Observations of an interplanetary switch-on shock driven by a magnetic cloud

[1] A possible interplanetary switch-on shock event prior to a trailing magnetic cloud was observed on August 1, 2002 at 1 AU. We fit the data with the Rankine-Hugoniot (R-H) relations based on both oblique and switch-on shock models. It is found that both models are consistent with the observed data, and the best fit solutions of the two models are close to one another. For the oblique shock model, the best fit upstream shock normal angle, θ_BN1 (= cos^-1(B_rl/B₁)), is as small as 5.55°. The shock has the following characteristics: (1) plasma density, plasma temperature, and the magnetic field strength all increase across the shock, (2) protons are thermalized very efficiently across the shock, but it is not the case for electrons, (3) the fast-mode Mach number is greater than unity in the preshock region and less than unity in the postshock region, and (4) from the oblique shock model we find that the normal Alfven Mach number is very close to unity in the postshock region, while from the switch-on shock model we obtain a solution of unity normal AlfVen Mach number. Our results clearly demonstrate the MHD character of a fast shock propagating along the ambient magnetic field.