Hans J. Fahr

Energetic neutral atom imaging of the heliospheric boundary region

Energetic neutral atom (ENA) imaging is a powerful technique, which can remotely probe the properties of distant hot plasmas. Hot plasmas are abundant at the heliospheric boundary, the region where the expanding solar wind meets the surrounding local interstellar cloud. Here we present a new concept for imaging this boundary in ENA fluxes. Heliospheric ENAs are born from charge exchange between energetic protons and the background interstellar atomic hydrogen gas. The technique is ideal for studying the asymmetric three-dimensional heliospheric interface region remotely, from 1 AU. We show that ENA imaging in the 0.2–6 keV energy range will establish the nature of the termination shock and properties of hot proton populations in the heliosheath. We also examine how the evolution of pickup proton populations at and beyond the shock can be explored. Global heliosphere ENA images will distinguish among the competing models of the interaction between the Sun and the local interstellar medium, and they will reveal the physics of important processes in the interface region. Heliospheric ENA fluxes are exceptionally weak, which makes imaging implementation difficult. Nonetheless, we show how single-pixel ENA sensors can image the heliosphere from a spinning spacecraft on a typical mission near 1 AU. The required instrumentation is briefly discussed.

The extraterrestrial UV-background and the nearby interstellar medium

Recent measurements of the extraterrestrial UV- and EUV-radiation, and the various theoretical approaches used in explaining the measured features of these radiations are reviewed. Whereas the structures and intensities of extraterrestrial EUV-radiation are essentially undetermined up to now, the observations of the extraterrestrial UV-sky give a clear indication of the existence of neutral interstellar hydrogen within the solar system.The effects of solar radiation pressure, and of temporal variations and spatial asymmetries in the solar radiations, on the structure of the extraterrestrial Lα sky are investigated in detail, and the various attempts to derive interstellar parameters from the interpretation of the measured Lα intensities are discussed.From these discussions the local interstellar medium is established as a tenuous hot intercloud H i-medium. The amount of its relative motion against the solar system cannot be reliably fixed. Further activities concerning the measurement of extraterrestrial UV- and EUV-radiation features are suggested that may be highly valuable in clarifying the outstanding problems.

The Interstellar Boundary Explorer (IBEX)

The Interstellar Boundary Explorer (IBEX) mission is exploring the frontiers of the heliosphere where energetic neutral atoms (ENAs) are formed from charge exchange between interstellar neutral hydrogen atoms and solar wind ions and pickup ions. The geography of this frontier is dominated by an unexpected nearly complete arc of ENA emission, now known as the IBEX ‘Ribbon’. While there is no consensus agreement on the Ribbon formation mechanism, it seems certain this feature is seen for sightlines that are perpendicular to the interstellar magnetic field as it drapes over the heliosphere. At the lowest energies, IBEX also measures the flow of interstellar H, He, and O atoms through the inner heliosphere. The asymmetric oxygen profile suggests that a secondary flow of oxygen is present, such as would be expected if some fraction of oxygen is lost through charge exchange in the heliosheath regions. The detailed spectra characterized by the ENAs provide time-tagged samples of the energy distributions of the underlying ion distributions, and provide a wealth of information about the outer heliosphere regions, and beyond.

Reflection of pre-accelerated pick-up ions at the solar wind termination shock: The seed for anomalous cosmic rays

It has been hypothesized for quite some time that interplanetary pick-up ions due to energization taking place in the region close to the solar wind termination shock, at some fraction and as an outcome of a complicated chain of processes, eventually are converted into species of the anomalous cosmic-ray particles. For the actual conversion efficiency it is of great importance to know the energy distribution of these pick-up ions upon their arrival at the shock. It turns out that pre-acceleration of these ions during their passage through the heliosphere shall substantially increase their chances to become reflected at the shock into the upstream direction which is a prerequisite for a further climb-up in energy by virtue of Fermi-1 acceleration processes. In this paper we start out from stochastically pre-accelerated pick-up ions and investigate their behaviour at the shock. With the use of adiabatic approaches in the de Hoffman-Teller frame of the shock, we calculate the energy distribution function of the reflected part of pick-up ions. From the calculated distribution functions it turns out that the reflected ions in the average suffer an energy increase by about a factor of 10, still not enough to let them move off the shock by spatial diffusion in the upstream direction. Thus, since converted back into the shock, they can undergo repeated reflections and energy gains till the diffusion-convection limit is reached. As we show in addition, the reflection probability for pick-up ions is about a factor of 10 higher than expected from the present literature and strongly varies with the off-axis angle, pointing to the fact that the termination shock represents a surface with a three-dimensionally varying source strength for the production of anomalous cosmic rays. The ACR source pattern is also expected to vary during the solar cycle and the relevant injection energies are expected to be larger by factors of 10 to 100 than the canonically adopted 1 keV nucl−1.

HST/GHRS Observations of the Velocity Structure of Interplanetary Hydrogen

We present high-resolution spectra of the emission-line profile of inflowing interplanetary hydrogen atoms along lines of sight with the Earth orbital motion upwind (into the flow), downwind, and across the flow to Doppler-shift the line from the geocoronal emission. The line-center positions, in comparison with hot-model profiles, confirm that the inflow speed of H atoms far from the Sun (~50 AU) is in the range 18-21 km s-1, which implies a decrease in the velocity distribution of 5-8 km s-1 for hydrogen within the solar system, relative to the He flow and to the local interstellar medium. Best-fit values are derived for the speed and effective solar gravity along the three lines of sight by comparison with model profiles convolved with the instrument line-spread function. For the assumed inflow direction, the cross-flow line profile requires that the μ-value be slightly less than unity near solar minimum, and a technique is presented for determining the exact inflow direction and μ-value independently of the other parameters. The line widths indicate a broadening along the flow direction in addition to the dynamical effects near the Sun expected from two different hot models, whereas the cross-flow line width is similar to the hot-model profiles. The altered velocity distribution in the inflow direction appears likely to be related to the crossing of the interstellar/interplanetary medium interface structure, although questions remain about the cumulative effects of changing solar activity on the timescale of the H atom flow through the solar system.

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

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

Heliopause imaging in EUV : Oxygen O+ ion 83.4-nm resonance line emission

We explore the possibility of remote, from 1 AU, study of the heliopause by an observer outside the geocorona. We argue that the heliopause, a boundary that separates the solar wind and the galactic plasma of the local interstellar medium (LISM), can be imaged by detecting solar extreme ultraviolet (EUV) radiation reflected by interstellar ions. Such EUV imaging would map the heliopause and provide important insight into its three-dimensional structure and the LISM parameters as well. We consider heliopause mapping in the oxygen O+ ion resonance line (83.4 nm); imaging in the helium He+ ion line (30.4 nm) will be considered in a future article. We show that the expected heliopause brightness map at 83.4 nm is essentially different from that of the foreground glow of the solar wind O+ pickup ions. The interstellar plasma glow is brighter in the upwind (with respect to the interstellar wind) direction, while the pickup ion glow dominates in the downwind direction. The spectral characteristics of the radiation scattered by the LISM plasma and by the pickup ions are significantly different. The all-sky images at 83.4 nm are highly sensitive to the ionization state of the LISM and would allow one to probe the asymmetry of the interstellar magnetic field. We briefly discuss the experimental requirements to heliopause EUV mapping, which would require 3 orders of magnitude improvement in instrumentation sensitivity. This is a challenging but not impossible task.

Modification of Properties and Dynamics of Distant Solar Wind Due to Its Interaction with Neutral Interstellar Gas

Neutral interstellar H atoms penetrate into the inner heliosphere and after ionization are converted into pickup ions (PUIS). The main loss process of these atoms is resonant charge transfer with solar wind (SW) protons. The appropriate method to describe this interaction is a kinetic approach based on the Boltzmann—Vlasov integro-differential equation (see, e.g., Ripken and Fahr, 1983; Fahr, 1991; Osterbart and Fahr, 1992; Baranov and Malama, 1993). Charge-exchange removal of keV-particles from SW connected with mass loading by new ions (nearly at rest) significantly affects the solar wind momentum flow. Incorporation of the new ions as suprathermal PUI’s into the SW bulk flow leads also to a non-adiabatic plasma behaviour recognized first by Fahr (1973), and Holzer and Leer (1973).

Access to the heliospheric boundary: EUV‐echoes from the heliopause

We argue that the heliopause, a boundary that separates the solar wind and the plasma of the Local Interstellar Medium (LISM), can be explored remotely, from 1 AU, by detecting solar extreme-ultraviolet (EUV) radiation reflected by the heliospheric interface region. The measurements of the solar EUV radiation echoes from the heliopause would map the heliopause and provide important insight into the LISM parameters. Heliopause mapping can be done in the oxygen O + resonance line (83.4 nm). We show that the expected heliopause brightness is higher than the expected major source of the background line radiation, viz. the glow of the solar wind O + pickup ions and discuss a way to remotely establish the ionization state of the LISM and to probe the LISM interstellar magnetic field.

Stationary field-aligned MHD flows at astropauses and in astrotails - Principles of a counterflow configuration between a stellar wind and its interstellar medium wind

Context. A stellar wind passing through the reverse shock is deflected into the astrospheric tail and leaves the stellar system either as a sub-Alfvenic or as a super-Alfvenic tail flow. An example is our own heliosphere and its heliotail. Aims. We present an analytical method of calculating stationary, incompressible, and field-aligned plasma flows in the astrotail of a star. We present a recipe for constructing an astrosphere with the help of only a few governing parameters, like the inner Alfven Mach number and the outer Alfven Mach number, the magnetic field strength within and outside the stellar wind cavity, and the distribution of singular points (neutral points) of the magnetic field within these flows. Methods. Within the framework of a one-fluid approximation, it is possible to obtain solutions of the governing MHD equations for stationary flows from corresponding static MHD equilibria, by using noncanonical mappings of the canonical variables. The canonical variables are the Euler potentials of the magnetic field of magnetohydrostatic equilibria. Thus we start from static equilibria determined by the distribution of magnetic neutral points, and assume that the Alfven Mach number for the corresponding stationary equilibria is finite. Results. The topological structure, i.e. the distribution of magnetic neutral points, determines the geometrical structure of the interstellar gas – stellar wind interface. Additional boundary conditions like the outer magnetic field and the jump of the magnetic field across the astropause allow determination of the noncanonical transformations. This delivers the strength of the magnetic field at every point in the astrotail/astrosheath region beyond the reverse shock. Conclusions. The mathematical technique for describing such a scenario is applied to astrospheres in general, but is also relevant for the heliosphere. It shows the restrictions of the outer and the inner magnetic field strength in comparison with the corresponding Alfven Mach numbers in the case of subalfvenic flows.