Application of historic datasets to understanding open solar flux and the 20th-century grand solar maximum. 1. Geomagnetic, ionospheric, and sunspot observations

Abstract

We updated annual mean reconstructions of near-Earth interplanetary conditions and (signed) open solar flux FS for the past 186 years. Furthermore, we added observations for solar cycle 24 to refine regressions and improved allowance for orthogardenhose and folded (a.k.a., switchback) heliospheric flux from studies using strahl electrons. We also improved the allowance made for the annual mean gardenhose angle of the interplanetary magnetic field. We used both multiple regression with interplanetary magnetic field B and solar wind speed VSW and linear regression with the function BVSWn and demonstrated that the latter gives correlations that are not significantly lower than those given by the former. We conducted a number of tests of the geomagnetic indices used, of which by far the most important is that all four usable pairings of indices produce almost identical results for B, VSW, and FS. All reconstructions were given full 2σ uncertainties using a Monte Carlo technique that generates an ensemble of 1 million members for each pairing of indices. The long-term variations of near-Earth interplanetary field B and open solar flux FS were found to closely match those of the international sunspot numbers but VSW show a significantly different variation. This result explains why of the two peaks of 20th-century grand solar maximum, the range geomagnetic indices give a larger second peak, whereas the diurnal variation indices give a first peak that is larger, as it is for sunspots. We found that the increase in solar cycle averages of FS was between 2.46 × 1014 Wb in 1906 and 4.10 × 1014 Wb in 1949, the peak of the grand maximum, and hence, the rise in open flux was by a factor of 67%.

Application of historic datasets to understanding open solar flux and the 20th-century grand solar maximum. 2. Solar observations

“…From the above, we conclude that the long-term variation of OSF deduced from geomagnetic activity, as reviewed in Paper 1 (Lockwood et al, 2022), is reflected in the sunspot-minimum structure of the solar corona as seen in eclipse events and (for recent minima) in images from the LASCO coronagraph on SoHO.…”

“…Figure 2 shows that the sunspot-minimum streamer belt width derived from the images, Λ SB , has an inverse relationship with the open solar flux (OSF), F S derived from geomagnetic activity and as presented in Paper 1 (Lockwood et al, 2022). This relationship is predicted by simple global flux-continuity modelling of both Λ SB and F S by Lockwood and Owens (2014).…”

“…The major difference in available solar observations of the rise and of the fall are accurate magnetograms from solar magnetographs: we here use synthetic magnetograms to interpret the rise and employ historic observations of Polar Crown Filaments to test them and verify their use. We show that eclipse images at sunspot minimum reveal the long-term variation of open flux deduced from geomagnetic observations in Paper 1 (Lockwood et al, 2022). We also make use of polar coronal hole fluxes derived from historic white light images of polar faculae, but have to consider the implications of the fact that these facular images do not tell us the polarity of the field.…”

“…Paper 1 (Lockwood et al, 2022) studied the signed OSF, F S defined as the magnetic flux of one polarity (toward or away from the Sun) threading the coronal source surface. This surface is often taken to be spherical and at heliocentric distance r = 2.5R ⊙ (where a mean photospheric radius, R ⊙ = 6.96 × 10 8 m).…”

“…The key difference between the two is the volume of the magnetic flux tube, which depends primarily on how far it extends into the heliosphere (or even beyond) and so on the time elapsed since it emerged through the coronal source surface. Paper 1 (Lockwood et al, 2022) detailed the quantification of the long-term variation in F S = F SB + F CH , as determined from historical geomagnetic activity records. In this paper we look at historic solar observations and recent flux transport modelling that help us understand the observed long-term variation in OSF.…”