Properties and origin of subproton‐scale magnetic holes in the terrestrial plasma sheet

Electron‐scale magnetic depressions in the terrestrial plasma sheet are studied using Cluster multispacecraft data. The structures, which have an observed duration of ~5–10 s, are approximately 200–300 km wide in the direction of propagation, and they show an average reduction in the background magnetic field of 10–20%. A majority of the events are also associated with an increase in the high‐energy high pitch angle electron flux, which indicates that the depressions are presumably generated by electrons with relatively high velocity perpendicular to the background magnetic field. Differences in the recorded electron spectra in the four spacecraft indicates a possible nongyrotropic structure. Multispacecraft measurements show that a subset of events are cylindrical, elongated along the magnetic field, and with a field‐parallel scale size of at a minimum 500 km. Other events seem to be better described as electron‐scale sheets, about 200–300 km thick. We find that no single formation mechanism can explain this variety of events observed. Instead, several processes may be operating in the plasma sheet, giving rise to similar magnetic field structures in the single‐spacecraft data, but with different 3‐D structuring. The cylindrical structures have several traits that are in agreement with the electron vortex magnetic holes observed in 2‐D particle‐in‐cell simulations of turbulent relaxation, whereas the sheets, which show nearly identical signatures in the multispacecraft data, are better explained by propagating electron solitary waves.