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Solar Physics

, 294:90 | Cite as

Solar Cycle Variation of the Heliospheric Plasma Sheet Thickness

  • Chin-Chun WuEmail author
  • Kan Liou
  • Ronald P. Lepping
Article

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.

Keywords

Solar cycle variation Heliospheric plasma sheet Heliospheric current sheet Heliospheric plasma-sheet inflation hypothesis Interplanetary magnetic field 

Notes

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.

Disclosure of Potential Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Copyright information

© This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2019

Authors and Affiliations

  1. 1.Naval Research LaboratoryWashingtonUSA
  2. 2.Applied Physics LaboratoryJohns Hopkins UniversityLaurelUSA
  3. 3.UMBC/NASA/GSFCGreenbeltUSA

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