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Multiple Reconnection X‐Lines at the Earth's Magnetopause in the Presence of Magnetosheath Flow.

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Title: Multiple Reconnection X‐Lines at the Earth's Magnetopause in the Presence of Magnetosheath Flow.
Authors: Fuselier, S. A.1,2 (AUTHOR) stephen.fuselier@swri.org, Trattner, K. J.3 (AUTHOR), Petrinec, S. M.4 (AUTHOR), Vines, S. K.1 (AUTHOR), LLera, K.1 (AUTHOR), Aunai, N.5 (AUTHOR), Michotte de Welle, B.6 (AUTHOR), Ghisalberti, A.5 (AUTHOR), Burch, J. L.1 (AUTHOR), Gershman, D. J.6 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. Mar2026, Vol. 131 Issue 3, p1-13. 13p.
Subject Terms: Magnetic reconnection, Magnetopause, Plasma flow
Company/Entity: Magnetospheric Multiscale Mission (U.S.) , United States. National Aeronautics & Space Administration
Abstract: Magnetic reconnection occurs continuously along long X‐lines at the Earth's magnetopause. The maximum magnetic shear model predicts the locations of these long X‐lines for a wide range of upstream solar wind conditions. One of the more perplexing observational results is that these X‐lines appear to be stationary, even in the presence of significant magnetosheath plasma bulk flow. A scenario is developed whereby an X‐line forms at the location predicted by the maximum magnetic shear model, but then immediately propagates with the magnetosheath plasma bulk flow away from this location. Before this X‐line can travel far, a new X‐line forms at the location of the original X‐line. If the X‐line reformation cadence is high enough, then these multiple, propagating X‐lines could appear as a single, quasi‐stationary X‐line. One observational test of this scenario is that a spacecraft crossing the magnetopause poleward of the original X‐line location would always observe multiple X‐lines. Magnetospheric Multiscale observations are used to perform this observational test. Results show that there are multiple X‐lines near the predicted location of the X‐line and therefore, this scenario may have merit. Plain Language Summary: Magnetic reconnection, a key process at Earth's magnetopause, allows energy and particles from the solar wind to enter Earth's space environment. It occurs along long "x‐lines," where magnetic fields break and reconnect. A model called the "maximum magnetic shear model" predicts where these x‐lines should form. However, observations show that x‐lines appear stationary, even though the surrounding plasma flows significantly. To explain this, researchers propose that an x‐line forms at the predicted location but quickly moves with the plasma flow. Before it drifts far, a new x‐line reforms in the original location. If this cycle of breaking and reforming happens rapidly, it would appear as a single stationary x‐line even though multiple x‐lines are forming and moving sequentially. NASA's Magnetospheric Multiscale (MMS) mission was used to test this idea. The hypothesis predicts that a spacecraft crossing near the boundary would detect multiple x‐lines clustered near the predicted location. Observations from MMS confirmed this: several x‐lines were found close together, supporting the proposed scenario. This suggests that what appears to be a stationary x‐line may instead be a rapid sequence of x‐lines constantly forming and moving, providing new insight into this fundamental plasma process. Key Points: A scenario is proposed whereby x‐lines at the magnetopause continuously form and propagate away from their initial locationIn this scenario, multiple x‐lines should always be observed for magnetopause crossing poleward of the initial x‐line locationMMS observations show that multiple x‐lines are observed nearly all the time and therefore this scenario has merit [ABSTRACT FROM AUTHOR]
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Abstract:Magnetic reconnection occurs continuously along long X‐lines at the Earth's magnetopause. The maximum magnetic shear model predicts the locations of these long X‐lines for a wide range of upstream solar wind conditions. One of the more perplexing observational results is that these X‐lines appear to be stationary, even in the presence of significant magnetosheath plasma bulk flow. A scenario is developed whereby an X‐line forms at the location predicted by the maximum magnetic shear model, but then immediately propagates with the magnetosheath plasma bulk flow away from this location. Before this X‐line can travel far, a new X‐line forms at the location of the original X‐line. If the X‐line reformation cadence is high enough, then these multiple, propagating X‐lines could appear as a single, quasi‐stationary X‐line. One observational test of this scenario is that a spacecraft crossing the magnetopause poleward of the original X‐line location would always observe multiple X‐lines. Magnetospheric Multiscale observations are used to perform this observational test. Results show that there are multiple X‐lines near the predicted location of the X‐line and therefore, this scenario may have merit. Plain Language Summary: Magnetic reconnection, a key process at Earth's magnetopause, allows energy and particles from the solar wind to enter Earth's space environment. It occurs along long "x‐lines," where magnetic fields break and reconnect. A model called the "maximum magnetic shear model" predicts where these x‐lines should form. However, observations show that x‐lines appear stationary, even though the surrounding plasma flows significantly. To explain this, researchers propose that an x‐line forms at the predicted location but quickly moves with the plasma flow. Before it drifts far, a new x‐line reforms in the original location. If this cycle of breaking and reforming happens rapidly, it would appear as a single stationary x‐line even though multiple x‐lines are forming and moving sequentially. NASA's Magnetospheric Multiscale (MMS) mission was used to test this idea. The hypothesis predicts that a spacecraft crossing near the boundary would detect multiple x‐lines clustered near the predicted location. Observations from MMS confirmed this: several x‐lines were found close together, supporting the proposed scenario. This suggests that what appears to be a stationary x‐line may instead be a rapid sequence of x‐lines constantly forming and moving, providing new insight into this fundamental plasma process. Key Points: A scenario is proposed whereby x‐lines at the magnetopause continuously form and propagate away from their initial locationIn this scenario, multiple x‐lines should always be observed for magnetopause crossing poleward of the initial x‐line locationMMS observations show that multiple x‐lines are observed nearly all the time and therefore this scenario has merit [ABSTRACT FROM AUTHOR]
ISSN:21699380
DOI:10.1029/2025JA034839