Tests of Sunspot Number Sequences: 2. Using Geomagnetic and Auroral Data

Abstract

We compare four sunspot-number data sequences against geomagnetic and terrestrial auroral observations. 5172, 2014a). Each sunspot number is fitted with terrestrial observations, or parameters derived from terrestrial observations to be linearly proportional to sunspot number, over a 30-year calibration interval of 1982 -2012. The fits are then used to compute test sequences, which extend further back in time and which are compared to R ISNv1 , R G , R BB , and R C . To study the long-term trends, comparisons are made using averages over whole solar cycles (minimum-to-minimum). The test variations are generated in four ways: i) using the IDV(1d) and IDV geomagnetic indices (for 1845 -2013) fitted over the calibration interval using the various sunspot numbers and the phase of the solar cycle; ii) from the open solar flux (OSF) generated for 1845 -2013 from four pairings of geomagnetic indices by Lockwood et al. (Ann. Geophys. 32, 383, 2014a) and analysed using the OSF continuity model of Solanki, Schüssler, and Fligge (Nature, 408, 445, 2000), which employs a constant fractional OSF loss rate; iii) the same OSF data analysed using the OSF continuity model of Owens and Lockwood (J. Geophys. Res. 117, A04102, 2012), in which the fractional loss rate varies with the tilt of the heliospheric current sheet and hence with the phase of the solar cycle; iv) the occurrence frequency of low-latitude aurora for 1780 -1980 from the survey of Legrand and Simon (Ann. Geophys. 5, 161, 1987). For all cases, R BB exceeds the test terrestrial series by an amount that increases as one goes back in time.Sunspot Number Recalibration

Tests of Sunspot Number Sequences: 1. Using Ionosonde Data

“…Given the daisy-chaining of intercalibrations involved in the construction of R BB , all values before 1945 need to be 12 % lower (relative to modern values) to make proper allowance for the Waldmeier discontinuity. However, the difference between R (or R C ) and R BB also grows increasingly large as one goes back in time (see Article 2, Lockwood et al, 2016a): from the study presented here we cannot tell if this trend has the same origin as the detected difference during Cycle 17; however, Cycle 17 is consistent with the longer-term trend. That an error as large as 12 % can be found in R BB as late as 1945 does not give confidence that there are not much larger errors in R BB at earlier times.…”

“…Figure 1a shows the sequences of R, R C , and R BB for the interval analysed in the present article (in blue, green, and red, respectively). As discussed in Article 2 (Lockwood et al, 2016a), while the differences over the interval in Figure 1 are relatively minor, they continue to grow as one goes back in time.…”

Tests of Sunspot Number Sequences: 4. Discontinuities Around 1946 in Various Sunspot Number and Sunspot-Group-Number Reconstructions

“…Larger differences, inferred from geomagnetic-activity data, low-latitude auroral sightings, and cos-mogenic isotope abundances in ice sheets, tree trunks, and meteorites, are found for earlier years, which are discussed in Article 2 (Lockwood et al, 2016b) and in the article by Asvestari et al (2016). Changes around 1946 are of interest as there has been discussion about a putative inhomogeneity in the calibration of the original Zürich sunspot-number data series [R ISNv1 ] that has been termed the "Waldmeier discontinuity", as discussed in Article 1 (Lockwood et al, 2016a).…”

“…It is now agreed that R ISNv1 needs correcting for this effect, but it is unclear if, why, and how it influences other data series. Tests comparing against ionospheric data (Lockwood et al, 2016a), auroral sightings, and geomagnetic data (Lockwood et al, 2016b) all suggest that, somehow, an excessive or inappropriate allowance for the Waldmeier discontinuity has been introduced into R BB .…”

Properties of sunspot cycles and hemispheric wings since the 19th century

“…Clette et al 2014), but this revision has been subject to some debate (e.g. Lockwood et al 2016;Usoskin et al 2016). Hoyt & Schatten (1998) introduced another index of solar activity, the group sunspot number (GSN).…”