Magnetic Reconnection Ion Outflows Observed at the Dayside Magnetopause: Observation‐Model Comparison and Influence of Solar Wind Structures

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Abstract
Magnetic reconnection is a fundamental process in magnetized plasmas that accelerates charged particles at the expense of magnetic energy. This study examines 122 dayside magnetopause crossing events characterized by accelerated ion plasma flows consistent with magnetic reconnection. We use in situ observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS, 73 events) and Magnetospheric Multiscale (MMS, 49 events) missions, covering various positions near the magnetic equator. The observed ion outflow directions are compared with those predicted by the reconnection X‐line model proposed by Gonzalez and Mozer (1974, https://doi.org/10.1029/ja079i028p04186 ) and implemented by Sibeck and Lin (2011, https://doi.org/10.1029/2011ja016560 ). The model's performance is evaluated using an angular aperture criterion, whereby the observed jet direction must fall within a cone centered on the direction locally perpendicular to the X‐line. The model predictions were consistent with the observations in 63% of the THEMIS and 57.1% of the MMS events. When the aperture angle is reduced to , the prediction accuracy decreases to 53% and 43%, respectively. Most events occur during southward interplanetary magnetic field conditions, which favor magnetopause reconnection. Solar wind flow speeds exceeding 500 km/s indicate a potential interaction between solar wind structures and the reconnection events under investigation. We also conclude that most of the studied events occurred during High Speed Streams (HSS), 47% (THEMIS) and 82% (MMS), as compared with their occurrence during Interplanetary Coronal Mass Ejections (ICMEs), most probably because an HSS passage typically lasts much longer than that of an ICME.

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