Solar Cycle Variation of the Heliospheric Plasma Sheet Thickness

Abstract

Past independent studies of the heliospheric plasma sheet (HPS) have shown that the thickness is highly variable, ranging from ≈ 3.8 × 10 5 to 8.9 × 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 [| B| ] at 2.5 solar radii [R⊙] was ≈ 40 % larger in 1995 than in 2007 (0.22 gauss versus 0.16 gauss). We also found a larger (≈ 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.