SH42A-01 10:20h
Energy Spectra and Anisotropies at a 2-D Shock: Implications for Voyager-1 and the Termination Shock
The enhancement of low-energy particle fluxes and large field-aligned anistropies detected by Voyager-1 indicate, most likely, the proximity of the termination shock. It is still debated if Voyager-1 crossed the shock or remained still upstream in the supersonic wind. It seems quite conceivable that Voyager-1 was still inside the shock, but its magnetic field line had already intersected the shock. This may occur naturally if the magnetic field line intersects the shock multiple times, which can occur in a number of plausible ways. We study the effects of such multiple intersection in the framework of diffusive acceleration. We consider a 2-dimensional, quasi-perpendicular planar shock, allowing the direction of the magnetic field to vary along the shock. Energy spectra and anisotropies are calculated by both analytical approximation and numerical solution of Parker's diffusive transport equation in 2-D. The predicted fluxes show quite remarkable spatial variation depending on the configuration of the magnetic field. Energy spectra and anisotropies exhibit cosiderable variability as well. We will discuss the implications for the Voyager-1 observations.
SH42A-02 10:35h
Effects of a Tilted Heliospheric Current Sheet in the Heliosheath: 3D MHD Modeling
Recent observations indicate that Voyager 1, now beyond 90 AU, is in a region unlike any encountered in it's 26 years of exploration. There is currently a controversy as to whether Voyager 1 has already crossed the Termination Shock, the first boundary of the Heliosphere (Krimigis et al. 2003; McDonald et al. 2003, Burlaga et al. 2003). An important aspect of this controversy is our poor understanding of this region. The region between the Termination Shock and the Heliopause, the Helisheath, is one of the most unknown regions theoretically. In the Heliosheath magnetic effects are crucial, as the solar magnetic field is compressed at the Termination Shock by the slowing flow. Therefore, to accurately model the Heliosheath the inclusion of the solar magnetic field is crucial. Recently, our simulations showed that the Heliosheath presents remarkable dynamics, with turbulent flows and a presence of a jet flow at the current sheet that is unstable due to magnetohydrodynamic instabilities (Opher et al. 2003; 2004). We showed that to capture these phenomena, spatial numerical resolution is a crucial ingredient, therefore requiring the use of an adaptive mesh refinement (AMR). These previous works assumed that the solar rotation and the magnetic axis were aligned. Here we present for the first time results including the tilt of the heliocurrent sheet using a 3D MHD AMR simulation, with BATS-R-US code. We discuss the effects on the global structure of the Heliosheath, the flows, turbulence and magnetic field structure. We assess the consequences for the observations measured by Voyager 1 since mid-2002.
http://butch.engin.umich.edu/~merav
SH42A-03 10:50h
Non-linear Cosmic-ray Transport in Enhanced Compressive Wave Turbulence at the Termination Shock
Provided that the termination shock is significantly modified by the anomalous cosmic-ray (ACR) component, strong ion-acoustic wave turbulence might be generated by the ACR cosmic-ray pressure gradient at the termination shock. Under such conditions standard quasi-linear theory (QLT) needs to be extended to a non-linear level to treat particle trajectories which are diffusive instead of being undisturbed gyro orbits on a turbulence correlation scale. We present such a non-linear diffusive kinetic theory for suprathermal particle transport along the large-scale magnetic field in the presence of strong compressive wave turbulence with small-scale Alfven waves. We show that the standard cosmic-ray transport equation needs to be modified at low suprathermal energies because of the fundamentally altered nature of spatial diffusion, modifications to convection and adiabatic energy changes due to the compressive wave turbulence, and the additional occurrence of effective stochastic acceleration by the compressive wave turbulence which produces particle spectra with a hard power law at low suprathermal energies and with a rollover at higher energies. This acceleration might lead to efficient injection of pickup ions into diffusive shock acceleration described by standard theory at the termination shock.
SH42A-04 11:05h
Timing of the 2-3 kHz Radio Events Observed by Voyager
Radio emissions observed at 2-3~kHz by the Voyager spacecraft occur when global merged interaction regions (GMIRs) cross the heliopause. The radiation is thought to occur when a GMIR enters a region close to the heliopause where the electron speed distribution is primed with a superthermal tail, produced by lower hybrid drive. Recently, emission mechanisms have been proposed for these events, and subsequent work has confirmed that these mechanisms predict dynamic spectra which closely resemble the steady component observed by the Voyager spacecraft. Here, these results are extended using simulations [Zank and Muller, 2003] in which the solar wind ram pressure varies with the solar cycle. We predict that strong emissions can occur 2-3 years after solar maximum, which is in close agreement with previous observed events.
SH42A-05 INVITED 11:20h
Future Prospects for ENA Imaging of the Outer Heliosphere
Recent advances in Energetic Neutral Atom (ENA) imaging have demonstrated the ability to remotely image a variety of space plasmas. Now ENA imaging is poised to let us globally image the plasmas beyond the termination shock, providing the first synoptic measurements of the interstellar boundaries at the edges of our heliosphere. These observations can be achieved with high sensitivity all-sky images, at a variety of energies, provided by large aperture ENA cameras on a simple spinning spacecraft. Such observations will achieve the objective of discovering the global interaction between the solar wind and the interstellar medium by answering four fundamental science questions: (1) What is the global strength and structure of the termination shock, (2) How are energetic protons accelerated 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? This objective is central to the NASA's plans as demonstrated by both the 2003 SEC Roadmap and 2002 NRC's Decadal Survey and is specifically identified in the 2003 NASA-wide Strategic Plan. This talk describes the science and general implementation of a mission to make these groundbreaking observations. Further, if the Interstellar Boundary Explorer (IBEX) Small Explorer (SMEX) mission, presently undergoing Phase A study, is selected by the time of the meeting, this talk will provide a more complete presentation of that mission.
SH42A-06 11:35h
The relation between the solar wind dynamic pressure at Voyager 2 and the energetic particle events at Voyager 1
Voyager 1 energetic particle data suggest the recent close approach to, or crossing of, the termination shock. Although the plasma experiment on Voyager 1 is not providing useful data, plasma data from Voyager 2 may shed light on the plasma conditions at Voyager 1. Before the first particle event, Voyagers 1 and 2 see similar particle signatures. Voyager 2 pressure and energetic particle flux profiles have similar structure. Changes in the energetic particle flux at Voyager 1 are associated with merged interaction regions (MIRs) passing the spacecraft. Many MIRs are observed at both spacecraft, so Voyager 2 dynamic pressures may predict energetic particle changes at Voyager 1 and the movement of the termination shock near Voyager 1. The magnetic field strength and dynamic pressure are correlated at Voyager 2 so that changes in the magnetic field strength may serve as a proxy for dynamic pressure changes at Voyager 1.
SH42A-07 11:50h
A Transition to Fast Flows and Its Effects on the Magnetic Fields and Cosmic Rays Observed by Voyager 2 near 70 AU
A transition from quasi-periodic flows to increasingly fast flows was observed by Voyager 2 (V2) near 70 AU from 2002.8 to 2004.4. The transition was related to the appearance of coronal holes near the solar equatorial plane on Carrington Rotation (CR)1994. Voyager 2 observed several important features (two cycles of quasi-periodic variations in V and B, a shock at 2003.37, a Merged Interaction Region (MIR) with strong magnetic fields in a region with increasing speed, and a shock at 2004.33. A 1-D multi-fluid MHD model (Wang, C., and J. Richardson, J. Geophys. Res., 106(A12) 29,401-29,408, 2001) shows that these features at V2 evolved from the flows observed by ACE at 1 AU. The changes in the cosmic ray intensity (CRI) greater than 70 MeV/n were related to the magnetic field strength at V2. The quasi-periodic variations were related to variations in CRI, but they produced no appreciable net decrease in the CRI. The first shock and the MIR produced a step-like decrease in the CRI at V2 during 2003. The shock observed by V2 at 2004.33 was followed by a MIR and a fast stream. These events were related to a system of transient flows associated with the October - November 2003 events at 1 AU, and they caused a large decrease in the cosmic ray intensity and a large enhancement of 2.5 MeV protons observed by V2.
SH42A-08 INVITED 12:05h
A Theorist's View of Recent Observations by Voyager 1 in the Outer Heliosphere
Recent observations of energetic particles on the Voyager 1 spacecraft in the outer heliosphere suggest new phenomena associated with its approach to the termination shock of the solar wind. Interpretations of the data have lead to controversy, with some claiming that the termination shock has been crossed and others not. I will review the published data from the relevant Voyager1 instruments and discuss their interpretation. No completely satisfactory model or theory which explains the basic data consistently has been produced. The various theoretical ideas advanced to explain the phenomena will be discussed. If the energetic particle data are taken to imply that the termination shock has indeed been crossed (Krimigis, et al, Nature, 2003) then we must understand why no compression of the magnetic field was observed (Burlaga, et al, GRL, 2003). However, if the termination shock has {\it not} been crossed (McDonald, et al, Nature, 2003), then the observed power-law energy spectrum down to very low energies reported by Krimigis, et al, is difficult to explain. Similarly, the radio data (Gurnett, et al, GRL, 2003) show no evidence of a shock crossing. These issues have been discussed, but there is, as yet, no generally accepted explanation of these data. The observed streaming anisotropies of the energetic particles provide valuable constraints on the physics, and they have been interpreted to support both the idea that the shock has been crossed and that it hasn't. Published theoretical analyses of particle acceleration and transport upstream of a shock (Jokipii and Giacalone, Ap. J. Lett 2004; Jokipii etal., Ap. J. Lett., 2004) suggest that the observed anisotropies are most-readily interpreted if Voyager 1 was near the termination shock, but that the shock was not crossed.