Physics – Plasma Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsh53d..04c&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SH53D-04
Physics
Plasma Physics
[2104] Interplanetary Physics / Cosmic Rays, [2124] Interplanetary Physics / Heliopause And Solar Wind Termination, [2126] Interplanetary Physics / Heliosphere/Interstellar Medium Interactions, [7845] Space Plasma Physics / Particle Acceleration
Scientific paper
The Voyager 1 (V1) spacecraft crossed the termination shock of the solar wind on 16 December 2004, just as energetic particles in the outer heliosphere had started to recover from solar maximum conditions. By the time Voyager 2 (V2) crossed the termination shock in late August of 2007, the intensities of anomalous cosmic rays (ACRs) were entering a stabler period of solar minimum conditions in the outer heliosphere, with slower intensity increases of low-energy ACRs. Both spacecraft are still in the heliosheath and neither has found the source region of ACRs, as the ACR intensities at the lowest energies measured continue to increase at V1. At the highest ACR energies, the V1/V2 gradient is ~0, suggesting we are observing the source intensity at these energies and that the mean-free path of these particles is very long, since the separation distance of the two spacecraft is ~130 AU. In addition, there is a strong solar-cycle polarity dependence of these high-energy ACRs, with the intensity at present being similar to that in the last A<0 period in 1987 but approximately a factor of 3.5 times greater than that in the intervening A>0 period in 1998. During their time in the heliosheath, the Voyagers have experienced periods when the modulation changes appear to have been beyond the location of V1, as well as times when both spacecraft appeared to be flying through a fixed radial gradient. Near the end of 2009, the solar minimum conditions near Earth appeared to be ending, as evidenced by, e.g., a sharp increase in the tilt angle of the heliospheric current sheet that was accompanied by decreases in intensities of ACRs and galactic cosmic rays. These conditions may take several years to reach the Voyager spacecraft, which are deep in the heliosheath where the radial solar wind velocity is low. This work was supported by NASA under contract NAS7-03001 and grant NNX10AE45G.
Cummings Alan C.
Heikkila Bryant
Lal Niranjan
McDonald Frank B.
Stone Edward C.
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