ULF Pulsations as a Driver of Relativistic Electrons: Pros and Cons

Ultralow frequency (ULF) waves in the Pc5 band have been suggested as a possible intermediary transferring energy from high‑speed streams of the solar wind to magnetospheric electrons. Although ULF waves are not the only mechanism of accelerating electrons up to relativistic energies, nonetheless they are an essential element of the electron energization process, though their role has not been finally established yet. Among observational facts regarding the interrelationships between Pc5 wave activity and electron dynamics, we discuss the following pro and con factors related to the ULF‑associated energization mechanisms: The correlation of electron fluxes at the geosta‑tionary orbit and the Pc5 wave power; The correspondence between the azimuthal phase velocities of toroidal and poloidal Pc5 waves and the electron magnetic drift; The correspondence between the latitudinal structure of Pc5 waves and the outer radiation belt. Consideration of these facts does not allow one to unambiguously resolve the issues concerning the role of ULF waves in the magnetospheric electron energization. We suggest that the acceleration of electrons by ULF disturbances may occur not in a regime of “geoserfotron” with Pc5 waves (match of the azimuthal velocities of waves and drifting electrons), but rather in a regime of “geosynchrotron” with transit‑time acceleration by substorm‑related Pi3 pulsations.