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Global solar activity variation mainly occurs over about an 11 yr cycle. However, both longer and shorter periodicities than the solar cycle are also present in many different solar activity indices. The longer timescales may be up to hundreds of years, while the shorter timescales for global solar variability could be within 0.5–2 yr, which include, e.g., from the Rieger-type periods (150–160 days) to quasi-biennial oscillations of 2 yr. The most likely origin of this short-timescale quasi-periodicity is attributed to magnetic Rossby waves, which have periods of 0.8–2.4 yr. In this work, we present findings of a unique evolution of identified shorter periodicities, like the Rieger-type, arising from magnetic Rossby waves, throughout Solar Cycles 19–25. We report further observational evidence of the strong relationship between the Rieger-type periodicity, magneto-Rossby waves, and major solar flare activity. Moreover, this study also reveals that the global solar magnetic field has a continuous periodic longitudinal conveyor belt motion along the solar equator, together with an up-and-down movement in the latitudinal directions. We found that when these longitudinal and latitudinal movements have Rieger-type periodicity and magneto-Rossby waves during the same period of a solar cycle, major flare activity is present.

The long-term variations of quasiperiodic solar flares are of great importance for a better understanding and accurate prediction of solar flare activity. To explore the quasiperiodic characteristics of different classes of solar flares and the hemispheric difference, we comprehensively analyzed the quasiperiodic variations of different classes of solar flares, including the whole solar disk and hemispheres during solar cycles (SC) 21–25. The main results show that: (1) Certain periods are observed only in one hemisphere or some specific solar cycles. For example, over the entire time interval, the period of 3.49 yr was detected exclusively in C-class, M-class, and X-class flares in the northern hemisphere. (2) The quasi-biennial oscillation periodicities were observed for B-class, C-class, M-class, and X-class flares during SC22, SC24, and SC25. Moreover, the quasi-biennial oscillation periodicities were more pronounced in C-class and X-class flares in SC21 and in B-class and M-class flares in SC23. (3) The occurrence of different classes of solar flares displayed statistically significant short- and medium-range oscillations in both hemispheres, with distinct periodic variations and asymmetric evolutionary features. Identifying features such as the 3.49 yr period and quasi-biennial oscillation periodicities may help predict the activity patterns of different classes of solar flares across hemispheres and solar cycles, potentially improving forecasts of solar flare impacts on Earth’s technological infrastructure and space weather systems.

A new daily composite of the solar flare index (SFI) and the hemispherically‐resolved versions (hSFI) are presented for 1937 to 2024. The data set confirms that the northern hemisphere (NH) dominated solar flare activity during Solar Cycles 17 to 21, but that the southern hemisphere has dominated from Solar Cycle 22 to present. That said, the highest SFI value occurred in the NH during the recent superstorm of May 2024. In sunspot activity, the “Gnevyshev‐Ohl rule” shows that the sum of sunspot numbers during even‐numbered cycles is related to those of adjacent odd‐numbered cycles. A similar rule appears to apply to SFI. The “Gnevyshev gap” phenomenon where solar maximum activity sometimes has two peaks separated by up to 1–2 years of a gap is confirmed for SFI. Although our data set represents the longest continuous daily data set for solar flare activity to‐date, it is known that stronger solar flare events occurred before 1937. Therefore, a brief discussion of earlier solar flare events in the historical record is also provided for context. The statistics of the SFI and hSFI series are compared to other solar and geomagnetic activity indices, including the May and October 2024 solar storms. Statistical analysis of past geomagnetic storms confirms they are more frequent during active cycles and less frequent during solar minima. Strong geomagnetic storms are also more likely to occur during the positive phase of a 1.7 year's quasi‐biennial oscillation in solar activity. The likelihood of low‐magnetic latitude aurorae seems to have a 30 year periodicity component.

We explore the association of non-neutralized currents with solar flare occurrence in a sizable sample of observations, aiming to show the potential of such currents in solar flare prediction. We used the high-quality vector magnetograms that are regularly produced by the Helioseismic Magnetic Imager, and more specifically, the Space weather HMI Active Region Patches (SHARP). Through a newly established method that incorporates detailed error analysis, we calculated the non-neutralized currents contained in active regions (AR). Two predictors were produced, namely the total and the maximum unsigned non-neutralized current. Both were tested in AR time-series and a representative sample of point-in-time observations during the interval 2012 – 2016. The average values of non-neutralized currents in flaring active regions are higher by more than an order of magnitude than in non-flaring regions and correlate very well with the corresponding flare index. The temporal evolution of these parameters appears to be connected to physical processes, such as flux emergence and/or magnetic polarity inversion line formation, that are associated with increased solar flare activity. Using Bayesian inference of flaring probabilities, we show that the total unsigned non-neutralized current significantly outperforms the total unsigned magnetic flux and other well-established current-related predictors. It therefore shows good prospects for inclusion in an operational flare-forecasting service. We plan to use the new predictor in the framework of the FLARECAST project along with other highly performing predictors.

Understanding the evolution of the Moon requires mapping the global distribution and abundance of major elements on the lunar surface. X-ray fluorescence exploration is a promising method for estimating the distribution of light elements. The lunar south pole has drawn increasing scientific and exploratory interest. The chemical composition of the polar region is crucial for evaluating potential landing sites and interpreting data from in situ exploration of the lunar south pole. However, X-ray fluorescence observations have not achieved a complete global map of the lunar elemental distribution. This is due to various factors, such as low solar flare activity periods and radiation damage to the detectors. In some case, the limited energy resolution of the detectors prevented adequate resolution of the spectra of light elements. Particularly in the polar regions, observing fluorescent X-rays has been difficult because solar X-rays are weakly incident. This paper proposes global fluorescence X-ray imaging observations using our novel ultra-compact and lightweight X-ray telescope. We conduct a numerical calculation to develop an emission model for lunar fluorescent X-rays under M1-class solar flare conditions. Next, we incorporate a response model that reflects the instrument’s quantum efficiency and field of view. Assuming a polar circular orbit and an annual occurrence of 300 M-class solar flare events, we evaluate the required mission duration for a signal-to-background ratio exceeding 10. The simulation results indicate that O, Fe, Mg, Al, and Si could be observed globally in two years. In this case, the spatial resolution is approximately 70 km × 70 km. Moreover, a spacecraft with 25 telescopes to 5 × 5 array could facilitate global observation of O, Mg, Al, Si, and Fe in a year and detect Na within 2 years. The spatial resolution is about 30 km × 30 km. This result suggests that this approach could assist in developing future global lunar elemental maps and evaluating the chemical abundance of these elements across the entire surface of the Moon. Graphical Abstract

Solar activity induces ionospheric irregularities that degrade Global Navigation Satellite System (GNSS) performance through amplitude and phase scintillation. Although numerous studies have investigated event‐driven or regional responses, the nonlinear coupling between solar radiation, ionospheric scintillation, and precise point positioning (PPP) degradation remains insufficiently quantified. This study presents a multi‐year observational analysis integrating GNSS measurements, ionospheric scintillation monitoring data, and solar activity indices from 2017 to 2024, covering high‐, mid‐, and low‐latitude regions. By jointly examining extreme ultraviolet (EUV) radiation, coronal mass ejection activity, scintillation indices (S4 and σφ${\\sigma }_{\\varphi }$ ), cycle‐slip occurrences, and PPP errors, a distinct nonlinear response of ionospheric scintillation to solar radiation is identified. Results reveal an approximately “inverted V‐shaped” dependence of scintillation intensity on EUV flux, with a critical threshold near 0.35 W/m2. In addition, enhanced solar radiation can suppress ionospheric scintillation, leading to transient improvements in positioning accuracy. On the day of peak solar flare activity, GNSS positioning accuracy exhibits a slight improvement as a result of reduced scintillation effects. Both scintillation occurrences and cycle slips aligned more closely with variations in EUV radiation flux. Results also indicate that low‐latitude regions exhibited a higher frequency of amplitude scintillation events (S4 > 0.2) than high‐latitude areas. The results refine the understanding of sun–ionosphere–positioning coupling and offer potential insights for assessing GNSS reliability and anti‐interference strategies during periods of elevated solar activity.

From April 2021 to October 2022, in the monitoring data obtained daily at the Big Scanning Antenna radio telescope (BSA LPI), 11 events were identified for which X-ray flares in the solar corona were followed by magnetic storms on Earth. Interplanetary scintillation monitoring data were considered together with data on solar flare activity and a simple kinematic model of ejection propagation. Based on the estimated ejection velocity between the Sun and the probed region, under the assumption of a constant velocity, the time of arrival of the ejection to the Earth was calculated. Of the 11 events considered, 7 are associated with solitary flares followed by a coronal mass ejection (CME) and 4 are more complex and possibly associated with corotating perturbations or a superposition of corotating and flare perturbations. For the entire set of events, the average time of the real onset of a magnetic storm after the time predicted by the model was 3.6 h and the average time between the onset of scintillation enhancement and the onset of a magnetic storm was 20.1 h. For events associated with solitary flares, the magnetic storm began, on average, 0.8 hours after the predicted time and 15.6 hours after the onset of scintillation enhancement. The delay of magnetic storms with respect to the predicted time is apparently related to the deceleration of the ejection between the probed region of the solar wind and the Earth’s orbit.

In the current investigation we have studied the distribution as well as the asymmetry of solar X-ray flares during the period 1976–2017 which corresponds to solar cycle 21, 22, 23 and the almost complete solar cycle 24. The study of the N–S distribution of soft X-ray flares during solar cycles 21, 22, 23 and 24 reveals that the 10°–20° latitude band produced maximum number of soft X-ray flares and is found to be southern hemisphere dominated during all the considered cycles. The soft X-ray flares events above 40° latitude is very rare. Most of the solar X-ray flare activities are observed to be concentrated in the 8°–30° latitude band. A time-latitude plot has been plotted to graphically represent the soft X-ray flare distribution at various latitudes over the course of solar cycle. Our analysis also reveals that solar cycles 21, 22 and 23 are southern hemisphere dominated and the corresponding probability value is statistically significant. The soft X-ray solar flare activity during solar cycle 24 is also found to be southern hemisphere dominated, similar to cycles 21, 22 and 23. Using cross-correlation analysis, we have found that the smoothed curves of the number of soft X-ray flares in northern hemisphere are ahead of the southern hemisphere by 10 months, 2 months, 8 months and 4 months during solar cycles 21, 22, 23 and 24 respectively. Also, for total time span (1976–2017), the smoothed curve of the frequency of soft X-ray flares in northern hemisphere is ahead of a similar curve from the southern hemisphere by 7 months.

Solar flare activity is characterized by different classification systems, both in optical and X-ray ranges. The most generally accepted classifications of solar flares describe important parameters of flares such as the maximum of brightness of the flare in the optical range – Hα flare class (change from F to B), area of the flare in Hα (change from S for areas less than 2 square degrees to 4 for areas more than 24.7 square degrees) and the maximum amplitude of the soft X-ray (SXR)-flux in the band 0.1–0.8 nm ( F0.1-0.8max ) – X-ray flares of classes from C to X. A new classification parameter of solar flares is proposed here – the X-ray index of flare XI, based on GOES measurements of solar radiation in the SXR-range. The XI-index has a clear physical interpretation associated with the total flare energy in the SXR-range. XI is easily calculated for each flare with the use of available GOES data. The XI-index can be used along with other geoeffective parameters of Solar activity to assess both flares and coronal mass ejections that are connected with them.

The Solar Influences Data analysis Center (SIDC) in Brussels at the Royal Observatory of Belgium (ROB) has been providing daily space weather forecasts for more than a decade. A verification analysis was applied to evaluate the performance of the SIDC forecasts of fundamental space weather parameters such as the F10.7 radio flux, solar flare activity, and local geomagnetic index. Strengths and weaknesses are determined compared to common numerical models. Descriptive model statistics, common verification measures, error analysis and conditional plots related to forecasts and observations are presented. The verification analysis methods have been designed such that future improvements and additions can easily be included, for example with new forecasting models. The SIDC forecast (together with the persistence model) achieves the best performance for forecasting F10.7 on day 1, but has potential for improvement for a larger lead time mainly by applying estimates from the persistence and corrected recurrence models. The persistence model is superior for the forecast of flares, though corrected recurrence models are slightly better in foreseeing M- and X-class flares and the SIDC forecast estimates B- and C-class flares very well. The SIDC forecast scores better than all models in forecasting the local K-index. It best reproduces observations in the range of K = 2–4, but underestimates larger K values. The SIDC forecast provides a distribution that best matches the observations of the K-index. The analysis presented here demonstrates the influence of solar activity on the confidence level of the forecasts, as well as the hinted influence of the forecaster on duty due to the subjective nature of forecasting. The output aids to identify the strong and weak points of the SIDC forecast as well as those of the models considered. Though the presented analysis needs further extension, it already illustrates the opportunity to regularly reevaluate space weather forecasts and to stimulate ideas for improvement and increase the reliability of space weather forecasting.