) has been substantially broadened by the direct detection of pickup ions and of neutral helium. The strength of these methods lies in the local measurement of the particles. By scanning the gravitational focusing cone of the interstellar wind, a consistent set of interstellar helium parameters, neutral density, temperature and relative velocity, has been derived. However, the accuracy of these parameters is still hampered by uncertainties in some of the crucial ionization rates and in the pickup ion transport. Recent observations have shown that the scattering mean free path of pickup ions is comparable with the large scale variation of the interplanetary magnetic field (IMF) in the inner heliosphere. This requires a substantial modification in the modeling of the ion distribution and more detailed measurements, tasks that can be addressed in the near future.

The modulation of galactic cosmic rays and other energetic particles by the solar wind produces a gradient in their pressure, which in turn influences the wind dynamics. The basic equations describing this interaction are presented in the 'hydrodynamic' approximation. A perturbation solution of the equations is presented for the case in which both the galactic cosmic ray pressure, and the pressure of the anomalous cosmic ray component accelerated at the wind termination shock, are small compared with the wind ram pressure. Analytical expressions for the deceleration of the wind and the modification of the shock and its location are derived in this case. These effects are estimated to have a relative magnitude of several percent in the solar wind. The interstellar neutral gas which penetrates the heliosphere is ionized, predominantly by photoionization and charge exchange with the wind, and may also have a significant dynamical effect on the wind. The basic equations describing this interaction are also presented, A perturbation solution is presented under the assumption that the mass loading and momentum-loading of the wind by the interstellar pickup ions is small compared with the wind ram pressure. Analytical expressions for the deceleration of the wind, the contribution of the pickup ions to the wind pressure, and the modification of the termination shock location are derived. Again, with the exception of pickup ion pressure which is large compared with solar wind thermal pressure, the effects are estimated to be several percent in relative magnitude in the solar wind.

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.

Recently, it was demonstrated that stochastic acceleration of particles going through a series of compressive plasma waves can be efficient and fast. It could be too fast so that the pressure built up by the accelerated particles may in turn modify the amplitude of waves to prevent the particles from having an exploding pressure. We call this condition pressure balance. In this paper, we take into account the fact that active acceleration of particles only occupies a limited volume of space due to a possible intermittent nature of plasma waves or turbulence. We develop a bimodal acceleration theory that treats the populations of particles in the active and inactive acceleration regions separately and allows the two populations to exchange particles efficiently. Under the requirement of the pressure balance condition, we show that the system automatically produces a solution of v –5 steady state distribution for the accelerated particles. It is found that the v –5 distribution is more robust and easier to achieve with a small volume of intense particle acceleration. These properties explain why the v –5 distribution is commonly observed in space. We apply our model to pickup ion propagation and acceleration throughout the entire heliosphere. Our results can reproduce various observations in some great detail. We also found that this mechanism could be responsible for producing anomalous cosmic rays deep in the heliosheath.

Energetic neutral atoms (ENAs) observed by the Interstellar Boundary Explorer (IBEX) provide powerful diagnostics about the origin of the progenitor ion populations and the physical mechanisms responsible for their production. Here we survey the fluxes, energy spectra, and energy dependence of the spectral indices of ~0.5-6?keV ENAs measured by IBEX-Hi along the lines of sight of Voyager 1 and 2. We compare the ENA spectra observed at IBEX with predictions of Zank et?al. who modeled the microphysics of the heliospheric termination shock to predict the shape and relative contributions of three distinct heliosheath ion populations. We show that (1) the ENA spectral indices exhibit similar energy dependence along V1 and V2 directions?the spectrum hardens to ? ~ 1 between ~1 and 2?keV and softens to ? ~ 2 below ~1?keV and above ~2?keV, (2) the observed ENA fluxes agree to within ~50% of the Zank et?al. predictions and are unlikely to be produced by core solar wind (SW) ions, and (3) the ENA spectra do not exhibit sharp cutoffs at ~twice the SW speed as is typically observed for shell-like pickup ion (PUI) distributions in the heliosphere. We conclude that ENAs at IBEX are generated by at least two types of ion populations whose relative contributions depend on the ENA energy: transmitted PUIs in the ~0.5-5?keV energy range and reflected PUIs above ~5?keV energy. The ~0.5-5?keV PUI distribution is probably a superposition of Maxwellian or kappa distributions and partially filled shell distributions in velocity space.

This special supplement issue of the Astrophysical Journal comprises six coordinated papers that provide the first detailed analyses of the direct sampling of interstellar neutral atoms by the Interstellar Boundary Explorer (IBEX). Interstellar atoms are the detritus of older stars—their stellar winds, novae, and supernovae—spread across the galaxy, which fill the vast interstellar space between the stars. The very local interstellar medium around the Sun is filled with both ionized and neutral atoms with approximately equal numbers, and occasional ionization, charge exchange, and recombination makes them a single interacting material over large distances. IBEX (McComas et al. 2009a) is a NASA Small Explorer mission with the sole, focused science objective to discover the global interaction between the solar wind and the interstellar medium; this objective has primarily been achieved by taking the first global energetic neutral atom (ENA) images, which provide detailed ENA fluxes and energy spectra over all look directions in space. IBEX was launched 2008 October 19 and subsequently maneuvered into a high-altitude, highly elliptical (~15,000 × 300,000 km), roughly week-long orbit. The payload comprises two very high sensitivity, single-pixel ENA cameras: IBEX-Hi (Funsten et al. 2009a), which measures ENAs from ~300 eV to 6 keV, and IBEX-Lo (Fuselier et al. 2009a), which measures ENAs from ~10 eV to 2 keV. The initial IBEX ENA results were published together in a special issue of Science magazine (McComas et al. 2009b; Funsten et al. 2009b; Fuselier et al. 2009b; Schwadron et al. 2009). Since then there have been numerous additional studies of the IBEX ENA observations of the heliosphere, as well as ENAs from the Moon and Earth's magnetosphere (see recent review by McComas et al. 2011 and references therein). Prior to IBEX, the only interstellar neutral atoms to be directly sampled were He, observed by the Ulysses spacecraft a decade ago (Witte et al. 1996; Witte 2004). The first paper published on IBEX observations of interstellar neutral atoms (Mobius et al. 2009) used observations from the spring of 2009, shortly after IBEX achieved its first long-term orbit; that study showed that IBEX is able to directly observe interstellar H and O in addition to He, but provided only limited analysis of these measurements. IBEX has now completed a second full annual season of neutral observations in 2010, which together with the independent 2009 observations provide data adequate to carry out the first round of detailed, quantitative analyses of the IBEX interstellar neutral observations. In this special supplement issue, the IBEX Science Team presents a coordinated series of six articles that focus on various synergistic aspects of these observations, their analyses, and their implications. A critical foundational paper in this volume, Hlond et al. (2012), analyzes the angular pointing knowledge of IBEX observations and demonstrates that the arrival direction knowledge of neutral atoms can be determined to ~0.1° in both spin angle and elevation. This is no mean feat for a Small Explorer mission designed to measure ENAs in 7° × 7° pixels, and largely at much higher energies than the direct interstellar neutrals. In addition, these authors demonstrate that the in-space (post-launch) bore sight of the IBEX-Lo instrument can achieve this accuracy with either the spacecraft's on board attitude control system or an independent Star Sensor that was designed and built directly into the IBEX-Lo instrument. Lee et al. (2012) derive the analytical solution for the hyperbolic trajectories of individual neutral atoms by using Liouville's theorem, including solar gravity and radiation pressure, photoionization and charge exchange, to produce interstellar neutral atom phase-space distributions. These distributions are then transformed into the IBEX reference frame and integrated over the IBEX-Lo instrumental acceptance to provide an analytic solution for the predicted fluid moments of the interstellar neutral atom distributions. This analytic solution for the interstellar neutral parameters provides the basis for a companion paper by Mobius et al. (2012), who analyze the IBEX He (and Ne+O) measurements using the Lee et al. analytic solutions. This approach allows for physical insights into the dominant physical processes, while in another related paper Bzowski et al. (2012) describe a detailed forward model of the interstellar helium from the edge of the heliosphere all the way through the IBEX instrument geometry. Together, these papers show that the prior values for the interstellar flow speed and direction from Ulysses are inconsistent with our new IBEX observations. Mobius et al. (2012) compare the He and O+Ne flow distributions for both 2009 and 2010 and find interstellar flow parameters of ecliptic longitude at ∞ = 79.0° + 3.0°/–3.5°, ecliptic latitude at ∞ = –4.9° ± 0.2°, ISM speed at ∞ = 23.5 + 3.0/–2.0 km s–1, and neutral He temperature = 5000-8200 K. They also find a combined O+Ne temperature of 5300-9000 K, consistent with an isothermal medium for He, O, and Ne. Bzowski et al. (2012) develop and extensively test a detailed forward model simulation of the interstellar He propagation, losses, and measurement in the IBEX-Lo instrument. These simulations start particles at 150 AU and include more detailed physics than the analytic solutions; they therefore complement the analytic method by allowing detailed mapping of the multi-dimensional space of possible solutions. These authors show that the IBEX results are not in statistical agreement with the Ulysses values and provide new best-fit values of ecliptic longitude 79.2°, ecliptic latitude of –5.1°, speed of ~22.8 km s–1, and He temperature is 6200 K. The values obtained with both complementary methods agree with each other and are in agreement with the flow vector of the local interstellar cloud obtained from studies of interstellar absorption (Redfield & Linsky 2008). Bzowski et al. also show evidence for a previously unknown and unanticipated secondary population of helium. Together, the Mobius et al. (2012) and Bzowski et al. (2012) results provide a new interstellar flow direction and a significantly lower velocity of the incoming gas and therefore significantly lower dynamic pressure on the heliosphere, which translates into a heliospheric interaction that is even less dominated by the external dynamic pressure and clearly lies squarely in the middle ground of astrospheres dominated by the external magnetic and dynamic pressures (McComas et al. 2009b). On another topic, Bochsler et al. (2012) report the first direct measurements of interstellar Ne and estimate the interstellar Ne/O abundance ratio, showing a gas-phase Ne/O ratio for the LISM of 0.27 ± 0.10. This value agrees with results obtained from pickup ion observations (Gloeckler & Geiss 2004; Gloeckler & Fisk 2007) and is significantly larger than the solar abundance ratio, indicating that the LISM is different than the Sun's formation region and/or that a substantial portion of the O in the LISM is tied up (and thus "hidden") in grains and/or ices. Finally, Saul et al. (2012) provide the first detailed analysis of the new interstellar H measurements from IBEX. These authors confirm that the arrival direction of interstellar H is offset from that of He. They further show a variation in the strength of the radiation pressure and thus a change in the apparent arrival direction of H penetrating to 1 AU between the first two years of IBEX observations; these results are consistent with solar cycle variations in the radiation pressure, which works opposite to the Sun's gravitational force to effect the penetration of H into the inner heliosphere. Together, these six studies provide the first detailed analyses of the multi-component local interstellar medium—a medium that both effects us by bounding and interacting with our heliosphere, and a medium that gives us a first direct glimpse of non-solar material from the rest of the galaxy.

[1] Recent efforts to improve and complete modulation theory are described, emphasizing factors that arise in attempts to understand the diffusion tensor based upon turbulence theory and the theory of charged particle scattering. Direct numerical solutions of the transport equations are used to study sensitivity of cosmic ray modulation to the correlation length. It is shown that modulation is strongly influenced by the radial variation of the correlation length. This variation is still poorly understood, in part owing to the uncertain impact of pickup ion driven turbulence in the outer heliosphere and to practical difficulties in measuring the parallel correlation length in the outer heliosphere.

We have used a one-dimensional hybrid code (macro-ions, massless electron fluid) in order to study the interaction of an interstellar pickup ion distribution with the heliospheric termination shock. The shock is generated by reflecting the solar plasma at a rigid wall. The interstellar pickup ions are additional populations modeled as spheres in velocity space with radii given by the solar wind speed, comoving with the solar wind. The pickup ions are located on the outer shell of the spheres. We have determined the reflection rates of the pickup ions, which have been defined as the ratio of the reflected to the incident pickup ions at the shock. The dependence of these reflection rates on the shock normal angle Θ Bn and on an upstream imposed turbulence has been investigated. No backscattered pickup ions are found for Θ Bn greater than 70° and the reflection rates decrease with increasing level of upstream imposed turbulence. The dependence of the reflection rate on the ratio of the upstream pickup proton density to the upstream solar wind density has been investigated. Since this ratio is proportional to the heliospheric distance of the termination shock we have been able to investigate the reflection rates of the pickup ions at different heliospheric locations of the termination shock. Based on these hybrid simulations a model for the acceleration of anomalous component of the cosmic rays has been developed which is able to explain the differential flux of anomalous helium at the termination shock needed in modulation calculations to fit observations in the inner heliosphere. In this model we have not included the influence of the anomalous component on the shock structure. From comparison with modulation calculations it is concluded that the location of the termination shock is at distances between 80 and 120 AU.

We present an analytical model for the density variations of interstellar pickup ions in the solar wind due to a time-dependent variation in the photoionization rate of the interstellar atoms. Following a sudden increase in the photoionization rate, our model predicts a pickup ion density enhancement lasting for a time of the order of the duration of the increase plus the solar wind convection time to the observation point. If the photoionization rate returns to its initial value, this enhancement is followed by a decreased pickup ion density resulting from a depleted interstellar neutral particle density. In the absence of further variations in the photoionization rate, the pickup ion density recovers on a time which scales as the radial position of the observation point divided by the inflow speed of the neutral particles. Gradual variations in the photoionization rate result in a pickup ion density which tends to track the ionization rate, though the density variations are smoothed and delayed in time due to the solar wind convection of ions picked up at points closer to the Sun.

Hybrid (kinetic ion/fluid electron) simulations have been used to study self-consistently the injection and initial stages of acceleration of interstellar pickup ions at the solar wind termination shock. Results are presented from one-dimensional simulations of high Mach number oblique (40 deg-50 deg) shocks with a 10% population of interstellar pickup hydrogen. In these simulations, the pickup ions, the solar wind ions, the shock fields and the waves are all treated self-consistently. Pickup ions reflected by the shock excite large amplitude (DELTA.B / B approximately 0.3) upstream magnetosonic waves. These waves, in turn, scatter the pickup hydrogen, as expected in the diffusive shock acceleration process. The spectrum of excited waves broadens in time. We find that, for the parameters studied, the termination shock efficiently injects and accelerates the interstellar pickup hydrogen, and thus this work represents a microscopic verification of this anomalous cosmic ray injection mechanism. Comparison of injected fluxes from the simulations with Voyager fluxes extrapolated to the termination shock suggests that injection at the termination shock is probably greater than injection by interplanetary shocks. A study of accelerated pickup ion orbits shows that the energy gain comes predominantly from shock drift acceleration in the shock front, with the upstream waves aiding the acceleration by allowing multiple encounters with the shock.

The deposition of energy into the solar wind by the pickup of interstellar neutrals is due to both the creation of hot, nonthermal ions and the associated generation of low frequency magnetohydrodynamic (MHD) waves. Dissipation of some fraction of the free wave energy released by ion pickup and isotropization is possible through nonlinear turbulent processes which may lead to heating of the core thermal solar wind proton distribution. Simple energy budget arguments are utilized to show that pickup ion generated wave dissipation may play a significant role in determining the solar wind radial temperature profile in the outer heliosphere. In particular, depending on the density of interstellar hydrogen in the heliosphere, there will be some radial distance beyond which the thermal solar wind core temperature increases steadily until the termination shock. Existing Pioneer and Voyager temperature profiles are consistent with this interpretation.

Acceleration of interstellar pickup H(+) and He(+) as well as of solar wind protons and alpha particles has been observed on Ulysses during the passage of a corotating interaction region (CIR) at approximately 4.5 AU. Injection efficiencies for both the high thermal speed interstellar pickup ions (H(+) and He(+)) and the low thermal speed solar wind ions (H(+) and He(++) are derived using velocity distribution functions of protons, pickup He(+) and alpha particles from less than 1 to 60 keV/e and of ions (principally protons) above approximately 60 keV. The observed spatial variations of the few keV and the few hundred keV accelerated pickup protons across the forward shock of CIR indicate a two stage acceleration mechanism. Thermal ions are first accelerated to speeds of 3 to 4 times the solar wind speed inside the CIR, presumably by some statistical mechanism, before reaching higher energies by a shock acceleration process. Our results also indicate that (1) the injection efficiencies for pickup ions are almost 100 times higher than they are for solar wind ions, (2) pickup H(+) and He(+) are the two most abundant suprathermal ion species and they carry a large fraction of the particle thermal pressure, (3) the injection efficiency is highest for protons, lowest for He(+), and intermediate for alpha particles, (4) both H(+) and He(+) have identical spectral shapes above the cutoff speed for pickup ions, and (5) the solar wind frame velocity distribution function of protons has the form F(w) = F(sub o)w(sup -4) for 1 is less than w is less than approximately 5, where w is the ion speed divided by the solar wind speed. Above w approximately 5-10 the proton spectrum becomes steeper. These results have important implications concerning acceleration of ions by shocks and CIRs, acceleration of anomalous cosmic rays, and particle dynamics in the outer heliosphere.

The shape of the velocity distribution of water group ions observed by the Giotto ion mass spectrometer on its approach to comet Halley is modeled to derive empirical values for the rates of ionization, energy diffusion, and loss in the midcometosheath. The model includes the effect of rapid pitch angle scattering into a bispherical shell distribution as well as the effect of the magnetization of the plasma on the charge exchange loss rate. It is found that the average rate of ionization of cometary neutrals in this region of the cometosheath appears to be of the order of a factor 3 faster than the `standard' rates approx. 1 x 10(exp -6)/s that are generally assumed to model the observations in most regions of the comet environment. For the region of the coma studied in the present work (approx. 1 - 2 x 10(exp 5) km from the nucleus), the inferred energy diffusion coefficient is D(sub 0) approx. equals 0.0002 to 0.0005 sq km/cu s, which is generally lower than values used in other models. The empirically obtained loss rate appears to be about an order of magnitude greater than can be explained by charge exchange with the `standard' cross section of approx. 2 x 10(exp -15)sq cm. However such cross sections are not well known and for water group ion/water group neutral interactions, rates as high as 8 x 10(exp -15) sq cm have previously been suggested in the literature. Assuming the entire loss rate is due to charge exchange yields a rate of creation of fast neutral atoms of the order of approx. 10(exp -4)/s or higher, depending on the level of velocity diffusion. The fast neutrals may, in turn, be partly responsible for the higher-than-expected ionization rate.

We present and discuss results of a new model for ion injections and acceleration at quasi-perpendicular collisionless shocks. We use the one-dimensional hybrid simulation (kinetic ions/fluid electrons) and impose an assumption on the ion motion so that diffusion across the magnetic field (normal to the shock front) is possible. These motions are otherwise suppressed by both one- and two-dimensional simulations. We find that, even in strictly perpendicular shocks, when scattering normal to the field is included, a fraction of the incident ions are accelerated to suprathermal energies. When reasonable scattering times are considered, only pickup ions are injected, whereas thermal solar wind ions are not. The acceleration of these ions is very rapid. We have found that a few of the initially low-energy pickup ions can reach many tens to a few hundred times the plasma ramming energy in less than 100 gyroperiods. Furthermore, highly field aligned energetic ions are found to exist upstream of the slightly oblique shock. The most direct applications of this study is toward the interpretation of observations of both solar wind and interstellar pickup ion distributions in the vicinity of interplanetary shocks which are most often quasi-perpendicular. This work also directly addresses a fundamental issue with regard to our current understanding of the anomalous component of cosmic rays.

The evolution of the Martian atmosphere, particularly with regard to water, is influenced by (1) “nonthermal” escape of oxygen atoms created by dissociative recombination and (2) by oxygen ion pickup by the solar wind. Both processes depend on the intensity of solar EUV radiation, which affects atmosphere temperatures (scale heights) and photoionization rates, and thereby the exosphere and the fluxes of escaping atoms and ions. This study involves the calculation, by the two-stream model method of Nagy and Cravens (1988), of the exospheric hot oxygen densities for “ancient” atmospheres and ionospheres (e.g., for different EUV fluxes), and the associated neutral escape fluxes. The ion production rates above nominal ionopause altitudes are also computed and are considered to be the upper limits to losses by direct solar wind pickup. Since we do not consider the pickup ion precipitation and additional neutral escape due to the sputtering process described by Luhmann and Kozyra (1991), the results presented here represent conservative estimates of the neutral escape fluxes, but somewhat generous estimates of ion loss rates. We find that when the inferred increased solar EUV fluxes of the past are taken into account, oxygen equivalent to that in several tens of meters of water, planet-wide, should have escaped to space over the last 3 Gyr.

Hybrid (kinetic ions/fluid electrons) plasma simulations are used to study the effects of a population of energetic interstellar pickup hydrogen ions on the solar wind termination shock. The pickup hydrogen is treated as a second ion species in the simulations, and thus the effects of the pick-ups on the shock, as well as the effects of the shock on the pickups, are treated in a fully self-consistent manner. For quasi-perpendicular shocks with 10-20 percent pickup hydrogen the pickup ions manifest themselves in a small foot ahead of the shock ramp caused by pickup ion reflection. For oblique shocks with smaller angles between the field and the shock normal, a large fraction of the pickup ions are reflected and move back upstream where they excite large amplitude magnetosonic waves which steepen into shocklets. These backstreaming pickup ions may provide advance warning of a spacecraft encounter with the termination shock.

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 authors present a 3-dimensional global hybrid simulation of the interaction of the solar wind with the entire dayside of Venus. The model obstacle is half the size of Venus, and planetary ion mass loading is included self-consistently. Results are compared to observations as well as to results from gasdynamic convected field modeling. Magnetic field magnitudes and bulk flow speeds along the planet-Sun line are comparable in both models, but only the hybrid model reproduces the experimentally observed magnetic barrier proton density depletions. The finite gyroradius of the planetary pickup ions causes a number density asymmetry in the direction of the convective ({minus}V {times} B) electric field, as predicted and observed. Mass addition consistent with photoionization of the planetary neutral hot oxygen corona has little effect on the geometry of the shock, including the subsolar and terminator shock altitudes. Mass addition rates well in excess of likely values are required to significantly affect the model shock geometry. The hybrid model results imply that oxygen ions originating deep within the dayside Venus magnetic barrier are nearly fluidlike while oxygen ions produced higher on the dayside, at much lower densities, behave more as test particles. Gasdynamic modeling incorporating both fluid and testmore » particle mass addition reproduces the O{sup +} terminator escape flux (a few times 10{sup 24} s{sup {minus}1}) found in the hybrid model and inferred from observations, but underestimates the escape region spatial extent. The hybrid model predictions include a shock asymmetry dependent on the upstream IMF orientation, asymmetries in the magnetic barrier position and field magnitude, an asymmetry in pickup ion speed altitude profiles, and a finite gyroradius effect asymmetry in pickup ion number density caused by field draping.« less

The fluxes and energy spectra of picked-up planetary O+ ions incident on the dayside atmospheres of Venus and Mars are calculated using the neutral exosphere models of Nagy and Cravens (1988) and the Spreiter and Stahara (1980) gasdynamic model of the magnetosheath electric and magnetic field. Cold (∼10 eV) O+ ions are launched from hemispherical grids of starting points covering the daysides of the planets and their trajectories are followed until they either impact the dayside “obstacle” or cross the terminator plane. The impacting, or precipitating, ion fluxes are weighted according to the altitude of the hemispherical starting point grid in a manner consistent with the exosphere density models and the local photoion production rate. Maps of precipitating ion number flux and energy flux show the asymmetrical distribution of dayside energy deposition expected from this source which is unique to the weakly magnetized planets. Although the associated heating of the atmosphere and ionosphere is found to be negligible compared to that from the usual sources, backscattered or sputtered neutral oxygen atoms are produced at energies exceeding that needed for escape from the gravitational fields of both planets. These neutral “winds,” driven by pickup ion precipitation, represent a possibly significant loss of atmospheric constituents over the age of the solar system.

The evolution of pitch angle anisotropy in initially isotropic spherical shell distribution of pickup ions subjected to a continuous influence of weak or moderate intrinsic solar wind turbulence is investigated using a physical model which assumes that the wave of the solar wind turbulence is propagating mainly in the direction parallel to the ambient magnetic field. It is shown that, because of the pitch angle dependence of the velocity diffusion process, a significant pitch angle anisotropy of pickup ion shell distribution develops in the early stage of the diffusion process. Although it is smeared out later on, the result can be of significant importance, especially if the pickup ion density is large, because the pitch angle anisotropy can excite collective instabilities and increase the wave level, thus accelerating the diffusion process.

'Pressure-balanced structures' (PBSs) in the heliosphere are microscale phenomena with a characteristic length along the radial dimension of the order of 0.05 AU across which the sum of the thermal pressures and the magnetic pressure is constant. PBSs have been identified in the Voyager data from 1 AU to 24 AU. If the density of pickup ions were proportional to the proton density across a PBS, and if the electron pressure were negligible, then from measurements of the magnetic field and the proton density and temperature across pressure balanced structures a pickup ion density is derived which is significantly smaller than the density predicted by a model based on the same assumption. This suggests that there are processes which 'smear out' the pressure of the pickup ions so that it is nearly constant on the scale of a PBS. If the pressure of the pickup ions is uniform across a PBS, then one can determine the electron temperature from the pressure balance condition.

It has been known for many years that neutral interstellar atoms enter the solar system from the upwind side and penetrate deep into the inner heliosphere. Helium atoms, in particular, advance towards very small solar distances before they are ionized and then again convected as He + pickup ions outwards with the solar wind. Since these ions were recently detected in space, we concentrate here on calculations of He + production rates and He + fluxes. It is shown that inside 1 a.u., the He + production is essentially determined both by solar e.u.v. photoionization and by electron impact ionization. We calculate He + production rates as a function of space coordinates, taking into account the core-halo structure of the energy distribution of solar wind electrons and their temperature distribution with distance according to relevant solar wind models. For this purpose, a newly developed program to compute He densities was used. In contrast to the production of H + , the He + production rates are found to be higher on the downwind axis than on the upwind axis by a factor of 5. We also determine partial and total He + ion fluxes as a function of solar distance and longitude. It is interesting to note that only the values for total fluxes agree well with the integrated He + fluxes measured by the SULEICA experiment aboard the AMPTE satellite. This indicates that pickup ions under the influence of the intrinsic MHD wave turbulence in the solar wind change their primary seed distribution function by rapid pitch-angle scattering and subsequent adiabatic cooling. To interpret the He + intensity profile along the orbit of the Earth in terms of LISM helium parameters, we point to the need to take into account carefully electron impact ionization in order to prevent misinterpretations.