Impact of sunspot activity on the rainfall patterns over Eastern Africa: a case study of Sudan and South Sudan

The relation between sunspots and rainfall patterns is still obscure in Africa, especially for Sudan and South Sudan. This research explores the response of rainfall to solar activity in eastern regions of Africa, with a case study in Sudan and South Sudan. Rainfall varies with time; therefore, skillful monitoring, predicting, and early warning of rainfall events is indispensable. Severe climatic events, such as droughts and floods, are critical factors in planning and managing all socioeconomic activities. Similar trends for the sunspot activity (sunspot number and sunspot groups) changes and rainfall variations for different stations in East Africa during the years 1910– 2018 were not found. Correlation analysis carried out for the above period indicated a weak negative correlation between the total rainfall and the average number of sunspots over the longterm scale for selected stations in Sudan and South Sudan. The overall result of the paper indicated no significant relationship between sunspot numbers and rainfall in temporal and spatial scales in Sudan and South Sudan.


INTRODUCTION
Rainfall is usually considered the most important meteorological variable throughout the world, and especially for African countries. It is the most important part of the hydrological cycle (Nazari-Sharabian et al. a). The surface runoff is majorly dependent on rainfall, among other factors (Nazari-Sharabian et al. , b). Rainfall is necessary to support the Sudan and South Sudan populations' livelihoods, where agriculture in the majority of these countries is heavily dependent on rainfall (El Tom ). Sudan's ecological conditions vary greatly, from the northern desert region, the central arid and semi-arid regions to the southern tropical rainforests. The average annual rainfall varies from almost zero in the north to more than 1,000 mm in the south (Rhodes ).
The study focuses on solar activity (SA) in terms of sunspot numbers. The solar cycle (SC) influence on climate is a research subject of high scientific importance due to the rigorous need to estimate and distinguish between natural variability and climate change. The lack of understanding of the atmosphere's physical mechanisms of the response to SA changes is a major challenge (Chiodo et al. ).
The overall measured change in solar radiation during SC of 11 years is about 0.1%. This change leads to a radiative forcing of intensity of about 0.2-0.3 W/m 2 in total solar radiation (TSI). Although the net forcing in TSI is small, space measurements indicated significant differences (4-8%) in the UV range of 200-250 nm solar minimum to maximum conditions (Lean et al. ). The ultraviolet light fabricates propellant gases from small amounts of water vapor, sulfur dioxide, and ozone. The increase in propellant gases made by gamma sources' ionization is fixed from nucleation to above 50 nm diameter. That is suitable for cloud condensation nuclei (Svensmark et al. ). The mass-flux of small ions may enhance the neutral molecules' condensation. The increase from ions may represent many per cents of the neutral increase (Svensmark et al. ). Solar radiation is one of the fundamental natural driving forces of the Earth's atmosphere. Due to this energy varying temporally and spatially, its impact on the climate is never zero. The climate response must be fully defined to improve our understanding of the climate system and the impact of human activities (El-Mahdy et al. ). However, despite all the efforts made, whether subtle differences in solar radiation affect climate and weather or not remains a mystery (Fan et al. ). One of the main elements often taken as evidence of response is the similarity of periodicities between several indicators of SA and different meteorological parameters (Nazari-Sharabian & Karakouzian ). Some previous studies include a long history of negative or positive correlations between climate and weather factors, such as temperature, rainfall, drought, etc. and SA cycles like the 27-day cycle, the prominent 11-year sunspot cycle, the 22-year Hale cycle and the Gleissberg cycle of 80-90 years (Xu et al. ). However, doubts about these relationships will remain as long as the indisputable physical mechanism, which may work to produce these relationships, is not available (Kakad et al. ; Michalek et al. ). All results show that the most visible feature of global temperature change in the 20th century was the sudden and strong warming that has occurred since the early 1970s and beyond (Tapley et al. ). Moreover, over the course of the 20th century, global warming did not occur steadily, as different periods of change were identified (Solomon et al. ). Climate change may occur in many remarkable aspects. The precipitation rate increased by 0.05% to 0.1% per annum in the 20th century in the majority of the middle and high latitudes in the northern hemisphere, while it increased by 0.02% to 0.03% per annum in tropical areas (10 S to 10 N). On the other hand, the precipitation rate decreased over most of the northern sub-tropical areas ( There is plenty of circumstantial evidence that SA changes influence climate over the longer term on regional scales worldwide (Solomon et al. ).
Recent research has focused primarily on the tropics of eastern Africa; however, the rainy season resulting from the convergence zone's passage between the tropics penetrates as far north as the Sahel, affecting the area that suffers from severe water stress (Audu & Okeke ).
The temperature gradient from the pole to the equator, which is maintained by differential absorption of solar radiation, is the driving force of the atmospheric dynamics and the general circulation (Haigh  The SC is also called 'the sunspot cycle' because it can be traced by the sunspot number (SSN). The sunspot is defined as a dark area seen on the surface of the Sun. The A connection among the Sun, cosmic rays, and rainfall exists worldwide (Svensmark et al. ). The UV in the stratosphere has no impact on rainfall; but, the Forbush decrease influence on rainfall is proven, possibly by ions (Svensmark et al. ). The occurrence of the geomagnetic storms and galactic cosmic rays' peak mostly occurs through the descending phase of the 11-year SC (Steinhilber et al.

EXTREME EVENTS
Drought is considered one of the worst natural phenomena that Sudan suffers from. The recurring consecutive dry years in the Sudano-Sahelian region have become usual. There are two types of droughts: the first is widespread drought, which is caused by below-normal rainfall across the country; the second is localized drought that affects only some parts of the country. Between 1961 and 1998, episodes of drought had various intensities in Sudan (Elagib & Elhag ). In this period, the two most severe widespread droughts in the century occurred from 1967 to 1973, and from 1980 to 1984, with the latter being the severest. A series of drought events occurred in the years 1987, 1989, 1990, 1991, and 1993, mostly 1962-1965, 1978-1979, 1988, 1994, and 1998. The most vulnerable groups to both forms of flooding are people who live in low lands and along the riverbanks. Of the most severe floods recorded for the River Nile (1878, 1946, 1988, 1994, 1998, and 2020), three occurred within the past 30 years. Between April and October, severe sandstorms, or 'Haboubs', blow frequently in the northern part of the country (ElSiddig ). variability. The specifications of stations are listed in Table 1 and

Simple linear regression model (SLRM)
Regression analysis is the most widely used statistical technique for examining and modeling the relationship

T-test for significance
The t-test is a statistic utilized to explore if the difference between the means of two datasets is significant (Helsel & Hirsch ). Rains in that quarter are concentrated in less than four months of the year with a relative standard deviation between 20 and 40%. In the southernmost quarter, the MAR exceeds 700 mm annually, as shown in Table 2. Therefore, the MAR over Sudan trend is to decrease gradually from south to north, as shown in Figure 5.

Trends analysis of annual rainfall over the study area
In this study, the long-term trend of rainfall was investigated at 18 stations in the study region with a full record of 108 years and a minimum record of 98 years. Results showed that there was a negative trend in rainfall in most of the stations. There were three trend zones for rainfall in the study region, i.e., positive trend, negative trend, and no  Table 3 and shown in Figure 6.     (2008) SSNmin (   As depicted in Table 5 Table 6 and Figure 8.
The trend analysis test result revealed a non-significant increasing trend for the mean annual sunspot number and sunspot groups through the studied period. The sunspot number and sunspot groups' rate of change were found to be 0.039 and 0.013 per decade, respectively.

Analysis of correlation between total rainfall and sunspot number
The total rainfall dataset in the study region from 1910 to 2018 was obtained from the Sudan Meteorological *P-value <0.05 (significant); **P-value <0.01 (significant); P-value >0.05 (non-significant). The analysis results are shown in Table 7 and Figure 9.
The CC between SSNs and TAR in the study region was 0.25). Figure 11 shows the correlation between the TAR and the total sunspot groups. The only significant CCs between the total annual rainfall and the total sunspots groups were found at the stations of El Obied, Khartoum,

DISCUSSION AND CONCLUSION
This study investigated the relation between sunspot numbers and groups on the rainfall patterns of eastern Africa, especially in Sudan and South Sudan. The correlation analysis results showed low significant relationships between rainfall and sunspot numbers and groups. A weak linkage between the average SSNs and seasonal rainfall pattern is attained from the analysis. Therefore, Figure 9 | Correlation between total annual rainfall and total annual sunspot number in the study area.
Figure 11 | Correlation between total annual rainfall and total sunspot groups in the study area.
the sunspot data alone cannot be used to predict rainfall over the study region. In general, the Sudan and South Sudan annual rainfall does not follow the same trend as the average number of sunspots. These less significant correlation results may be due to the likelihood of rains depending more strongly on processes internal to the Earth-atmosphere system such as ENSO, El Niño, La Niña, and atmospheric-oceanic circulation mechanisms.
Further analysis of other phenomena and their relation with sunspots and rainfall is required. Linking the current study with El Niño and La Niña studies will enhance both.

DATA AVAILABILITY STATEMENT
Data cannot be made publicly available; readers should contact the corresponding author for details.