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A Deflection of the Location of Earth's Dayside X‐Line: Knee Events and the Maximum Magnetic Shear Model.

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Title: A Deflection of the Location of Earth's Dayside X‐Line: Knee Events and the Maximum Magnetic Shear Model.
Authors: Trattner, K. J.1 (AUTHOR) karlheinz.trattner@lasp.colorado.edu, Petrinec, S. M.2 (AUTHOR), Fuselier, S. A.3,4 (AUTHOR), Burch, J. L.3 (AUTHOR), Ergun, R.1 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. Jun2026, Vol. 131 Issue 6, p1-17. 17p.
Subject Terms: *Geomagnetism, Magnetic reconnection, Magnetopause, Magnetospheric physics, Solar wind
Abstract: Magnetic reconnection is a process that occurs at the boundary between the Earth's magnetic field and the interplanetary magnetic field (IMF) called the magnetopause. Magnetic reconnection allows solar wind plasma to enter Geospace and precipitate along the newly opened geomagnetic field lines into the cusp regions. To fully understand the interaction due to magnetic reconnection between these two disparate plasma regimes, it is important to understand the location along the magnetopause where reconnection occurs. The location of the dayside X‐line at Earth's magnetopause has been well described with the maximum magnetic shear model, which correctly predicts the location within 2 RE (Petrinec et al., 2014, https://doi.org/10.1002/2014ja020209) for 82% (e.g., Trattner, Burch, et al., 2017, https://doi.org/10.1002/2017ja024488). Events where the model fails are not randomly distributed throughout the parameter space, rather they cluster in clearly defined parameter spaces. One of these event groups are so called "Knee" events, wherein one of the two anti‐parallel reconnection regions along the dayside magnetopause includes a sharp bend (kink or "knee") such that this region maps through both the northern and southern hemispheres. This scenario causes a deflection of the entire dayside X‐line, which is controlled by the motion of the anchor point (the transition between the component and the anti‐parallel sections along the X‐lines) to where the last draped IMF line crosses the anti‐parallel reconnection region. Key Points: "Knee" events are classified as an anti‐parallel reconnection region featuring a sharp kink in the X‐line at the magnetopauseThe primary dayside component X‐line that crosses the noon meridian is often deflected during "Knee" eventsThe local magnetic shear difference between the deflected and max shear X‐line locations is small [ABSTRACT FROM AUTHOR]
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Description
Abstract:Magnetic reconnection is a process that occurs at the boundary between the Earth's magnetic field and the interplanetary magnetic field (IMF) called the magnetopause. Magnetic reconnection allows solar wind plasma to enter Geospace and precipitate along the newly opened geomagnetic field lines into the cusp regions. To fully understand the interaction due to magnetic reconnection between these two disparate plasma regimes, it is important to understand the location along the magnetopause where reconnection occurs. The location of the dayside X‐line at Earth's magnetopause has been well described with the maximum magnetic shear model, which correctly predicts the location within 2 RE (Petrinec et al., 2014, https://doi.org/10.1002/2014ja020209) for 82% (e.g., Trattner, Burch, et al., 2017, https://doi.org/10.1002/2017ja024488). Events where the model fails are not randomly distributed throughout the parameter space, rather they cluster in clearly defined parameter spaces. One of these event groups are so called "Knee" events, wherein one of the two anti‐parallel reconnection regions along the dayside magnetopause includes a sharp bend (kink or "knee") such that this region maps through both the northern and southern hemispheres. This scenario causes a deflection of the entire dayside X‐line, which is controlled by the motion of the anchor point (the transition between the component and the anti‐parallel sections along the X‐lines) to where the last draped IMF line crosses the anti‐parallel reconnection region. Key Points: "Knee" events are classified as an anti‐parallel reconnection region featuring a sharp kink in the X‐line at the magnetopauseThe primary dayside component X‐line that crosses the noon meridian is often deflected during "Knee" eventsThe local magnetic shear difference between the deflected and max shear X‐line locations is small [ABSTRACT FROM AUTHOR]
ISSN:21699380
DOI:10.1029/2026JA035167