Fig. 3. Electron velocity distribution function (Event 1).

a Gyro-averaged electron velocity distribution function (28.3–2112 eV) with the cyclotron resonance velocity (V_res) shown as a red dashed line. b–d histograms of electron phase space density (PSD) in $\zeta$ direction at 3 energy bins (fan-shaped area surrounded by a gray curve in Fig. 3a). The nongyrotropy significantly exceeded 2σ error bars (see Methods, subsection Electron and ion measurements by FPI) around V_res, while the electrons did not exhibit clear nongyrotropy around the pitch angle of about 130° below about 550 eV. This indicates confinement of this electron nongyrotropy around V_res. Electron data from 9 temporal bins (270 ms) around 15:59:19.644 UT (Fig. 4) from each spacecraft were used (see Methods, subsection Electron and ion measurements by FPI). The fan-shaped area surrounded by a gray curve corresponds to the pitch angle and energy ranges for the calculation of the resonant current (J_res) for Event 1.