Fig. 7

Schematic diagram of the energetic particle sounding technique. In panel a a spacecraft (purple dot) is located near a scattering boundary. In the plane perpendicular to the magnetic field, the particle detector needs to collect electrons from all directions to get a full perpendicular PSD (PSD_⊥). However, the measured PSD_⊥ is not gyrotropic because electrons outside the boundary can be scattered away by the boundary. Two critical looking directions (ϕ₁ and ϕ₂), corresponding to the gyro-orbits tangential to the boundary (two cyan circles), can be recognized by sharp PSD declines. The two critical directions as well as the particle gyro-radius can be used to calculate the boundary orientation (β_i) and distance (R_i) to the spacecraft. Panel b shows an example of boundary sounding. Using PSD_⊥ data for a certain energy channel, the sounding technique generates a data point (red) at each moment, leading to a series of orientations (β_i−3,…, β_i + 3) and distances (R_i−3,…, R_i+3) throughout the whole traversal. The corresponding data points (red crosses) are obtained based on the known spacecraft location. The boundary geometry can be obtained by connecting these points