Solar Cycle Variation of the Heliospheric Plasma Sheet Thickness
Journal Article - Open Access
NAVAL RESEARCH LAB WASHINGTON DC WASHINGTON United States
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Past independent studies of the heliospheric plasma sheet HPS have shown that the thickness is highly variable, ranging from approximate to 3.8x105 to 8.9x106 km. Here we conduct a survey of the previous results and find a solar cycle dependence- where the HPS tends to be wider during solar-minimum years and narrower during solar-maximum years. The HPS is thicker near solar minimum than near solar maximum by a factor of 1.6 in Solar Cycle 23 and8 in Solar Cycle 24. We also found that the average HPS thickness in 2007 near the minimum of Solar Cycle 2324 was almost ten times larger than that in 1995 near minimum of Solar Cycle 2223, and it was associated with a weak polar magnetic field in 2007. Based on the solar-surface-field measurements, we found that the average solar magnetic-field strength B at 2.5 solar radii R-circle dot was approximate to 40 larger in 1995 than in 2007 0.22 gauss versus 0.16 gauss. We also found a larger approximate to 27 magnetic pressure-gradient force in 1995 than in 2007. Because this magnetic gradient force points toward the Equator in the corona which is probably also true farther out, a wider HPS is expected to occur in 2007 than in 1995, at least close to the Sun. This result supports the so-called heliospheric plasma-sheet inflation hypothesis, i.e. the HPS is wider if the Suns polar field is weaker and narrower if the Suns polar field is stronger.