Interplanetary Shock Impact Angles Control Magnetospheric ULF Wave Activity: Wave Amplitude, Frequency, and Power Spectra

We present the first clear observational connection between interplanetary shock impact angles and magnetospheric ultralow frequency (ULF) wave activity. We perform a comparative study of two solar wind shocks with relatively similar strengths, but one being nearly frontal and, the other, highly inclined. We utilize multipoint observations with magnetometers based in space and on the ground for comparisons. For satellites and ground stations occupying similar local time positions, we find that the ULF waves have larger amplitudes and tend to be more narrowband in the case of the nearly head-on impact, confirming previous simulation results. Additionally, we provide evidence that, due to their symmetric compression nature, nearly frontal shocks can excite only odd mode waves, while asymmetric compressions caused by inclined shocks can excite both odd and even mode waves. These results suggest that shock impact angles play crucial roles in mediating ULF wave-particle interactions in the inner magnetosphere. Plain Language Summary The interaction of solar perturbations with the Earth's magnetic field is of paramount importance in space weather investigations. The subsequent response is almost always characterized by magnetic field disturbances measured in space and on the ground, which in turn can modulate energetic particle populations that damage satellite electronics and cause electric current surges in large-scale power transmission lines. These disturbances are also manifested as geomagnetic pulsations which in part are responsible for the distribution of energy in the near-Earth space environment. In this work, we connect, for the first time, observational properties of these pulsations or waves with the impact angle of the perturbation. We find that nearly head-on impacts are associated with stronger wave response in comparison to inclined impacts. These results show that space weather forecasters should take the perturbation impact angle into account when predicting geomagnetic effects caused by solar perturbations.