An extended catalog of long-periodic terms of the Sun tide-generating potential (STGP ver.2) is released. It contains 2.5 times of such terms than in the first version of the STGP catalog (Cionco et al. 2023). The analysis of the new data is done in order to re-examine the existence of tidal forces of ≈ 11.0-yr and ≈ 22.0-yr period able to excite the observed solar-activity cycles. In several recent studies, such tidal forcings are claimed to be a result of certain combinations of Venus, Earth and Jupiter mean motions with the multipliers (6, −10, 4) and (3, −5, 2), respectively. So, in this contribution, we specifically look for the periodic terms related to these combinations. As a result of these additional investigation we do not confirm any noticeable tidal forcing factors of ≈ 11.0-yr nor ≈ 22.0-yr period in the extended STGP spectrum either.

Using tree-ring radiocarbon 14C data, solar cycles are now reconstructed for the last millennium, more than doubling the previously known statistic of direct solar observations and giving a new opportunity to validate basic empirical rules connecting solar cycle lengths, strengths and intensities. This includes the Waldmeier rule relating the cycle’s strength to the length of its ascending phase, and the Gnevyshev-Ohl rule suggesting that cycles are paired so that the intensity of an odd solar cycle is higher than that of the preceding even cycle. Using the extended solar-cycle statistic, we found that the Waldmeier rule remains robust for the well-defined solar cycles implying that it is an intrinsic feature of the solar cycle. However, the validity of the Gnevyshev-Ohl rule is not confirmed at any reasonable statistical level, indicating that either the insufficient accuracy of the reconstructed solar cycles or that this rule is not a robust feature.

Geomagnetic storms can be modeled as stochastic processes with log-normal probability distribution function over their minimum Dst index value measured during the main phase of each event. Considering a time series of geomagnetic storm events between 1957 and 2019 we have analyzed the probability of occurrence of small, moderate, strong and extreme events. The data were separated according to solar cycle (SC) and solar cycle phases and fitted through maximum likelihood method in order to compare rates of occurrence of the last Solar Cycle (SC24) with previous ones. Our results show that for Dst < – 100 nT events in SC24 are similar to those in SC20, obtaining ⁓42 vs 21 median rate storms per cycle with 95% confidence intervals using Bootstrap Method. As SC24 has been the least active solar cycle in over 200 years, we conclude that this method tends to overestimate geomagnetic storms occurrence rates even for small events.

The sunspot cycle is quite variable in duration and amplitude, yet in the long term, it seems to return to solar minimum on schedule, as if guided by a clock with an average period of close to 11.05 years for the sunspot number cycle and 22.1 years for the magnetic cycle. This paper provides a brief review of the sunspot number cycle since 1750, discusses some of the processes controlling the solar dynamo, and provides clues that may add to our understanding of what controls the cadence of the solar clock.

In this study, using the data of Istanbul University Observatory, general features of Solar Cycle 24 are presented.

Forecasting the solar activity is of great importance not only for its effect on the climate of the Earth but also on the telecommunications, power lines, space missions and satellite safety. In the present work, machine learning using Artificial Neural Networks (ANNs) called Nonlinear Autoregressive Network (NAR) with Exogenous Inputs (NARX) have been applied for the prediction of future evolution of the present sunspot cycle. NARX network is able to combine the performance of ANN algorithm with nonlinear autoregressive method to handle problems such as finding dependencies among solar indices and prediction of solar cycle evolution.

The meteoroid ablation is an important source of upper atmosphere metal atoms. Many meteoroids ablate between 70 - 110 km and form an ionized plasma trail which is detected by radar technique. It is also known that the ablation heights of the meteors depend on various factors such as velocity, mass, and its composition, etc. The meteor ablation height provides new opportunities to gather information on the neutral atmosphere in the Mesosphere and Lower Thermosphere (MLT) region. In this study, we analysed the 11 years of meteor radar data (2005 - 2015), i.e., descending phase of solar cycle 23, and ascending phase of solar cycle 24, detected by all sky meteor radar at Thumba. We found that the solar activity influences the meteor ablation height, here, during the solar maxima meteor peak detection height rise to few hundred meters higher altitudes. We also examined the long term pattern of the meteor count rate which shows a decreasing trend and has good agreement with the sunspot number (SSN).

Hemispheric asymmetry is one of the significant parameters related to the action of solar dynamo. Comparison of hemispheric activities during various phases are found out for solar cycles 12 to 23. Asymmetry of solar activity shows extremum values during the cycles 14 and 19. Lowest and highest levels of north-south asymmetry are mainly observed during minimum and maximum phases respectively of solar cycles. A change of phase is found to be existing between the asymmetries at solar maxima and the whole cycle, after solar cycle 15 and 18. Also, for cycles 17-19, the behaviour of the asymmetry is observed to be peculiar and different from that of the other cycles. Periodic behaviour of north-south asymmetry mainly occurs in 8.8 years and noticed very high during the cycles 18-22.

Sunspots are active regions on the surface of the Sun having strong magnetic fields. Activity level of the Sun shows long-time scale phenomena known as grand episodes-Grand maxima and Grand minima. Present study examines grand episodes shown by sunspot numbers (1090-2017), using methods of wavelet transform and sinusoidal regression. Time interval analysed includes two grand maxima and four grand minima. Interval in between grand episodes are regular oscillations. Phase changes found from periodicity analysis clearly show the presence of upcoming grand episodes. The forthcoming grand episodes are suggested to be two grand minima which are likely to occur between the years 2100-2160 and 2220-2300.

We investigated the prominence eruptions and disappearances observed with the Nobeyama Radioheliograph during over 20 years for studying the anomaly of the recent solar cycle. Although the sunspot number of Cycle 24 is smaller than the previous one dramatically, the occurrence rate, size and radial velocity of the prominence activities are not changed significantly. We also found that the occurrence of the prominence activities in the northern hemisphere is normal from the duration of the cycle and the migration of the producing region of the prominence activities. On the other hand, the migration in the southern hemisphere significantly differs from that in the northern hemisphere and the previous cycles. Our results suggest that the anomalies of the global magnetic field distribution started at the solar maximum of Cycle 23.

We have examined solar polar fields during the solar cycles 21–23 using both ground and space based synoptic magnetograms. It has been shown that the unsigned polar fields in the latitude range (78°–90°) show a steady decline starting ~1995–1996 till the maximum of cycle 23. We also find that the long term changes in the unsigned polar fields at higher latitudes are well correlated with changes in the surface meridional flow speed that have been reported for the cycle 23.

Independent of the normal solar cycle, a decrease in the sunspot magnetic field strength has been observed using the Zeeman-split 1564.8nm Fe I spectral line at the NSO Kitt Peak McMath-Pierce telescope. Corresponding changes in sunspot brightness and the strength of molecular absorption lines were also seen. This trend was seen to continue in observations of the first sunspots of the new solar Cycle 24, and extrapolating a linear fit to this trend would lead to only half the number of spots in Cycle 24 compared to Cycle 23, and imply virtually no sunspots in Cycle 25.

We examined synoptic observations from the NSO Kitt Peak Vacuum Telescope and initially (with 4000 spots) found a change in sunspot brightness which roughly agreed with the infrared observations. A more detailed examination (with 13,000 spots) of both spot brightness and line-of-sight magnetic flux reveals that the relationship of the sunspot magnetic fields with spot brightness and size remain constant during the solar cycle. There are only small temporal variations in the spot brightness, size, and line-of-sight flux seen in this larger sample. Because of the apparent disagreement between the two data sets, we discuss how the infrared spectral line provides a uniquely direct measurement of the magnetic fields in sunspots.

The numerical method developed by Veselovsky & Ivanov (2006), together with magnetograms of the Sun obtained at the photospheric level were used to calculate the coronal magnetic field with open, closed and intermittent topology during March-December 2007. The results of the modelling are compared with stereoscopic images and movies of the corona observed by EUV telescopes onboard STEREO and SOHO spacecraft. The sources of the permanent and transient high speed solar wind streams as well as the sector structure and the heliospheric plasma sheet observed at the Earth's orbit by the ACE and STEREO spacecraft are discussed.

The heliospheric magnetic field and the solar wind are behaving differently in the current solar minimum, compared to the previous minimum. The radial component of the heliospheric magnetic field, and thus the average value of the component of the solar magnetic field that opens into the heliosphere, the so-called open magnetic flux of the Sun, is lower than it was in the previous solar minimum; in fact, lower than in any previous solar minimum for which there are good spacecraft observations. The mass flux, the ram pressure, and the coronal electron temperature as measured by solar wind charge states are also lower in the current minimum compared to the previous one. This situation provides an opportunity to test some of the concepts for the behavior of the heliospheric magnetic field and the solar wind that have been developed; to improve these theories, and to construct a theory for the solar wind that accounts for the observed behavior throughout the solar cycle, including the current unusual solar minimum.

The solar magnetic cycle affects all levels of the Sun including the convection zone, photosphere, chromosphere and corona. Recent advances in solar space missions (Yohkoh, SOHO and others) and, also, ground-based observations provide us an excellent opportunity to investigate solar magnetic activity in detail, and to draw a new picture of the solar magnetic cycle. Magnetic field appears on the solar surface as a result dynamo processes in the convection zone, and forms bipolar complexes of solar activity. These complexes can be seen in the photosphere as dark sunspots surrounded by the bright plages extended into chromosphere, with arcades of coronal loops best observed in EUV and soft X-rays. The coronal loops reflect the large-scale magnetic structure of complexes of activity. The new data reveal fundamental changes in the magnetic topology during the solar cycle, and details of the polar magnetic field reversals occurring near the sunspot maximum. The solar synoptic maps obtained from the photospheric and coronal data display a close correlation between the erupted magnetic flux and coronal emissions and show large-scale magnetic connectivities. The brief review of solar cycle studies is presented.

It is suggested that we are already in the weak solar cycles series since the start of cycle 23. The interplanetary magnetic filed and the solar wind speed and density are expected to drop considerably during the approaching second weak cycle number 24 and the following ones leading to inflation of the magnetosphere. The corona is also expected to cool down relative to normal cycles corona. A daily background coronal index is proposed. The mechanism of production of weak cycles is tied to the rapid rotation of the photospheric layer which is deeply rooted in the bottom of the convection zone. This rapid surface and subsurface rotation implies slower rotation of the tachocline. Slower dynamo rotation leads to reduction of the strength of the magnetic cycle. One of the very important sequences of the weak cycles, is the expected cooling of the Earths air and sea surface temperatures which would have negative effects on agriculture with increased drought-flood hazards. The reduction of solar UV flux can lead to the closure of the ozone hole on the long run.