Global Structure of Magnetotail Reconnection Revealed by Mining Space Magnetometer Data

Reconnection in the magnetotail occurs along so-called X-lines, where magnetic field lines tear and detach from plasma on microscopic spatial scales (comparable to particle gyroradii). In 2017-2020, the Magnetospheric MultiScale (MMS) mission detected X-lines in the magnetotail enabling their investigation on local scales. However, the global structure and evolution of these X-lines, critical for understanding their formation and total energy conversion mechanisms, remained virtually unknown because of the intrinsically local nature of observations and the extreme sparsity of concurrent data. Here, we show that mining a multi-mission archive of space magnetometer data collected over the last 26 yr and then fitting a magnetic field representation modeled using flexible basis-functions faithfully reconstructs the global pattern of X-lines; 24 of the 26 modeled X-lines match (B-z = 0 isocontours are within similar to 2 Earth radii or R-E) or nearly match (B-z = 2 nT isocontours are within similar to 2R(E)) the locations of the MMS encountered reconnection sites. The obtained global reconnection picture is considered in the context of substorm activity, including conventional substorms and more complex events. Plain Language Summary Magnetic reconnection is a fundamental process in plasmas which couples microscopic scales (similar to electron to proton gyroradii) to explosive macroscopic phenomena many orders of magnitude larger, such as solar flares and geomagnetic storms/substorms. Reconnection forms along "X-lines", rifts where oppositely directed magnetic field lines are forced together. In the Earth's magnetosphere, reconnection has been observed by satellites at isolated locations; however, the large-scale structure of X-lines and their time evolution remains unknown because of the rarity and local nature of observations. Here, ground based measurements of geomagnetic activity and solar wind measurements are used to data-mine 26 yr of magnetometer data from 22 Earth-orbiting satellites, which are then utilized to reconstruct the global magnetic field associated with X-lines in Earth's magnetosphere. We show that these reconstructions pinpoint the reconnection locations by verifying their consistency with direct spacecraft observations.