Pitch angle evolutions of oxygen ions driven by storm time ULF poloidal standing waves

We present the first systematic observational study on the pitch angle evolutions of O⁺ ions associated with ULF Pc5 poloidal standing waves excited during geomagnetic storms. The O⁺ ion measurements are made on board the CLUSTER satellites with the Composition Distribution Function (CODIF) instrument, which covers energies from 1 to 40 keV, a low-energy portion of the ring current. We find that the nature of the ion flux oscillation strongly depends on the magnetic latitude of observation. Near the magnetic equator, the flux oscillation appears only around 0° and 180° pitch angles with no phase delay, which can result from wave-particle interactions in a fundamental mode standing wave with a strong poloidal component. Away from the equator, however, the flux oscillation appears in a wide range of pitch angles with strong pitch angle dispersion that reverses sign from the Southern Hemisphere to the Northern Hemisphere. The latitude dependence of the dispersion signature is explained by combining the ion energy modulation near the equator and the time of flight effect of ion bounce motion. The analysis technique shown in this study can be used to diagnose the field line mode structure of ULF waves.