Clustering Wind data at 1 AU to contextualize magnetic reconnection in the solar wind

¹ KU Leuven, Department of Mathematics, Celestijnenlaan 200b, Leuven 3001, Belgium
² National Institute for Astrophysics, Astrophysical Observatory of Turin, Via Osservatorio 20, Pino Torinese, 10025 Turin, Italy
³ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA
⁴ Institut für Theoretische Physik, Ruhr-Universität Bochum, Universitätstraße 150, 44801 Bochum, Germany
⁵ Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium
⁶ Dipartimento di Fisica, Università di Torino, via Pietro Giuria 1, 10125 Turin, TO, Italy

^⋆ Corresponding author: francesco.carella@kuleuven.be

Received: 25 February 2025
Accepted: 25 July 2025

Abstract

Context. Magnetic reconnection events are frequently observed in the solar wind. Understanding the patterns and structures within the solar wind is crucial to put observed magnetic reconnection events into context, since their occurrence rate and properties are likely influenced by solar wind conditions.

Aims. We employed unsupervised learning techniques such as self-organizing maps (SOM) and K-Means to cluster and interpret solar wind data at 1 AU for an improved understanding of the conditions that lead to magnetic reconnection in the solar wind.

Methods. We collected magnetic field data and proton density, proton temperature, and solar wind speed measurements taken by the Wind spacecraft. After preprocessing the data, we trained a SOM to visualize the high-dimensional data in a lower-dimensional space and applied K-Means clustering to identify distinct clusters within the solar wind data. We then compare the results with the Xu, F., & Borovsky, J. E. (2015, J. Geophys. Res. Space Phys., 120, 70) classification of the solar wind.

Results. Our analysis revealed that the reconnection events are distributed across five different clusters: (a) slow solar wind, (b) compressed slow wind, (c) highly Alfvénic wind, (d) compressed fast wind, and (e) ejecta. Compressed slow and fast wind and ejecta are clusters associated with solar wind transients such as stream interaction regions and interplanetary coronal mass ejections. The majority of the reconnection events are associated with the slow solar wind, followed by the highly Alfvénic wind, compressed slow wind, and compressed fast wind, and a small fraction of the reconnection events are associated with ejecta.

Conclusions. Unsupervised learning approaches with SOM and K-Means lead to physically interpretable solar wind clusters based on their transients and allow for the contextualization of magnetic reconnection exhausts’ occurrence in the solar wind.