Tests of Sunspot Number Sequences: 1. Using Ionosonde Data

Abstract

More than 70 years ago it was recognised that ionospheric F2-layer critical frequencies $foF2$ had a strong relationship to sunspot number. Using historic datasets from the Slough and Washington ionosondes, we evaluate the best statistical fits of $foF2$ to sunspot numbers (at each Universal Time [UT] separately) in order to search for drifts and abrupt changes in the fit residuals over Solar Cycles 17 - 21. Polynomial fits are made both with and without allowance for the white-light facular area, which has been reported as being associated with cycle-to-cycle changes in the sunspot number - $foF2$ relationship. Over the interval studied here, the ISN, $R$, the backbone group number $Rbb$, and the corrected number $Rc$ largely differ in their allowance for the 'Waldmeier discontinuity' around 1945 (the correction factor for which for $R$, $Rbb$ and $Rc$ is, respectively, zero, effectively over 20%, and explicitly 11.6%). It is shown that for Solar Cycles 18 - 21, all three sunspot data sequences perform well, but that the fit residuals are lowest and most uniform for $Rbb$. We here use $foF2$ for those UTs for which $R$, $Rbb$, and $Rc$ all give correlations exceeding 0.99 for intervals both before and after the Waldmeier discontinuity. The error introduced by the Waldmeier discontinuity causes $R$ to underestimate the fitted values based on the $foF2$ data for 1932 - 1945 but $Rbb$ overestimates them by almost the same factor, implying that the correction for the Waldmeier discontinuity inherent in $Rbb$ is too large by a factor of two. Fit residuals are smallest and most uniform for $Rc$ and the ionospheric data support the optimum discontinuity multiplicative correction factor derived from the independent Royal Greenwich Observatory (RGO) sunspot group data for the same interval.

arXiv comment35 pages, 11 figures

References in corpus (8)

• Reconstruction of the Sunspot Group Number: the Backbone Method
• A New Calibrated Sunspot Group Series Since 1749: Statistics of Active Day Fractions
• Reconstruction of solar UV irradiance since 1974
• Which solar EUV indices are best for reconstructing the solar EUV irradiance ?
• Tests of sunspot number sequences: 3. Effects of regression procedures on the calibration of historic sunspot data
• Power law relating 10.7 cm flux to sunspot number
• Tests of Sunspot Number Sequences: 2. Using Geomagnetic and Auroral Data
• Tests of sunspot number sequences: 4. Discontinuities around 1946 in various sunspot number and sunspot group number reconstructions

Cited by in corpus (10)

• The new Sunspot Number: assembling all corrections
• New reconstruction of the sunspot group number since 1739 using the direct calibration and "backbone" methods
• An Assessment of Sunspot Number Data Composites over 1845-2014
• Tests of sunspot number sequences: 3. Effects of regression procedures on the calibration of historic sunspot data
• Uncertainties in the Sunspot Numbers: Estimation and Implications
• Iwahashi Zenbei's Sunspot Drawings in 1793 in Japan
• Tests of Sunspot Number Sequences: 2. Using Geomagnetic and Auroral Data
• Tests of sunspot number sequences: 4. Discontinuities around 1946 in various sunspot number and sunspot group number reconstructions
• Revisiting Kunitomo's Sunspot Drawings during 1835-1836 in Japan
• A Transit of Venus Possibly Misinterpreted as an Unaided-Eye Sunspot Observation in China on 9 December 1874