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Characteristics of Magnetic Helicity of Flux Transfer Events at the Magnetopause.

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Title: Characteristics of Magnetic Helicity of Flux Transfer Events at the Magnetopause.
Authors: Bai, Hua1 (AUTHOR), Sun, Tianran2,3 (AUTHOR) trsun@spaceweather.ac.cn, Huang, Shiyong1 (AUTHOR) shiyonghuang@whu.edu.cn, Wang, Chi2,3 (AUTHOR), Yuan, Zhigang1 (AUTHOR), Jiang, Kui1 (AUTHOR), Tang, Yaotian1 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. Mar2026, Vol. 131 Issue 3, p1-16. 16p.
Subject Terms: Magnetopause, Interplanetary magnetic fields, Magnetohydrodynamics, Magnetosphere, Magnetic properties, Plasma transport processes, Solar wind
Abstract: Flux transfer events (FTEs) transport solar wind energy and plasma into the Earth's magnetosphere. Their topological twisting features can be described by magnetic helicity, a crucial quantity for understanding the large‐scale magnetic field structure and evolution. The interplanetary magnetic field (IMF) orientation significantly influences the properties and global behaviors of FTEs. Recent observations revealed the relation between FTE formation and helicity sign and its dependence on IMF orientations. However, the detailed characteristics of FTE helicity magnitude and its relations with FTE evolution as influenced by IMF orientation are still poorly understood. Using global MHD simulations with different IMF clock angles arctan(By/Bz), we designed a novel method to calculate FTE helicity magnitude for the first time and investigated its relations with other FTE properties. We find that magnetic helicity has positive correlations with the FTE core field and lifetime. The helicity and lifetime are the largest when IMF has comparable By and Bz, implying that as IMF turns from near‐purely southward to duskward, the helicity and lifetime first increase as a function of increasing By/Bz but then decrease once By/Bz exceeds a threshold. This is attributed to the increasing magnetic tension force generated by the FTE core field, which can suppress kink instability or straighten the flux rope depending on the IMF orientation regime. Interplanetary magnetic field By generally controls FTE helicity signs, with exceptions occurring when IMF By is small even without the Hall effect. These findings enhance our understanding of global‐scale FTE structures and evolution, aiding interpretation of future satellite data such as SMILE and TRACERS. Plain Language Summary: Flux transfer events (FTEs) at the Earth's dayside magnetopause help transfer energy and plasma from the solar wind into the magnetosphere. Magnetic helicity quantifying FTE twisting features is critical for understanding FTE structure and evolution. Using global MHD simulations with varying IMF orientations, we designed a novel method to calculate FTE magnetic helicity magnitude for the first time and studied its connections to other FTE properties and how these are influenced by IMF orientations. We find that the helicity is positively related to the strength of an FTE's core field and its lifetime. As the IMF rotates from near‐purely southward to near‐purely duskward, the helicity and lifetime have a trend of first increasing but then starting to decrease after a certain degree of IMF rotation. While the IMF By orientation usually governs an FTE's magnetic twist direction, exceptions occur during small IMF By even without the Hall effect. Key Points: Magnetic helicity has positive correlations with both the core field and lifetime of flux transfer events (FTEs)As the interplanetary magnetic field (IMF) turns from near‐purely southward to duskward, the FTE helicity and lifetime have a trend of first increasing but then decreasingThe signs of FTE magnetic helicity are not strictly controlled by IMF By when it is small due to local disturbances near the magnetopause [ABSTRACT FROM AUTHOR]
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Abstract:Flux transfer events (FTEs) transport solar wind energy and plasma into the Earth's magnetosphere. Their topological twisting features can be described by magnetic helicity, a crucial quantity for understanding the large‐scale magnetic field structure and evolution. The interplanetary magnetic field (IMF) orientation significantly influences the properties and global behaviors of FTEs. Recent observations revealed the relation between FTE formation and helicity sign and its dependence on IMF orientations. However, the detailed characteristics of FTE helicity magnitude and its relations with FTE evolution as influenced by IMF orientation are still poorly understood. Using global MHD simulations with different IMF clock angles arctan(By/Bz), we designed a novel method to calculate FTE helicity magnitude for the first time and investigated its relations with other FTE properties. We find that magnetic helicity has positive correlations with the FTE core field and lifetime. The helicity and lifetime are the largest when IMF has comparable By and Bz, implying that as IMF turns from near‐purely southward to duskward, the helicity and lifetime first increase as a function of increasing By/Bz but then decrease once By/Bz exceeds a threshold. This is attributed to the increasing magnetic tension force generated by the FTE core field, which can suppress kink instability or straighten the flux rope depending on the IMF orientation regime. Interplanetary magnetic field By generally controls FTE helicity signs, with exceptions occurring when IMF By is small even without the Hall effect. These findings enhance our understanding of global‐scale FTE structures and evolution, aiding interpretation of future satellite data such as SMILE and TRACERS. Plain Language Summary: Flux transfer events (FTEs) at the Earth's dayside magnetopause help transfer energy and plasma from the solar wind into the magnetosphere. Magnetic helicity quantifying FTE twisting features is critical for understanding FTE structure and evolution. Using global MHD simulations with varying IMF orientations, we designed a novel method to calculate FTE magnetic helicity magnitude for the first time and studied its connections to other FTE properties and how these are influenced by IMF orientations. We find that the helicity is positively related to the strength of an FTE's core field and its lifetime. As the IMF rotates from near‐purely southward to near‐purely duskward, the helicity and lifetime have a trend of first increasing but then starting to decrease after a certain degree of IMF rotation. While the IMF By orientation usually governs an FTE's magnetic twist direction, exceptions occur during small IMF By even without the Hall effect. Key Points: Magnetic helicity has positive correlations with both the core field and lifetime of flux transfer events (FTEs)As the interplanetary magnetic field (IMF) turns from near‐purely southward to duskward, the FTE helicity and lifetime have a trend of first increasing but then decreasingThe signs of FTE magnetic helicity are not strictly controlled by IMF By when it is small due to local disturbances near the magnetopause [ABSTRACT FROM AUTHOR]
ISSN:21699380
DOI:10.1029/2025JA034913