Impact Angle Control of Interplanetary Shock Geoeffectiveness

TL;DR

This study uses global MHD simulations to analyze how the impact angle of interplanetary shocks affects Earth's magnetospheric response, revealing that shock orientation significantly influences geomagnetic activity.

Contribution

It demonstrates that shock normal orientation, not just Mach number or upstream conditions, determines the geoeffectiveness of interplanetary shocks.

Findings

Frontal shocks cause stronger geomagnetic responses than inclined shocks.

Shock impact angle influences magnetotail deflection and substorm triggering.

Similar upstream conditions can lead to different geomagnetic effects depending on shock orientation.

Abstract

We use OpenGGCM global MHD simulations to study the nightside magnetospheric, magnetotail, and ionospheric responses to interplanetary (IP) fa st forward shocks. Three cases are presented in this study: two inclined oblique shocks, here after IOS-1 and IOS-2, where the latter has a Mach number twice stronger than the former. Both shocks have impact angles of 30$^o$ in relation to the Sun-Earth line. Lastly, we choose a frontal perpendicular shock, FPS, whose shock normal is along the Sun-Earth line, with the same Mach number as IOS-1. We find that, in the IOS-1 case, due to the north-south asymmetry, the magnetotail is deflected southward, leading to a mild compression. The geomagnetic activity observed in the nightside ionosphere is then weak. On the other hand, in the head-on case, the FPS compresses the magnetotail from both sides symmetrically. This compression triggers a substorm allowing a larger amount of stored energy in the magnetotail to be released to the nightside ionosphere, resulting in stronger geomagnetic activity. By comparing IOS-2 and FPS, we find that, despite the IOS-2 having a larger Mach number, the FPS leads to a larger geomagnetic response in the nightside ionosphere. As a result, we conclude that IP shocks with similar upstream conditions, such as magnetic field, speed, density, and Mach number, can have different geoeffectiveness, depending on their shock normal orientation.