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Forecasting of solar activity is extremely important, due to its impact on our space-based technology. In recent years, we have observed a continuous decline in the peak sunspot number for Solar Cycles (SCs) 21 to 24. We are more curious about peak activity of SC 25 because if the solar activity weakens further, then it may be an indication of new extended minima. So far, the daily sunspot-number data make up the longest observational series of the solar activity available, and recently this has been replaced with the corrected new Version 2.0 sunspot-number series. In general, different prediction models are available to forecast the peak smoothed sunspot number (SSN) of the upcoming SC but they are based on the older Version 1.0 sunspot data. Therefore, it is necessary to check the applicability of earlier proposed models to predict the peak SSN using the Version 2.0 sunspot-number series. Here, we re-evaluate one such earlier proposed prediction model, which is based on the estimates of the Shannon entropy, a measure of randomness, for Version 2.0 sunspot data. We find that this prediction model is applicable to the Version 2.0 sunspot-number series. To verify the robustness of the prediction model, we used the histogram and additionally the kernel density estimator (KDE) method to calculate the probability distribution function (PDF). The estimate of the PDF is a prerequisite to compute the Shannon entropy. We found that the prediction model is robust and the correlation coefficients between model parameters are 0.93 and 0.92, respectively, for these two approaches. This exercise provides a new prediction model for the peak SSN based on the Version 2.0 sunspot-number series. The model forecasts the peak smoothed sunspot number of 136.9 ± 24 using a histogram-derived PDF and 150.7 ± 25 using a KDE-derived PDF for the upcoming SC 25. These predictions for SC 25 are more reliable as up to date (December 2020) sunspot-number data have been utilized to get the entropy in the end phase of SC 24. It suggests that SC 25 would be similar or slightly stronger than SC 24. [ABSTRACT FROM AUTHOR]

Recent solar cycles (SCs) 21–24 have experienced a gradual decrease in their activity with considerable weakening during current SC 24. This is a unique opportunity to examine the long-term response of Earth's low-latitude ring-current and high latitude auroral electrojet current systems during such systematically decreasing solar activity. With the advancement in technology, continuous recordings of ground/space magnetic field are available for the last few decades that allow us to explore the behaviour of probability distribution functions (PDFs) linked with the ring-current and auroral electrojet current systems for past five SCs (20–24). Also, PDFs linked with solar wind parameters that drive these current systems like magnetic field and velocity at Earth's bow shock are examined. We noticed the significant narrowing of PDF of ring-current and auroral electrojet during SC 24. The number of one-hour intervals with Dst < - 150 nT are less than 600 for SCs 20–23, which constitutes less than 0.7% of respective PDF, and number of one-hour intervals with Dst < - 250 nT are less than 100 for SCs 20–23, which corresponds to less than 0.1% of respective PDF. But for SC 24 the Dst < - 150 nT encountered only for 58 h, which corresponds to 0.06% of PDF and there are no intervals when Dst was < −250 nT. For auroral electrojet, the number of one-hour intervals with AE > 750 nT and AE > 1500 nT are less than 3060 and 70, respectively for SCs 20–23, which corresponds to < 4% and < 0.06% of respective PDFs. But for SC 24 the AE > 750 nT encountered only for 1398 h, which corresponds to 1.7% of PDF and there are only 9 intervals when AE increased above 1500 nT, which is 0.01% of PDF. It implies that the probability of intense ring-current and auroral electrojet current during SC 24 was unusually low. Such narrowing is seen in PDFs of the interplanetary magnetic field and solar wind velocity as well. This fair quiet space weather experienced during SC 24 is attributed to the weakening of solar activity, which has subsequently influenced the strength of the interplanetary magnetic field and solar wind velocity at Earth's bow shock. [ABSTRACT FROM AUTHOR]

We present a statistical study of electromagnetic ion cyclotron (EMIC) waves observed at Antarctic station (geographic 70.7 ∘ S , 11.8 ∘ E , L = 5 ) on quiet and disturbed days during 2011–2017. The data span a fairly good period of both ascending and descending phases of the solar cycle 24, which has witnessed extremely low activity. We noted EMIC wave occurrence by examining wave power in different frequency ranges in the spectrogram. EMIC wave occurrence during ascending and descending phases of solar cycle 24, its local time, seasonal dependence and durations have been examined. There are total 2367 days for which data are available. Overall, EMIC waves are observed for 3166.5 h ( ≈ 5.57 % of total duration) which has contributions from 1263 days. We find a significantly higher EMIC wave occurrence during the descending phase (≈ 6.83%) as compared to the ascending phase (≈ 4.08%) of the solar cycle, which implies nearly a twofold increase in EMIC wave occurrence. This feature is attributed to the higher solar wind dynamic pressure during descending phase of solar activity. There is no evident difference in the percentage occurrence of EMIC waves on magnetically disturbed and quiet days. On ground, EMIC waves show marginally higher occurrence during winter as compared to summer. This seasonal tendency is attributed to lower electron densities and conductivities in the ionosphere, which can affect the propagation of EMIC waves through ionospheric ducts. In local time, the probability distribution function of EMIC wave occurrence shows enhancement during 11.7–20.7 LT (i.e., afternoon–dusk sector). Daily durations of EMIC waves are in the range of 5–1015 min and it is noted that the longer duration (240–1015 min) events are prevalent on quiet days and are mostly seen during the descending phase of solar cycle. [ABSTRACT FROM AUTHOR]

A new model is proposed to forecast the peak sunspot activity of the upcoming solar cycle (SC) using Shannon entropy estimates related to the declining phase of the preceding SC. Daily and monthly smoothed international sunspot numbers are used in the present study. The Shannon entropy is the measure of inherent randomness in the SC and is found to vary with the phase of an SC as it progresses. In this model each SC with length Tcy is divided into five equal parts of duration Tcy/5. Each part is considered as one phase, and they are sequentially termed P1, P2, P3, P4, and P5. The Shannon entropy estimates for each of these five phases are obtained for the nth SC starting from n = 10 - 23. We find that the Shannon entropy during the ending phase (P5) of the nth SC can be efficiently used to predict the peak smoothed sunspot number of the (n + 1)th SC, i.e. Sn+1 max. The prediction equation derived in this study has a good correlation coefficient of 0.94. A noticeable decrease in entropy from 4.66 to 3.89 is encountered during P5 of SCs 22 to 23. The entropy value for P5 of the present SC 24 is not available as it has not yet ceased. However, if we assume that the fall in entropy continues for SC 24 at the same rate as that for SC 23, then we predict the peak smoothed sunspot number of 63±11.3 for SC 25. It is suggested that the upcoming SC 25 will be significantly weaker and comparable to the solar activity observed during the Dalton minimum in the past. [ABSTRACT FROM AUTHOR]

The knowledge about future solar activity is necessary to plan our space-based missions. As yet, several prediction models (statistics- or dynamo-based) have been developed to forecast the peak smoothed sunspot number (SSN) of the upcoming solar cycle. Many of these data-based prediction models require sunspot number until the end of n th solar cycle to predict the peak of the solar cycle n + 1. However, one prefers to have the predictions well in advance. We propose a new data-based model that can provide information about the peak SSN (or amplitude) of solar cycle n ​+ ​1, and sum of peak SSN of solar cycle n ​+ ​2 and n ​+ ​3 ​at the end of each n th solar cycle. The solar cycles are paired using even-odd cycles, therefore in this model n is allowed to take even number. We used recently updated, Version-2, daily and monthly sunspot-number data. The area under the curve of each n th solar cycle [ A n ] is estimated and used in the model together with its length and peak SSN [ S max n ]. We noticed that difference in the area under the curve of solar cycles n and n + 1 can be used to predict the summation of peak SSN of n + 2 and n + 3 solar cycles (i.e., S max n + 2 + S max n + 3). We have tested this prediction model with both daily and monthly sunspot numbers. Our model predicts; S max 24 + S max 25 ​= ​219.7 ​± ​31 and S max 26 + S max 27 ​= ​229.4 ​± ​31. As the peak SSN of solar cycle 24 is known, we get S max 25 ​= ​103.3 ​± ​15. Further, model suggest that the solar cycle 26 and 27 would be similar or slightly stronger than solar cycle 24 and 25. [Display omitted] • A new model of peak sunspot number is presented, which is based on the recently updated Version-2 sunspot-number data. • This model provides the summation of peak smooth sunspot number (SSN) of even-odd pair of solar cycles. • Our model predicts the peak SSN of solar cycle 25 as 103 ​± ​15, and sum of peak SSN of solar cycle 26 and 27 as 229.4 ​± ​31. [ABSTRACT FROM AUTHOR]