AUTHOR=Lockwood Mike , Owens Mathew J. , Barnard Luke A. , Scott Chris J. , Frost Anna M. , Yu Bingkun , Chi Yutian 

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

JOURNAL=Frontiers in Astronomy and Space Sciences

VOLUME=9

YEAR=2022

URL=https://www.frontiersin.org/journals/astronomy-and-space-sciences/articles/10.3389/fspas.2022.960775

DOI=10.3389/fspas.2022.960775

ISSN=2296-987X

ABSTRACT=<p>We updated annual mean reconstructions of near-Earth interplanetary conditions and (signed) open solar flux <italic>F</italic><sub><italic>S</italic></sub> 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 <italic>B</italic> and solar wind speed <italic>V</italic><sub><italic>SW</italic></sub> and linear regression with the function <inline-formula id="inf1"><mml:math id="m1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>B</mml:mi><mml:mi>V</mml:mi></mml:mrow><mml:mrow><mml:mi>S</mml:mi><mml:mi>W</mml:mi></mml:mrow><mml:mrow><mml:mi>n</mml:mi></mml:mrow></mml:msubsup></mml:math></inline-formula> 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 <italic>B</italic>, <italic>V</italic><sub><italic>SW</italic></sub>, and <italic>F</italic><sub><italic>S</italic></sub>. All reconstructions were given full 2<italic>σ</italic> 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 <italic>B</italic> and open solar flux <italic>F</italic><sub><italic>S</italic></sub> were found to closely match those of the international sunspot numbers but <italic>V</italic><sub><italic>SW</italic></sub> 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 <italic>F</italic><sub><italic>S</italic></sub> was between 2.46 × 10<sup>14</sup> <italic>Wb</italic> in 1906 and 4.10 × 10<sup>14</sup> <italic>Wb</italic> in 1949, the peak of the grand maximum, and hence, the rise in open flux was by a factor of 67%.</p>