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It is known that the solar radio flux is strongly correlated to the sunspot cycle and that the flux is nearly the same at every minimum. If 64 sfu is taken as the baseline for all cycles, then we can calculate the proxy sunspot number from the flux, and vice versa. Furthermore, we find that the fitted adjusted flux divided by the fitted sunspot number gives a strong marker for the start of Solar Cycle 25 in October 2019. The high resolution 2K (based on 2048x2048 pixels SDO images) sunspot number currently indicates a start in June 2019.

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Recent research has demonstrated the existence of a new type of solar event, the “terminator.” Unlike the Sun’s signature events, flares and coronal mass ejections, the terminator most likely originates in the solar interior, at or near the tachocline. The terminator signals the end of a magnetic activity cycle at the Sun’s equator and the start of a sunspot cycle at mid-latitudes. Observations indicate that the time difference between these events is very short, less than a solar rotation, in the context of the sunspot cycle. As the (definitive) start and end point of solar activity cycles the precise timing of terminators should permit new investigations into the meteorology of our star’s atmosphere. In this article we use a standard method in signal processing, the Hilbert transform, to identify a mathematically robust signature of terminators in sunspot records and in radiative proxies. Using a linear extrapolation of the Hilbert phase of the sunspot number and F10.7 cm solar radio flux time series we can achieve higher fidelity historical terminator timing than previous estimates have permitted. Further, this method presents a unique opportunity to project, from analysis of sunspot data, when the next terminator will occur, May 2020 (\(+4\), −1.5 months), and trigger the growth of Sunspot Cycle 25.

Regression analysis and neural network training procedures are used to study the relationship between sunspot number (SSN) and solar flux at 10.7 cm (F10.7). Measurements of SSN and F10.7 for the periods from December 1963 to January 2019 (including the periods from Solar Cycles 20 to 24) were used. The value of correlation coefficient between SSN and F10.7 for the entire data used in the study is 0.95, showing that the parameters are well correlated. Results from the study reveal that there are remarkable differences on the relationship between SSN and F10.7 during the four solar cycle phases of low (-activity), high (-activity), ascending, and descending. The relationships are identical for the ascending and descending phases when the SSNs are lower than about 150, but the regression curves diverge as the SSNs increase beyond that limit. A conspicuously changed relationship between SSN and F10.7 is also observed for years 2014 and 2015 in which the annual SSN-versus-F10.7 plots for those years are visibly above those of the prior years. The results indicate that F10.7 values can be predicted from SSNs using neural networks, with root-mean-square errors of about 13.68 solar flux units. Results from the neural network procedure also indicate that a newly introduced solar cycle phase index (defined to indicate the phase of the solar cycle in which given observations belong) was effective in improving the neural network predictions.

Sunspot number series are subject to various uncertainties, which are still poorly known. The need for their better understanding was recently highlighted by the major makeover of the international Sunspot Number [Clette et al., Space Science Reviews, 2014]. We present the first thorough estimation of these uncertainties, which behave as Poisson-like random variables with a multiplicative coefficient that is time- and observatory-dependent. We provide a simple expression for these uncertainties, and reveal how their evolution in time coincides with changes in the observations, and processing of the data. Knowing their value is essential for properly building composites out of multiple observations, and for preserving the stability of the composites in time.

Two independent methods for estimating basic parameters of the solar cycle are presented. The first of them, the ascending-descending triangle method, is based on a previous work by Tritakis (Astrophys. Space Sci.
82, 463, 1982), which described how the fundamental parameters of a certain solar cycle could be predicted from the shape of the previous one. The relation between the two cycles before and after a specific 11-year solar cycle is tighter than between the two cycles belonging to the same 22-year solar cycle (even-odd cycle). The second is the MinimaxX method, which uses a significant relation in the international sunspot number between the maximum value of a solar cycle and its value 2.5 or 3 years (depending on the enumeration of the even or odd cycle) before the preceding minimum. The tests applied to Cycles 12 to 24 indicate that both methods can estimate the peak of the 11-year solar radio flux at a high confidence level. The data used in this study are the 10.7 cm solar radio flux since 1947, which have been extrapolated back to 1848 from the strong correlation between the monthly international sunspot numbers and the adjusted values of the 10.7 cm radio flux.

The Sunspot Number, created by R.Wolf in 1849, provides a direct long-term
record of solar activity from 1700 to the present. In spite of its central role
in multiple studies of the solar dynamo and of the past Sun-Earth relations, it
was never submitted to a global critical revision. However, various
discrepancies with other solar indices recently motivated a full re-calibration
of this series. Based on various diagnostics and corrections established in the
framework of several Sunspot Number Workshops and described in Clette et al.
2014, we assembled all corrections in order to produce a new standard version
of this reference time series. In this paper, we explain the three main
corrections and the criteria used to choose a final optimal version of each
correction factor or function, given the available information and published
analyses. We then discuss the good agreement obtained with the Group sunspot
Number derived from a recent reconstruction. Among the implications emerging
from this re-calibrated series, we also discuss the absence of a rising secular
trend in the newly-determined solar cycle amplitudes, also in relation with
contradictory indications derived from cosmogenic radionuclides. As conclusion,
we introduce the new version management scheme now implemented at the World
Data Center - SILSO, which reflects a major conceptual transition: beyond the
re-scaled numbers, this first revision of the Sunspot Number also transforms
the former locked data archive into a living observational series open to
future improvements.

The correlation coefficients of the linear regression of six solar indices versus 10.7 cm radio flux F
10.7 were analysed in solar cycles 21, 22 and 23. We also analysed the interconnection between these indices and F
10.7 with help of approximation by polynomials of second order. The indices we have studied in this paper are: the relative sunspot numbers – SSN, 530.3 nm coronal line flux – F
530, the total solar irradiance – TSI, Mg II 280 nm core-to-wing ratio UV-index, the Flare Index – FI and the counts of flares. In most cases the regressions of these solar indices vs. F
10.7 are close to the linear regression except the moments of time near the minimums and maximums of the 11-year activity. For the linear regressions, we found that correlation coefficients K
corr(t) for the solar indices vs. F
10.7 and SSN dropped to their minimum values twice during each 11-year cycle.

This article is an update of a study (Tapping and Valdès in Solar Phys.272, 337, 2011) made in the early part of Cycle 24 using an intercomparison of various solar activity indices (including sunspot number and the 10.7 cm solar radio flux), in which it was concluded that a change in the relationship between photospheric and chromospheric/coronal activity took place just after the maximum of Cycle 23 and continued into Cycle 24. Precursors (short-term variations) were detected in Cycles 21 and 22. Since then the sunspot number index data have been substantially revised. This study is intended to be an update of the earlier study and to assess the impact of the revision of the sunspot number data upon those conclusions. This study compares original and revised sunspot number, total sunspot area, and 10.7 cm solar radio flux. The conclusion is that the transient changes in Cycles 21 and 22, and the more substantial change in Cycle 23, remain evident. Cycle 24 shows indications that the deviation was probably another short-term one.

During the last solar cycle Earth's cloud cover underwent a modulation more closely in phase with the galactic cosmic ray flux than with other solar activity parameters. Further it is found that Earth's temperature follows more closely decade variations in galactic cosmic ray flux and solar cycle length, than other solar activity parameters. The main conclusion is that the average state of the heliosphere affects Earth's climate.