A forward-reverse shock pair in the solar wind driven by over-expansion of a coronal mass ejection: Ulysses observations

A previously unidentified type of solar wind forward‐reverse shock pair has been observed by Ulysses at 4.64 AU and S32.5°. In contrast to most solar wind forward‐reverse shock pairs, which are driven by the speed difference between fast solar wind plasma and slower plasma ahead, this particular shock pair was driven purely by the over‐expansion of a coronal mass ejection, CME, in transit from the Sun. A simple numerical simulation indicates that the overexpansion was a result of a high initial internal plasma and magnetic field pressure within the CME. The CME observed at 4.64 AU had the internal field structure of a magnetic flux rope. This event was associated with a solar disturbance in which new magnetic loops formed in the corona almost directly beneath Ulysses ∼11 days earlier. This association suggests that the flux rope was created as a result of reconnection between the “legs” of neighboring magnetic loops within the rising CME.