SH11A-01 INVITED 

Voyager 2 in the Vicinity of the Termination Shock with Voyager 1 Well Beyond

* Stone, E C (ecs@srl.caltech.edu), California Institute of Technology, 220-47 Downs Laboratory, Pasadena, CA 91125,

Voyager 2 began observing intense inward streaming of MeV ions from the termination shock in 2005 and has recently observed plasma oscillations excited by electron beams from the shock. Such beams occur on field lines that are tangential to planetary bow shocks, and Gurnett and Kurth proposed a similar topology for oscillations observed by Voyager 1 upstream of the termination shock. This suggests that Voyager 2 is now directly connected along the magnetic field to the shock, consistent with increasing intensities of sub-MeV ions. MHD models by Opher et al. and Pogorelov et al. indicate that direct connection along the spiral magnetic field to the non-spherical shock occurs when Voyager 2 is within ~3 AU from the shock. This suggests that if the shock moves steadily inward at >30 km/s, Voyager 2 may enter the heliosheath this year at a radial distance of <85 AU. The Voyager 2 data at the shock combined with Voyager 1 data at 105 AU in the heliosheath should help address questions such as the plasma flow immediately upstream of the shock and in the heliosheath, the source of anomalous cosmic rays and their relationship to the lower energy ions from the shock, the gradients of cosmic rays in the heliosheath, the effects of transients at the shock and beyond, and the north-south asymmetries in the shock and heliosheath

SH11A-02 INVITED 

New Observations of the Heliospheric Magnetic Field from the Voyager Spacecraft

* Burlaga, L F (Leonard.F.Burlaga@nasa.gov), NASA/Goddard Space Flight Center, Geospace Physics Laboratory Code 673, Greenbelt, MD 20771, United States

We review recent observations of variations of the heliospheric magnetic field B(t) made by Voyager 1 and 2 (V1 and V2), and we discuss the boundary conditions needed for models to explain the observations. Usually, observations from a spacecraft close to the Sun, such as ACE, WIND or Ulysses are used as input to a time- dependent model. Generally, the predicted profile B(t) can be compared directly with the observed profile only when either V1 or V2 is approximately radially aligned with a near-Sun spacecraft; this happens rarely and only for a brief time interval. The Bastille Day events illustrate this situation. In the absence of radial alignment of the spacecraft it is possible to predict the development of a global structure (a GMIR) with data from ACE or WIND, if they obtain a representative sample the flows that merge to form a GMIR. When latitudinal gradients are small and when there is statistical homogeneity in the azimuthal direction, it is possible to predict the statistical properties of the large-scale fluctuations of B(t) observed by V1 or V2 during a year or so. We illustrate this situation with observations from the recent solar maximum and the declining phase of the solar cycle. Predictions of detailed observations made by V1 and V2 under general conditions (e.g., when there is a large latitudinal gradient) require boundary conditions as a function of time on a surface, such as a Sun-centered sphere with a radius of 1 AU. These conditions can only be provided by global solar observations. We suggest the feasibility of such an approach, using V2 observations for 2005 and 2006. The prediction of observations in the heliosheath requires the solution of the 3-D boundary problem for the supersonic solar wind and propagation of solar wind through the termination shock into the heliosphere. The properties of B(t) observed in the heliosheath have not yet been predicted.

SH11A-03 

The Solar Wind in the Foreshock Upstream of the Termination Shock

* Richardson, J D (jdr@space.mit.edu), M.I.T., M.I.T. 37-655, Cambridge, MA 02139, United States

Voyager 2 is observing streaming energetic particles which indicate that it is in the foreshock region which extends several AU in front of the termination shock. The density obsserved by V2 in July and August 2007 has been very low and the speed has decreased, so the dynamic pressure is low and the termination shock should be moving inward. We show recent solar wind data and compare with other data from Voyager. The speeds on Voyager have fallen below 300 km/s, the lowest speeds since the beginning of the mission, suggesting a rapid slowdown of the solar wind is occurring before the shock. This decrease is more rapid than predicted from the interaction with interstellar neutrals, and we discuss the implications of these data.

SH11A-04 

Voyager 2 Observations of Langmuir Waves Upstream of the Termination Shock

* Kurth, W S (william-kurth@uiowa.edu), University of Iowa, Dept. of Physics and Astronomy, Iowa City, IA 52242, United States Gurnett, D A (donald-gurnett@uiowa.edu), University of Iowa, Dept. of Physics and Astronomy, Iowa City, IA 52242, United States Decker, R B), Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, United States Krimigis, S M), Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, United States Richardson, J D), Massachusetts Inst. of Technology, Center for Space Research, Cambridge, MA 02139, United States Stone, E C), CalTech, Space Radiation Laboratory, Pasadena, CA 91125, United States

Electrons from a collisionless shock beamed upstream into the supersonic flow are often unstable to Langmuir waves, also called electron plasma oscillations. These were expected to be precursors to crossings of the termination shock and Voyager 1 observed these occasionally for about 10 months prior to that spacecraft encountering the shock on Dec. 16, 2004. Analysis of the solar wind conditions leading up to the Voyager 1 crossing suggested increasing pressure in the supersonic wind probably resulting in the spacecraft chasing the shock for a good portion of the 10-month interval during which the Langmuir waves were observed. Beginning on Aug. 1, 2007, the Voyager 2 plasma wave instrument recorded Langmuir waves at 311 Hz in the most extensive such activity since the Neptune encounter. Additional activity was observed briefly on August 26 and extensively on Aug. 29-30, 2007. In this most recent event, some wave activity was also found in the 562 Hz channel. For a plasma frequency at the center of the 311 Hz channel, the electron density is about 0.0012 cm^-3. A plasma frequency of 562 Hz corresponds to a density of about 0.0039 cm^-3. Measurements of the plasma density by the Voyager 2 plasma instrument are consistent with this range of plasma frequencies. The Langmuir waves appear to occur when large fluxes of ~20 keV ions are observed by the Voyager 2 LECP instrument. While the ions are not directly responsible for the Langmuir waves, this association strongly suggests a magnetic connection to the termination shock, especially with an abrupt energy cutoff in the ion anisotropies, as is observed. The plasma instrument indicated decreasing solar wind pressure during August 2007 and even earlier. Hence, contrary to the situation with the Voyager 1 crossing, it may be that the termination shock is moving inward and a crossing by Voyager 2 will occur on a time scale much shorter than the 10-month upstream interval observed by Voyager 1.

SH11A-05 

The Voyagers at the Termination Shock: Low-energy Charged Particle Measurements

* Decker, R B (robert.decker@jhuapl.edu), Johns Hopkins Univ. Applied Physics Lab., 11100 Johns Hopkins Rd, Laurel, MD 20723- 6099, United States Krimigis, S M (tom.krimigis@jhuapl.edu), Johns Hopkins Univ. Applied Physics Lab., 11100 Johns Hopkins Rd, Laurel, MD 20723- 6099, United States Roelof, E C (edmond.decker@jhuapl.edu), Johns Hopkins Univ. Applied Physics Lab., 11100 Johns Hopkins Rd, Laurel, MD 20723- 6099, United States Hill, M E (matt.hill@jhuapl.edu), Johns Hopkins Univ. Applied Physics Lab., 11100 Johns Hopkins Rd, Laurel, MD 20723- 6099, United States

As of 06 Sep. 2007, Voyager 2 (V2: 84.7 AU, S27.5°) was upstream of the termination shock (TS) in its foreshock region, and Voyager 1 (V1: 103.8 AU, N34.3°) was downstream in the heliosheath (HSH). We focus on measurements made by the Low Energy Charged Particle Experiments (LECP) on V2 and V1 during the past year. At V1, intensities of ions >0.04 MeV in the HSH remained relatively flat during 2007, varying by factors ≤2. The plasma flow velocity in the R-T plane (the LECP scan plane), estimated from angular distributions of ions in three energy channels 0.04-0.14 MeV, continues to show large variations in speed and azimuth. For example, starting in July 2007, the flow speed began increasing from ~40 km/s, reaching ~100 km/s in the most recent data, with the flow direction rotating from mainly toward +R to mainly toward -T. Recent data from several instruments on V2 indicate that V2 may soon encounter the TS. V2 has been measuring termination foreshock (TFS) particles since at least Jan. 2005, shortly after the TS moved radially inward, crossing V1 at 94.0 AU on 16 Dec. 2004. During its 2.5-year traversal of the TFS (mid-2002 through 2004), V1 measured an energy spectrum of low-energy ions that extended smoothly down to at least 0.04 MeV. In contrast, during the past 2.5 years at V2 (2005 to about mid-2007) the TFS spectrum at V2 has rolled over at ~0.2 MeV, dropping to background below ~0.1 MeV. The V2 ion energy spectrum began to unfold in mid-July 2007, and now is above background at 0.028 MeV, but contains large temporal fluctuations on a scale of hours. In the most recent data, the V2 ion intensities above 0.04 MeV exceed those measured by V1 in the TFS during 2004, and above 0.5 MeV exceed those measured by V1 in the HSH during 2005 onward. The recent V2 data also show impulsive intensity increases of electrons >0.022 MeV. Although ion beaming anisotropies at V2 continue to be directed mostly toward +T (in the usual RTN system), there are short bursts (~few days) toward -T. We will discuss these points in more detail and describe the most recent results.

SH11A-06 

Evolution of the Energy Spectra of Low-Energy Particles at Voyagers 1 and 2 in the Outer Heliosphere and Implications for the Source Location of Anomalous Cosmic Rays

* Cummings, A C (ace@srl.caltech.edu), California Institute of Technology, Mail Code 220-47, Pasadena, CA 91125, United States Stone, E C), California Institute of Technology, Mail Code 220-47, Pasadena, CA 91125, United States McDonald, F B), University of Maryland, Computer and Space Science Bldg 3245, College Park, MD 20742, United States Heikkila, B C), Goddard Space Flight Center, Code 612.4, Greenbelt, MD 20771, United States Lal, N), Goddard Space Flight Center, Code 612.4, Greenbelt, MD 20771, United States Webber, W R), New Mexico State University, P.O. Box 30001/Dept. 4500, Las Cruces, NM 88003, United States

As of this writing, 4 September 2007, the Voyager 1 (V1) spacecraft is at 103.8 AU and 34.3 deg N. It is in the heliosheath, presumably about 10 AU beyond the solar wind termination shock. The Voyager 2 (V2) spacecraft is at 83.7 AU and 27.5 deg S. It has not crossed the termination shock but may be close, having been observing low-energy termination shock particles since ~2005.0. In addition, very recently the intensity of protons with ~2-8 MeV at V2 has grown to exceed that at V1 by a factor of ~3. From ~10 to 100 MeV, the V2 intensity is lower than that at V1. For He, from ~10-25 MeV/nuc, the V2 energy spectrum is below that of V1 but well above that at V1 when it crossed the shock in late 2004. Energy spectra at both spacecraft have recently been evolving. The spectral observations at V2 when it crosses the termination shock compared to the spectra at V1 at the same time and at the time V1 crossed may help determine the source location of the anomalous cosmic rays. Finally, the energy spectra of H and O scale to that of He with multiplicative factors in energy/nucleon and intensity that suggest that the diffusion coefficient is proportional to particle speed times rigidity to the power ~1.4. This work was supported by NASA under contract NAS7-03001.

SH11A-07 

Energy Spectra of Energetic Particles Upstream of the Termination Shock

* Jokipii, J R (jokipii@lpl.arizona.edu), University of Arizona Planetary Sciences, Kuiper Space Sciences, Tucson, AZ 85721, United States Giacalone, J (giacalon@lpl.arizona.edu), University of Arizona Planetary Sciences, Kuiper Space Sciences, Tucson, AZ 85721, United States Decker, R B (robert.decker@jhuapl.edu), Applied Physics Lab Johns Hopkins University, 11100 Johns Hopkins Rd, Laurel, MD 20723-6099, United States

Energetic particles observed upstream of the solar-wind termination shock provide valuable remote information concerning physical processes occurring at the shock. In this paper we discuss the implications of the energy spectra of ions observed at Voyagers 1 and 2, for the energy range .04-18 MeV, for the time periods when the spacecraft were upstream of the shock but very close to it. The upstream events at the two Voyagers were similar in many respects in that they were highly variable and anisotropic. However, the Voyager 2 spectra exhibit an evolving low-energy turnover at approximately 1 MeV, whereas the Voyager 1 spectra do not show any such turnover. This asymmetry in the behavior at the two spacecraft is likely to be a manifestation of a large-scale spatial asymmetry of the heliosphere and/or the termination shock. We present possible interpretations of the observed energy spectra in terms of the propagation of the particles from the shock to the spacecraft, in the stochastic upstream magnetic field, and a blunt, asymmetric termination shock. The interpretations are consistent with the observations and allow us to place limits on the large-scale shape and any lateral asymmetries of the shock. They also constrains the propagation modes of the energetic particles.

SH11A-08 

Voyager Observation of Galactic Cosmic Ray Elctrons in the Heliosheath

* McDonald, F B (fmcdonal@umd.edu), Institute for Physical Science and Technology, Univeristy of Maryland, 4211 CSS Building, College Park, MD 20742, United States Webber, W R (bwebber@nmsu.edu), Department of Physics and Astronomy, New Mexico State University, P.O. Box 30001, Las Cruces, NM 88003, United States Stone, E C (ecs@srl.caltech.edu), California Institute of Technology, 1200 East California Blvd., Pasadena, CA 91109, United States Cummings, A C (ace@citsrl.srl.caltech.edu), California Institute of Technology, 1200 East California Blvd., Pasadena, CA 91109, United States Heikkila, B C (bryant@mail630.gsfc.nasa.gov), NASA/Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, United States Lal, N (nand.lal@nasa.gov), NASA/Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, United States

Voyager 1 observations of low-energy, (2.5-140 MeV), highly relativistic, galactic cosmic ray (GCR) electrons are important in studying the transport of energetic particles and the nature of transient disturbances in the heliosheath. These electrons originate as directly accelerated primaries, as interstellar secondaries from the decay of charged pions, or at low energies as knock-on electrons produced by the passage of higher energy cosmic rays through the interstellar medium. Inside 10 AU below some 60 MeV, Jovian electrons are the dominant electron component. Except for solar (out to 30 AU) and Jovian electrons there was no increase in the electron channels above the background level until V1 approached the termination shock in mid 2002. Since crossing the termination shock there has been a strong increase in the electron intensity with radial gradients of 6 – 75 MeV electrons that are inferred to be on the order of 18%/AU. The 10 MeV component appears to be especially sensitive to the passage of transient disturbances through the heliosheath. As V2 began observing an extended termination shock particle event in late 2004 there have been accompanying increases of 2.5 – 14 MeV electrons that are probably related to the interaction between interplanetary disturbances, the termination shock and the existing GCR electrons.

Author(s) (2007), Title, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract #####-##.