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

Abstract

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.

Impact Angle Control of Local Intense dB/dt Variations During Shock‐Induced Substorms

“…By carefully analyzing two isolated substorms triggered by similarly strong shocks with very different inclinations, we found that the general geomagnetic activity represented by geomagnetic indices, energetic particle injection, auroral poleward expansion, westward auroral electrojet current, ground dB/dt variations, and ULF wave activity following the NFS were overall higher than the same effects following the HIS. These findings are supported by many experimental (Oliveira et al, 2016(Oliveira et al, , 2020Oliveira & Raeder, 2015;Shi et al, 2019;Takeuchi et al, 2002;Wang et al, 2006) and simulation (Guo et al, 2005;Oliveira & Raeder, 2014;Samsonov, 2011;Samsonov et al, 2015;Selvakumaran et al, 2017) works in the literature.…”

“…Although the results of wave activity on 5 April 2010 seem to agree with the results reported by Ngwira et al (2018), the results observed during the 20 September 2015 are quite different. The later and weaker wave activity during the latter event are consistent with results showing that inclined shocks trigger less intense substorm activity (Oliveira & Raeder, 2014 and ULF wave activity (Oliveira et al, 2020) in comparison to shocks with small impact angles. Ultra-low frequency wave analyses with White Horse (WHIT) data for the nearly frontal shock (left column) and with Fort Simpson (FSIM) data for the highly inclined shock (right column).…”

Impact angle control of local intense d$B$/d$t$ variations during shock-induced substorms

“…Although the results of wave activity on 5 April 2010 seem to agree with the results reported by Ngwira et al (2018), the results observed during the 20 September 2015 are quite different. The later and weaker wave activity during the latter event are consistent with results showing that inclined shocks trigger less intense substorm activity (Oliveira & Raeder, 2014 and ULF wave activity (Oliveira et al, 2020) in comparison to shocks with small impact angles.…”

“…These periods are also marked on the bottom panel by the regions with enhanced ion energy flux represented by the first narrow region (single pulse) and the following two narrow regions (double pulse). Later, at 0640:21 UT, MMS definitely crossed the magnetopause boundary entering the magnetosheath, as is clearly shown by the polarity inversion of B z , increase in plasma density and decrease in plasma temperature, and enhanced ion energy fluxes, which are well-known magnetopause crossing signatures (e.g., Dong et al, 2018;Oliveira et al, 2020). Minimum variation analysis with the four MMS spacecraft yields θ xn = 137.67 • ± 2.2 • for the first structure, and θ xn = 143.74 • ± 0.53 • for the other two structures.…”

“…We use the SuperMAG dataset because the SuperMAG indices are usually more likely to capture geomagnetic activity triggered by shocks due to their larger and more effective geographic coverage (e.g. Oliveira et al, 2016;Oliveira & Raeder, 2015;Oliveira & Samsonov, 2018;Rudd et al, 2019;Shi et al, 2019;Oliveira et al, 2020).…”