Dynamic Motion of the Bow Shock and the Magnetopause and the Magnetospheric Response - THEMIS Observations

We present an observational study of the dynamic motion of the bow shock and the magnetopause and suggest that the dynamic motion is due to the interaction of an interplanetary shock with the Earth's bow shock. THEMIS B spacecraft crossed the magnetopause, a discontinuity and the bow shock successively in 5 minutes during its outbound journey on July 10, 2007. Following THEMIS B, THEMIS C, D, E and A consecutively crossed the magnetopause and the discontinuity but not the bow shock. Timing analysis shows that the magnetopause and the discontinuity were moving earthward with speeds of ~47 km/s and ~90 km/s respectively. There is a trend that the discontinuity decelerates as it propagates towards the magnetopause. We suggest that the dynamic motion and the discontinuity are results of the interaction of a weak (MA=1.2) interplanetary shock with the Earth's bow shock. After the interaction, the transmitted interplanetary shock took the form of a discontinuity where total magnetic field and density increase and the temperature decreases. The rotation of the magnetic field across this discontinuity was similar to that of the interplanetary shock. The expected fast shock ahead of the discontinuity for shock-shock interaction was not observed. Ground stations recorded compression over a wide range of MLT and latitudes.