Fig. 8

Different types of the URA acceleration depending on the interrelation between \( v_{\parallel } \) and \( V_{g} \). a Trajectory of the most successful electron in case 3 (5.7 MeV-86°) is again taken as an example. The electron travels slower than the chorus wave propagation (\( v_{\parallel } \) < \( V_{g} \)). The electron is accelerated by URA when it is overtaken by the chorus wave approaching from the behind (overtaken URA). b Trajectory in case 3-1 (5.7 MeV-62°). The electron travels with the velocity \( v_{\parallel } \) > \( V_{g} \), and is accelerated by URA when it catches up with the chorus wave going ahead (catching up URA). c Trajectory in case 3-2 (5.7 MeV-74°). The electron travels with \( v_{\parallel } \approx V_{g} \) and is neither overtaken nor catching up with the chorus wave, thus the efficient URA acceleration does not occur