SH13C-01 INVITED
Voyager Observations of the Termination Shock and in the Heliosheath
The two Voyager spacecraft have crossed the solar wind termination shock and are in the heliosheath where the subsonic wind is deflected as it interacts with the local interstellar medium. Voyager 1 is at 108 AU and ~17 AU beyond the expected current location of the shock at 34° degrees north latitude, while Voyager 2 is at 88 AU, just 4 AU beyond where it crossed the shock at a latitude of -28° degrees. Comparing conditions in these two different regions will help separate spatial and temporal changes as the two spacecraft probe more deeply into the heliosheath during a period of increasing solar activity. The diversion of the plasma flow and compression of the magnetic field will increase as the heliopause is approached, with transients revealing the propagation of solar disturbances in the heliosheath. Changes in the energy dependent gradients of anomalous cosmic rays will be a signature of the location of their source and their propagation in the heliosheath. The evolution of the spectrum of low energy ions from the termination shock at Voyager 2 toward the steady spectrum observed at Voyager 1 over the last three years suggests a common shock source mechanism that uniformly fills the heliosheath beyond ~86 AU. Although galactic cosmic rays are thought to be modulated in the heliosheath, their gradient is also small, indicating larger gradients beyond Voyager 1. These and other observations will inform our understanding of the complex interaction of the solar wind and the surrounding interstellar medium.
SH13C-02
Energetic Particle Populations in the Heliosheath Measured at Voyagers 1 and 2
Voyagers 1 (V1) and 2 (V2) have been in the inner heliosheath behind the solar wind termination shock (TS) since 2004/351 and 2007/242, respectively. In this report we compare energetic ion (~0.03-20 MeV) and electron (~0.03-1.5 MeV) intensities, energy spectra, and anisotropies, and the variability of these quantities, measured by the Low Energy Charged Particle instruments on V1 (108 AU, N34°) and V2 (87 AU, S28°). Among the results from recent data taken through Aug. 2008 are the following. (1) Starting around 2008/120 at V2, intensities of heliosheath ions above ~1 MeV and electrons above 0.02 MeV began to decrease, reached minima at pre-TS levels around 2008/145, and since then have increased gradually. The largest drop (factor ~10) occurred in electron intensities, which after 1/3 of a year have recovered to only 1/3 of their pre-drop levels. (2) Large intensity depressions associated with the 2008/120 disturbance did not extend to ions below ~1 MeV at V2. However, these lower energy ions did begin to show more rapid intensity fluctuations (time scale ~hours) comparable to those observed soon after the TS crossing. The net effect after 2008/120 on suprathermal ions 0.028-3.5 MeV was a transition of their energy spectrum from a single power-law with spectral index ~1.4, to one with index varying from ~1.2 at 0.04 MeV to ~1.9 at 3 MeV. By contrast, the intensities of ions 0.04- 4.0 MeV at V1 remain flat and steady, and the energy spectrum is well fit by a power-law with spectral index 1.6±0.1. (3) During 2005.5 to 2008.5 at V1, the component of the heliosheath plasma flow velocity in the R-T plane, estimated from analysis of low-energy ion angular distributions, declined slowly in amplitude from 80-100 km/s to 40-60 km/s. The velocity direction also continued to slowly rotate in the R-T plane, as the angle between the flow velocity and the radial direction increased from ~15° in mid-2005 to ~45° in mid-2008. This resulted mainly from a relatively large decrease in the R-component of the velocity from ~80 to 40 km/s, while the T-component remained relatively steady at about -40 km/s.
SH13C-03 INVITED
Are Charged Particles Accelerated by the Solar Wind Termination Shock?
The termination shock of the solar wind has long been thought to be the source of anomalous cosmic rays arising from the acceleration of interstellar pickup ions by diffusive shock acceleration. This has been the standard paradigm since the 1970's. However, since the two Voyagers have now both crossed the shock, it is clear that this picture needs to be revisited. The primary challenge to the standard paradigm is that the ACR energy spectrum was seen to be "modulated" in the heliosheath, whereas the expectation was that it would be a power law with a spectral index that depends on the jump in plasma density across the shock. Another challenge to the paradigm is the observation of highly anisotropic particle beams prior to the shock crossings. This suggests that the scattering mean-free path of these particles is very large making acceleration at the shock problematic. New ideas have emerged to explain the observations, including the recognition of the importance of the large-scale shape of the shock itself (blunt as opposed to spherical), and the effect of large-scale turbulence on the physics of shock acceleration. These will be discussed in this talk. The new observations also provide valuable information about the acceleration of low-energy ions and the injection process itself. I will present results from new simulations that show that the termination shock is a rapid and efficient accelerator of suprathermal pickup ions and the simulations results agree well with the low-energy part of the ACR spectrum (up to about 100 keV).
SH13C-04 INVITED
Stochastic and compression acceleration of energetic particles in the heliosheath
One of the biggest surprising results from Voyager observations in the inner heliosheath is that anomalous cosmic rays do not appear to be accelerated at the termination shock region where the Voyager spacecraft have encountered. Various models of particle acceleration have been put forward to explain the production and transport of anomalous cosmic rays. Some of the models are based on the idea that plasma in the heliosheath is undergoing some strong compressions, either large scale systematic compression or stochastic compression in the form of turbulence. According to Parker transport equation, charged particles are accelerated any time when they pass through a region of compression. Repeated passages of compression region through spatial diffusion can make particles reach very high energies, which is a mechanism similar to shock acceleration. In this talk, results of detailed model calculation involving diffusive compression acceleration by large scale compression or stochastic turbulent compression of plasma in the heliosheath will be presented. Model requirements will be placed under the constraints of Voyager observations of anomalous cosmic rays and the power spectrum of an index close to -5 at low energies.
SH13C-05
The Acceleration of Anomalous Cosmic Rays by Stochastic Acceleration in the Heliosheath
There is growing evidence that Anomalous Cosmic Rays (ACRs) are accelerated by stochastic acceleration in the heliosheath. However, most stochastic acceleration mechanisms are not appropriate. The energy density in the ACRs and in the interstellar pickup ions out of which the ACRs are accelerated greatly exceeds the energy density in the turbulence in the heliosheath. Thus, a traditional stochastic acceleration mechanism in which particles are accelerated by damping the turbulence will not work. We have developed a stochastic acceleration mechanism in which the total energy of the pickup ions and the ACRs is conserved. Energy is redistributed from the core pickup ions into a suprathermal tail to create the ACRs. A model for the acceleration of the ACRs in the heliosheath, based on this stochastic acceleration mechanism, is presented, which provides reasonable fits to the spectra of suprathermal particles and ACRs observed by Voyager.
SH13C-06
Acceleration of Anomalous Cosmic Rays at a Blunt Termination Shock with Varying Strength along the Shock Face
Voyagers' crossing of the termination shock (TS) in 2004 and 2007 brought several surprises. The spectrum of anomalous cosmic rays (ACRs) did not unfold to a smooth power law at the shock. ACR fluxes continued to increase into the heliosheath and ACR spectra seen by V-1 unfolded gradually. The recent observation of Stereo, indicating a double peak in the flux of energetic neutral atoms (ENAs) (Wang et al., Nature, 454, p 81, 2008) poses further challenge for theoretical modelers. We consider a two-dimensional model of the global heliosphere with an offset circle representing the blunt termination shock. Field lines wound in a Parker spiral cross the TS multiple times. Parker's diffusive transport equation is solved including anisotropic diffusion, convection adiabatic cooling and shock acceleration. In our earlier simulations we assumed the same strength everywhere along the shock. It is quite likely, however, that the TS is stronger at the nose region and less strong toward the flanks and tail, rendering acceleration less effective at the flanks and tail. We present numerical simulation results and discuss implications.
SH13C-07
Galactic cosmic ray transport in the three-dimensional heliosheath
Voyager observations made during the past few years revealed that galactic cosmic ray (GCR) intensities near the termination shock are well below their interstellar values. Specifically, the heliosheath region accounts for about half of the total modulation of galactic cosmic ray protons with energies below 500 MeV. We study the process of GCR modulation in the heliosheath using a fully four-dimensional (3D space + momentum) cosmic-ray transport model. Our model is a product of a coupling between a 3D MHD-neutral plasma code and a new 4D Parker transport code. The model captures the essential physics of GCR transport, including drifts along the neutral sheet and the termination shock. The model incorporates essential 3D features, including the variability of the solar wind speed with heliolatitude and asymmetries of the termination shock and the heliosheath imposed by the tilt of the interstellar magnetic field. Simulations of the 2008 solar minimum modulation are presented and other solar-cycle modulation patterns are discussed.
SH13C-08
Effects of Fluid Velocity Shear on Energetic-Particle Transport
The effects of viscosity and shear can play a significant role in the heliosphere and have been little studied. We will discuss some of the expected effects that have been published and go on to discuss some recent, new ideas which appear likely to play important roles as we study the heliosphere in greater detail. We find that shear layers may provide significant barriers to charged-particle transport. For example, we find that the heliopause may be a significant barrier to galactic cosmic rays entering the heliosphere. The shear flow near the heliopause should produce both an hancement of the magnetic-field magnitude and reduce field-line mixing or random walk, both of which will reduce transport across the heliiopause. The same effect may reduce the access of galactic cosmic rays. These factors may be closely related to the magnetic wall" discussed by earlier authors in connection with numerical simulations of the heliosphere. We will also discuss some possible effects of shear on particle transport in the inner heliosphere.