Evolution of Three Geoeffective Shock-CME pairs in September 2011

Three sizable geomagnetic storms were recorded in September 2011. The intensity of geomagnetic storms (Dstmin: minimum Dst) are -69, -70, -101 nT and the storms' onset time are September 9, 17, and 26, respectively. A sequence of coronal mass ejections (CMEs) correspond causing these three geomagnetic storms. The severe geomagnetic storm (Dstmin < -100 nT) on 26 September was caused by a couple of CMEs erupted on 24 September. Wind spacecraft detected an interplanetary (IP) shock at ~11:18 UT on 26 September but no magnetic cloud was recorded behind the IP shock. A severe geomagnetic storm was recorded ~6 hours after the IP shock passed through the Wind spacecraft. Geomagnetic index (Dst) dropped to -101 nT which was due to the z-component of interplanetary magnetic field (Bz) dropped to ~ -20 nT. Both September 9th and 17th IP shocks have followed by a magnetic hole with a very sharp change in both magnetic field and density. Inside the magnetic holes, both solar wind velocity and temperature are almost constant, and the peak of density and dip of magnetic field occurred near the centre of the magnetic field hole. Peak densities were close to ~94, ~60 cm-3 near the centre of the hole on Sept. 09, 17, respectively. A global, three-dimensional (3-D) magnetohydrodynamic (MHD) numerical model with inputs based on actual solar observations (e.g., velocity of the CME) is used to simulate the responses of the 3-D heliosphere. These velocity pulses are deduced from STEREO-A which are used to minic the initiation of the observed 15 CMEs at lower boundary (2.5 Rs) to investigate the CME evolution from the Sun to the Earth during September 03-30, 2011.Simulated background solar wind parameters (velocity, density, magnetic field, and temperature) are matched well with 1 AU in-situ measurement from Wind spacecraft. In summary, we have successfully simulated these CMEs' evolution and the IP shocks arrival time at 1 AU by comparison with Wind measurement.It is found that background solar wind is an important factor on the propagation of IP shocks and CMEs. The simulation results are also useful for explaining "How were the magnetic holes formed behind the IP shocks?" *Work of CCW was supported by ONR 6.1 program