Magnetospheric responses to the passage of the interplanetary shock on 24 November 2008

The passage of an interplanetary (IP) shock was detected by Wind, ACE, Geotail, and THEMIS-B in the solar wind on 24 November 2008. From the propagation time of the IP shock at the spacecraft, it is expected that the IP shock front is aligned with the Parker spiral and strikes the postnoon dayside magnetopause first. Using multipoint observations of the sudden commencement (SC) at THEMIS probes, GOES 11, and ETS in the dayside magnetosphere, we confirmed that the magnetospheric response to the IP shock starts earlier in the postnoon sector than in the prenoon sector. We found that the estimated normal direction of the SC front is nearly aligned with the estimated IP shock normal. We also found that the SC front normal speed is much slower than the fast mode speed and is about 22-56% of the IP shock speed traveling in the solar wind. Thus, we suggest that the major field changes of the SC in the dayside magnetosphere are not due to the magnetic flux carried by hydromagnetic waves but to the increased solar wind dynamic pressure behind the shock front sweeping the magnetopause. The SC event appears as a step-like increase in the H component at the low-latitude Bohyun station and a negative-then-positive variation in the H component at the high-latitude Chokurdakh (CHD) station in the morning sector. During the negative perturbation at CHD, the SuperDARN Hokkaido radar detected a downward motion in the ionosphere, implying westward electric field enhancement. Using the THEMIS electric field data, it is confirmed that the westward electric field corresponds to the inward plasma motion in the dayside magnetosphere due to the magnetospheric compression.