Impact Angle Control of Interplanetary Shock Geoeffectiveness

Wednesday, 17 December 2014
Denny Oliveira and Joachim Raeder, Space Science Ctr, Durham, NH, United States
We use OpenGGCM global MHD simulations to study the nightside magnetospheric/ magnetotail/ ionospheric responses to interplanetary (IP) fast foward shocks. Three cases are presented in this study: two inclined oblique shocks, hereafter IOS-1 and IOS-2, where the latter has a Mach number twice stronger than the former. Both shocks have impact angles of 30o in relation to the Sun-Earth line. Lastly, we choose a frontal perpendicular shock, FPS, whose shock normal is along th 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 on 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 a larger geomagnetic activity there. By comparing IOS-2 and FPS, we find that, despite the IOS-2 having a larger Mach number, the FPS leads to larger geomagnetic responses in the ionosphere nightside. As a result, we conclude that IP shocks with similar upstream conditions, such as magnetic field, speed, density, and even Mach number, can be differently geoeffective, depending on their shock normal orientation.