Solar Cycle 25 is Currently Very Similar to Solar Cycle 24

The study of solar activity in the past (Usoskin 2017; Muñoz-Jaramillo & Vaquero 2019) and its prediction (Pesnell 2008; Nandy 2021) is a task with important theoretical and practical consequences. On the one hand, the study of past solar activity is crucial to our understanding of the physical processes underlying in the stars, in general, and in the Sun, in particular (Charbonneau 2020). On the other hand, reliable predictions of solar activity are key to meet the new needs of our society due to the potential problems that space weather can cause in our technology (Pulkkinen 2007).

The onset of the current Solar Cycle 25 occurred in 2019 December according to the sunspot Index and Long-term Solar Observations (SILSO, https://wwwbis.sidc.be/silso), responsible for the sunspot number index (Clette & Lefèvre 2016). One year and a half later, we already have the first year with values of the smoothed monthly sunspot number of Solar Cycle 25. In this short note, we compare the evolution of Solar Cycle 25 with past solar cycles.

We defined solar cycles from minimum to minimum according to the smoothed monthly sunspot number values provided by SILSO. Figure 1 (top and middle panel) shows a comparison of the evolution of the solar cycles, defined as previously indicated, from Solar Cycle 1 to Solar Cycle 25 (Clette et al. 2014).

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In order to find the solar cycle more similar to Solar Cycle 25, we calculate the sum of the squared differences between the smoothed monthly sunspot number value of each month since the beginning of each cycle with that corresponding of Solar Cycle 25. That is:

$$\begin{eqnarray*}&&{D}^{2}\left({\mathrm{SC}}_{i}\right)=\displaystyle \sum _{j=0}^{N}{\left({\mathrm{SN}}_{j}\left({\mathrm{SC}}_{i}\right)-{\mathrm{SN}}_{j}\left({\mathrm{SC}}_{25}\right)\right)}^{2}\end{eqnarray*}$$

where j goes from 0 (first month since the beginning of the cycle) to N months, SC_i from Solar Cycle 1 to 24 and SN is the smoothed monthly sunspot number value. Applying this simple method of similarity, the most similar cycles to Solar Cycle 25 are those with the lowest values of those sums (D²). Thus, we obtained that the most similar cycle to Solar Cycle 25 is Solar Cycle 24 (D² = 48.2) and the second one is Solar Cycle 7 (D² = 58.2).

The smoothed monthly sunspot number in 2020 December (15.3) was 12.5% larger than the value for 2020 November. This is slightly lower that the increment in November (14.3%) but similar as in previous months: 10.5% in September and 13.3% in October 2020. Furthermore, at this point of the cycle, the value of the smoothed monthly sunspot number corresponding to SC25 (15.3) is just below of that corresponding for Solar Cycle 7 (17.0) and 10 (17.6) and above Solar Cycle 24 (12.7). We can stress that (i) Solar Cycle 25 slightly abandon the wake of Solar Cycle 7 and 10 that just followed in the previous months, and (ii) only three solar cycles had lower sunspot number values than Solar Cycle 25 at this point, namely Solar Cycle 5 and 6 (in the Dalton Minimum) and 24 (the previous solar cycle).

Regarding the monthly sunspot number (no smoothing), 2021 April, May, and June (in addition to 2020 November and December) were the most active months of Solar Cycle 25. In fact, the maximum daily sunspot number value in 2021 June was 57 (the same as on 2021 April 20) and this value represents the highest of this solar cycle, only behind those values for the period 2020 November 26–30 when the daily sunspot number value was between 58 and 96. Moreover, the minimum value of the daily sunspot number in 2021 June was 8. 2021 June and March are the only months without spotless days in Solar Cycle 25. Hence, the next smoothed monthly sunspot number predictably continue increasing. Thus, Solar Cycle 25 could overpass in the next few months to Solar Cycle 7 and 1 in sunspot number values because Solar Cycle 1 and 7 underwent a decrease in the smoothed monthly sunspot number during its second year after the start of the cycle.

The smoothed monthly sunspot number in 2020 December (15.3) was slightly lower than the predicted value by SILSO using both its standard (16.0) and combined (16.6) model. Instead, it is within the range of sunspot number values provided by the National Oceanic and Atmospheric Administration (NOAA) (14.9–15.9). The same behavior in the predicted values occurred in the previous months. More information on different predictions published for the maximum amplitude of Solar Cycle 25 can be consulted in Nandy (2021).

The maximum amplitude of Solar Cycle 25 provided by NOAA is in the range of sunspot number values of 114.6–124.6. Furthermore, the maximum amplitudes of sunspot number values of Solar Cycle 1, 7, 10 and 24, mentioned previously as more similar to Solar Cycle 25, were 144, 119, 186 and 116, respectively. Therefore, this simple analysis using the smoothed sunspot number values of the first year of Solar Cycle 25 suggests that this cycle will have a small-moderate amplitude, similar to Solar Cycle 24. However, these provisional results should be used with caution. We must follow the evolution of Solar Cycle 25 in the next months until reaching greater certainty.

This research was supported by the Economy and Infrastructure Counselling of the Junta of Extremadura through grant GR18097 (co-financed by the European Regional Development Fund) and by the Ministerio de Economía y Competitividad of the Spanish Government (CGL2017-87917-P).

The authors declare that they have no conflicts of interest.