Average Thickness of Magnetosheath Upstream of Magnetic Clouds at 1 AU versus Solar Longitude of Source

Details Average Thickness of Magnetosheath Upstream of Magnetic Clouds at 1 AU versus Solar Longitude of Source Average Thickness of Magnetosheath Upstream of Magnetic Clouds at 1 AU versus Solar Longitude of Source

Mar 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008soph..248..125l&link_type=abstract

Solar Physics, Volume 248, Issue 1, pp.125-139

Physics

5

Magnetic Clouds, Solar Wind, Shock Waves, Magnetosheath

Scientific paper

Starting with a large number ( N=100) of Wind magnetic clouds (MCs) and applying necessary restrictions, we find a proper set of N=29 to investigate the average ecliptic plane projection of the upstream magnetosheath thickness as a function of the longitude of the solar source of the MCs, for those cases of MCs having upstream shock waves. A few of the obvious restrictions on the full set of MCs are the need for there to exist a driven upstream shock wave, knowledge of the MC’s solar source, and restriction to only MCs of low axial latitudes. The analysis required splitting this set into two subsets according to average magnetosheath speed: slow/average (300 - 500 km s-1) and fast (500 - 1100 km s-1) speeds. Only the fast set gives plausible results, where the estimated magnetosheath thickness (Δ S) goes from 0.042 to 0.079 AU (at 1 AU) over the longitude sector of 0° (adjusted source-center longitude of the average magnetic cloud) to 40° off center (East or West), based on N=11 appropriate cases. These estimates are well determined with a sigma ( σ) for the fit of 0.0055 AU, where σ is effectively the same as sqrt{} (chi-squared) for the appropriate quadratic fit. The associated linear correlation coefficient for Δ S versus |Longitude| was very good (c.c.=0.93) for the fast range, and Δ S at 60° longitude is extrapolated to be 2.7 times the value at 0°. For the slower speeds we obtain the surprising result that Δ S is typically more-or-less constant at 0.040±0.013 AU at all longitudes, indicating that the MC as a driver, when moving close to the normal solar wind speed, has little influence on magnetosheath thickness. In some cases, the correct choice between two candidate solar-source longitudes for a fast MC might be made by noting the value of the observed Δ S just upstream of the MC. Also, we point out that, for the 29 events, the average sheath speed was well correlated with the quantity Δ V[=(< V MC>-< V UPSTREAM>)], and also with both < V MC> and < V MC,T>, where < V MC> is the first one-hour average of the MC speed, < V MC,T> is the average MC speed across the full MC, and < V UPSTREAM> is a five-hour average of the solar wind speed just upstream of the shock.

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