Duskward displacement of plasmoids and reconnection in the magnetotail

Abstract Magnetic reconnection in the near-Earth magnetotail is responsible for explosive releases of energy during auroral breakups. This near-tail reconnection was previously assumed to occur around the midnight meridian, where earthward flows were observed. The Geotail spacecraft mission discovered that the reconnection location was displaced toward dusk, based on observations of tailward-moving plasmoids. This dusk preference is possibly caused by the Hall electric field, as was suggested in recent simulations. However, recent spacecraft observations suggested that the reconnection was displaced towards dawn, not dusk, in Mercury’s magnetotail. The reason for the difference between Earth and Mercury remains unclear. This study reinterprets and integrates previous statistical results on the dawn–dusk location of fast plasma flows in the near-Earth magnetotail to address the controversy surrounding the dawn–dusk displacement of reconnection location. Consequently, we confirmed that dusk preference is generally evident for tailward-moving structures but found that location preference is variable between studies of earthward-moving structures. These results indicate that the statistical results of earthward flows are sensitive to event selection criteria. We conclude that the dawn–dusk location of earthward flow is statistically unclear at the time of substorm onset. Accordingly, reconnection does not necessarily occur on the meridian where planetward-moving structures are observed. Tailward-moving plasmoids and reconnection may be predominantly located on the dusk side also in the Mercury’s magnetotail.