Interplanetary Pick-Up Ion Acceleration A Study of Anisotropic Phase-Space Diffusion

Interplanetary pick-up ions originate from ionizations of neutral interstellar atoms in the heliosphere. Over the past periods it was generally expected that after pick-up by the frozen-in solar wind magnetic fields these ions quickly isotropize in velocity space by strong pitch- angle scattering, they do, however, not assimilate to the ambient solar wind ions. Meanwhile careful investigations of pick-up ion data obtained with the plasma analyzers on AMPTE and ULYSSES could clearly reveal that, especially at periods of flow-aligned fields, noticeably anisotropic distributions must prevail. To better understand the evolutionary tracks of pick-up ions in interplanetary phase-space we carried out an injection study which takes into account all relevant convection and diffusion processes, i.e. describing pitch angle scattering, adiabatic cooling, drifts and energy diffusion. As demonstrated here particles injected at 1 AU establish a distribution function with substantial anisotropies up to distances beyond 6 AU. Only under the action of fairly strong isotropic turbulence levels a trend towards isotropy can be recognized. The bulk velocity of the injected pick-up ions turns out to be remarkably smaller than the solar wind velocity. It also is obvious that pick-ups are strongly spread out from that solar wind plasma parcel into which they were originally implanted. As one consequence it must be concluded that the derivation of interstellar He gas parameters, using He pick-up ion flux data, require appreciable caution. Due to anisotropic spatial diffusion the location of the LISM helium cone axis, i.e. the LISM wind vector, and the LISM helium temperature are hidden in the associated He⁺pick-up ion flux patterns. This revised version was published online in July 2006 with corrections to the Cover Date.     

Signatures of the Interplanetary Helium Cone Reflected by Pick-up Ions

As known for a long time, interstellar wind neutral helium atoms deeply penetrate into the inner heliosphere and, when passing through the solar gravity field, form a strongly pronounced helium density cone in the downwind direction. Helium atoms are photoionized and picked-up by the solar wind magnetic field, but as pick-up ions they are not simply convected outwards with the solar wind in radial directions as assumed in earlier publications. Rather they undergo a complicated diffusion-convection process described here by an appropriate kinetic transport equation taking into account adiabatic cooling and focusing, pitch angle scattering and energy diffusion. In this paper, we solve this equation for He⁺pick-up ions which are injected into the solar wind mainly in the region of the helium cone. We show the resulting He⁺pick-up ion density profile along the orbit of the Earth in many respects differs from the density profile of the neutral helium cone: depending on solar-wind-entrained Alfvénic turbulence levels, the density maximum when looking from the Earth to the Sun is shifted towards the right side of the cone, the ratio of peak-densities to wing-densities varies and a left-to-right asymmetry of the He⁺-density profile is pronounced. Derivation of interstellar helium parameters from these He⁺-structures, such as the local interstellar medium (LISM) wind direction, LISM velocity and LISM temperature, are very much impeded. In addition, the pitch-angle spectrum of He⁺pick-up ions systematically becomes more anisotropic when passing from the left to the right wing of the cone structure. All effects mentioned are more strongly pronounced in high velocity solar wind compared to the low velocity solar wind.     

On the state of ionization of the local interstellar medium

The abundance ratio of neutral hydrogen to helium, as deduced from interplanetary observations of Lyman-alpha and He 584 Å radiation by Mariner 10, is significantly lower than the cosmic abundance ratio of these elements, thus showing that the local interstellar medium (LISM) is partly ionized. The effect of various sources of ionization — especially electron impact and EUV photon flux — on hydrogen and helium is discussed. It is shown that the observational data on the temperature of the LISM, on the diffuse EUV flux and on the neutral and electron densities in the nearby interstellar medium (NISM) are not all compatible. However, if the diffuse EUV flux below 912 Å as deduced from the preliminary analysis of Voyager observations is not representative, then it is easy to reconcile all observations. In this case an important source of ionization of the LISM would be electron impact, yielding an ionization degree of about 50% for the hydrogen component.     

An axisymmetric magnetohydrodynamic model for the interaction of the solar wind with the local interstellar medium

We numerically analyze a magnetohydrodynamic, steady-state model for the interaction of a spherically symmetric solar wind with a three-component local interstellar medium (LISM), which is composed of plasma, hydrogen atoms, and a magnetic field. The magnetic field is assumed to be parallel to the velocity in the LISM. In this case, the model is axisymmetric. We study the effects of magnetic field on the plasma-flow geometry and on the distribution of hydrogen-atom parameters. In particular, we show that the presence of hydrogen atoms does not affect the qualitative change in the shape of the bow shock, the heliopause, and the solar-wind shock with increasing strength of the interstellar magnetic field. The presence of a magnetic field in the LISM can strongly affect the parameters of the energetic hydrogen atoms originated in the solar wind, although its effect on the “hydrogen wall” observed with the GHRS instrument onboard the HST spacecraft (Linsky and Wood 1996) is marginal.     

Interstellar heliospheric probe/heliospheric boundary explorer mission—a mission to the outermost boundaries of the solar system

The Sun, driving a supersonic solar wind, cuts out of the local interstellar medium a giant plasma bubble, the heliosphere. ESA, jointly with NASA, has had an important role in the development of our current understanding of the Suns immediate neighborhood. Ulysses is the only spacecraft exploring the third, out-of-ecliptic dimension, while SOHO has allowed us to better understand the influence of the Sun and to image the glow of interstellar matter in the heliosphere. Voyager 1 has recently encountered the innermost boundary of this plasma bubble, the termination shock, and is returning exciting yet puzzling data of this remote region. The next logical step is to leave the heliosphere and to thereby map out in unprecedented detail the structure of the outer heliosphere and its boundaries, the termination shock, the heliosheath, the heliopause, and, after leaving the heliosphere, to discover the true nature of the hydrogen wall, the bow shock, and the local interstellar medium beyond. This will greatly advance our understanding of the heliosphere that is the best-known example for astrospheres as found around other stars. Thus, IHP/HEX will allow us to discover, explore, and understand fundamental astrophysical processes in the largest accessible plasma laboratory, the heliosphere.     

Proton Irradiation of Centaur,Kuiper Belt,and Oort Cloud Objects at Plasma to Cosmic Ray Energy

Times for accumulation of chemically significant dosages on icy surfaces of Centaur, Kuiper Belt, and Oort Cloud objects from plasma and energetic ions depend on irradiation position within or outside the heliosphere. Principal irradiation components include solar wind plasma ions, pickup ions from solar UV ionization of interstellar neutral gas, energetic ions accelerated by solar and interplanetary shocks, including the putative solar wind termination shock, and galactic cosmic ray ions from the Local Interstellar Medium (LISM). We present model flux spectra derived from spacecraft data and models for eV to GeV protons at 40 AU, a termination shock position at 85 AU, and in the LISM. Times in years to accumulate dosages ～100 eV per molecule are computed from the spectra as functions of sensible surface depth less than one centimeter at unit density.The collisional resurfacing model of Luu and Jewitt is reconsidered in thecontext of depth-dependent dosage rates from plasma, suprathermal,and higher energy protons, and global exposure, by micrometeoroiddust grain impacts, of moderately irradiated red material below athin crust of heavily irradiated neutral material. This material should be more visible on dynamically `cold’ objects in the ～40 AU region.     

The Interstellar Heliopause Probe: Heliospheric Boundary Explorer Mission to the Interstellar Medium

The Sun, driving a supersonic solar wind, cuts out of the local interstellar medium a giant plasma bubble, the heliosphere. ESA, jointly with NASA, has had an important role in the development of our current understanding of the Suns’ immediate neighborhood. Ulysses is the only spacecraft exploring the third, out-of-ecliptic dimension, while SOHO has allowed us to better understand the influence of the Sun and to image the glow of interstellar matter in the heliosphere. Voyager 1 has recently encountered the innermost boundary of this plasma bubble, the termination shock, and is returning exciting yet puzzling data of this remote region. The next logical step is to leave the heliosphere and to thereby map out in unprecedented detail the structure of the outer heliosphere and its boundaries, the termination shock, the heliosheath, the heliopause, and, after leaving the heliosphere, to discover the true nature of the hydrogen wall, the bow shock, and the local interstellar medium beyond. This will greatly advance our understanding of the heliosphere that is the best-known example for astrospheres as found around other stars. Thus, IHP/HEX will allow us to discover, explore, and understand fundamental astrophysical processes in the largest accessible plasma laboratory, the heliosphere.     

On the model of the solar wind-interstellar medium interaction with two shock waves

The model of the solar wind interaction with interstellar medium suggested by Baranovet al. (1970) is developed. In this model (TSM) the presence of two shock waves is assumed, through which the solar wind and interstellar gas pass, the latter moving relative to the Sun at supersonic speed (20 km s^–1).The distance between shocks was considered earlier (Baranovet al., 1970; Baranov and Krasnobaev, 1971) to be small compared with their distance from the Sun, due to the hypersonic character of the flow. The structure of the subsonic flow portion may not be taken into account.In the present paper the distribution of the gas parameters in the region between shocks is calculated which, in particular, allows us to estimate the possibility of its experimental detection, observing radio-scintillation on interstellar irregularities (Baranovet al., 1975).The possible influence on the model of galactic hydrogen neutral atoms penetrating into interplanetary medium is estimated.     

Charge transfer reactions at interfaces between neutral gas and plasma: Dynamical effects and X‐ray emission

Charge‐transfer is the main process linking neutrals and charged particles in the interaction regions of neutral (or partly ionized) gas with a plasma. In this paper we illustrate the importance of charge‐transfer with respect to the dynamics and the structure of neutral gas‐plasma interfaces. We consider the following phenomena: (1) the heliospheric interface ‐ region where the solar wind plasma interacts with the partly‐ionized local interstellar medium (LISM) and (2) neutral interstellar clouds embedded in a hot, tenuous plasma such as the million degree gas that fills the so‐called “Local Bubble”. In (1), we discuss several effects in the outer heliosphere caused by charge exchange of interstellar neutral atoms and plasma protons. In (2) we describe the role of charge exchange in the formation of a transition region between the cloud and the surrounding plasma based on a two‐component model of the cloud‐plasma interaction. In the model the cloud consists of relatively cold and dense atomic hydrogen gas, surrounded by hot, low density, fully ionized plasma. We discuss the structure of the cloud‐plasma interface and the effect of charge exchange on the lifetime of interstellar clouds. Charge transfer between neutral atoms and minor ions in the plasma produces X‐ray emission. Assuming standard abundances of minor ions in the hot gas surrounding the cold interstellar cloud, we estimate the X‐ray emissivity consecutive to the charge transfer reactions. Our model shows that the charge‐transfer X‐ray emission from the neutral cloud‐plasma interface may be comparable to the diffuse thermal X‐ray emission from the million degree gas cavity itself (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interstellar pickup protons and solar wind heating in the outer heliosphere

The ionization of hydrogen atoms that penetrate into the heliosphere from the interstellar medium gives rise to a peculiar population of energetic protons (interstellar pickup protons) in the solar wind. The short-wavelength Alfvènic turbulence in the outer heliosphere is entirely attributable to the source associated with the instability of the initial anisotropic pickup proton velocity distribution. The bulk of the generated turbulent energy is subsequently absorbed by the pickup protons themselves through the cyclotron-resonance particle-wave interaction, and only an insignificant fraction of this energy can be transferred to the solar wind protons and heat them up.     

Formation of higher harmonics of the galactic cosmic-ray distribution function

We derive equations for the multipole moments of the distribution function of Galactic cosmic rays with energies 1–20 TeV that experience random scattering by turbulence with a power-law spectrum. We take into account the irregularity of the local interstellar medium (LISM) in the neighborhood of the solar system due to the presence of interstellar clouds, the interstellar wind flow around the heliomagnetosphere, and preceding supernova explosions in the local superbubble. The amplitudes of the second and third harmonics of the cosmic-ray distribution function are expressed in terms of the amplitude of the first harmonic without assuming them to be small compared to the first harmonic. Reconciling their values in magnitude and phase with the observed values requires a significant LISM irregularity, which is consistent with other observational data on the LISM structure. Our model is consistent with the assumption that supernova remnants in the Galactic disk located at distances from the Solar system much larger than the particle transport mean free path are the sources of the particles under consideration.     

Physics and Gasdynamics of the Heliospheric Interface

During 30 years, a big theoretical effort to understand the physical processes in the heliospheric interface has followed the pioneer papers by Parker (1961) and Baranov et al. (1971). The heliospheric interface is a shell formed by the solar wind interaction with the ionized component of the circumsolar local interstellar medium (LISM). For fully ionized supersonic interstellar plasma two-shocks (the termination shock and the bow shock) and a contact discontinuity (the heliopause) are formed in the solar wind/LISM interaction. However, LISM consists of at least of three components additional to plasma: H-atoms, galactic cosmic rays and magnetic field. The interstellar atoms that penetrate into the solar wind, are ionized there and form pickup ions. A part of the pickup ions is accelerated to high energies of anomalous cosmic rays (ACRs). ACRs may modify the plasma flow upstream the termination shock and in the heliosheath. In this short review I summarize current understanding of the physical and gasdynamical processes in the heliospheric interface, outline unresolved problems and future perspectives. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of the heliospheric interface on the distribution of interstellar hydrogen atom inside the heliosphere

This paper deals with the modeling of the interstellar hydrogen atoms (H atoms) distribution in the heliosphere. We study influence of the heliospheric interface, that is the region of the interaction between solar wind and local interstellar medium, on the distribution of the hydrogen atoms in vicinity of the Sun. The distribution of Hatoms obtained in the frame of the self-consistent kinetic-gasdynamicmodel of the heliospheric interface is compared with a simplified model which assumes Maxwellian distribution of H atoms at the termination shock and is called often as “hot” model. This comparison shows that the distribution of H atoms is significantly affected by the heliospheric interface not only at large heliocentric distances, but also in vicinity of the Sun at ∼1–5 AU. Hence, for analysis of experimental data connected with direct or undirect measurements of the interstellar atoms one necessarily needs to take into account effects of the heliospheric interface. In this paper we propose a new model that is relatively simple but takes into account all major effects of the heliospheric interface. This model can be applied for analysis of backscattered La-alpha radiation data obtained on board of different spacecraft.     

Revised interstellar neutral helium/hydrogen density ratios and the interstellar UV-radiation field

The data deduced from the UV-spectroscope on theCopernicus satellite strongly suggest that the most important ionization source in interstellar space near the solar system is a UV radiation field originating from B-stars. Adopting this hypothesis, we have used the ionization state of several elements in the interstellar medium observed byCopernicus to determine the required radiation field. From this, the degree of ionization of elements that could not be observed byCopernicus is estimated.It is shown that this interpretation of thecopernicus data can be made consistent with neutral interstellar hydrogen densities inferred from extraterrestrial L observations and with electron densities deduced from pulsar dispersion measures. Furthermore, it is shown that the ratio of neutral interstellar helium to neutral interstellar hydrogen is likely to be 2 to 3 times as large as the cosmic abundance ratio of these elements. The possibility that this ratio is about 10 times as large, meaning equal interstellar neutral hydrogen and helium densities near the solar system, cannot be ruled out. It would, however, require an interstellar radiation temperature near 9000 K. A comparison of the intensity of the interplanetary back scattered He 584 Å and the H 1216 Å radiation would lead to a direct determination of this ratio provided the solar radiation at these lines is known.     

Heliospheric magnetic fields, energetic particles, and the solar cycle

The heliosphere is the region filled with magnetized plasma of mainly solar origin. It extends from the solar corona to well beyond the planets, and is separated from the interstellar medium by the heliopause. The latter is embedded in a complex and still unexplored boundary region. The characteristics of heliospheric plasma, fields, and energetic particles depend on highly variable internal boundary conditions, and also on quasi-stationary external ones. Both galactic cosmic rays and energetic particles of solar and heliospheric origin are subject to intensity variations over individual solar cycles and also from cycle to cycle. Particle propagation is controlled by spatially and temporally varying interplanetary magnetic fields, frozen into the solar wind. An overview is presented of the main heliospheric components and processes, and also of the relevant missions and data sets. Particular attention is given to flux variations over the last few solar cycles, and to extrapolated effects on the terrestrial environment.     

Neutral gases of interstellar origin in interplanetary space

The distribution of interplanetary neutral gases of interstellar origin is recalculated. Several improvements over earlier solutions of the problem are presented. This is especially true for the upwind and the downwind axes, which are of importance in the analysis of the observations of the back-scattered solar resonance radiation.     

The heliospheric plasma tail under the influence of charge exchange processes with interstellar H atoms

The relative motion of the solar system with respect to the ambient interstellar medium is known to form a plasma interface region where the subsonic interstellar and solar wind plasma flows adapt to a pressure equilibrium surface, called the heliopause. Inside this discontinuity surface the solar plasma is deflected from the upwind to the downwind side, finally escaping from the solar system along a heliospheric tail. Due to continuous charge exchange interactions with interstellar H atoms entering from the tailward flanks of the heliopause tail plasma, originating from shocked solar wind, changes its thermodynamic character by cooling and deceleration while passing along the tail to larger downstream distances. Here we describe this charge-exchange-induced modification of the tail plasma up to a final assimilation into the interstellar plasma. On the other hand neutral H atoms are produced by means of charge exchange interactions in the heliotail with velocities by which these atoms are shot back into the inner heliosphere. We calculate the velocity distribution of such H atoms entering the inner heliosphere from the downwind direction and study their contribution to the H-pick-up ion production in the downwind region. As we show in this paper, total H-pick-up ion production rates in the downwind region are dominated by ionization of such anti-tailward H atoms within the orbit of the earth. They also dominate the pick-up ion energy spectrum beyond 4keV at distances between 1 and 10AU.     

The Influence of Fluctuations of the Solar Emission Line Profileon the Doppler Shift of Interplanetary HLyα Lines Observed by the Hubble–Space–Telescope

In an earlier research the employment of a radiation transport model with angle-dependent partial frequency redistribution, self-absorption by interplanetary hydrogen, realistic solar H_Lyαemission profile, and a time dependent `hot' hydrogen model to analyze 5 interplanetary H_Lyα glow spectra obtained with theHubble–Space–Telescope–GHRS spectrometer, has not resulted in unequivocal determination of a set of thermodynamical parameters of the interstellar hydrogen The residual discrepancies between the model and the data concern the observations performed within an interval of 1 year close to the solar minimum from very similar lines of sight. In this paper we investigate by calculating interplanetary H_Lyα lines with the use of a one hydrogen distribution and several solar H_Lyα line profiles whether this residual may be caused by possible variations in time of the shape of the solar H_Lyα emission line profile which cause variable illuminations of the interplanetary gas. These variations of illuminations cause variations in Doppler shift of the resonant interplanetary H_Lyα line that can amount to ≃ 4 km s^-1in the line peak. Consequently we conclude that without adequate knowledge of the solar H_Lyα emission line profile during spectral observations of the interplanetary hydrogen gas it is impossible to obtain an agreement between models and observations better than by this value. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Multifluid Character of the `Baranov' Interface

Since about three decades now it is clearly recognized that the interaction of the solar system with the ambient interstellar medium flow mainly is characterized by its hydrodynamic nature invoking structures like the inner shock, the heliopause and the outer shock with plasma sheath regions in between. After the pioneering works by Eugene Parker and Vladimir Baranov the main outlines of this interaction scenario were established, while some discussion on location and geometry of these structures is still going on till now. Fundamentally new aspects of this interaction problem have meanwhile appeared calling for new and more consistent calculations. The revisions of the earlier interaction concept starts with the neutral LISM gas component passing through the solar system. At the occasion of ionizations of this component a medium-energetic plasma component in form of keV-energetic pick-up ions is created. This component changes the distant solar wind properties by mass-, momentum-, and energy-loading, by wave generation and lowering the solar wind Mach numbers. Furthermore pick-up ions serve as a seed population for a high-energetic plasma population with energies between 10 and 100 MeV/nuc called anomalous cosmic rays. This latter component by means of its pressure gradient not only modifies the solar wind flow but also modulates its termination shock. In this paper it is shown how the main features of the enlarged interaction scenario change if the above mentioned multifluid character of the scenario is taken into account. While now we present a `multicolour vision' of the interacting heliosphere, it should never be forgotten that these modern views only were possible due to the fundamental `black-and-white vision' already presented by Baranov in the seventieths. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of the heliosheath and standing termination shock on galactic cosmic ray propagation in a stationary heliosphere model

We consider a stationary model of the propagation of galactic cosmic rays (GCR) in the heliosphere and adjacent interstellar space. The heliosphere is assumed to be a two-layer medium consisting of two adjacent regions that are spherically symmetric relative to the sun. The solar wind velocity is supersonic in the inner heliosphere bounded by the standing termination shock, and this velocity is subsonic in the outer heliosphere bounded by the heliosheath. The GCR scattering in these regions is due to different factors characterized by relevant diffusion coefficients. The solar wind velocity is assumed to be zero in the interstellar medium, where the scattering becomes weaker. No particle sources are presumed to exist at the boundaries between the layers. An exact analytical solution of the corresponding mathematical problem can be obtained without essential difficulties, although it is extremely cumbersome. Analytical expressions for the GCR spectra of particles with very high energies (>2500 MeV) and very low energies (<1400 MeV) are obtained for each region of particle propagation. The low-energy particle distribution corresponds to the data obtained by the Voyager spacecraft. It is shown that the low-energy particle density continuously increases from the sun toward the heliospheric boundary, regardless of the scattering mode in the inner and outer parts of the heliosphere.