Observations of a subcritical switch-on shock

Context. The conditions that lead to subcritical switch-on shocks are very rare in interplanetary space. Aims. In this work, we report the identification of a subcritical switch-on shock driven by an interplanetary coronal mass ejection. Methods. The subcritical switch-on shock is identified by fitting the Rankine-Hugoniot (R-H) conditions. Results. It is found that the shock has the following characteristics. (1) Proton density, temperature, and total magnetic field strength all increase from the upstream region to the downstream region. (2) Observed parameters meet the R-H relations closely. (3) The fast-mode Mach number is greater than unity in the preshock state and less than unity in the postshock state. (4) The normal Alfven-Mach number is almost equal to 1 in the postshock state, while the upstream tangential magnetic component is around zero but the downstream tangential magnetic component is not zero (it is "switched on"). Conclusions. Considering that the shock parameters satisfy all the properties of subcritical switch-on shocks, we conclude that the discontinuity is a subcritical switch-on shock.