The solar wind, which emanates from the Sun and flows outward, envelops the Earth in a constantly moving stream of plasma and magnetic fields. This persistent solar wind has a significant influence on the behaviour of Earth’s magnetosphere. For instance, particle precipitation from the solar wind and plasma accumulation in the magnetotail are primary drivers of auroral activity [1] . These energetic particles transfer energy to atmospheric particles, producing the visually striking phenomenon known as the aurora.

Between 1996 and 2008, distinct characteristics emerged during both the ascending and descending phases of Solar Cycle 23. Notably, the unusually prolonged solar minimum (e.g., Kane 2005) [2] has drawn considerable attention, as it plays a critical role in the interpretation of geomagnetic indices. This period is thus considered crucial for the study of solar-terrestrial interactions.

Numerous studies have investigated the relationship between geomagnetic indices—such as the Kp index and solar activity, particularly in relation to sunspot counts and sunspot area ( Figure 1 ). Solar Cycle 23 stands out for its extended minimum phase marked by significantly low sunspot numbers. This unique condition presents a valuable opportunity to examine how diminished solar activity correlates with geomagnetic behaviour. While previous research has laid the groundwork for understanding these dynamics, much of it has focused on periods of heightened solar activity, leaving a gap in knowledge regarding the effects of low solar activity on geomagnetic indices [3] .

Our study highlights the seasonal fluctuations in Kp index behaviour and examines the broader relationship between solar activity and geomagnetic indices. Previous research has demonstrated that geomagnetic activity can vary significantly with the seasons, particularly around the equinoxes. While these seasonal patterns have been acknowledged, further investigation is needed to understand how the unique features of Solar Cycle 23 may alter or amplify these trends. Understanding these seasonal fluctuations is crucial for improving the accuracy of geomagnetic storm forecasts and mitigating their potential impacts on communication and technological infrastructure [5] [6] .

Gleason and Aboy noted the erratic fluctuations of the Kp index—a key component of geomagnetic activity—across different seasons, suggesting that these variations require further study. Research has also shown that annual trends in geomagnetic activity are most pronounced during the equinoctial periods, when the alignment of Earth’s geomagnetic field with the interplanetary magnetic field (IMF) allows for more effective coupling with solar wind energy. This alignment makes Earth’s magnetosphere more susceptible to disturbances, increasing geomagnetic activity during these times. Although seasonal dependencies in geomagnetic indices like the Kp index are well documented, the distinctive nature of Solar Cycle 23 may either amplify or suppress these variations. As such, this cycle presents a valuable opportunity to investigate how periods of low solar activity influence deviations from established seasonal patterns.

By analysing the Solar Cycle 23 period, our study addresses a notable gap in the literature concerning how low levels of solar activity influence seasonal changes in geomagnetic behaviour. This research contributes to the growing body of knowledge on the relationship between solar activity and geomagnetic indices, emphasizing that even atypical solar cycles can significantly affect these interactions. Recognizing and quantifying these effects is essential for the development of robust forecasting models. Such models are increasingly important in today’s technologically dependent world, where even minor disturbances in Earth’s geomagnetic field can disrupt navigation systems, communications, and power grids [7] [8] .

The analysis of solar wind magnetosphere interaction is as old as space physics, with pioneering works demonstrating the importance of solar activity for geomagnetic activity. Chapman and Ferraro (1941) appeared to be pioneers and the first to provided concepts about the effects of solar storms on the magnetic environment of the Earth and energy transfer from solar wind particles to the magnetosphere causes geomagnetic storms. These basic investigations formed the basis for further research into the complexity of the solar-terrestrial connection, where it has been established that solar wind velocity, density and magnetic fields have a substantial effect on geomagnetic activity as described by the Kp index. Since then many other researches have been done to explore these solar-terrestrial relations, with particularly stressing necessity of taking into account various phases of the solar cycle [9] .

Among different indices that characterize the level of geomagnetic activity, the Kp index proved to be one of the most important indices used in space weather studies. Bartels (1932) already created the measurement for geomagnetic storm intensity as Kp index which varies from 0 to 9. Since then, the Kp index is used routinely, and is applied broadly in space weather research for operating and prospecting geomagnetic storms [10] . Tsurutani et al., [11] have investigated this fact when pointing to the frequently observed solar wind conditions as well as the strong interplanetary magnetic field leading to the Kp index enhancement. However, most of the initial attempts were carried out during periods of high intensity solar storms, aimed at determining their effect on the geomagnetic environment. Since this focus has placed much importance on particular periods, there is inadequate information available on how minimum solar conditions such as those of the Solar Cycle 23 extended minimum impact certain values of the geomagnetic indices [11] .

Solar Cycle 23 that started in 1996 up to 2008 has been of interesting history because it was characterized by an unexpected extra-long solar minimum period. As Kane pointed out in his recent article, this cycle was characterized by an unusually lengthy and dormant solar minimum, one that has been named as having one of the longest records of low levels of sunspot activity. This long minimum has since formed the basis of extended research because it offered scientists the chance to study how the magnetosphere of Earth reacts to lasting low solar activity. A few researchers claim that this protracted minimum might affect the typical response of the Earth’s magnetic field to conditions in a manner that has been detected during other low-solar-activity intervals. The effect of such extended interval of silence on the parameters such as Kp index, however, merits further research.

There are many cases reported in previous studies that phenomena associated with geomagnetic activity are most intense around equinoctial periods only. Equinoctial hypothesis has been expressed by Russell and McPherron [12] stating that geomagnetic field of the Earth gets closely aligned to IMF during equinox period which enhanced energy coupling from the solar wind into the magnetosphere [12] . These seasonal effects have also been supported by numerous studies, such as the work of Cliver et al. (2000) who probe increased geomagnetic activity during equinox periods because of appropriate IMF configurations. Despite such previous explorations of climatic annual variations, behaviors of Solar Cycle 23, which is differently featured from those that came before it, may well engage or change these annually predictable traits, hence, a need to investigate the effects of seasons taking into consideration the contested protracted solar minimum phase [13] .

In spite of the significant progress made in the study of GM and its source, the prior achievements and numerous studies have primarily focused the articles and research on high solar activity periods, means maximum phases with high sunspot and flaring activity [4] . It has been the active periods that have attracted the interest of many researchers because during intense geomagnetic storms, satellites, communication and power systems can be affected. Nevertheless, fewer studies have been made with low activity periods and the behavior of different indices such as the Kp index during the absence of solar activity. Since Solar Cycle 23 experienced a lengthy low level of solar activity, what we propose is that low-Kp conditions may demonstrate other patterns of geomagnetic activity and seasonal responses [14] .

Besides emphasizing the reaction to solar and seasonal stimuli, it is witnessing the emergence of a demand for further investigations on the predictability of geomagnetic indexes, such as Kp index, under various solar conditions. Research by Richardson and Cane (2012) clearly reveals that many space weather forecasts have higher variability, correlation and prediction error rates of geomagnetic storms while the sun is quiet. These restrictions are suggesting the need to research geomagnetic indices in various solar conditions in order to enhance calibration declaration. Investigating the dependency of the Kp index on seasonal changes and its low activity phase behavior during Solar Cycle 23 plays significant roles for enhancing the uncertainties of Kp prediction models under the other vital conditions necessary for vital infrastructures that require credible space weather forecasts [15] .

I. Tsagouri [16] discusses the significant impact that geomagnetic storms have on the Earth’s upper atmosphere, particularly emphasizing how the highly variable solar wind energy input into the magnetosphere alters ionospheric structure. During such storm events, interactions between ionospheric plasma and atmospheric neutrals result in substantial fluctuations in peak electron density, manifesting as positive or negative ionospheric storms. These large-scale electron density variations have critical implications for the performance and reliability of technological systems, especially those reliant on radio signal propagation and satellite-based navigation.

Ionospheric storms have therefore been the subject of extensive research in recent decades. A comprehensive understanding of their behavior has emerged, positioning them as a crucial component in the broader solar-terrestrial interaction system. Despite these advances, ionospheric storms remain a dynamic area of research due to ongoing developments in geospace modeling, real-time monitoring capabilities, and evolving technological requirements.

The paper offers a concise survey of the current understanding of ionospheric storm responses, particularly at mid-latitudes, focusing on the morphological characteristics and occurrence patterns of these disturbances. Special attention is given to the triggering role of solar wind conditions, which continue to present interpretive challenges and remain central to improving forecasting and mitigation strategies in space weather research.

Toriumi et al. [17] emphasized that the formation of extremely hot outer stellar atmospheres is one of the most prominent manifestations of magnetic activity in late-type dwarf stars, including the Sun. These outer layers—the chromosphere, transition region, and corona—are widely believed to be heated by the dissipation of energy transported upward from the stellar surface via magnetic fields. This heating process is reflected in spectral line fluxes at various wavelengths, which exhibit power-law relationships with surface magnetic flux over a wide range of formation temperatures. These relationships appear to be universal among the Sun and Sun-like stars, regardless of their age or magnetic activity level.

In thier study, the authors compiled a comprehensive catalog of power-law indices correlating solar activity proxies with various spectral line fluxes. Compared to previous research, this work significantly expands the scope by:

In their research the extended dataset enables detailed investigation of flux–flux scaling laws across spectral lines originating from different temperature regions, from the corona (log(T/K) ≈ 6 - 7) to the chromosphere (log(T/K) ≈ 4). Furthermore, the catalog facilitates the reconstruction of historical solar spectral fluxes and can be applied to studies of F-, G-, and K-type dwarfs as well as modeled stellar atmospheres.

A. Espuña Fontcuberta et al. [6] emphasize that predicting the solar magnetic cycle is of critical importance for humanity. In this context, a novel development is the application of machine learning algorithms for solar cycle forecasting. Various approaches have been developed for this purpose, though no consensus has yet emerged among different techniques, including both data-driven and physics-based methods.

In their study, the authors evaluate the performance of four machine learning algorithms, all belonging to the class of Recurrent Neural Networks (RNNs), for predicting simulated sunspot cycles based on a well-established, stochastically forced, nonlinear time-delay solar dynamo model. Among the models tested, the Echo State Network (ESN) demonstrated the best performance. However, its predictability is limited to only one future sunspot cycle, which aligns with current physical understanding.

The authors then trained the ESN and a modified version (MESN) using historical solar cycle observations to forecast Solar Cycles 22 - 25. Their models produced accurate hindcasts for Solar Cycles 22 through 24. For Solar Cycle 25, the ESN forecasts a peak amplitude of 131 ± 14 sunspots around July 2024, with a cycle length of approximately 10 years. The MESN predicts a peak of 137 ± 2 sunspots around April 2024, with a similar cycle duration.

Qualitatively, both models suggest that Cycle 25 will be slightly stronger than Cycle 24, but weaker than Cycle 23. This research presents a hybrid approach that bridges the gap between physics-based models and machine learning methods, achieving promising consistency across diverse forecasting techniques.

B. L., M. I. Desai et al. [5] reported on the annual variation of quiet-time suprathermal ion composition for elements ranging from carbon (C) through iron (Fe), using data from the Advanced Composition Explorer (ACE)/Ultra-Low Energy Isotope Spectrometer (ULEIS). Their analysis covered the energy range 0.3 MeV/nucleon to 1.28 MeV/nucleon over the period 1998 to 2019, encompassing the rising phase of Solar Cycle 23 through the declining phase of Solar Cycle 24.

Their findings include:

From these observations, the authors infer that quiet-time suprathermal ions are likely remnants of CIR-related activity during solar minima, and residuals from GSEP events during solar maxima. Additionally, and somewhat unexpectedly, the study finds that sulfur (S) behaves like a low first ionization potential (FIP) ion in the suprathermal regime. This behavior suggests it originates from low-FIP solar sources, offering new insights into the compositional dynamics of suprathermal ion populations.

Eid A. Amin et al. [18] investigated the relationship between geomagnetic storms and solar events, compiling a comprehensive catalog of multi-source geomagnetic storms spanning the period from August 1996 to December 2019. Their study focuses on assessing how solar activity influences geomagnetic storm characteristics, particularly during the minimum and maximum phases of Solar Cycles 23 and 24. The results indicate that geomagnetic activity was more intense during Cycle 23 compared to Cycle 24.

A total of 104 geomagnetic storms were identified, each associated with a minimum Dst index of ≤–100 nT, classifying them as intense. A strong correlation was found between the number of storm events and average sunspot numbers (correlation coefficient CC = 0.73), underscoring the direct relationship between solar activity and geomagnetic disturbances. The majority of these storms were associated with coronal mass ejections (CMEs) and solar flares, with relatively few linked to other interplanetary sources.

The study determined that the average CME speed for storms contributing to geomagnetic disturbances was approximately 876 km/s, and a moderate correlation was observed between CME speed and Dst index (R = 0.61). Among the CME-driven events, 63% were full halo CMEs, while 37% were partial or narrower halo CMEs. Notably, seven of the most severe storms during Solar Cycle 23 exhibited a magnetic H-component exceeding 400 nT, reflecting their extreme intensity.

The analysis further incorporated multiple geophysical and solar wind parameters, including solar wind speed, Dst index, Ap index, Kp index, auroral electrojet (AE) index, and the north–south component of the interplanetary magnetic field (IMF-Bz). These parameters were statistically examined to establish the connection between storm occurrences and specific solar drivers such as CMEs, solar flares, and corotating interaction regions (CIRs). This multi-dimensional approach provides a robust characterization of geomagnetic storm behavior during two contrasting solar cycles.

This research adopts a quantitative operational framework to analyze the correlation between solar activity and geomagnetic activity during Solar Cycle 23, covering the period from 1996 to 2008. The methodology integrates data from multiple sources, including ensemble average daily sunspot area, F10.7 solar radio flux, and the Kp index—widely used as a global measure of geomagnetic activity—to assess both long-term and short-term behavioural trends.

At the methodological level, the study leverages the advantages of a seasonal and, more specifically, a fine-grained (microscopic) approach, enabling detailed observation of how geomagnetic indices respond to solar activity across different time scales. The research process involves data cleaning, feature selection and transformation, exploratory data analysis, correlation analysis, and predictive modelling.

This multilevel strategy facilitates a comprehensive analysis of Solar Cycle 23, with particular attention to the extended minimum phase. It also seeks to uncover patterns and insights that could enhance the accuracy of space weather forecasting during periods of low solar activity, contributing valuable guidance for future solar cycles.

To investigate the relationship between solar activity and the Kp index during Solar Cycle 23, we focused our analysis on periods where the sunspot area was greater than or equal to 500 millionths of a solar hemisphere (MH). This threshold was selected because larger sunspot areas are typically associated with stronger solar magnetic fields and heightened solar activity, including solar flares and coronal mass ejections (CMEs). These phenomena intensify solar wind parameters, which in turn influence the Earth’s magnetosphere and often lead to elevated Kp index values, indicating geomagnetic storms.

By concentrating on sunspot areas ≥ 500 MH, the analysis isolates periods of significant solar activity, ensuring that the impact on geomagnetic indices can be assessed with greater statistical relevance. This targeted approach allows for a clearer examination of how intense solar events drive geomagnetic variability, thereby enhancing our understanding of the dynamic interaction between solar and geomagnetic activity.

This study investigates the relationship between various solar activity indicators (including sunspot area, F10.7 index) and the behavior of the Kp index during Solar Cycle 23 using multiple analytical and statistical methods.

We conclude that there is a direct correlation between the F10.7 index for sunspot areas greater than or equal to 500 millionths of a solar hemisphere (MH) and the Kp index during Solar Cycle 23. A time lag is observed in the response of the Kp index to solar activity changes during the ascending and descending phases of the cycle. However, during the solar maximum, this relationship becomes almost synchronous.

Seasonal variations of the Kp index and F10.7 index were analyzed, revealing distinct patterns:

These results contribute new insights into Solar Cycle 23, particularly regarding its impact on the Kp index during solar minimum. Notably, our findings highlight short-term fluctuations in the Kp index during low solar activity periods and emphasize asymmetric seasonal variations in geomagnetic responses, particularly between the spring and autumn equinoxes. These insights add a new dimension to previous research on the solar-terrestrial interaction, especially in relation to solar minimum conditions.

Furthermore, we confirm that solar activity has significant effects on Earth in various ways. Increased solar activity leads to enhanced emissions of X-rays and extreme ultraviolet radiation from the Sun, which cause dramatic changes in the Earth’s upper atmosphere. This atmospheric heating increases both the temperature and density of the atmosphere at high spacecraft altitudes.

In addition, the frequency of coronal mass ejections (CMEs) and solar flares increases, raising the likelihood of spacecraft instrument damage due to energetic particles accelerated during these events. These solar energetic particles also pose a health risk to astronauts in space and airline passengers traveling on high-altitude polar routes.

Overall, the present study underscores the complex relationship between solar activity and geomagnetic disturbances, offering valuable insights into how variations in solar behavior influence space weather phenomena and their impact on both spacecraft systems and human health.