Determination of shock parameters for the very fast interplanetary shock on 29 October 2003

We study a very fast interplanetary shock (IP shock) event observed on 29 October 2003 based on the Geotail particle and field measurements in the solar wind. During this event the intensity of high-energy solar energetic particles (greater than several to several tens of MeV) was quite high, causing a serious background problem for plasma particle measurements on Geotail as well as. on the other spacecraft. The magnetic/electric field measurements and the plasma wave measurement aboard Geotail, on the other hand, were free from such a background problem and provided a reliable estimate for the local plasma parameters including the plasma density. From these measurements, our best estimation for the local shock velocity is ∼2000 km/s in the observer's rest frame or ∼1400 km/s in the upstream plasma rest frame. The corresponding Alfvén Mach number is ∼12. It is found that the timing analysis of the shock arrivals at ACE and Geotail gives a shock velocity significantly lower than the above value. We argue that this difference is due to the shock surface rippling by 15-20 deg. We also comment that this IP shock had a property of "cosmic-ray-mediated" shock, namely a shock having a spatial structure affected by pressures exerted by nonthermal particles accelerated by the shock itself.