Fast Acceleration of Inner Magnetospheric Hydrogen and Oxygen Ions by Shock Induced ULF Waves

The interaction between interplanetary shocks and the Earth's magnetosphere manifests many important space physics phenomena including particle acceleration. We investigated the response of the inner magnetospheric hydrogen and oxygen ions to a strong interplanetary shock impinging on the Earth's magnetosphere. Both hydrogen and oxygen ions are found to be heated/accelerated significantly with their temperature enhanced by a factor of two and three immediately after ∼1 min and ∼12 min of the shock arrival. The accelerated hydrogen ions are found to be exhibiting energy dispersion signatures both in the field-aligned (0^o) and anti-field-aligned (180^o) direction immediately after the interplanetary shock impact. Tracing back the dispersed ion signatures, the acceleration source region can be estimated at the magnetic equator region. It is found that the energy spectrum from 10 ev to ∼ 40 keV are highly correlated with the cross product of observed ULF wave electric and magnetic field (V=(E×B)/B²), which indicated that both cold plasmaspheric plasma and hot thermal ions (10eV to ∼ 40 keV) are accelerated and decelerated with the various phases of ULF wave electric field. Then, we demonstrated that ion acceleration due to the interplanetary shock compression on the Earth's magnetic field is rather limited, whereas the major part of contribution to acceleration comes from the electric field carried by ULF waves via drift-bounce resonance for both the hydrogen and oxygen ions. The integrated hydrogen and oxygen ion flux with the poloidal mode ULF waves are highly correlated (above 0.9) whereas the correlation with the toroidal mode ULF waves is negligible, implying that the poloidal mode ULF waves are much more efficient to accelerate hydrogen and oxygen ions in the inner magnetosphere than the toroidal mode ULF waves. The duration of high coherence for oxygen ions with the poloidal mode ULF wave is longer than that for hydrogen ions, indicating oxygen ions can be heated/accelerated more efficiently by the poloidal mode ULF wave induced by the interplanetary shock.