Role of symmetry in the geo-effectiveness of interplanetary shocks

In this study we investigate the interaction of interplanetary (IP) shocks with Earth's magnetosphere using OpenGGCM global simulations. In particular we are interested in the effects of IP shocks on the tail and the ionosphere. We find that the IP shock angle, that is, the angle between the shock normal and the sun-Earth line, plays a major role determining the geo-effectiveness of the IP shock. For most IP shocks this angle is large. In such cases, the shock hits one side of the magnetosphere first, pushing the tail either sideways or in the north or south direction. Waves generated by the shock - magnetosphere interaction travel through the magnetosphere without much effect. The response of the magnetosphere is thus rather mild, because the shock does not effectively compress the magnetosphere. In cases where the shock angle is small, the magnetosphere response is much stronger. Because of the symmetry, shock-induced waves converge in the tail, producing a strong compression. The compression, in turn, triggers reconnection, and possibly a substorm, with large effects on the ionosphere. We thus conclude that the geo-effectiveness of interplanetary shocks is not just a function of their strength, i.e., Mach number, but also, and possibly much more so, a function of their impact angle.