An interplanetary shock interacts with Earth’s bow shock before it encounters the magnetosphere. The interaction of an interplanetary shock with the bow shock launches a fast shock into the magnetosheath and forms a new discontinuity where the magnetic field strength and density increase, the temperature decreases and the velocity remains unchanged. The bow shock moves earthward immediately after the interplanetary shock crossing. Earthward followed by sunward motion of the bow shock due to the interaction of an interplanetary shock with the bow shock has been observed.

Most of the previous studies have been focused on strong interplanetary shocks. Observations of the interaction of a weak interplanetary shock with the bow shock by five THEMIS spacecraft have been reported recently [Zhang et al. (2009), Dynamic motion of the bow shock and the magnetopause observed by THEMIS spacecraft, J. Geophys. Res., 114, A00C12, doi:10.1029/2008JA013488].

Observations

Figure 1 shows the trajectory of the THEMIS spacecraft during its outbound journey from 0500 UT to 0800 UT on 10 July 2007. The positions of the five THEMIS probes at 0800 UT are marked by five different symbols in Figure 1. THEMIS B was leading on this outbound pass and was followed by THEMIS C, D, E, and A. THEMIS C and D are very close to each other.

Figure 1. THEMIS trajectory projected in GSM XY plane from 0500 UT to 0800 UT on 10 July 2007. Timing analysis shows that both the magnetopause and the discontinuity are moving earthward. The velocities in the X direction are 47 km/s and 90 km/s, respectively.

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THEMIS observations showed that no fast shock has been observed after the interaction of a weak interplanetary shock with the Earth’s bow shock. Instead, the transmitted interplanetary shock took the form of a discontinuity which is characterized by increases in the magnitude of the magnetic field and plasma density and a decrease in the plasma ion temperature as shown in Figure 2.

Figure 2. The discontinuity produced by an interaction of a weak interplanetary shock and the Earth’s bow shock.

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The magnetopause and discontinuity crossings have been observed by all five THEMIS probes. Figure 3 shows the magnetic field components and the ion spectrum from all five THEMIS probes. The blue and red arrows mark the magnetopause and discontinuity crossings, respectively. The velocity of the magnetopause and discontinuity motion can be obtained by timing analysis based on multispacecraft observations of the same structure. Timing analysis shows that both the magnetopause and the discontinuity are moving earthward. The velocities in X direction are 47 km/s and 90 km/s, respectively.

Figure 3. The magnetopause and discontinuity observed by all five THEMIS probes. The top panels show the magnetic field components and the ion spectrum from THEMIS B. The following panels show the same parameters from THEMIS C, D, E and A, respectively. The blue and red arrows mark the magnetopause and discontinuity crossings, respectively.

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Conclusion

With five spacecraft in a row in the magnetosheath on July 10, 2007, THEMIS offered a remarkable opportunity to track the propagation of the transmitted interplanetary shock. Observations from THEMIS spacecraft demonstrated that after interacting with the Earth’s bow shock, the transmitted interplanetary shock took the form of a discontinuity, where total magnetic field and density increase and the temperature decreases, and propagated earthward with a speed of 90 km/s.

Biographical Note

Hui Zhang is a postdoctoral fellow at NASA Goddard Space Flight Center. Her research interests focus on structure and dynamics of the Earth’s dayside magnetosphere including the magnetosheath, magnetopause, boundary layer, cusp, interaction of solar wind discontinuities (including shocks) with the earth’s bow shock and the global effects resulting from the interaction; and multiple spacecraft data analysis.