Abstract
This study examines the variation of different energies linked with the Sun and the Earth’s magnetosphere-ionosphere systems for solar cycles (SCs) 22–24 for which the gradual decrease in the solar activity is noticed. Firstly, we investigated the variation of solar magnetic energy density (SMED) for SCs 21–24 and its relation to the solar activity. We observed distinct double peak structures in SMED for the past four SCs, 21–24. This feature is consistent with noticeable asymmetry in their two peaks. For SCs 22–24 a significant decrease is observed in the integrated SMED of each SC. This reduction is 37% from SCs 22 to 23 and 51% from SCs 23 to 24, which indicates substantial weakening of Sun’s magnetic field for SC 24. Also, the magnetic, kinetic, and thermal energy densities at the Earth’s bow-shock nose are found to be considerably low for the SC 24. We examined the solar wind Alfven speed, magnetosonic Mach number, solar wind-magnetosphere energy coupling parameter (ε), and the Chapman-Ferraro magnetopause distance (LCF) for the SCs 22–24. The estimated maximum stand-off magnetopause distance is larger for SC 24 (LCF ≤ 10.6 RE) as compared to SC 23 (LCF ≤ 10.2 RE) and SC 22 (LCF ≤ 9.8 RE). The solar wind Alfven speeds during SCs 22 and 23 are in the same range and do not exceed ≈73 km/s whereas, it is below 57 km/s for SC 24. A lower bound of solar wind magnetosonic Mach number for SC 24 is larger (M ≥ 6.9) as compared to SC 22 (M ≥ 5.9) and SC 23 (M ≥ 6). We noticed weakening in the energy coupling parameter for SC 24, which resulted in substantial (15%–38%) decrease in average strength of high latitude ionospheric (AE), low latitude magnetospheric (Dst) and equatorial ionospheric (EEJ) current systems in comparison with SC 23. Subsequently, a reduction of ≈30% is manifested in the high latitude Joule heating for SC 24. Overall this study indicates the significant step down in various energies at Sun, Earth’s bow-shock, and near Earth environment for current SC 24, which will have important implication on our Earth’s atmosphere-ionosphere-magnetosphere system.
Highlights
The Sun is the main source of energy for our Earth, and its variability influences the Earth’s atmosphere, ionosphere, magnetosphere systems, and climate, which is evident from several studies (McComas et al, 2008; Ermolli et al, 2013; Dudok de Wit & Bruinsma, 2017)
To understand the response of Earth’s current systems, we looked into the proxies representing high latitude currents like the auroral electrojet (AE), low latitude magnetospheric ring current (Dst), and equatorial ionospheric E-region current system known as equatorial electrojet (EEJ)
The two-peak structure is seen in the monthly smooth sunspot number (SSN), but the difference between them is only marginal. The presence of such dual-peak at solar maximum was seen by Gnevyshev (1967) and the interval between these two peaks is called as Gnevyshev-gap (Gnevyshev, 1977; Feminella & Storini, 1997; Norton & Gallagher, 2010)
Summary
The Sun is the main source of energy for our Earth, and its variability influences the Earth’s atmosphere, ionosphere, magnetosphere systems, and climate, which is evident from several studies (McComas et al, 2008; Ermolli et al, 2013; Dudok de Wit & Bruinsma, 2017). Their study conclude that this observed decrease cannot be explained by existing magnetosphere– ionosphere coupling knowledge but rather it might be related to the unidentified mechanism connected to the long-term solar activity. In such a scenario, it is worth exploring the overall impact of the recent diminishing solar activity on the near Earth environment. The expected decrease may not be linear This raises the question of how do Earth’s ionosphere– magnetosphere systems configure themselves during periods of such extremely low solar activity and can we quantify these changes?.
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