SH43A-1080 1340h
Recent Results from the Voyager 1 and 2 Plasma Instruments
Voyager 1 and Voyager 2, launched in 1978, are now at radial distances of 94 AU and 75 AU and continue to return data daily. The plasma (PLS) instruments onboard both spacecraft were designed and built by M.I.T. The PLS instrument on V1 ceased normal operation after the Saturn fly-by in 1980 but is still able to return ``DC Mode" current measurements integrated over all energies when fluxes are high enough. Eventually the instrument was turned off for power-sharing reasons and remained off until May 2003 when, following the controversial termination shock event at V1 in August 2002, the V1 PLS instrument was turned back on. The V2 PLS instrument is undamaged and continues to work perfectly. In this paper we introduce interpretation of the recent DC Mode data from the V1 PLS instrument. Alongside this we present V2 plasma data from the largest events of recent years, notably the 2000 Bastille day event and the Halloween events of 2003. Using V2 plasma data we are able to estimate likely DC mode readings at V1 and we compare these predictions to actual measurements. Finally, a survey of the V2 data set shows the V2 Bastille and Halloween events may be members of a new class of events observed near interplanetary shocks. The events are characterised by pulses followed by "troughs" in the plasma density, which coincide with increases in plasma temperature and magnetic field strength.
SH43A-1081 1340h
Long-wavelength velocity oscillations in the outer heliosphere
Voyager 2 detected unusual long-wavelength, low frequency velocity oscillations in the solar wind with a period of 2.3 days and peak to trough amplitudes that ranged from 10 km/s - 20 km/s [Paularena et al., 1996]. Since the observed oscillatory features had not yet merged, Paularena et al concluded that the waves had been generated comparatively near Voyager 2 in the outer heliosphere. Three possibilities for the excitation of these waves were suggested, none of which were convincing except that it was somehow associated with pickup ions. We present a simple model for wave excitation driven by pickup ion pressure in the outer heliosphere that appears to account for the observations. Examples of the theoretical model are compared directly to the observed velocity oscillations and we find very reasonable agreement.
SH43A-1082 1340h
Possible Effects at Voyager 1 and Voyager 2 of Solar Events
Recent Voyager observations are analyzed in the context of the propagation of solar events to the outer heliosphere. The three-dimensional HAFv2 model results show that solar events can asymmetrically propagate to the outer heliosphere. Both longitudinal and latitudinal asymmetries may be important. We compare the preliminary HAFv2 model results with in-situ solar wind plasma, interplanetary magnetic field, and energetic particle observations from Voyager. We also compare the HAFv2 predictions with those from other models.
SH43A-1083 1340h
Pioneer Observations of ICMEs in the Outer Heliosphere
Interplanetary coronal mass ejections (ICMEs) are an important physical process throughout the heliosphere. They are associated with a broad range of phenomena, such as particle acceleration and cosmic ray modulation. They may also play an important role in the dynamics and evolution of the solar wind during periods of high solar activity. Until recently, there have been few direct observations of ICMEs at heliocentric distances greater than 1 AU. Since then, Paularena et al. [2001], Wang and Richardson [2004], and others have demonstrated that it is possible to use alpha density enhancements on proton temperature depressions to perform surveys of ICMEs at the Voyager and Ulysses spacecraft during Solar Cycles 22 and 23. Solar wind spectra from the Pioneer 10 spacecraft are examined to perform a comparable survey of ICMEs at this spacecraft. Preliminary results suggest that existing alpha particle and proton temperature observations are sufficiently sensitive to identify the signatures of possible ICMEs out to heliocentric distances of 5-10 AU and 10-15 AU respectively. These observations allow us to extend our knowledge of the radial and temporal variation of ICMEs in the outer heliosphere during the declining phase of Solar Cycle 20 and the ascending phase of Solar Cycle 21.
SH43A-1084 1340h
A Physically Motivated Stochastic-Integration Method for Energetic Particles at Shocks and Other Discontinuities
Stochastic integration is a well-known method for solving diffusion problems. We present a physically motivated algorithm for solving Parker's diffusion-convection transport equation for energetic charged particles in the presence of arbitrarily thin discontinuities such as shocks and current sheets. We consider as a specific example of a plane, steady collisionless shock such as the solar-wind termination shock. We follow pseudo-particles injected at the termination shock with a monoenergetic spectrum. The random motion of the particles are determined from the diffusion coefficient, with a specified energy change when the particle crosses the shock. The resulting solution agrees well with the corresponding analytic solution. We will also discuss the application of these ideas to the singular drift motions at a shock and at the interplanetary current sheet.
SH43A-1085 1340h
PIC simulations of reforming perpendicular shocks- implications for ion acceleration at SNRs and the heliospheric termination shock
Recent particle-in-cell (PIC) simulations have revealed time-dependent shock solutions for parameters relevant to astrophysical and heliospheric shocks [e.g. 1,2,3]. These solutions are characterised by a shock which cyclically reforms on the spatio-temporal scales of the incoming protons. Whether a shock solution is stationary or reforming depends not only upon the model adopted for electron dynamics, but also on the plasma parameters, notably the upstream beta. For the heliospheric termination shock, these parameters are not well determined: some estimates suggest that the termination shock may be in a parameter regime such that it is time-dependent. It has been pointed out [3] that this may terminate some acceleration processes, for example shock surfing, which have been proposed for time-stationary shock solutions. The introduction of time-dependent electromagnetic fields intrinsic to the shock does however introduce the possibility of new mechanisms for the acceleration of protons. We will discuss the prospects for local ion acceleration at reforming quasiperpendicular shocks, in the presence of pickup ions as seen in selfconsistent PIC simulations with parameters relevant to both SNRs and the heliospheric termination shock. [1] Shimada, N., and M. Hoshino, Astrophys. J, 543, L67, 2000. [2] Schmitz, H., S.C. Chapman and R.O. Dendy, Astrophys. J, 570, 637, 2002 [3] Scholer, M., I. Shinohara and S. Matsukiyo, J. Geophys. Res., 108, 1014, 2003
SH43A-1086 1340h
Momentum Diffusion of Suprathermal Ions in Compressional Solar Wind Turbulence
The theories on momentum diffusion in non-compressional waves (Alfv\'{e}n waves) in the solar wind are well-developed while momentum diffusion of ions in compressional waves is explored to less extent. We present a model on stochastic acceleration of ions in compressional solar wind turbulence and compare the results to data from the ACE, SOHO, WIND, and Ulysses spacecraft. The experimentally determined acceleration time scales are used to model the evolution of the spectra of suprathermal ions convected through the heliosphere to the solar wind termination shock.
SH43A-1087 1340h
Heliopause stability in the presence of charge exchange
The heliopause is a surface separating the tenuous hot plasma of the inner heliosheath and the dense cold plasma of the interstellar origin in the outer heliosheath. It has been suggested previously that the heliopause may become Rayleigh-Taylor unstable under the influence of charge transfer collisions between ions and neutral hydrogen atoms. Here we present a first detailed analytic model of this phenomena based on the dispersion analysis of the linearized equation of motion for the plasma in the vicinity of the stagnation point on the surface of the heliopause. We investigate two scenarios which differ in the way the neutral populations are treated. In the first, we include only the interaction with interstellar neutrals. The resulting cubic dispersion relation admits an imaginary solution for all range of wavenumbers, implying that heliopause is unconditionally Rayleigh-Taylor unstable to small perturbations propagating parallel to the discontinuity surface. The second model includes the population of secondary hot neutrals produced as a result of charge exchange between interstellar atoms and the hot inner heliosheath ions. This second population is shown to significantly reduce the growth rate of the long-wavelength modes. We confirm our analysis by following the nonlinear development of the instability with a time-dependent numerical simulation using two- and four-fluid numerical models. In agreement with preceding studies we obtain cyclical motion of the upwind heliopause with a period of the order of 100 years. We find that adding hot heliosheath atoms reduces the magnitude of the oscillations, as predicted by the theory. Possible stabilizing effect of a strong LISM magnetic field on the heliopause dynamics is also discussed. Finally, we identify two areas of heliospheric physics where the instability phenomenon could be important. The first is X-ray generation in the interface region from enhanced charge exchange due to mixing between the solar wind plasma, rich in highly charged heavy ions, and neutrals of the hydrogen wall. The second is turbulence generation in the outer heliosheath resulting in further impediment of galactic cosmic rays propagation in the outer heliosphere.
SH43A-1088 1340h
Self-consistent Interaction of Neutrals and Plasma at Shock Waves
The heliosphere creates a bow shock as it moves through the local interstellar medium (LISM), assuming it does so supersonically. Since the LISM is partially ionized and neutral hydrogen is coupled to the plasma through charge exchange, the bow shock might be modified in structure by this interaction. To explore the interaction of neutral gas and a collisionless plasma shock, we have developed a semi-analytical model which describes plasma-neutral gas-interaction in a self-consistent manner, combining the gas-dynamical equations of a plasma with the Boltzmann equation describing the neutral gas. Solving the interaction analytically is rather complicated since the plasma drift velocity and plasma velocity distribution function (VDF) enter into the Boltzmann equation of the neutral gas. To solve the system, the neutral VDF is expanded first in terms of Legendre polynomials and then in Laguerre harmonics. Retaining only the first few harmonics, which contribute the most, we are able to study the shock structure semi-analytically, thereby revealing a more basic understanding of shock processes in a partially ionized medium and illustrating the mediation of the plasma shock by neutral hydrogen.
SH43A-1089 1340h
The Role of Collisions and Charge Exchange in Determining Heliospheric Structure
The interaction of the solar wind with the interstellar medium is strongly mediated by the presence of interstellar hydrogen atoms. The coupling of atoms to plasma is generally assumed to be via the process of charge exchange. However the cross-sections of H-H and H-p collisions are of similar magnitude to the charge-exchange cross-section, so that including collisions may affect the overall solution. We describe a newly developed Monte-Carlo approach of modeling interstellar hydrogen atoms impinging on the heliosphere is discussed. We present some preliminary results of simulations here charge exchange and collisions are considered and compare these to charge exchange only runs.
SH43A-1090 1340h
Self-Consistent Model of the Outer Solar Wind Including Turbulence Transport
Based on models describing the transport of low-frequency MHD turbulence in the outer heliosphere, which include the effects of driving and turbulent dissipation in a simple way, one can reproduce the observed profiles for magnetic fluctuation energy and temperature out to ~ 40 AU [Zank et al. 1996; Matthaeus et al. 1999; Smith et al. 2001]. These models however do not take into account the back reaction of the heating and velocity changes, etc induced by the dissipation of fluctuations, and by pickup processes. Here, we develop a fully self-consistent model that includes conservation of mass, momentum and energy for the solar wind plasma and the pickup ions, as well as a turbulence transport model. We solve for a radial supersonic solar wind from 1 AU to 100 AU. The resulting theoretical profiles for magnetic fluctuation energy, magnetic correlation length and temperature will be compared with observations from Voyager. This consistent description of pickup ions and turbulence in the solar wind is expected to strengthen dissipation of magnetic energy as a good candidate for heating the solar wind and causing the observed non-adiabatic temperature profile.
SH43A-1091 1340h
Coupling of the interstellar and interplanetary magnetic fields at the heliospheric interface: three-dimensional multifluid model
We analyze the three-dimensional structure of the solar wind (SW) in its interaction with the local interstellar medium (LISM) on the basis of a newly developed, substantially revised multifluid model that treats plasma and different populations of neutral particles as separate components interacting via charge exchange. Special attention is paid to the coupling between the interstellar and interplanetary magnetic fields (ISMF and IMF) at the heliospheric interface. The vector of the LISM velocity is chosen to be perpendicular to the ISMF vector, which is inclined at $60^\circ$ to the ecliptic plane. This orientation of the magnetic field agrees with observational data, including the kilohertz radio emission that is believed to originate in the outer heliosheath ahead of the heliopause. Numerical simulations show the deformation of the heliopause and its rotation with respect to the solar wind meridional plane. Of interest is the bending and rotation of the heliospheric current sheet with respect to its original orientation in the inner regions of the solar wind behind the termination shock. It is shown that regardless of its obvious limitations the multifluid model of Pauls and Zank (1996) is capable of capturing several important features of the SW--LISM interaction, such as the formation of the hydrogen wall ahead of the heliopause, heating and deceleration of the solar wind ahead of the termination shock, the interstellar plasma deceleration due to its charge exchange with the fast secondary neutrals that originate in the subsonic solar wind region. It also ensures proper confinement of the IMF within the heliopause, which is crucial for adequate modeling of cosmic ray transport into the heliosphere.
SH43A-1092 1340h
Mars Express ASPERA-3 Observations of Neutral Atom Streams in the Interplanetary Medium - of yet Unknown Origin
The Analyzer of Space Plasmas and Energetic Atoms, ASPERA-3, onboard the Mars Express mission carries a neutral particle detector, NPD, measuring energetic neutral atom flux in the energy range 0.1 - 10 keV, resolving velocity and mass (H and O). Two NPD heads have total field of view is $10\deg\times180\deg$ divided into 6 pixels. The angular response in each pixel has a FWHM (Full Width at Half Maximum) of $5\deg\times30\deg$. The total efficiency is 1 - 15 % depending on energy. The ENA detection technique is based on the atom - surface interaction. During the cruise phase as well as at Mars NPD detected flux of neutrals with a peak energy of 800 eV and spreading of 100% (FWHM) obtained from time-of-flight spectrum assuming hydrogen. The number flux is of the order of $10^{4} cm^{-2}s^{-1}sr^{-1}$ but highly variable between different observations. Each observation covered up to one hour on selected days. During the period from the first NPD switch-on in July 2003 and up to mid summer 2004, the NPD field of view covered the entire range of heliospheric longitudes being mostly within $15\deg$ about the ecliptic plane. For a particular observation date the stream angular spreading exceeds one NPD pixel in agreement with the spread in energy assuming it is thermal. No preferential direction of the flux has been so far identified through the entire period of observations. Since the NPD sensor becomes "blind" as soon as the Sun is within its field of view, the neutral component of the solar wind can be fully ruled out. Moreover, the signal is seeing even in the Marian eclipse that rules out the possibility of somehow neutralized solar wind protons reaching the sensor. We review the latest status of the observations in detail making only tentative speculations concerning the origin of these streams.
SH43A-1093 1340h
Understanding heliospheric structure: A comparison of multi-fluid, PIC kinetic, and Direct Simulation Monte-Carlo kinetic approaches to the modeling of neutral interstellar hydrogen
The global structure of the heliosphere is very strongly influenced by the inclusion of interstellar neutral hydrogen. The charge exchange coupling of neutral hydrogen and plasma and the large charge exchange mean free path within the heliosphere ensure that the neutral hydrogen distribution is not equilibrated. Consequently, a simple single fluid model for the neutral hydrogen is inappropriate. We have developed three approaches to model the highly non-equilibrated neutral hydrogen distribution that interacts with the heliosphere: a multi-fluid model [Zank et al., 1996], a particle-in-cell kinetic approach [Mueller et al., 2000], and a Direct Simulation Monte-Carlo kinetic model [Heerikhuisen et al., 2004]. A detailed comparison of the three approaches is presented, and the limitations of the various techniques are discussed.
SH43A-1094 1340h
Consequences of Variations in the Galactic Environment of the Sun on the Inner Heliosphere
The physical properties of the cloud surrounding the solar system vary over timescales of $<<10^5$ years because both the Sun and interstellar clouds move through space. Within the past $10^3$ to $10^5$ years the Sun emerged from a large void ($>$80 parsecs radius)in interstellar gas and entered an outflow of material from the Scorpius-Centaurus Association. The absence of neutral gas in the void indicates that anomalous cosmic rays (E $~<$ 0.5 GeV/nucleon) would have been absent from the inner heliosphere when the Sun was in the void. Other temporal variations in the galactic environment of the Sun as affecting the heliosphere are discussed.
http://xxx.lanl.gov/abs/astro-ph/0405167
SH43A-1095 1340h
Variability of interstellar medium: Impact on heliospheric structure
Interstellar cloud types within several hundred pc of the Sun vary in their density, temperature, degree of ionization, and galactic velocity. This suggests that during its lifetime, the heliosphere has been exposed to a variety of different interstellar environments. In particular, the Sun was once located in the hot, ionized, low-density plasma interior of the Local Bubble, and may occasionally be battered by high-velocity interstellar shock fronts. By means of numerical modeling, the interaction of the solar wind (assumed constant) with a variety of partially ionized interstellar media is investigated. A range of ISM densities, relative velocities, and temperatures is considered, including the three cases of the interior of the Local Bubble; an encounter with a high-density cloud; and immersion in a high velocity medium. The resulting changed heliospheric structures are discussed, as are the altered distributions of neutral atoms and galactic cosmic ray spectra.
SH43A-1096 1340h
Ab Initio Solar Modulation of Cosmic Rays Using Improved Models of Perpendicular Diffusion and Heliospheric Turbulence
A solar wind modulation model is discussed, implementing several recent advances, including a perpendicular diffusion coefficient computed from Nonlinear Guiding Center Theory (NLGC) [Matthaeus et al., ApJ, 590, L53, 2003], a turbulence model that includes cross helicity effects, and latitudinal variations of the boundary conditions for magnetic variance, correlation length, cross helicity and plasma temperature. The model spans from 0.3 AU to 100 AU. Also varied in this simulation are the observationally based latitudinal profiles of solar wind speed, mass flux density, and temperature [McComas et al., J. Geophys. Res., 105, 10419, 2000]. A simple model of pick-up ions is employed. In the ab initio modulation approach, the computed turbulence properties determine diffusion coefficients that appear in the numerical integration of the Parker's transport equation. A significant improvement in the cosmic ray latitudinal gradient is accomplished by using a numerical solution of the integral equation for the NLGC perpendicular diffusion coefficient. The run time for this calculation is lengthy. The modulation result thus obtained is tested with another version that implements an analytical approximation [Shalchi et al., ApJ, 604, 675, 2004] of the NLGC result. The latter implementation gives very similar results and takes significantly less time to run. Supported by NASA grant NNG04GF81G.
SH43A-1097 1340h
The effect of the changing solar system environment on galactic cosmic ray propagation through the heliosphere: Consequences for cosmogenic isotope production in the Earth's atmosphere.
The solar system is traveling through highly inhomogeneous interstellar medium. Our galactic environment (the Local Bubble) is a vast region formed by supernova explosions filled with extremely tenuous fully ionized gas at a temperature of over a million K. Embedded in the Local Bubble are interstellar clouds ranging from cold ($T<100$ K), dense ($n\sim 5000$ cm$^{-3}$) molecular clouds to warm ($T\sim 10^4$ K) and relatively tenuous ($n\sim 0.3$ cm$^{-1}$) partially ionized clouds, such as the Local Cloud where the Sun is currently located. The properties of the cloud control the size and shape of the heliosphere and, consequently, affect the propagation of galactic cosmic rays (GCRs) between the boundary of the modulation region (the heliopause) and Earth. GCRs with energies above several hundred MeV initiate nuclear reactions in the Earth's upper atmosphere producing radioactive isotopes of Beryllium and Carbon that are precipitated on the surface and eventually incorporated into sediments. It is then quite plausible that the history of the variability of the solar environment may be preserved in cosmogenic isotope records available from ice and sea sediment cores dating back more than 100,000 years. Previously, we showed that increasing the density of the cloud surrounding the solar system by a factor of 30 leads to an increase in 1 AU GCR fluxes by a factor of 1.5--3, and that cloud encounters may have been responsible for the observed peaks in $^{10}$Be records 35 and 60 thousand years ago. Extending our early model, we now calculate GCR distribution from the solution of the 2D Parker equation using the global model-calculated plasma and magnetic field parameters as a background to determine the diffusion coefficients. Initial results from a more comprehensive investigation of the global structure of the heliosphere embedded in clouds of varying density, from the present conditions in the Local Cloud to the extreme case of dense molecular clouds, are discussed.
SH43A-1098 1340h
Local and Global Aspects of Galactic Cosmic Ray Modulation
Galactic cosmic rays (GCR) are modulated by the solar wind in the heliosphere. A part of the modulation arises from local effects at earth's orbit due to the dynamics of the source regions of the interplanetary magnetic field and the solar wind while the rest of it arises from happenings in the outer heliosphere. The differences in the two kinds of contributions need to be clarified to understand the correlation between GCR modulation and the time variations of geomagnetic index Ap. We discuss our insights based on analyses of cosmic ray data obtained with the detectors on ground and on board the balloons and near earth satellites; they respond to a broad range of GCR spectrum at earth's orbit spanning rigidities of 1 GV to 100 GV. The physical significance of our findings will be discussed.
SH43A-1099 1340h
The Gradient/Curvature Drift of Cosmic Rays in the Heliospheric Magnetic Fields with Turbulence
Although various observations of cosmic ray modulation indicate that the effects of gradient/curvature particle drift should be reduced from its full drift in the average Parker heliospheric magnetic field, there is no theory for the calculation of particle drift in a magnetic field with turbulence. We present here a new approach to this problem, using an orbit tracing method for the calculation of drift. The solution is a direct numerical integration of the basic expression for drift as presented in Rossi and Olbert [1970]. The same technique was previously used to calculate the drifts in the Parker field for particles with an isotropic distribution. The drift speed agrees with the first order drift approximation when the particle gyroradius is much less than the scale size of magnetic field gradient. In this case, we include a more complicated magnetic field which contains both slab and 2D components of turbulence [Bieber et al, 1994; Qin et al, 2002]. It is expected that the turbulence will alter the particle trajectory, thus changing the behavior of the particle drift. Its implications to cosmic ray modulation will also be discussed
SH43A-1100 1340h
V-Shaped Heliosphere and Dependence of Solar-Cycle-Variations on Trajectories of Galactic Cosmic Rays
It has long been believed that the global structure of the heliopause under the interaction of the solar wind and the LISM is a simple nose-cone type. However, once the equatorial magnetic neutral sheet of the solar wind along the equatorial plane is taken into account, the situation changes drastically. The ram-pressure of the solar wind is no longer isotropic in the interplanetary space in this case. As a result, it is not easy to find a new stationary state because MHD simulation analyses tend to face non-stationary states of the heliopause, where the solar wind plasma coming from the inner side region extends from the nose cone surface by forming long-lived vortices. By using our MHD simulation, we found that a 3D stationary heliopause in this case is a V-shaped gutter on the nose-cone surface along the equatorial plane, and that the high ram-pressure equatorial sheet plasma is vented to a high-latitudinal region along the termination shock surface in the outer heliosphere. The self-consistency of this new structure will be discussed. Trajectories of cosmic-rays in the global 3-D magnetic heliosphere, which includes the effect of the solar-cycle-variations in the above MHD simulation model, are traced numerically. To simulate the propagation of high-energy galactic cosmic-ray protons to the earth, we eject anti-protons from the earth in various directions and trace their orbits back to the interstellar space by using the fourth order Runge-Kutta method. We make calculations mainly for 500 GeV cosmic ray protons, and find in our analyses that there are two major groups of particles. One group of particles is arriving at the earth from a wide range of the heliotail with their orbits significantly deflected at the magnetic wall formed in the upstream side of the heliosheath. The other group of particles also comes from the heliotail, meandering along the neutral surface of the heliopause sandwiched between the heliosheath magnetic field and the LISM field, which are opposite to each other in direction. We also find that cosmic rays in the inner heliosphere are arriving from the polar region or along the equatorial sheet due to the gradient and curvature drift, which is strongly dependent upon the solar-cycle-variations of the magnetic field. Preliminary results of a particle simulation for the study of how the galactic cosmic rays come across the heliopause surface will be also discussed.