Abstract
Past independent studies of the heliospheric plasma sheet (HPS) have shown that the thickness is highly variable, ranging from \(\approx 3.8 \times 10^{5}\) to \(8.9 \times 10^{6}\) 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) and 8 (in Solar Cycle 24). We also found that the average HPS thickness in 2007 (near the minimum of Solar Cycle 23/24) was almost ten times larger than that in 1995 (near minimum of Solar Cycle 22/23), 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 [\(| \boldsymbol{B}|\)] at 2.5 solar radii [R⊙] was \(\approx 40\)% larger in 1995 than in 2007 (0.22 gauss versus 0.16 gauss). We also found a larger (\(\approx 27 \)%) 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 Sun’s polar field is weaker and narrower if the Sun’s polar field is stronger.
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Acknowledgments
We thank the World Data Center SILSO (Sunspot Index and Long-term Solar Observations), Royal Observatory of Belgium, Brussels for proving sunspot data, and Y.-M. Wang of the Naval Research Laboratory for providing the derived solar magnetic-field data at 2.5 solar radii. The work of C.-C. Wu was supported partially by the Chief of Naval Research, and the NASA 80HQTR18T0023, HSWO2R17-0005, and 80HQTR19T0062 grants. The work of K. Liou was supported by the NSF grant 1743118 to the Johns Hopkins University Applied Physics Laboratory.
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Wu, CC., Liou, K. & Lepping, R.P. Solar Cycle Variation of the Heliospheric Plasma Sheet Thickness. Sol Phys 294, 90 (2019). https://doi.org/10.1007/s11207-019-1464-0
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DOI: https://doi.org/10.1007/s11207-019-1464-0