Interhemispheric Asymmetries in the Ground Magnetic Response to Interplanetary Shocks: The Role of Shock Impact Angle

Abstract

Interplanetary (IP) shocks drive magnetosphere‐ionosphere (MI) current systems that in turn are associated with ground magnetic perturbations. Recent work has shown that IP shock impact angle plays a significant role in controlling the subsequent geomagnetic activity and magnetic perturbations; for example, highly inclined shocks drive asymmetric MI responses due to interhemispherical asymmetric magnetospheric compressions, while almost head‐on shocks drive more symmetric MI responses. However, there are few observations confirming that inclined shocks drive such asymmetries in the high‐latitude ground magnetic response. We use data from a chain of Antarctic magnetometers, combined with magnetically conjugate stations on the west coast of Greenland, to test these model predictions (Oliveira & Raeder, 2015, https://doi.org/10.1002/2015JA021147; Oliveira, 2017, https://doi.org/10.1007/s13538-016-0472-x). We calculate the time derivative of the magnetic field ( ∂Bfalse/∂t) in each hemisphere separately. Next, we examine the ratio of Northern to Southern Hemisphere ∂Bfalse/∂t intensities and the time differences between the maximum ∂Bfalse/∂t immediately following the impact of IP shocks. We order these results according to shock impact angles obtained from a recently published database with over 500 events and discuss how shock impact angles affect north‐south hemisphere asymmetries in the ground magnetic response. We find that the hemisphere the shock strikes first usually has (1) the first response in ∂Bfalse/∂t and (2) the most intense response in ∂Bfalse/∂t. Additionally, we show that highly inclined shocks can generate high‐latitude ground magnetic responses that differ significantly from predictions based on models that assume symmetric driving conditions.

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

“…Numerical simulations show that shocks with small impact angles usually lead the MI system to faster and stronger responses in comparison to shocks with large impact angles (Guo et al, 2005;Oliveira & Raeder, 2014;Samsonov et al, 2015). These results have been confirmed by many experimental studies as well (Oliveira & Raeder, 2015;Oliveira, Arel, et al, 2018;Rudd et al, 2019;Shi et al, 2019;Takeuchi et al, 2002;Wang et al, 2006;Xu et al, 2020). Oliveira and Raeder (2014) studied the effects caused by shock orientations on the Earth's magnetosphere through numerical simulations.…”

“…With the shock normal vector given by in geocentric solar ecliptic (GSE) coordinates and , , ) in solar magnetic (SM) coordinates, we compute the following: where is the angle the shock normal performs with the Sun‐Earth line, is the angle the normal vector performs with the y axis in the yz plane, and is the angle between the shock normal vector and the SM xy plane (Xu et al, 2020). The SM angle ( ) is slightly more inclined than the GSE angle ( …”

“…where 𝜃 x n is the angle the shock normal performs with the Sun-Earth line, 𝜑 𝑦 n is the angle the normal vector performs with the y axis in the yz plane, and 𝜃 d n is the angle between the shock normal vector and the SM xy plane (Xu et al, 2020). The SM angle (𝜃 d n ) is slightly more inclined than the GSE angle (𝜃 x n ) because of the Earth's dipole tilt (Hapgood, 1992).…”

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

“…All panels in Figure 2 indicate positive jumps in the IMF and plasma parameter magnitudes (velocity, number density, and temperature), which are fast forward shock signatures (Priest, 1981;Oliveira, 2017). Additionally, the HIS shows more variability in the y and z components of the IMF and solar wind speed due to its normal inclinations with respect to the xy and xz planes (Xu et al, 2020). Table 2 also shows that the average southward component of the IMF (B z2 ) was slightly more negative in the downstream region of the HIS (-14 nT) than in the downstream region of the NFS (-12 nT).…”

“…Consequently, when shocks hit Earth, different levels of geomagnetic activity may follow, including ground sudden impulses, field-aligned current and auroral intensifications, and wave response in the magnetosphere-ionosphere system (e.g., see review by Oliveira & Samsonov, 2018). Observations and simulations show that geomagnetic activity following the impact of an inclined shock is usually weaker and slower than the geomagnetic activity following a frontal shock as a result of asymmetric magnetospheric compressions (Takeuchi et al, 2002;Guo et al, 2005;Grib & Pushkar, 2006;Wang et al, 2006;Samsonov, 2011;Samsonov et al, 2015;Selvakumaran et al, 2017;Shi et al, 2019;Xu et al, 2020). Numerical simulations conducted by Oliveira and Raeder (2014) showed that a frontal shock triggered intense substorm activity, whereas an inclined shock triggered moderate substorm activity, even though the inclined shock was stronger.…”