Voyager Observations of Galactic Cosmic Ray Transport in the Heliosheath and their Reacceleration at the Termination Shock F.B. McDonald 1, W.R. Webber 2, E.C. Stone 3, A.C. Cummings 3, B.C. Heikkila 4, N. Lal 4 1 Institute for Physical Science and Technology, Univ. of Maryland, College Park, MD, USA 2 Dept. of Physics and Astronomy, New Mexico State Univ., Las Cruces, New Mexico, USA 3 California Institute of Technology, Pasadena, CA, USA 4 NASA/Goddard Space Flight Center, Greenbelt, MD, USA 2 nd Heliospheric Network Workshop Island of Kefalonia, Greece 6-9 May 2008

V1 TSX:Dec 16, 2004 (94 AU) Near Solar Maximum Moved ~16 AU beyond TS V2 TSX:Aug 30, 2007 (83.7 AU) Near Solar Minimum CRS Exp. Energetic Particle Coverage H: MeV He: MeV/n Z = 1-28 (Resolves Isotopes) E: 2.5 – 140 MeV INTRODUCTION 28.6 cm A Major Task is to Separate Temporal and Spatial Effects Very Small GCR Radial Gradients 10 MeV Electrons Increasing at ~50%/Year at V1 Large Increase of 2.5 – 16 MeV Electrons at V2 TSX with Maximum Occurring Beyond the TS.

Low Energy Galactic Cosmic Ray Electrons (2 – 160 MeV)

Origin: Directly Accelerated primaries Interstellar secondaries from the decay of charged pions Knock-on electrons produced by the passage of higher energy cosmic rays through the interstellar medium At energies below 200 MeV: These electrons are the source of the lower energy diffuse gamma, x-ray and synchrotron radio emission from the galaxy. May play a major role in ionizing and heating the interstellar medium. Difficult to observe at 1AU: Large Jovian electron intensity Strongly modulated In the heliosheath, their very low rigidity should make them especially sensitive to the passage of transient disturbances.

Low Energy Galactic Cosmic Ray Electrons (2 – 160 MeV)

Voyager 2 electron data time shifted so that the time of the TSX coincides with that of Voyager1. Next Step Determine the Voyager 2 electron spectra at the peak following the TSX. Determine the radial intensity gradients in the heliosheath using: i.V12/V2 data ii.V1 data alone

Low Energy Galactic Cosmic Ray Electrons (2 – 160 MeV)

Effect of Transients on GCR electrons in the Heliosheath and in the Foreshock Region

Low Energy Galactic Cosmic Ray Electrons (2 – 160 MeV)

ELECTRON CONCLUSIONS Essentially all of the electron modulation in this energy range occurs in the heliosheath Large fluctuations in this very low rigidity component reveals a turbulent heliosheath Expect electron intensities to continue to increase toward interstellar intensities as V1 approaches the heliopause. The large increase in MeV electrons at V2 near the TS and after the TSX is probably related to their re-acceleration at the TS. Both spatial and temporal effects are still important at V1.

MODULATIN SECTION

Reduction of MeV/n GCR He Solar Min to Solar Max in Cycle 23. IMP8 (1AU) factor of 4.4 V2 (63.5 AU)33% V1 (81 AU)22% Essentially all of the modulation associated with the 11 year solar activity cycle occurs in the region of the supersonic solar wind. The Current State of the Heliosphere as Defined by Galactic Cosmic Rays

COSMIC RAY MODULATION

MODULATION CONCLUSIONS Over the 11 year modulation cycle changes in propagation conditions occur mainly between 15AU and the TS. Propagation conditions in the inner heliosphere appear to change significantly less from solar minimum to solar maximum than in the outer heliosphere. Observations suggest that GCR modulation in the heliosheath has remained essentially constant over cycle 23. Based on the latest estimate of the local interstellar galactic cosmic ray intensity (Webber and Lockwood) and a heliosheath thickness of 40 AU, we expect to observe a radial intensity gradient of ~1.7%/AU. The observed gradient is 0.2 ± 0.2 %/AU The amount of the modulation of GCR ions in the heliosheath requires a more accurate estimate of the LIS GCR energy spectra. V1/V2 265 MeV/n GCR He data is consistent with modest reacceleration at the TS. However, the magnitude of the latitudinal gradients are not known.