Physics – Plasma Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsh13c..03d&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SH13C-03
Physics
Plasma Physics
[2114] Interplanetary Physics / Energetic Particles, [2124] Interplanetary Physics / Heliopause And Solar Wind Termination, [2126] Interplanetary Physics / Heliosphere/Interstellar Medium Interactions, [7807] Space Plasma Physics / Charged Particle Motion And Acceleration
Scientific paper
This talk will focus on recent measurements from the LECP instruments on Voyager 1 and Voyager 2. Voyager 1 (118 AU, N34 deg.) crossed the termination shock at 94.0 AU in Dec. 2004 and is roughly 24 AU into the inner heliosheath. Voyager 2 (96 AU, S30 deg) crossed the shock at 83.5 AU in Sep. 2007 and is roughly 13 AU into the heliosheath. Large variations in the intensities, energy spectra, and angular distributions of low-energy heliosheath ions and electrons have occurred during the past year, evidently in response to an increase in the number of solar active regions in January 2010. For example, starting around 2010.8, ion intensities at Voyager 1 began a two-step exponential decrease. The intensities of ions below 0.5 MeV fell by 40%, reaching minima around 2011.5 that were the lowest measured since mid-2005. During the intensity drop at Voyager 1 the ion energy spectrum evolved to a single power-law with an index -1.5. Similar intensity drops began somewhat earlier at Voyager 2, and the intensity minima of the low-energy ions occurred on 2011.2, one-third of a year earlier than at Voyager 1. In addition, after the ion intensities had recovered at Voyager 2, they were deficient at lower energies compared to the pre-drop levels, causing the energy spectrum to flatten with decreasing energy. During the intensity drop at Voyager 2, ion angular distributions evolved from convective-like, consistent with the measured plasma flow velocity, to azimuthal, more consistent with unidirectional streaming along the mean magnetic field. We will also summarize the heliosheath plasma flow velocity in the R-T (instrument scan) plane that is estimated using angular data from the three lowest energy Voyager 1 LECP ion channels, which cover 40-139 keV. The estimated radial component of flow decreased from about 60 km/s to about 0 km/s at a rate of -19 km/s/yr during 2008.7-2010.3, was consistent with 0 km/s from 2010.3-2011.0, and then went negative, averaging about -13 km/s during 2011.0-2011.5. From 2005.5 to 2010.7 the tangential component of flow had averaged about -40 km/s, with large variations of about 20 km/s. The tangential flow changed during 2010.7-2011.5 to an average of about -17 km/s. Analysis of LECP angular data taken thus far during spacecraft rolls, which enable angular measurements in the N-R plane, indicate that an N-component of plasma flow must be less than about 30 km/s.
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