Electron-scale Magnetic Peaks Upstream of the Terrestrial Bow Shock Observed by the Magnetospheric Multiscale Mission

The solar wind has rich wave activity and various magnetic structures. Here we report on a new type of magnetic structure in the solar wind using the unprecedented high temporal resolution data from the Magnetospheric Multiscale Mission. We find that a train of magnetic peaks with a size less than 1 ion inertial length exists upstream of the terrestrial bow shock. The electron number density and the perpendicular electron temperature have a slight decrease inside the magnetic peaks, leading to a decrease of the electron thermal pressure in the structure. These structures are pressure-balanced, and they are approximately stationary in the ambient electron flow. These electron-scale magnetic peaks are identified as magnetic bottle-like, and their cross sections are roughly circular. The electron velocity has a bipolar feature relative to the ambient flow in the cross section, indicating the existence of an electron vortex. The current density is mainly contributed by electrons. The peaks occur in a marginally mirror-stable environment; thus they are not locally generated by mirror instabilities. We suggest that the origin of the electron vortex might help to shed light on the formation of electron-scale magnetic peaks in the solar wind.