Density Derivation Using Controlled Spacecraft Potential in Earth's Magnetosheath and Multi‐Scale Fluctuation Analysis

In situ measurements from the Magnetospheric Multiscale (MMS) mission are used to estimate electron density from spacecraft potential and investigate compressive turbulence in the Earth's magnetosheath. During the MMS Solar Wind Turbulence Campaign in February 2019, the four MMS spacecraft were arranged in a logarithmic line constellation enabling the study of measurements from multiple spacecraft at varying distances. We estimate the electron density from spacecraft potential for a time interval in which the ion emitters actively control the potential. The derived electron density data product has a higher temporal resolution than the plasma instruments, enabling the examination of fluctuation for scales down to the sub‐ion range. The inter‐spacecraft separations range from 132 to 916 km; this corresponds to scales of 3.5–24.1 ion inertial lengths. As an example, the derived density and magnetic field data are used to study fluctuations in the magnetosheath through time lags on a single spacecraft and spatial lags between pairs of spacecraft over almost one decade in scale. The results show an increase in anisotropy as the scale decreases, similar for the density and the magnetic field. This suggests different drivers in the strongly compressive magnetosheath and the weakly compressive solar wind. Compressive structures such as magnetic holes, compressive vortices and jets might play key roles.