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Hans-jörg Fahr - One of the best experts on this subject based on the ideXlab platform.

  • the electron distribution function downstream of the Solar Wind Termination shock where are the hot electrons
    Astronomy and Astrophysics, 2015
    Co-Authors: Hans-jörg Fahr, J D Richardson, Daniel Verscharen
    Abstract:

    In the majority of the literature on plasma shock waves, electrons play the role of “ghost particles”, since their contribution to mass and momentum flows is negligible, and they have been treated as only taking care of the electric plasma neutrality. In some more recent papers, however, electrons play a new important role in the shock dynamics and thermodynamics, especially at the Solar-Wind Termination shock. They react on the shock electric field in a very specific way, leading to suprathermal nonequilibrium distributions of the downstream electrons, which can be represented by a kappa distribution function. In this paper, we discuss why this anticipated hot electron population has not been seen by the plasma detectors of the Voyager spacecraft downstream of the Solar-Wind Termination shock. We show that hot nonequilibrium electrons induce a strong negative electric charge-up of any spacecraft cruising through this downstream plasma environment. This charge reduces electron fluxes at the spacecraft detectors to nondetectable intensities. Furthermore, we show that the Debye length λ κ grows to values of about λ κ /λD � 10 6 compared to the classical value λD in this hot-electron environment. This unusual condition allows for the propagation of a certain type of electrostatic plasma waves that, at very large wavelengths, allow us to determine the effective temperature of the suprathermal electrons directly by means of the phase velocity of these waves. At moderate wavelengths, the electron-acoustic dispersion relation leads to nonpropagating oscillations with the ion-plasma frequency ωp, instead of the traditional electron plasma frequency.

  • Entropy generation at multi-fluid magnetohydrodynamic shocks with emphasis to the Solar Wind Termination shock
    Astronomy and Astrophysics, 2015
    Co-Authors: Hans-jörg Fahr, Mark Siewert
    Abstract:

    In a series of earlier papers, we developed expressions for ion and electron velocity distribution functions and their velocity moments at the passage over the Solar Wind Termination shock. As we have shown there, with the introduction of appropriate particle invariants and the use of Liouville’s theorem one can get explicit solutions for the resulting total downstream pressure by adding up from partial pressure contributions of Solar Wind protons, Solar Wind electrons and pick-up protons. These expressions are the first step toward delivering the main contributions to the total plasma pressure in the downstream plasma flow and consistently determine the shock compression ratio. Here we start from these individual fluid pressures downstream of the shock and thereafter evaluate for the first time the shock-induced entropy production of the different fluids, when they are passing over the shock to the downstream side. As shown here, the resulting ion entropy production substantially deviates from earlier calculations using a pseudo-polytropic reaction of the ions to the shock compression, with polytropies selected to describe fluid-specific reactions at the shock passage similar to those seen by the Voyagers. From these latter models, ion entropy jumps are derived that depend on the pick-up ion abundance, while our calculations deliver an abundance-independent ion entropy production that only depends on the shock compression ratio and the tilt angle between the upstream magnetic field and the normal to the shock surface. We also show here that the thermodynamically permitted upper limit in the entropy production is only reached when strongly heated electrons are included in the entropy balance.

  • Entropy generation at the multi-fluid MHD Solar Wind Termination shock
    arXiv: Solar and Stellar Astrophysics, 2014
    Co-Authors: Hans-jörg Fahr, Mark Siewert
    Abstract:

    In a series of earlier papers, we have developed expressions for ion and electron velocity distribution functions and theirvelocity moments at the passage over the Solar Wind Termination shock. As we have shown there, with introduction of appropriate particle invariants and the use of Liouville`s theorem one can get explicit solutions for the resulting total downstream pressure adding up from partial pressure contributions of Solar Wind protons, Solar Wind electrons and pick-up protons. These expressions deliver in a first step the main contributions to the total plasma pressure in the downstream plasma flow and consistently determine the shock compression ratio. Here now we start out from these individual fluid pressures downstream of the shock and thereafter evaluate for the first time the shock-induced entropy production of the different fluids, when they are passing over the shock to the downstream side. As is shown here, the resulting ion entropy production substantially deviates from earlier calculations using a pseudo-polytropic reaction of the ions to the shock compression, with polytropies selected to describe fluid-specific reactions at the shock passage similar to those seen by the VOYAGERs. From these latter models ion entropy jumps are derived that depend on the pick-up ion abundance, while our calculations, to the opposite, deliver an abundance-independent ion entropy production which only depends on the shock compression ratio and the tilt angle between the upstream magnetic field and the shock surface normal. We also do show here that only when including the strongly heated electrons into the entropy balance one then arrives at the total entropy production that just fulfills the thermodynamically permitted limit.

  • The multi-fluid pressures downstream of the Solar Wind Termination shock
    Astronomy and Astrophysics, 2013
    Co-Authors: Hans-jörg Fahr, Mark Siewert
    Abstract:

    In this paper we consider a multi-fluid plasma that describes the upstream Solar Wind at its passage over the Solar Wind Termination shock. In one respect, the plasma at the shock reacts like a joint fluid that is described by a single compression ratio. This ratio depends on all upstream and downstream pressures of the magnetohydrodynamic (MHD) plasma. In another respect, the distinguished plasma fluids in their downstream properties show fluid-specific reactions, thet we describe by using additional kinetic information on the plasma constituents, such as the Liouville theorem, the conservation of typical particle invariants, and the species-specific influence of the electric shock ramp. We thus obtain the resulting distribution functions of the seperate fluid particles and their associated velocity moments for the downstream region, especially their separate fluid pressures. We show that the different fluid pressures in different forms depend on the shock compression ratio and on the tilt angle between the upstream magnetic field and the shock surface normal. The dominant downstream pressures are connected with the pick-up protons and with the Solar Wind electrons, one dominating under some given shock conditions, the other dominating under some other shock conditions. Since the downstream distributions of Solar Wind protons and pick-up protons partly overlap in velocity space, we look for a joint distribution of the joint proton population in the form of a joint Kappa distribution and find that the associated Kappa index and the “Gaussian velocity width” are functions of the pick-up ion abundance, of the joint compression ratio, and of the tilt angle. Owing to the strongly heated electrons the energy-per-mass density ratio of the downstream plasma turns out to be fairly different from all that was expected up to now. This might also give a hint as to why the heliosheath plasma flow lines seen by Voyagers are different from all MHD simulations so far.

  • On the electron temperature downstream of the Solar Wind Termination shock
    Annales Geophysicae, 2013
    Co-Authors: I. V. Chashei, Hans-jörg Fahr
    Abstract:

    Abstract. In this paper we study the temperatures of electrons convected with the Solar Wind to large Solar distances and finally transported over the Solar Wind Termination shock. Nearly nothing, unless at high energies in the cosmic ray regime, is known about the thermodynamical behaviour of these distant electrons from in~situ plasma observations. Hence it is tacitly assumed these electrons, due to their adiabatic behaviour and vanishing heat conduction or energization processes, have rapidly cooled off to very low temperatures once they eventually arrive at the Solar Wind Termination shock (at about 100 AU). In this paper we show that such electrons, however, at their passage over the Termination shock due to the shock–electric field action undergo an over-adiabatic heating and therefore appear on the downstream side as a substantially heated plasma species. Looking quantitatively into this heating process we find that Solar Wind electrons achieve temperatures of the order of 2–4 × 106 K downstream of the Termination shock, depending on the upstream Solar Wind bulk velocity and the shock compression ratio. Hence these electrons therewith play an important dynamical role in structuring this shock and determining the downstream plasma flow properties. Furthermore, they present an additional ionization source for incoming neutral interstellar hydrogen and excite X-ray emission. They also behave similar to cosmic ray electrons and extend to some limited region upstream of the shock of the order of 0.1 AU by spatial diffusion and thereby also modify the upstream Solar Wind properties.

William R. Webber - One of the best experts on this subject based on the ideXlab platform.

  • An asymmetric Solar Wind Termination shock
    Nature, 2008
    Co-Authors: E. C. Stone, A. C. Cummings, Frank B. Mcdonald, Bryant C. Heikkila, Nand Lal, William R. Webber
    Abstract:

    The supersonic (with respect to the interstellar medium) Solar Wind creates a heliospheric bubble around the Sun. A Termination shock occurs where the Wind abruptly slows and forms the heliosheath. Voyager 2 crossed the Termination shock at 83.7 au in the southern hemisphere. The intensity of 4–5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. Voyager 2 crossed the Solar Wind Termination shock at 83.7  au in the southern hemisphere, ∼10  au closer to the Sun than found by Voyager 1 in the north^ 1 , 2 , 3 , 4 . This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field^ 5 , 6 , from transient-induced shock motion^ 7 , or from the Solar Wind dynamic pressure. Here we report that the intensity of 4–5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma^ 8 , magnetic field^ 9 , plasma-wave^ 10 and lower energy particle^ 11 observations at the shock.) Voyager 2 did not find the source of anomalous cosmic rays at the shock, suggesting that the source is elsewhere on the shock^ 12 , 13 , 14 or in the heliosheath^ 15 , 16 , 17 , 18 , 19 . The small intensity gradient of Galactic cosmic ray helium indicates that either the gradient is further out in the heliosheath^ 20 or the local interstellar Galactic cosmic ray intensity is lower than expected^ 21 . On 30 August 2007 Voyager 2 began to cross the Termination shock, a boundary produced by the inter-action of the Sun with the rest of the Galaxy, where the supersonic Solar Wind abruptly slows as it presses outward against the surrounding interstellar matter. Five Letters in this issue present the data that the probe sent back. The Voyager 2 crossings occurred about 1.5 billion kilometres closer to the Sun than those of Voyager 1, illustrating the asymmetry of the heliosphere. The results from the plasma experiment, low-energy particle, cosmic ray, magnetic field and plasma-wave detectors reveal a complex and dynamic shock, reforming itself in hours rather than days. The cover graphic of Voayer 2 on the brink of entering interstellar space is by Henry Kline of JPL. It may be decades before another probe crosses the Termination shock but remote observations can now bridge the gap — as shown by Wang et al . who report measurements of energetic neutral atoms in the heliosheath from the STEREO A and B spacecraft that complement the Voyager in situ observations made at the same time. In News & Views, J R Jokipii puts the Voyager findings into context. For more on the on Voyager odyssey, see page 24, and the Author page, and go to the movie on http://www.nature.com/nature/videoarchive/voyager .

  • An asymmetric Solar Wind Termination shock
    Nature, 2008
    Co-Authors: E. C. Stone, A. C. Cummings, Frank B. Mcdonald, Bryant C. Heikkila, Nand Lal, William R. Webber
    Abstract:

    Voyager 2 crossed the Solar Wind Termination shock at 83.7 au in the southern hemisphere, ~10 au closer to the Sun than found by Voyager 1 in the north. This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field, from transient-induced shock motion, or from the Solar Wind dynamic pressure. Here we report that the intensity of 4–5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma, magnetic field, plasma-wave and lower energy particle observations at the shock.) Voyager 2 did not find the source of anomalous cosmic rays at the shock, suggesting that the source is elsewhere on the shock or in the heliosheath. The small intensity gradient of Galactic cosmic ray helium indicates that either the gradient is further out in the heliosheath or the local interstellar Galactic cosmic ray intensity is lower than expected.

  • Voyager observations of energetic particles near the Solar Wind Termination shock
    2007
    Co-Authors: E. C. Stone, Frank B. Mcdonald, Bryant C. Heikkila, Nand Lal, A. C. Cummings, William R. Webber
    Abstract:

    Voyager 1 is observing the energy spectra of anomalous cosmic ray H, He, and O in the heliosheath. The relative abundance of H/He is 20 at 1-1.5 MeV/nuc, indicating that H pickup ions are more efficiently accelerated than was modeled from observations at higher energies. Voyager 2 began observing upstream energetic ions from the shock in late 2004 at ∼75 AU, about 10 AU closer to the Sun than observed by Voyager 1. Voyager 2 will be at 84.7 AU at the end of 2007 and may have crossed the Termination shock.

  • Characteristics of the Solar Wind Termination Shock Region from Voyager 1 Observations
    2005
    Co-Authors: A. C. Cummings, E. C. Stone, Frank B. Mcdonald, Bryant C. Heikkila, Nand Lal, William R. Webber
    Abstract:

    The Voyager 1 spacecraft crossed the Solar Wind Termination shock on 16 December 2004 at a distance of 94.0 AU from the Sun and at a heliolatitude of 34 N. Up to that time, since mid-2002, Voyager 1 had been on interplanetary magnetic field lines intermittently connected to a source of low-energy particles at the Termination shock. The energy spectra after the crossing, and often prior to the crossing as well, resemble the energy spectra expected for anomalous cosmic rays (ACRs) at the shock, with one important difference. Although the spectra have a power-law dependence at low energies and a roll off to a much steeper power-law spectrum at higher energies, similar to that expected for the ACR spectrum at the shock, the energy of the roll off is much lower than observed in the ACR spectrum at the same time. Hence, the ACRs must be originating from a different part of the shock. We find that the roll-off energy for protons is about 3 MeV, rather than ~100 MeV expected for ACR protons. The power-law spectral index at low energies is approximately –1.5, implying a shock strength of ~2.5.

  • Voyager 1 Observations of the Anisotropies of Enhanced MeV Ion Fluxes at 85 AU
    2003
    Co-Authors: A. C. Cummings, Leonard F. Burlaga, E. C. Stone, Norman F. Ness, F. B. Mcdonald, William R. Webber
    Abstract:

    We report on the diffusive flow anisotropies observed in low-energy protons during intensity increases at Voyager 1 in 2001 and 2002. We propose that the unusual increased intensities after mid-2002 are due to particles accelerated by the Solar Wind Termination shock or by the turbulent interface between the high-speed and low-speed Solar Wind flows.

S. V. Chalov - One of the best experts on this subject based on the ideXlab platform.

  • The role of Solar Wind electrons at the Solar Wind Termination shock
    Monthly Notices of the Royal Astronomical Society, 2013
    Co-Authors: S. V. Chalov, Hans-jörg Fahr
    Abstract:

    Voyager 2 plasma observations have recently revealed that, as predicted by theory, there exists a Solar Wind Termination shock at 87 au. However, it is evidently different from classical expectations, for instance revealing the downstream Solar Wind protons still to be in a supersonic mode. In this paper we show that in order to explain the non-classical structure and facts of this shock, one has to start from a multi-fluid magnetohydrodynamic (MHD) approach describing the shock transition. Different from our earlier attempt, here we consider Solar Wind electrons as an additional extra fluid and allow for preferential heating of electrons compared to thermal Solar Wind protons at the shock. As we can then show, with this enlargement of the MHD theory by a separate electron fluid we are able to describe observed features of the Solar Wind Termination shock. We arrive at the conclusion that the downstream Solar Wind thermal plasma is energetically dominated by the pressure of quasi-mass-less electrons.

  • Shock-drift acceleration of interstellar pickup protons at the Solar Wind Termination shock
    Journal of Physics: Conference Series, 2013
    Co-Authors: S. V. Chalov, Dmitry Alexashov, Yury G. Malama, Vladislav V. Izmodenov
    Abstract:

    The numerical model of the shock-drift acceleration of interstellar pick-up protons at the Solar Wind Termination shock is presented taking into account multiple reflections of the particles at and passing through the shock. The processes of the multiple interactions with the shock are possible due to pitch-angle scattering in the upstream and downstream parts of the Solar Wind flow. The model takes into account variations of the magnetic field direction near the shock front connected with its three-dimensional shape. The main advantage of the model is the self-consistent treatment of protons and interstellar hydrogen atoms, which interact with each other through the resonant charge exchange process. The model gives very simple and natural resolution of so-called injection problem for anomalous cosmic rays. The shock-drift acceleration can explain the fluxes of the anomalous cosmic rays measured at the Voyager 1/2 spacecraft after the Termination shock crossings.

  • The multifluid character of the Solar Wind Termination shock explaining the downstream supersonic Solar Wind ion flow
    2010
    Co-Authors: S. V. Chalov, Hans-jörg Fahr
    Abstract:

    The Voyager‐2 observations at the recent crossing of the Solar Wind Termination shock show that the downstream thermal protons still move with supersonic speed. Obviously it is due to their inefficient shock‐heating and that the surpathermal ions absorb most of the upstream kinetic Solar Wind energy. In this paper we present a three‐fluid approach of the Solar Wind plasma consisting of a thermal, a suprathermal and a high‐energetic fluid. Within a consistent set of conservation equations for this three‐fluid plasma we derive solutions for the observed properties of the upstream precursor and downstream plasma, assuming that the conservation of the magnetic moment of suprathermal ions in the jump conditions is fulfilled.

  • On the injection problem at the Solar Wind Termination shock
    Astronomy and Astrophysics, 2005
    Co-Authors: Reinald Kallenbach, S. V. Chalov, Jakobus A. Le Roux, Martin Hilchenbach, K. Bamert
    Abstract:

    This article presents an integrated analytical model on the injection efficiencies of the different ion species of the Anomalous component of the Cosmic Rays (ACRs) at the Solar Wind Termination shock. We find that the injection into diffusive (first-order Fermi) acceleration is dominated by parallel ion diffusion and not by perpendicular diffusion unless the angle  Ψ between the shock normal and the heliospheric magnetic field is almost exactly 90° ($89.3^\circ < \Psi \approx 90^\circ$). In steady state the threshold speed for injection into first-order Fermi acceleration at a not exactly perpendicular Solar Wind Termination shock – with the Parker shock angle $\Psi \approx 89.3^\circ$ – adjusts itself self-consistently. Increased anisotropic ACR flux amplifies Alfvenic turbulence which in turn suppresses parallel diffusion. It therefore increases the injection threshold and decreases the ACR flux until equilibrium is reached. For this equilibrium situation, we estimate the injection efficiencies of different species of suprathermal ions at the Termination shock. We consider the following pre-acceleration processes: 1) momentum diffusion in compressional (ion-acoustic and magnetosonic) turbulence in the upstream supersonic Solar Wind and adiabatic cooling during convection to the Termination shock; 2) reflection, transmission, and acceleration in the electric potential of the Termination shock; and 3) momentum diffusion (stochastic or second-order Fermi acceleration) in the subsonic Solar Wind downstream of the Termination shock in the inner heliosheath region. Our model results are compared to data from instruments on board the SOHO, ACE, Ulysses, and Voyager spacecraft.

  • Acceleration of Pick-up Ions at the Solar Wind Termination Shock
    Astrophysics and Space Science, 2000
    Co-Authors: S. V. Chalov
    Abstract:

    It is generally accepted that pick-up ions act as a seed population for anomalous cosmic rays originating at the Solar Wind Termination shock. We believe that the ion pre-acceleration process operating in the heliosphere up to the Termination shock can be very important to inject the ions into the shock acceleration process. The pick-up ions pre-accelerated by Solar Wind turbulences have already a pronounced high energy tail when they reach the shock. Some fraction of these ions can experience further acceleration up to energies of anomalous cosmic rays by means of shock drift and diffusive acceleration. In the present paper the shock drift acceleration of pick-up ions suffering multiple reflection due to abrupt changes in both the strength and direction of the magnetic field through the shock is considered. The reflection process operates for high velocity particles different from the reflection by the electric cross-shock potential. During the first reflection the mean kinetic energy of pick-up ions increases by approximately a factor of 10. Reflected particles have highly anisotropic velocity distribution. Subsequent excursion of the particles in the turbulent upstream flow leads to diffusion in pitch-angle space and, as a result, the particles can return to the shock again suffering, thus, multiple encounters. In order to describe the motion of particles in the upstream and down streamparts of the flow we solve the Fokker-Plank transport equation for anisotropic velocity distribution function.

L J Lanzerotti - One of the best experts on this subject based on the ideXlab platform.

  • mediation of the Solar Wind Termination shock by non thermal ions
    Nature, 2008
    Co-Authors: R Decke, S M Krimigis, E C Roelof, M E Hill, T P Armstrong, G Gloeckle, D C Hamilto, L J Lanzerotti
    Abstract:

    Broad regions on both sides of the Solar Wind Termination shock are populated by high intensities of non-thermal ions and electrons. The pre-shock particles in the Solar Wind have been measured by the spacecraft Voyager 1 (refs 1-5) and Voyager 2 (refs 3, 6). The post-shock particles in the heliosheath have also been measured by Voyager 1 (refs 3-5). It was not clear, however, what effect these particles might have on the physics of the shock transition until Voyager 2 crossed the shock on 31 August-1 September 2007 (refs 7-9). Unlike Voyager 1, Voyager 2 is making plasma measurements. Data from the plasma and magnetic field instruments on Voyager 2 indicate that non-thermal ion distributions probably have key roles in mediating dynamical processes at the Termination shock and in the heliosheath. Here we report that intensities of low-energy ions measured by Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. We conclude that these ions are the >0.028 MeV portion of the non-thermal ion distribution that determines the Termination shock structure and the acceleration of which extracts a large fraction of bulk-flow kinetic energy from the incident Solar Wind.

  • Mediation of the Solar Wind Termination shock by non-thermal ions
    Nature, 2008
    Co-Authors: R. B. Decker, S M Krimigis, E C Roelof, M E Hill, T P Armstrong, G. Gloeckler, D. C. Hamilton, L J Lanzerotti
    Abstract:

    Data from the plasma and magnetic field instruments on Voyager 2 indicate that non-thermal ion distributions probably play key roles in mediating dynamical processes at the Termination shock and in the heliosheath. Intensities of low-energy ions measured at Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. The acceleration of ions extracts a large fraction of bulk flow kinetic energy from the incident Solar Wind. Broad regions on both sides of the Solar Wind Termination shock are populated by high intensities of non-thermal ions and electrons. The pre-shock particles in the Solar Wind have been measured by the spacecraft Voyager 1 (refs 1–5 ) and Voyager 2 (refs 3 , 6 ). The post-shock particles in the heliosheath have also been measured by Voyager 1 (refs 3–5 ). It was not clear, however, what effect these particles might have on the physics of the shock transition until Voyager 2 crossed the shock on 31 August–1 September 2007 (refs 7–9 ). Unlike Voyager 1, Voyager 2 is making plasma measurements^ 7 . Data from the plasma^ 7 and magnetic field^ 8 instruments on Voyager 2 indicate that non-thermal ion distributions probably have key roles in mediating dynamical processes at the Termination shock and in the heliosheath. Here we report that intensities of low-energy ions measured by Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. We conclude that these ions are the >0.028 MeV portion of the non-thermal ion distribution that determines the Termination shock structure^ 8 and the acceleration of which extracts a large fraction of bulk-flow kinetic energy from the incident Solar Wind^ 7 . On 30 August 2007 Voyager 2 began to cross the Termination shock, a boundary produced by the inter-action of the Sun with the rest of the Galaxy, where the supersonic Solar Wind abruptly slows as it presses outward against the surrounding interstellar matter. Five Letters in this issue present the data that the probe sent back. The Voyager 2 crossings occurred about 1.5 billion kilometres closer to the Sun than those of Voyager 1, illustrating the asymmetry of the heliosphere. The results from the plasma experiment, low-energy particle, cosmic ray, magnetic field and plasma-wave detectors reveal a complex and dynamic shock, reforming itself in hours rather than days. The cover graphic of Voayer 2 on the brink of entering interstellar space is by Henry Kline of JPL. It may be decades before another probe crosses the Termination shock but remote observations can now bridge the gap — as shown by Wang et al . who report measurements of energetic neutral atoms in the heliosheath from the STEREO A and B spacecraft that complement the Voyager in situ observations made at the same time. In News & Views, J R Jokipii puts the Voyager findings into context. For more on the on Voyager odyssey, see page 24, and the Author page, and go to the movie on http://www.nature.com/nature/videoarchive/voyager .

M. S. Potgieter - One of the best experts on this subject based on the ideXlab platform.

  • Modeling the acceleration and modulation of anomalous cosmic ray oxygen
    Advances in Space Research, 2011
    Co-Authors: R. D. Strauss, M. S. Potgieter, Stefan Ferreira
    Abstract:

    Abstract After the Solar Wind Termination shock crossings of the Voyager spacecraft, the acceleration of anomalous cosmic rays has become a very contentious subject. In this paper we examine several topics pertinent to anomalous cosmic ray oxygen acceleration and transport using a numerical cosmic ray modulation model. These include the effects of drifts on a purely Fermi I accelerated spectra, the effects of introducing higher charge states of oxygen into the modulation model, examining the viability of momentum diffusion as a re-acceleration process in the heliosheath and examining energy spectra, and intensity gradients, in the inner heliosphere during consecutive drift cycles.

  • The heliospheric transport and modulation of multiple charged anomalous oxygen revisited
    Astronomy and Astrophysics, 2010
    Co-Authors: R. D. Strauss, M. S. Potgieter, Stefan Ferreira
    Abstract:

    Context. Since the crossings of the Solar Wind Termination shock by the Voyager 1 and 2 spacecraft, much speculation has surrounded the acceleration mechanism and region where the anomalous cosmic ray component is accelerated. A peculiar, and mostly overlooked feature of the observed anomalous oxygen spectrum near the Termination shock, is the power law form of the roll-over (cut-off )a t the high energy range of this spectrum. Aims. We investigate, using a numerical model, why this deviation from the expected exponential form of the cut-off part of the anomalous oxygen spectrum occurs, and if the observed power law form can be explained in terms of the acceleration of multiple charged anomalous oxygen. Methods. Multiple charged anomalous cosmic rays are incorporated in a numerical model, based on the standard Parker transport equation, including acceleration at the Solar Wind Termination shock. This is done by specifying an energy dependent charge state, constrained by observations. Results. Comparing computational results with spacecraft observations, it is found that the inclusion of multiply charged anomalous cosmic rays in the modulation model can explain the observed spectrum of anomalous oxygen in the energy range from 10−70 MeV per nucleon. The more effective acceleration of these multiple charge anomalous particles at the Solar Wind Termination shock causes a significant deviation from the usual exponential cut-off spectrum to display instead a power law decrease up to 70 MeV per nucleon where galactic oxygen starts to dominate. In addition, the model reproduces the features of multiple charged oxygen at Earth so that a good comparison is obtained between computations and observations.

  • The role of radial perpendicular diffusion and latitude dependent acceleration along the Solar Wind Termination shock
    Advances in Space Research, 2007
    Co-Authors: U. W. Langner, M. S. Potgieter
    Abstract:

    A numerical model, based on Parker’s transport equation, describing the modulation of anomalous cosmic rays and containing diffusive shock acceleration is applied. The role of radial perpendicular diffusion at the Solar Wind Termination shock, and as the dominant diffusion coefficient in the outer heliosphere, is studied, in particular the role it plays in the effectiveness of the acceleration of anomalous protons and helium when its latitude dependence is changed. It is found that the latitudinal enhancement of radial perpendicular diffusion towards the heliospheric poles and along the Termination shock has a prominent effect on the acceleration of these particles. It results in a ‘break’ in the energy spectrum for anomalous protons at ∼6.0 MeV, causing the spectral index to change from E−1.38 to E−2.23, but for anomalous helium at ∼3.0 MeV, changing the spectral index from E−1.38 to E−2.30. When approaching the simulated TS, the changes in the modulated spectra as they unfold to a ‘steady’ power law shape at energies below 50 MeV are much less prominent as a function of radial distances when radial perpendicular diffusion is increased with heliolatitude.

  • Possible explanations of anomalous spectra observed with Voyager 1 crossing the Solar Wind Termination shock
    AIP Conference Proceedings, 2006
    Co-Authors: U. W. Langner, M. S. Potgieter
    Abstract:

    This study was done with a numerical modulation model containing an asymmetrical heliosphere, a heliosheath, and diffusive shock acceleration of cosmic rays as applied to the Solar Wind Termination shock (TS). In order to give possible explanations for the recently observed spectra by the Voyager 1 spacecraft of the TS particles, galactic and anomalous cosmic rays, simultaneously, it was necessary to modify the model by changing: The latitudinal dependence of the compression ratio at the TS, the latitudinal dependence of the injection efficiency of pick‐up ions at the TS, and allowing a stronger decrease with radial distance r of the Solar Wind speed V in the heliosheath than the usual V ∝ 1/r2. Combining these scenarios have prominent effects on the acceleration of charged particles at and beyond the TS and can explain to a large extent the spectra observed at the position of the Voyager 1 spacecraft in the heliosheath.

  • Modulation of Galactic Protons in an Asymmetrical Heliosphere
    The Astrophysical Journal, 2005
    Co-Authors: U. W. Langner, M. S. Potgieter
    Abstract:

    A previously used two-dimensional model of the heliospheric modulation of cosmic rays including a Solar Wind Termination shock is extended to include an outer modulation boundary that is asymmetrically shaped with respect to the Sun. The modulation process is described kinetically using the Parker transport equation. The model includes drifts, adiabatic energy changes, diffusion, convection, a Solar Wind Termination shock, and a heliosheath and is used here to compute modulation differences between a symmetrical and an asymmetrical modeled heliosphere, as well as differences between an asymmetrical heliosphere including a Termination shock and one without such a shock for Galactic cosmic-ray protons. The solutions are for Solar minimum and moderate maximum conditions for both heliospheric magnetic field polarity cycles. It is found that the modulation produced for cosmic-ray protons with an asymmetrical heliospheric model differs from that produced with a symmetrical model, but significantly mostly for the A < 0 polarity cycle, especially in the tail region of the heliosphere. The results of the symmetrical model are, however, surprisingly reasonable for studies of the heliospheric nose region, measured against the asymmetrical model.