As known for a long time, interstellar wind neutral helium atoms deeply penetrate into the inner ... more 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-entrain...
The Sun moves through the local interstellar medium (ISM), continuously emitting ionized, superso... more The Sun moves through the local interstellar medium (ISM), continuously emitting ionized, supersonic solar wind plasma and carving out a cavity in interstellar space called the heliosphere. The border of this cavity traces regions where the interstellar gas first interacts with the solar wind. The Interstellar Boundary Explorer (IBEX) spacecraft has just completed the first all-sky maps of this interstellar interaction at the edge of the heliosphere, by imaging energetic neutral atoms (ENAs) emanating from this region. The IBEX all-sky maps and energy spectra provide detailed information about this interaction. Our observations show globally distributed ENA fluxes ordered by the solar wind structure, superimposed on an unexpected feature. This poster summarizes the IBEX observations, shares our unexpected results, and discusses the new information on stellar wind interactions with the ISM that we have gleaned from these data. Acknowledgments: We thank the IBEX team members. This work is primarily funded by the NASA Explorer Program.
Astrophysics and Space Sciences Transactions, 2009
We investigate the well accepted idea that anomalous cosmic ray (ACR) ions originate from suprath... more We investigate the well accepted idea that anomalous cosmic ray (ACR) ions originate from suprathermal halo ions by means of diffusive shock acceleration. Usually the seed ions are taken to be the interplanetary pickup ions, but here we want to enlarge this idea by taking in addition as seed candidates also into account normal solar wind ions which according to our recent calculations are reflected from the termination shock into the upstream solar wind flow and then are also picked-up as suprathermal ions. We start out from an ideally planar shock approximation and fix the ACR spectrum by absolute spectral intensities and maximum ACR energies, taking a precalculated fraction of the suprathermal ion flow as ACR injection rate. Comparison of our calculated spectral intensities with ACR measurements near 94 AU, shows that satisfying data fits only can be achieved, if about three percent of the suprathermal ions swept into the shock structure enter into the Fermi-1 acceleration process. We also show that the spectral slope of the ACR spectra is decreasing and the spectral intensities are increasing with increasing shock compression ratios. As maximum energies available from an ideally operating diffusive Fermi-1 acceleration process we find, depending on the shock compression ratio, ion energies ranging from a few MeV up to 10 3 MeV. Compared to observations this seems to be a little on the high side and may point to the fact that injection into Fermi-1 acceleration near the termination shock is occurring only sporadically due to variable upstream magnetic field orientations with respect to the shock normal vector, i.e. due to variations of the obliquity of the local shock surface with respect to the local upstream magnetic field.
ABSTRACT Our numerical analyses of the velocity and spatial distributions of pickup interstellar ... more ABSTRACT Our numerical analyses of the velocity and spatial distributions of pickup interstellar helium ions in the region of the solar gravitational cone in the ecliptic plane at a distance of 1 AU show that the ion density maximum must be displaced relative to the neutral helium cone axis in the direction of the Earth’s revolution around the Sun. The solar wind parameters in the numerical model correspond to their observed values during the crossing of the helium cone by the ACE spacecraft in 1998. At these parameters, the calculated angular displacement is 5°. The absence of a similar displacement in the ACE measurements is shown to stem from the fact that the spectrometer onboard ACE records and identifies only a fraction of the pickup helium ions with fairly high magnitudes and certain directions of the velocities.
The ionization of hydrogen atoms that penetrate into the heliosphere from the interstellar medium... more 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 Alfvnic 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
It is well known that newly created pickup ions are first injected into a toroidal distribution i... more It is well known that newly created pickup ions are first injected into a toroidal distribution in the velocity space which comoves with the solar wind. This distribution is, however, unstable with respect to excitation of resonant MHD waves, and thus by wave-driving some specific amount of the initial kinetic energy of pickup ions is handed over to the ambient wave fields, while the pickup ion velocity distribution thereby is rearranged. In this paper we describe the developement of the wavenumber spectrum of pickup proton-generated turbulence and the pickup proton energy spectrum resulting from resonant reabsorption of this turbulence, by solving simultaneously the coupled system of two differential equations for the spectral turbulence power and the pickup proton spectral intensity. We show that the majority of pickup generated wave energy in the range of wavenumbers above the wavenumber at which the waves are produced is reabsorbed by the pickup protons themselves.
A coordinated effort to combine all three methods that are used to determine the physical paramet... more A coordinated effort to combine all three methods that are used to determine the physical parameters of interstellar gas in the heliosphere has been undertaken. In order to arrive at a consistent parameter set that agrees with the observations of neutral gas, pickup ions and UV backscattering we have combined data sets from coordinated observation campaigns over three years from 1998 through 2000. The key observations include pickup ions with ACE and Ulysses SWICS, neutral atoms with Ulysses GAS, as well as UV backscattering at the He focusing cone close to the Sun with SOHO UVCS and at 1 AU with EUVE. For the first time also the solar EUV irradiance that is responsible for photo ionization was monitored with SOHO CELIAS SEM, and the He I 58.4 nm line that illuminates He was observed simultaneously with SOHO SUMER. The solar wind conditions were monitored with SOHO, ACE, and WIND. Based on these data the modeling of the interstellar gas and its secondary products in the heliosphere has resulted in a consistent set of interstellar He parameters with much reduced uncertainties, which satisfy all observations, even extended to earlier data sets. It was also established that a substantial ionization in addition to photo ionization, most likely electron impact, is required, with increasing relative importance closer to the Sun. Furthermore, the total combined ionization rate varies significantly with solar latitude, requiring a fully three dimensional and time dependent treatment of the problem.
The Solar System during its life has travelled more than 10 times through dense interstellar clou... more The Solar System during its life has travelled more than 10 times through dense interstellar clouds with particle concentrations of 10 2 −10 3 cm −3 and more, compressing the heliosphere to heliopause dimensions smaller than 1 AU and thus bringing the Earth in immediate contact with the interstellar matter. For cloud concentrations greater than of 10 2 cm −3 , the flowing interstellar material even at the Earth's orbit remains completely shielded from solar wind protons and would only be subject to solar photoionization processes. We have developed a 2D-two-fluid gas-dynamical numerical code to describe the hydrodynamical behavior of the incoming interstellar gas near the Earth, taking into account both the photoionization and the gravity of the Sun. As we show, the resulting strongly increased neutral hydrogen fluxes ranging from 10 9 to 10 11 cm −2 s −1 cause substantial changes in the terrestrial atmosphere. During the phase of the immersion into the cloud the resulting flux of neutral hydrogen incident on the terrestrial atmosphere in the steady state would be balanced by the upward escape flux of H-atoms and the downward flux of water molecules, which is the product of the atmospheric hydrogen-oxygen chemistry via even-odd reaction schemes. In that case hydrogen acts as a chemical agent to remove oxygen atoms and to cause ozone concentration reductions above 50 km by a factor of 1.5 at the stratopause to about a factor of 1000 and more at the mesopause. Thus, depending on the specific encounter parameters the high mixing ratio of hydrogen in the Earth's atmosphere may substantially decrease the ozone concentration in the mesosphere and may trigger an ice age of relatively long duration.
The problem of passages of the Solar System through dense, dusty and cold interstellar gas clouds... more The problem of passages of the Solar System through dense, dusty and cold interstellar gas clouds is discussed. A considerable number of such encounters must have already occurred in the past and these events must have pushed the heliosphere inward to the region of the terrestrial planets, provided a number density of the cloud's material of the order of, or greater than, 1000 cm −3 , and relative velocities of about 20 km s −1 prevail. Under these conditions it will happen that the Earth is directly exposed to the interstellar flow. Neutral interstellar hydrogen and proton densities at the Earth's orbit are calculated under conditions of the heliosphere with the heliopause reduced to dimensions smaller than 1 AU. A simple twofluids treatment of the incoming flow, ionized by the solar UV only, is proposed here and the amount of neutral interstellar hydrogen is assessed which is accreted by the Earth's atmosphere during one passage. The behavior of the flow variables is investigated by a 2D-hydrodynamic approach to model the interaction processes. Because of a reduction of the heliospheric dimension down to 1 AU, direct influence of the cloud's matter to the terrestrial environment and atmospheric chemistry must be envisaged. This will be briefly discussed in the paper.
Up until the recent past, it was generally believed that the solar wind termination shock (TS) is... more Up until the recent past, it was generally believed that the solar wind termination shock (TS) is the favourite site to accelerate ions from the keV-to the MeV-energy levels by means of Fermi-1 processes. When Voyager 1 was crossing the TS at the end of 2004, the registrations of this spacecraft showed, however, that beyond the shock passage fluxes of anomalous cosmic rays kept increasing with time. This obviously called for an acceleration site further downstream of the shock in the heliosheath which had not been identified before. In this paper we thus investigate the process of energy diffusion due to wave-particle interactions (Fermi-2) operating on pickup ions which are convected downstream of the TS with the subsonic solar wind. We investigate the continuous effect of stochastic acceleration processes suffered by pickup ions at their interaction with heliosheath turbulences, while they are slowly convected with the subsonic solar wind towards the heliotail. As we can show, the inner heliosheath region, with an extent of about 100 AU around the solar wind stagnation point, is specifically favourable for the energy processing of pickup ions by Fermi-2 processes up to MeV energies. In addition, we claim that this region is the origin of multiply-charged anomalous cosmic ray particles that have been registered in recent times.
A new technique to directly detect low energy neutral helium has been developed and successfully ... more A new technique to directly detect low energy neutral helium has been developed and successfully used in space for the first time. It makes possible the in-situ measurement of the local angular distribution of the flow of interstellar neutral helium in the inner heliosphere. Based on the transformation of a velocity distribution, using Liouville's theorem, a straightforward model has been developed to determine, from these measurements, the flow parameters of the interstellar helium at infinity. From twelve measurements at radial distances between 1 and 5 AU, average values of the flow parameters were determined, namely: v~= 26 ±1 km s 1, downstream direction: .\~= 72 ±2.4°,/~=-2.5 ± 2.7°,temperature T,~,= 6700 ±1500 K. These values are in agreement with results found by completely different methods (such as, backscattered UV-light, pickup ions), although in detail there are significant differences.
The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October... more The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October 2008 with the sole, focused science objective to discover the global interaction between the solar wind and the interstellar medium. IBEX is designed to achieve this objective by answering four fundamental science questions: (1) What is the global strength and structure of the termination shock, (2) How are energetic protons accelerated
In measured pickup ion spectra a special spectral feature has been identified on the high side of... more In measured pickup ion spectra a special spectral feature has been identified on the high side of the pickup ion injection energy which cannot be explained as due to energy diffusion of primary pickup protons originating from ionized interstellar H-atoms. As we can show, however, in the following article, this feature can be explained as a secondary pickup ion feature due to ions originating from the charge exchange ionization of energetic neutral H-atoms entering the inner heliosphere and coming from their birth places in the heliosheath. It may thus be the case that energetic neutral H-atoms from the heliosheath region, searched for by experimenters for quite some time now, have already been seen as secondary pickup ions in special features of measured distribution functions.
The helium gravitational focusing cone has been observed using pickup He + , first during the sol... more The helium gravitational focusing cone has been observed using pickup He + , first during the solar minimum in 1984-1985 with the AMPTE/IRM spacecraft, and again in more detail from 1998 to 2002 with ACE and in 2000 with Nozomi. Five traversals of the cone allow us to obtain an accurate determination of the ecliptic longitude of the interstellar wind flow direction, λ = 74.43 • ± 0.33 • , while observations of pickup He ++ with Ulysses give us an estimate, relatively free of instrumental systematic uncertainties, of the neutral He density, n He = 0.0151 ± 0.0015 cm −3 , in the Local Interstellar Cloud. From best fits to the measured velocity distributions of pickup He + using time-stationary models we deduce the radial dependence and magnitude of electron-impact ionization rates that cannot presently be measured, and find this to be an important ionization process in the inner (< ∼ 0.5 AU) heliosphere. We obtain excellent model fits to the 1998 cone profile using measured or deduced rates and known interstellar He parameters, and from this conclude that cross-field diffusion of pickup He + is small. Furthermore, we find no evidence for extra sources of He in or near the cone region. Best fits to the velocity distributions of He + are obtained assuming isotropic solar-wind-frame distributions, and we conclude from this that the scattering mean free path for pickup He + in the turbulent slow solar wind is small, probably less than 0.1 AU. We argue that application of 3D, time-dependent models for computation of the spatial distribution of interstellar neutral helium in the inner heliosphere may lead to excellent fits of short-term averaged pickup He + data without assuming loss rates that are significantly different from production rates.
The solar corona produces a plasma flow which expands as the supersonic solar wind to distances m... more The solar corona produces a plasma flow which expands as the supersonic solar wind to distances much beyond the region of the solar system planets. Due to the motion of the solar system relative to the ambient interstellar medium a very complex form of a magnetohydrodynamic interaction between the two counterstreaming media of solar and interstellar origin is esta blished. As we will show this interaction can only be adequately described in the frame of a multifluid simulation code which takes into account in a consistent form the dynamical coupling of protons, pick-up ions, H-atoms, and cosmic rays of anomalous and galactic type. The resulting conditions, especially for the inner heliosphere are highly sensitive to the physical state of the interstellar medium. As we will show, even the Earth and the terrestrial environment are characterized by strongly enhanced intensities of cosmic particle radiations at the event of a solar system crossing through thick interstellar cloud materi...
The Interstellar Boundary Explorer (IBEX) mission is exploring the frontiers of the heliosphere w... more 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.
The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October... more The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October 2008 with the sole, focused science objective to discover the global interaction between the solar wind and the interstellar medium. IBEX is designed to achieve this objective by answering four fundamental science questions: (1) What is the global strength and structure of the termination shock, (2) How are energetic protons accelerated 12 D.J. McComas et al. at the termination shock, (3) What are the global properties of the solar wind flow beyond the termination shock and in the heliotail, and (4) How does the interstellar flow interact with the heliosphere beyond the heliopause? The answers to these questions rely on energyresolved images of energetic neutral atoms (ENAs), which originate beyond the termination shock, in the inner heliosheath. To make these exploratory ENA observations IBEX carries two ultra-high sensitivity ENA cameras on a simple spinning spacecraft. IBEX's very high apogee Earth orbit was achieved using a new and significantly enhanced method for launching small satellites; this orbit allows viewing of the outer heliosphere from beyond the Earth's relatively bright magnetospheric ENA emissions. The combination of full-sky imaging and energy spectral measurements of ENAs over the range from ∼10 eV to 6 keV provides the critical information to allow us to achieve our science objective and understand this global interaction for the first time. The IBEX mission was developed to provide the first global views of the Sun's interstellar boundaries, unveiling the physics of the heliosphere's interstellar interaction, providing a deeper understanding of the heliosphere and thereby astrospheres throughout the galaxy, and creating the opportunity to make even greater unanticipated discoveries.
As the Sun moves through the local interstellar medium, its supersonic, ionized solar wind carves... more As the Sun moves through the local interstellar medium, its supersonic, ionized solar wind carves out a cavity called the heliosphere. Recent observations from the Interstellar Boundary Explorer (IBEX) spacecraft show that the relative motion of the Sun with respect to the interstellar medium is slower and in a somewhat different direction than previously thought. Here, we provide combined consensus values for this velocity vector and show that they have important implications for the global interstellar interaction. In particular, the velocity is almost certainly slower than the fast magnetosonic speed, with no bow shock forming ahead of the heliosphere, as was widely expected in the past.
As known for a long time, interstellar wind neutral helium atoms deeply penetrate into the inner ... more 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-entrain...
The Sun moves through the local interstellar medium (ISM), continuously emitting ionized, superso... more The Sun moves through the local interstellar medium (ISM), continuously emitting ionized, supersonic solar wind plasma and carving out a cavity in interstellar space called the heliosphere. The border of this cavity traces regions where the interstellar gas first interacts with the solar wind. The Interstellar Boundary Explorer (IBEX) spacecraft has just completed the first all-sky maps of this interstellar interaction at the edge of the heliosphere, by imaging energetic neutral atoms (ENAs) emanating from this region. The IBEX all-sky maps and energy spectra provide detailed information about this interaction. Our observations show globally distributed ENA fluxes ordered by the solar wind structure, superimposed on an unexpected feature. This poster summarizes the IBEX observations, shares our unexpected results, and discusses the new information on stellar wind interactions with the ISM that we have gleaned from these data. Acknowledgments: We thank the IBEX team members. This work is primarily funded by the NASA Explorer Program.
Astrophysics and Space Sciences Transactions, 2009
We investigate the well accepted idea that anomalous cosmic ray (ACR) ions originate from suprath... more We investigate the well accepted idea that anomalous cosmic ray (ACR) ions originate from suprathermal halo ions by means of diffusive shock acceleration. Usually the seed ions are taken to be the interplanetary pickup ions, but here we want to enlarge this idea by taking in addition as seed candidates also into account normal solar wind ions which according to our recent calculations are reflected from the termination shock into the upstream solar wind flow and then are also picked-up as suprathermal ions. We start out from an ideally planar shock approximation and fix the ACR spectrum by absolute spectral intensities and maximum ACR energies, taking a precalculated fraction of the suprathermal ion flow as ACR injection rate. Comparison of our calculated spectral intensities with ACR measurements near 94 AU, shows that satisfying data fits only can be achieved, if about three percent of the suprathermal ions swept into the shock structure enter into the Fermi-1 acceleration process. We also show that the spectral slope of the ACR spectra is decreasing and the spectral intensities are increasing with increasing shock compression ratios. As maximum energies available from an ideally operating diffusive Fermi-1 acceleration process we find, depending on the shock compression ratio, ion energies ranging from a few MeV up to 10 3 MeV. Compared to observations this seems to be a little on the high side and may point to the fact that injection into Fermi-1 acceleration near the termination shock is occurring only sporadically due to variable upstream magnetic field orientations with respect to the shock normal vector, i.e. due to variations of the obliquity of the local shock surface with respect to the local upstream magnetic field.
ABSTRACT Our numerical analyses of the velocity and spatial distributions of pickup interstellar ... more ABSTRACT Our numerical analyses of the velocity and spatial distributions of pickup interstellar helium ions in the region of the solar gravitational cone in the ecliptic plane at a distance of 1 AU show that the ion density maximum must be displaced relative to the neutral helium cone axis in the direction of the Earth’s revolution around the Sun. The solar wind parameters in the numerical model correspond to their observed values during the crossing of the helium cone by the ACE spacecraft in 1998. At these parameters, the calculated angular displacement is 5°. The absence of a similar displacement in the ACE measurements is shown to stem from the fact that the spectrometer onboard ACE records and identifies only a fraction of the pickup helium ions with fairly high magnitudes and certain directions of the velocities.
The ionization of hydrogen atoms that penetrate into the heliosphere from the interstellar medium... more 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 Alfvnic 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
It is well known that newly created pickup ions are first injected into a toroidal distribution i... more It is well known that newly created pickup ions are first injected into a toroidal distribution in the velocity space which comoves with the solar wind. This distribution is, however, unstable with respect to excitation of resonant MHD waves, and thus by wave-driving some specific amount of the initial kinetic energy of pickup ions is handed over to the ambient wave fields, while the pickup ion velocity distribution thereby is rearranged. In this paper we describe the developement of the wavenumber spectrum of pickup proton-generated turbulence and the pickup proton energy spectrum resulting from resonant reabsorption of this turbulence, by solving simultaneously the coupled system of two differential equations for the spectral turbulence power and the pickup proton spectral intensity. We show that the majority of pickup generated wave energy in the range of wavenumbers above the wavenumber at which the waves are produced is reabsorbed by the pickup protons themselves.
A coordinated effort to combine all three methods that are used to determine the physical paramet... more A coordinated effort to combine all three methods that are used to determine the physical parameters of interstellar gas in the heliosphere has been undertaken. In order to arrive at a consistent parameter set that agrees with the observations of neutral gas, pickup ions and UV backscattering we have combined data sets from coordinated observation campaigns over three years from 1998 through 2000. The key observations include pickup ions with ACE and Ulysses SWICS, neutral atoms with Ulysses GAS, as well as UV backscattering at the He focusing cone close to the Sun with SOHO UVCS and at 1 AU with EUVE. For the first time also the solar EUV irradiance that is responsible for photo ionization was monitored with SOHO CELIAS SEM, and the He I 58.4 nm line that illuminates He was observed simultaneously with SOHO SUMER. The solar wind conditions were monitored with SOHO, ACE, and WIND. Based on these data the modeling of the interstellar gas and its secondary products in the heliosphere has resulted in a consistent set of interstellar He parameters with much reduced uncertainties, which satisfy all observations, even extended to earlier data sets. It was also established that a substantial ionization in addition to photo ionization, most likely electron impact, is required, with increasing relative importance closer to the Sun. Furthermore, the total combined ionization rate varies significantly with solar latitude, requiring a fully three dimensional and time dependent treatment of the problem.
The Solar System during its life has travelled more than 10 times through dense interstellar clou... more The Solar System during its life has travelled more than 10 times through dense interstellar clouds with particle concentrations of 10 2 −10 3 cm −3 and more, compressing the heliosphere to heliopause dimensions smaller than 1 AU and thus bringing the Earth in immediate contact with the interstellar matter. For cloud concentrations greater than of 10 2 cm −3 , the flowing interstellar material even at the Earth's orbit remains completely shielded from solar wind protons and would only be subject to solar photoionization processes. We have developed a 2D-two-fluid gas-dynamical numerical code to describe the hydrodynamical behavior of the incoming interstellar gas near the Earth, taking into account both the photoionization and the gravity of the Sun. As we show, the resulting strongly increased neutral hydrogen fluxes ranging from 10 9 to 10 11 cm −2 s −1 cause substantial changes in the terrestrial atmosphere. During the phase of the immersion into the cloud the resulting flux of neutral hydrogen incident on the terrestrial atmosphere in the steady state would be balanced by the upward escape flux of H-atoms and the downward flux of water molecules, which is the product of the atmospheric hydrogen-oxygen chemistry via even-odd reaction schemes. In that case hydrogen acts as a chemical agent to remove oxygen atoms and to cause ozone concentration reductions above 50 km by a factor of 1.5 at the stratopause to about a factor of 1000 and more at the mesopause. Thus, depending on the specific encounter parameters the high mixing ratio of hydrogen in the Earth's atmosphere may substantially decrease the ozone concentration in the mesosphere and may trigger an ice age of relatively long duration.
The problem of passages of the Solar System through dense, dusty and cold interstellar gas clouds... more The problem of passages of the Solar System through dense, dusty and cold interstellar gas clouds is discussed. A considerable number of such encounters must have already occurred in the past and these events must have pushed the heliosphere inward to the region of the terrestrial planets, provided a number density of the cloud's material of the order of, or greater than, 1000 cm −3 , and relative velocities of about 20 km s −1 prevail. Under these conditions it will happen that the Earth is directly exposed to the interstellar flow. Neutral interstellar hydrogen and proton densities at the Earth's orbit are calculated under conditions of the heliosphere with the heliopause reduced to dimensions smaller than 1 AU. A simple twofluids treatment of the incoming flow, ionized by the solar UV only, is proposed here and the amount of neutral interstellar hydrogen is assessed which is accreted by the Earth's atmosphere during one passage. The behavior of the flow variables is investigated by a 2D-hydrodynamic approach to model the interaction processes. Because of a reduction of the heliospheric dimension down to 1 AU, direct influence of the cloud's matter to the terrestrial environment and atmospheric chemistry must be envisaged. This will be briefly discussed in the paper.
Up until the recent past, it was generally believed that the solar wind termination shock (TS) is... more Up until the recent past, it was generally believed that the solar wind termination shock (TS) is the favourite site to accelerate ions from the keV-to the MeV-energy levels by means of Fermi-1 processes. When Voyager 1 was crossing the TS at the end of 2004, the registrations of this spacecraft showed, however, that beyond the shock passage fluxes of anomalous cosmic rays kept increasing with time. This obviously called for an acceleration site further downstream of the shock in the heliosheath which had not been identified before. In this paper we thus investigate the process of energy diffusion due to wave-particle interactions (Fermi-2) operating on pickup ions which are convected downstream of the TS with the subsonic solar wind. We investigate the continuous effect of stochastic acceleration processes suffered by pickup ions at their interaction with heliosheath turbulences, while they are slowly convected with the subsonic solar wind towards the heliotail. As we can show, the inner heliosheath region, with an extent of about 100 AU around the solar wind stagnation point, is specifically favourable for the energy processing of pickup ions by Fermi-2 processes up to MeV energies. In addition, we claim that this region is the origin of multiply-charged anomalous cosmic ray particles that have been registered in recent times.
A new technique to directly detect low energy neutral helium has been developed and successfully ... more A new technique to directly detect low energy neutral helium has been developed and successfully used in space for the first time. It makes possible the in-situ measurement of the local angular distribution of the flow of interstellar neutral helium in the inner heliosphere. Based on the transformation of a velocity distribution, using Liouville's theorem, a straightforward model has been developed to determine, from these measurements, the flow parameters of the interstellar helium at infinity. From twelve measurements at radial distances between 1 and 5 AU, average values of the flow parameters were determined, namely: v~= 26 ±1 km s 1, downstream direction: .\~= 72 ±2.4°,/~=-2.5 ± 2.7°,temperature T,~,= 6700 ±1500 K. These values are in agreement with results found by completely different methods (such as, backscattered UV-light, pickup ions), although in detail there are significant differences.
The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October... more The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October 2008 with the sole, focused science objective to discover the global interaction between the solar wind and the interstellar medium. IBEX is designed to achieve this objective by answering four fundamental science questions: (1) What is the global strength and structure of the termination shock, (2) How are energetic protons accelerated
In measured pickup ion spectra a special spectral feature has been identified on the high side of... more In measured pickup ion spectra a special spectral feature has been identified on the high side of the pickup ion injection energy which cannot be explained as due to energy diffusion of primary pickup protons originating from ionized interstellar H-atoms. As we can show, however, in the following article, this feature can be explained as a secondary pickup ion feature due to ions originating from the charge exchange ionization of energetic neutral H-atoms entering the inner heliosphere and coming from their birth places in the heliosheath. It may thus be the case that energetic neutral H-atoms from the heliosheath region, searched for by experimenters for quite some time now, have already been seen as secondary pickup ions in special features of measured distribution functions.
The helium gravitational focusing cone has been observed using pickup He + , first during the sol... more The helium gravitational focusing cone has been observed using pickup He + , first during the solar minimum in 1984-1985 with the AMPTE/IRM spacecraft, and again in more detail from 1998 to 2002 with ACE and in 2000 with Nozomi. Five traversals of the cone allow us to obtain an accurate determination of the ecliptic longitude of the interstellar wind flow direction, λ = 74.43 • ± 0.33 • , while observations of pickup He ++ with Ulysses give us an estimate, relatively free of instrumental systematic uncertainties, of the neutral He density, n He = 0.0151 ± 0.0015 cm −3 , in the Local Interstellar Cloud. From best fits to the measured velocity distributions of pickup He + using time-stationary models we deduce the radial dependence and magnitude of electron-impact ionization rates that cannot presently be measured, and find this to be an important ionization process in the inner (< ∼ 0.5 AU) heliosphere. We obtain excellent model fits to the 1998 cone profile using measured or deduced rates and known interstellar He parameters, and from this conclude that cross-field diffusion of pickup He + is small. Furthermore, we find no evidence for extra sources of He in or near the cone region. Best fits to the velocity distributions of He + are obtained assuming isotropic solar-wind-frame distributions, and we conclude from this that the scattering mean free path for pickup He + in the turbulent slow solar wind is small, probably less than 0.1 AU. We argue that application of 3D, time-dependent models for computation of the spatial distribution of interstellar neutral helium in the inner heliosphere may lead to excellent fits of short-term averaged pickup He + data without assuming loss rates that are significantly different from production rates.
The solar corona produces a plasma flow which expands as the supersonic solar wind to distances m... more The solar corona produces a plasma flow which expands as the supersonic solar wind to distances much beyond the region of the solar system planets. Due to the motion of the solar system relative to the ambient interstellar medium a very complex form of a magnetohydrodynamic interaction between the two counterstreaming media of solar and interstellar origin is esta blished. As we will show this interaction can only be adequately described in the frame of a multifluid simulation code which takes into account in a consistent form the dynamical coupling of protons, pick-up ions, H-atoms, and cosmic rays of anomalous and galactic type. The resulting conditions, especially for the inner heliosphere are highly sensitive to the physical state of the interstellar medium. As we will show, even the Earth and the terrestrial environment are characterized by strongly enhanced intensities of cosmic particle radiations at the event of a solar system crossing through thick interstellar cloud materi...
The Interstellar Boundary Explorer (IBEX) mission is exploring the frontiers of the heliosphere w... more 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.
The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October... more The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October 2008 with the sole, focused science objective to discover the global interaction between the solar wind and the interstellar medium. IBEX is designed to achieve this objective by answering four fundamental science questions: (1) What is the global strength and structure of the termination shock, (2) How are energetic protons accelerated 12 D.J. McComas et al. at the termination shock, (3) What are the global properties of the solar wind flow beyond the termination shock and in the heliotail, and (4) How does the interstellar flow interact with the heliosphere beyond the heliopause? The answers to these questions rely on energyresolved images of energetic neutral atoms (ENAs), which originate beyond the termination shock, in the inner heliosheath. To make these exploratory ENA observations IBEX carries two ultra-high sensitivity ENA cameras on a simple spinning spacecraft. IBEX's very high apogee Earth orbit was achieved using a new and significantly enhanced method for launching small satellites; this orbit allows viewing of the outer heliosphere from beyond the Earth's relatively bright magnetospheric ENA emissions. The combination of full-sky imaging and energy spectral measurements of ENAs over the range from ∼10 eV to 6 keV provides the critical information to allow us to achieve our science objective and understand this global interaction for the first time. The IBEX mission was developed to provide the first global views of the Sun's interstellar boundaries, unveiling the physics of the heliosphere's interstellar interaction, providing a deeper understanding of the heliosphere and thereby astrospheres throughout the galaxy, and creating the opportunity to make even greater unanticipated discoveries.
As the Sun moves through the local interstellar medium, its supersonic, ionized solar wind carves... more As the Sun moves through the local interstellar medium, its supersonic, ionized solar wind carves out a cavity called the heliosphere. Recent observations from the Interstellar Boundary Explorer (IBEX) spacecraft show that the relative motion of the Sun with respect to the interstellar medium is slower and in a somewhat different direction than previously thought. Here, we provide combined consensus values for this velocity vector and show that they have important implications for the global interstellar interaction. In particular, the velocity is almost certainly slower than the fast magnetosonic speed, with no bow shock forming ahead of the heliosphere, as was widely expected in the past.
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Papers by H. Fahr