Fig. 2

Electron distributions on the electron separatrix. a–c Electron velocity distribution functions (eVDFs) in the plane perpendicular to the field lines. The locations to get the eVDFs are marked in Fig. 1a, b by small white rectangles. d electron spectra within the pitch-angles of [80°, 100°], calculated at the white vertical line in Fig. 1a. The red inverted triangle marks the position of the peak D _ng. The electron spectra show that the maximum perpendicular energy drops sharply at the enhanced D _ng region, implying that the enhanced D _ng region marks the boundaries for the energetic electrons accelerated in the diffusion region. e Sketch map for the generation of nongyrotropy due to the finite gyroradius effect. The black circles with arrows present low-energy electrons. The red circles with arrows present accelerated electrons. The thick purple line is the boundary of the accelerated electrons, which presents the electron separatrix. The right plot is the relevant eVDF calculated on the electron separatrix in the region represented by the green box stretched across the boundary. The gray population in the eVDF is composed of low-energy electrons, while the pink nongyrotropic population is composed of the accelerated electrons