Detailed understanding of reduced geoeffectiveness of solar cycle 24 in association with Geomagnetic storms

Selvakumaran R^1*Sneha Gokani^1*Shirsh Soni²

• ¹Amity Center of Excellence in Astrobiology, Amity Institute of Biotechnology, Amity University Maharashtra, Mumbai, India., Mumbai, Maharashtra, India
• ²Department of Climate and Space Sciences and Engineering, College of Engineering, University of Michigan, Ann Arbor, Michigan, United States

Solar Cycle 24 has been observed as the weakest solar cycle in over a century. It commenced with an unusually deep and prolonged minimum following Cycle 23, characterized by a weak polar magnetic field and asynchronous polar field reversal, leading to hemispheric asymmetry (e.g., Manoharan, 2012;Basu, 2016;Pesnell, 2016;Cliver and von Steiger, 2017). Several observations indicate that the Sun's overall magnetic field was significantly weaker in Cycle 24 compared to previous cycles (e.g., Hathaway, 2015 and references therein). The interplanetary magnetic field also exhibited a substantial decline, leading to the lowest recorded solar wind power. Due to the weak polar magnetic field during the Cycle 23/24 minimum, sunspot activity in Cycle 24 was also diminished, with the sunspot number declining by approximately 30% compared to Cycle 23.However, the occurrence rate of coronal mass ejections (CMEs) and the number of halo CMEs, which are among the most energetic CMEs, did not show a significant decline in Cycle 24. A notable difference, however, was an 80% reduction in intense geomagnetic storms and a 40% reduction in moderate storms compared to Cycle 23 (Selvakumaran et al., 2016). Gopalswamy et al. (2014) suggested that the reduced geoeffectiveness of CMEs in Cycle 24 was due to lower heliospheric pressure, which caused CMEs to undergo anomalous expansion in the inner heliosphere. This expansion is supported by observations indicating increased CME width beyond a certain distance from the Sun, along with a decrease in total heliospheric pressure (plasma + magnetic). Further, the compression of CMEs closer to the Sun and their subsequent expansion in the inner heliosphere were analyzed to confirm this behavior.To understand the reduced geoeffectiveness of geomagnetic storms, we analyzed the changes in solar-source flux ropes and their transformation into interplanetary flux ropes, alongside the energy input to the magnetosphere. Utilizing observations from SOHO, STEREO, ACE, and WIND, we identified the solar sources (CMEs) responsible for individual geomagnetic storms in Solar Cycles 23 and 24. This study investigates the characteristics of these sources and their efficiency in transferring energy to the magnetosphere, helping to establish the reasons behind the weaker geomagnetic storm activity in Cycle 24.