Context. Previous studies show significant north-south asymmetries for various features and indicators of solar activity. These findings suggest some decoupling between the two hemispheres over the solar cycle evolution, which is in agreement with dynamo theories. For the most important solar activity index, the sunspot numbers, so far only limited data are available for the two hemispheres independently. Aims. The aim of this study is to create a continuous series of daily and monthly hemispheric sunspot numbers (HSNs) from 1874 to 2020, which will be continuously expanded in the future with the HSNs provided by SILSO. Methods. Based on the available daily measurements of hemispheric sunspot areas from 1874 to 2016 from Greenwich Royal Observatory and National Oceanic and Atmospheric Administration (NOAA), we derive the relative fractions of the northern and southern activity. These fractions are applied to the international sunspot number (ISN) to derive the HSNs. This method and obtained data are validated against published HSNs for the period 1945-2004 and those provided by SILSO for 1992 to 2016. Results. We provide a continuous data series and catalogue of daily, monthly mean, and 13-month smoothed monthly mean HSNs for the time range 1874-2020 -fully covering solar cycles 12 to 24-that are consistent with the newly calibrated ISN (Clette et al. 2014). Validation of the reconstructed HSNs against the direct data available since 1945 reveals a high level of consistency, with Pearson correlation coefficients of r = 0.94 (0.97) for the daily (monthly mean) data. The cumulative hemispheric asymmetries for cycles 12-24 give a mean value of 16%, with no obvious pattern in north-south predominance over the cycle evolution. The strongest asymmetry occurs for cycle no. 19, in which the northern hemisphere shows a cumulated predominance of 42%. The phase shift between the peaks of solar activity in the two hemispheres may be up to 28 months, with a mean absolute value over cycles 12-24 of 16.4 months. The phase shifts reveal an overall asymmetry of the northern hemisphere reaching its cycle maximum earlier (in 10 out of 13 cases), with a mean signed phase shift of −7.6 months. Relating the ISN and HSN peak growth rates during the cycle rise phase with the cycle amplitude reveals higher correlations when considering the two hemispheres individually, with r ≈ 0.9. Conclusions. Our findings provide further evidence that to some degree the solar cycle evolves independently in the two hemispheres, and demonstrate that empirical solar cycle prediction methods can be improved by investigating the solar cycle dynamics in terms of the HSN evolution.
Context. Forecasting the solar cycle amplitude is important for a better understanding of the solar dynamo as well as for many space weather applications. Different empirical relations of solar cycle parameters with the peak amplitude of the upcoming solar cycle have been established and used for solar cycle forecasts, as e.g. the Waldmeier rule relating the cycle rise time with its amplitude, the polar fields at previous minimum, etc. Recently, a separate consideration of the evolution of the two hemispheres revealed even tighter relations. Aims. We introduce the maximal growth rate of sunspot activity in the ascending phase of a cycle as a new and reliable precursor of a subsequent solar cycle amplitude. We investigate whether the suggested precursor provides benefits for the prediction of the solar cycle amplitude when using the sunspot indices (sunspot numbers, sunspot areas) derived separately for the two hemispheres compared to the total sunspot indices describing the entire solar disk. Methods. We investigate the relationship between the maximal growth rate of sunspot activity in the ascending phase of a cycle and the subsequent cycle amplitude on the basis of four data sets of solar activity indices: total sunspot numbers, hemispheric sunspot numbers from the new catalogue from 1874 onwards (Veronig et al. 2021), total and hemispheric sunspot areas.Results. For all the data sets, a linear regression based on the maximal growth rate precursor shows a significant correlation. Validation of predictions for cycles 1-24 shows high correlations between the true and predicted cycle amplitudes reaching r = 0.93 for the total sunspot numbers. The lead time of the predictions varies from 2 to 49 months, with a mean value of 21 months. Furthermore, we demonstrated that the sum of maximal growth rate indicators determined separately for the North and the South hemispheric sunspot numbers provides more accurate predictions than that using total sunspot numbers. The advantages reach 27% and 11% on average in terms of rms and correlation coefficient, respectively. The superior performance is also confirmed with hemispheric sunspot areas with respect to total sunspot areas. Conclusions. The maximal growth rate of sunspot activity in the ascending phase of a solar cycle serves as a reliable precursor of the subsequent cycle amplitude. Furthermore our finding provide a strong foundation for supporting regular monitoring, recording, and predictions of solar activity with hemispheric sunspot data, which capture the asymmetric behaviour of the solar activity and solar magnetic field and enhance solar cycle prediction methods.
<p>The sun&#8217;s magnetic field drives the 11-year solar cycle, and predicting its strength has practical importance for many space weather applications. Previous studies have shown that analysing the solar activity of the two hemispheres separately instead of the full sun can provide more detailed information on the activity evolution. However, the existing Hemispheric Sunspot Number data series (1945 onwards) was too short for meaningful solar cycle predictions. Based on a newly created hemispheric sunspot number catalogue for the time range 1874-2020 (Veronig et al. 2021, http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/652/A56) that is compatible with the International Sunspot Number from World Data Centre SILSO, we investigate the evolution of the solar cycle for the two hemispheres and develop a novel method for predicting the solar cycle amplitude. We demonstrate a steady relationship between the maximal growth rate of activity in the ascending phase of a cycle and its subsequent amplitude and form a 3rd order regression for the predictions. Testing this method for cycles 12-24, we show that the forecast made by the sum of the maximal growth rate from the North and South Hemispheric Sunspot number is more accurate than the same forecast from the Total Sunspot Number: The rms error of predictions is smaller by 27%, the correlation coefficient r is higher by 11% on average reaching values in the range r = 0.8-0.9 depending of the smoothing window of the monthly mean data. These findings demonstrate that empirical solar cycle prediction methods can be enhanced by investigating the solar cycle dynamics in terms of the hemispheric sunspot numbers, which is a strong argument supporting regular monitoring, recording, and analysing solar activity separately for the two hemispheres.</p>
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