Abstract
AbstractWe analyze the widely used international/Zürich sunspot number record, R, with a view to quantifying a suspected calibration discontinuity around 1945 (which has been termed the “Waldmeier discontinuity”). We compare R against the composite sunspot group data from the Royal Greenwich Observatory network and the Solar Optical Observing Network, using both the number of sunspot groups, NG, and the total area of the sunspots, AG. In addition, we compare R with the recently developed interdiurnal variability geomagnetic indices IDV and IDV(1d). In all four cases, linearity of the relationship with R is not assumed and care is taken to ensure that the relationship of each with R is the same before and after the putative calibration change. It is shown the probability that a correction is not needed is of order 10−8 and that R is indeed too low before 1945. The optimum correction to R for values before 1945 is found to be 11.6%, 11.7%, 10.3%, and 7.9% using AG, NG, IDV, and IDV(1d), respectively. The optimum value obtained by combining the sunspot group data is 11.6% with an uncertainty range 8.1–14.8% at the 2σ level. The geomagnetic indices provide an independent yet less stringent test but do give values that fall within the 2σ uncertainty band with optimum values are slightly lower than from the sunspot group data. The probability of the correction needed being as large as 20%, as advocated by Svalgaard (2011), is shown to be 1.6 × 10−5.
Highlights
This is the first of a series of three papers to model the long-term variation in open solar flux and the variation in the latitudinal width of the coronal streamer belt
The development of open solar flux reconstructions and their modeling has recently been reviewed by Lockwood [2013] and Owens and Forsyth [2013] and the modeling in paper 3 reproduces the reconstructions by Lockwood et al [2014b]
We make use of both indices after 1874, when they are extremely similar despite considerable differences in how they are compiled and the different data sets used. We chose these indices because both depend primarily on the strength of the near-Earth interplanetary magnetic field (IMF) without the strong dependence on solar wind speed found in range indices such as aa and Ap: this means that they have a simpler relationship to sunspot number
Summary
This is the first of a series of three papers to model the long-term variation in open solar flux and the variation in the latitudinal width of the coronal streamer belt. We make use of both indices after 1874, when they are extremely similar despite considerable differences in how they are compiled and the different data sets used We chose these indices because both depend primarily on the strength of the near-Earth interplanetary magnetic field (IMF) without the strong dependence on solar wind speed found in range indices such as aa and Ap (see review by Lockwood [2013]): this means that they have a simpler relationship to sunspot number. There is no obvious major change in the behavior of the sunspot number, the sunspot group data, or the geomagnetic indices around this time; it lies on the steep initial rising phase of a solar cycle when a change in a scaling factor would be hardest to detect
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