Vladimir A. Florinski

Particle acceleration at perpendicular shock waves: Model and observations

Received 7 November 2005; revised 17 February 2006; accepted 27 February 2006; published 23 June 2006. [1] On the basis of a recently developed nonlinear guiding center theory for the perpendicular spatial diffusion coefficient k? used to describe the transport of energetic particles, we construct a model for diffusive particle acceleration at highly perpendicular shocks, i.e., shocks whose upstream magnetic field is almost orthogonal to the shock normal. We use k? to investigate energetic particle anisotropy and injection energy at shocks of all obliquities, finding that at 1 AU, for example, parallel and perpendicular shocks can inject protons with equal facility. It is only at highly perpendicular shocks that very high injection energies are necessary. Similar results hold for the termination shock. Furthermore, the inclusion of self-consistent wave excitation at quasiparallel shocks in evaluating the particle acceleration timescale ensures that it is significantly smaller than that for highly perpendicular shocks at low to intermediate energies and comparable at high energies. Thus higher proton energies are achieved at quasiparallel rather than highly perpendicular interplanetary shocks within 1 AU. However, both injection energy and the acceleration timescale at highly perpendicular shocks are sensitive to assumptions about the ratio of the two-dimensional (2-D) correlation length scale to the slab correlation length scale l2D/lk. Model proton spectra and intensity profiles accelerated by a highly perpendicular interplanetary shock are compared to an identical but parallel interplanetary shock, revealing important distinctions. Finally, we present observations of highly perpendicular interplanetary shocks that show that the absence of upstream wave activity does not inhibit particle acceleration at a perpendicular shock. The accelerated particle distributions closely resemble those expected of diffusive shock acceleration, and observed at oblique shocks, an example of which is shown.

The Effects of a κ-Distribution in the Heliosheath on the Global Heliosphere and ENA Flux at 1 AU

In this paper we investigate heliosheath energetic neutral atom (ENA) fluxes at keV energies, by assuming that the heliosheath proton distribution can be approximated using a κ-distribution. The choice of the κ-parameter derives from observational data of the solar wind (SW). Our work has direct applications to the upcoming IBEX mission, since we generate all-sky ENA maps within the IBEX energy range (10 eV-6 keV), as well as ENA energy spectra in several directions. We find that the use of κ, as opposed to a Maxwellian, gives rise to greatly increased ENA fluxes above 1 keV, while medium-energy fluxes are somewhat reduced. We show how IBEX data can be used to estimate the spectral slope of the proton distribution in the heliosheath, and that the use of κ reduces the differences between ENA maps at different energies. We also investigate the effect that introducing a κ-distribution has on the global interaction between the SW and the local interstellar medium (LISM), and find that there is generally an increase in energy transport from the heliosphere into the LISM, due to the modified profile of ENA energies. This results in a termination shock that moves out by 4 AU, a heliopause that moves in by 9 AU, and a bow shock 25 AU farther out, in the nose direction.

Termination Shock Asymmetries as Seen by the Voyager Spacecraft: The Role of the Interstellar Magnetic Field and Neutral Hydrogen

We show that asymmetries of the termination shock due to the influence of the interstellar magnetic field (ISMF) are considerably smaller in the presence of neutral hydrogen atoms, which tend to symmetrize the heliopause, the termination shock, and the bow shock due to charge exchange with charged particles. This leads to a much stronger restriction on the ISMF direction and its strength. We demonstrate that in the presence of the interplanetary magnetic field the plane defined by the local interstellar medium (LISM) velocity and magnetic field vectors does not exactly coincide with the plane defined by the interstellar neutral helium and hydrogen velocity vectors in the supersonic solar wind region, which limits the accuracy of the inferred direction of the ISMF. We take into account the tilt of the LISM velocity vector with respect to the ecliptic plane and show that magnetic fields as strong as 3 μG or greater may be necessary to account for the observed asymmetry. Estimates are made of the longitudinal streaming anisotropy of energetic charged particles at the termination shock caused by the nonalignment of the interplanetary magnetic field with its surface. By investigating the behavior of interplanetary magnetic field lines that cross the Voyager 1 trajectory in the inner heliosheath, we estimate the length of the trajectory segment that is directly connected by these lines to the termination shock. A possible effect of the ISMF draping over the heliopause is discussed in connection with radio emission generated in the outer heliosheath.

Heliospheric Response to Different Possible Interstellar Environments

At present, the heliosphere is embedded in a warm, low-density interstellar cloud that belongs to a cloud system flowing through the local standard of rest with a velocity near ~18 km s-1. The velocity structure of the nearest interstellar material (ISM), combined with theoretical models of the local interstellar cloud (LIC), suggest that the Sun passes through cloudlets on timescales of ≤103-104 yr, so the heliosphere has been, and will be, exposed to different interstellar environments over time. By means of a multifluid model that treats plasma and neutral hydrogen self-consistently, the interaction of the solar wind with a variety of partially ionized ISM is investigated, with the focus on low-density cloudlets such as are currently near the Sun. Under the assumption that the basic solar wind parameters remain/were as they are today, a range of ISM parameters (from cold neutral to hot ionized, with various densities and velocities) is considered. In response to different interstellar boundary conditions, the heliospheric size and structure change, as does the abundance of interstellar and secondary neutrals in the inner heliosphere, and the cosmic-ray level in the vicinity of Earth. Some empirical relations between interstellar parameters and heliospheric boundary locations, as well as neutral densities, are extracted from the models.

On the Possibility of a Strong Magnetic Field in the Local Interstellar Medium

We analyze the consequences of the local interstellar magnetic field being almost 3 times larger than the Galactic average (ordered) field on the structure of the heliospheric interface in the axisymmetric case when the field is parallel to the relative direction of motion between the local interstellar medium (LISM) and the Sun. A field of such strength is expected to exist in the Local Interstellar Cloud, if the latter condensed from material inside a magnetic flux tube rebounding from the wall of the Local Bubble cavity. The analysis is performed using a newly developed multifluid neutral MHD model. We show that the bow shock ahead of the heliopause still exists for supersonic and sub-Alfvenic LISM parameters. Our results agree well with the observations of the Lyα absorption spectra and yield positions of the termination shock and the heliopause similar to those obtained from the standard super-Alfvenic model.

Comparing various multi-component global heliosphere models

Context. Modeling of the global heliosphere seeks to investigate the interaction of the solar wind with the partially ionized local interstellar medium. Models that treat neutral hydrogen self-consistently and in great detail, together with the plasma, but that neglect magnetic fields, constitute a sub-category within global heliospheric models. Aims. There are several different modeling strategies used for this sub-category in the literature. Differences and commonalities in the modeling results from different strategies are pointed out. Methods. Plasma-only models and fully self-consistent models from four research groups, for which the neutral species is modeled with either one, three, or four fluids, or else kinetically, are run with the same boundary parameters and equations. They are compared to each other with respect to the locations of key heliospheric boundary locations and with respect to the neutral hydrogen content throughout the heliosphere. Results. In many respects, the models’ predictions are similar. In particular, the locations of the termination shock agree to within 7% in the nose direction and to within 14% in the downwind direction. The nose locations of the heliopause agree to within 5%. The filtration of neutral hydrogen from the interstellar medium into the inner heliosphere, however, is model dependent, as are other neutral results including the hydrogen wall. These differences are closely linked to the strength of the interstellar bow shock. The comparison also underlines that it is critical to include neutral hydrogen into global heliospheric models.

DO ANOMALOUS COSMIC RAYS MODIFY THE TERMINATION SHOCK

This work extends our previous two-dimensional self-consistent model of the cosmic rays interacting with the solar wind to include anomalous cosmic rays. As before, energetic particles are described kinetically using a Parker equation. The model includes diffusion, convection, and drift effects, as well as shock and compression acceleration and expansion cooling by nonuniform solar wind flow. A new numerical model has been developed featuring an adaptive-mesh refinement algorithm to accommodate small diffusive length scales of low-energy shock-accelerated particles. We show that anomalous cosmic rays have only a minor effect on the termination shock during the time near solar minima. Specifically, cosmic-ray gradients cause the subshock to move away from the Sun by about 1 AU with its compression ratio decreasing by about 5% compared to the reference case without cosmic-ray effects. We also study the effect of solar wind slowdown by charge exchange downstream of the termination shock, producing compressive flow in this region and resulting in additional acceleration of anomalous cosmic rays in the heliosheath. For the first time, spectra calculated with our self-consistent model show a good agreement with the cosmic-ray data from the two Voyager spacecraft, giving more confidence in the model predictions than the previous parametric studies.

A FOCUSED TRANSPORT APPROACH TO PICKUP ION SHOCK ACCELERATION: IMPLICATIONS FOR THE TERMINATION SHOCK

The discovery of low-energy suprathermal ions coming directly from the solar wind termination shock by the Voyager 1 spacecraft revealed strong beaming of these particles along the direction of the large-scale magnetic field. Since such large anisotropies exclude the use of standard cosmic-ray transport theory for nearly isotropic particle distributions in modeling the acceleration of these particles at the termination shock, we developed a shock acceleration model based on the numerical solution of the standard focused kinetic transport equation. After investigating the appropriateness of the physical content of this equation for shock acceleration, and applying the model to shocks of varying shock obliquity, we find that (1) this approach is a viable alternative to more sophisticated particle codes such as hybrid codes for simulating the shock acceleration of pickup ions and (2) that the model has the potential to reproduce many of the interesting features of energetic ions observed by the Voyager spacecraft at the termination shock.

The Effects of Global Heliospheric Asymmetries on Energetic Neutral Atom Sky Maps

The relationship between the all-sky distribution of energetic neutral atoms (ENAs) observable at 1 AU and the distribution and structure of plasma in the outer heliosphere is a promising tool for probing the structure of the heliospheric boundaries. The impetus for this work stems from the anticipated 2008 launch of the Interstellar Boundary Explorer, which will directly measure ENA fluxes from Earths orbit. We find, on the basis of global simulations, that the ENA flux depends sensitively on the shape of the heliopause as well as the velocity, density, and temperature of the postshock solar wind it envelopes. This Letter is intended as a preliminary, qualitative study of how ENA maps can help us identify heliospheric asymmetries.

The global heliosphere: theory and models

The solar system moves through interstellar space populated by three principal particle species: plasma with magnetic field, neutral atoms, and galactic cosmic rays. Their interaction with the solar wind creates what is known as the heliospheric interface, containing two shocks and a tangential discontinuity. The 21st century view of the heliosphere is emerging as a highly dynamic, non‐stationary environment with plasma flows carrying imprints of the solar cycle variations. Instabilities produced by interactions between non‐equilibrium flows add to the complexity and may have important implications for charged particle transport near the heliopause. Proliferation of MHD models brought discoveries of new effects caused by the heliospheric magnetic field geometry and a realization of the importance of the magnitude and direction of the interstellar field on the properties of the bow shock. Finally, cosmic ray evolution in the global heliosphere was found to be a more complex process with a variety of modula...