Spectral analysis of interplanetary and geomagnetic indices using fourier transform for high solar activity years

Space weather variability is typically characterized by a set of solar and geomagnetic indices and proxies, which are complex functions of space and time. To observe and classify this variability, solar indices such as Sun Spot Number (SSN), Solar Flux at 10.7 cm wavelength (F10.7), components of the Interplanetary Magnetic Field (IMF-B), and geomagnetic disturbance indices such as Disturbance Storm Time (Dst), Planetary K (Kp), and Auroral Electrojet (AE) are employed. In this study, we introduce a novel random field model that can be decomposed into a sum of primary periodic trends and secondary disturbances. These disturbances, convolved onto the trend, are among the primary causes of geomagnetic storms, which can impact satellite health and the functionality of positioning, navigation, guidance, and communication systems. Therefore, to predict the potential risks associated with variability in solar and geomagnetic indices, it is crucial to accurately identify the trend structure. This study investigates the spectral properties of the periodic trend using a tailored Fourier Transform for the complex space-time signals during the high solar activity years of the 23rd and 24th solar cycles, namely, 2000, 2001, 2002, 2013, and 2014. IMF-Bz, serving as a major coupling element of the space-Earth environment, carries periodicities related to both solar rotation, linked to sunspot distribution and intensities, and the spiral rotation of charges in the solar wind as it traverses the interplanetary medium. For the first time in the literature, it is shown that the periodic trend structure of the z-component of IMF (IMF-Bz) has a stochastic modulating function convolved with solar periodicities in the spectral domain. For the geomagnetic indices Dst, Kp, and AE, the stochastic trend is found to consist of geomagnetic and geographic modulating functions influenced by solar signals. While the periodicities observed in IMF-Bz originate from solar activities and solar wind structure, the dominant periodicities in geomagnetic indices reflect the Earth’s monthly, seasonal, and annual cycles in both lower and upper harmonics. Evidence supporting the convolutional shifts in the spectral domain is observed during solar maximum years, where the spectral peak of IMF-Bz shifts from 26.41 days to 27.76 days for Dst, 27.83 days for Kp, and 27.86 days for AE. For all investigated high solar years, IMF-Bz’s 24.64-day periodicity shifts to 26.80 days for Dst and 26.85 days for