Solar Wind Suprathermal Electron Strahl Width from 1.3 to 5.4 AU

Details Solar Wind Suprathermal Electron Strahl Width from 1.3 to 5.4 AU Solar Wind Suprathermal Electron Strahl Width from 1.3 to 5.4 AU

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsh34a..02g&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SH34A-02

Physics

[2114] Interplanetary Physics / Energetic Particles, [2162] Interplanetary Physics / Solar Cycle Variations, [2164] Interplanetary Physics / Solar Wind Plasma

Scientific paper

The solar wind suprathermal electron population typically includes an anisotropic anti-sunward field-aligned beam component, referred to as the strahl. As strahl electrons propagate anti-sunward in a decreasing interplanetary magnetic field, magnetic focusing acts to narrow the strahl beam width. At the same time, scattering processes work against the focusing and maintain strahl beams of finite width. The observed strahl width in the heliosphere is the result of the competition between focusing and scattering. The suprathermal electron strahl width and intensity observed by Ulysses from 1991 - 2008 have been newly examined. These observations cover radial distances between 1.3 and 5.4 AU, and span more than a solar cycle. The strahl width and intensity are characterized by fitting pitch angle distributions to a function consisting of a Gaussian, peaked parallel (or anti-parallel) to the interplanetary field, plus a constant term. Approximately 50 - 65% of the Ulysses pitch angle spectra yield reasonable fits in preliminary analysis, indicating distributions that are well-described by this simple function. For most of the Ulysses observations at energies below 429 eV, the strahl width lies between 20o and 90o. The Ulysses results contrast with previously reported ACE observations at 1 AU. In particular, the more distant Ulysses results appear shifted towards larger strahl widths, indicating that scattering becomes relatively more important than focusing beyond 1 AU. The Ulysses strahl widths are generally broader at heliospheric distances just beyond 2.5 AU than inside 2.5 AU. Between about 2.5 AU and 4.5 AU, the strahl width distribution varies little. Beyond 4.5 AU the strahl width again narrows, indicating that focusing begins to overcome scattering at these large distances. The distribution of strahl widths during the 1st (1992-1998) and 2nd (1998 - 2004) Ulysses polar orbits were compared, with little difference found. However a comparison of strahl widths during 1994, a year dominated by coronal hole high-speed solar wind, to 2000, a year dominated by slow solar wind, revealed a notable difference, with widths generally narrower during 1994 than in 2000. Here we present a comprehensive analysis regarding the variability of the strahl width and intensity with heliocentric distance, as well as with the estimated electron field line path length. In addition, we examine the occurrence of unidirectional and counterstreaming strahl electrons, as indicated by the fitting algorithm.

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