Generation and evolution of interplanetary slow shocks

It is well known that most MHD shocksobserved within 1 AU are MHD fast shocks. Only a very limited number of MHD slowshocks are observed within 1 AU. In order to understand why there are only a fewMHD slow shocks observed within 1 AU, we use a one-dimensional, time-dependentMHD code with an adaptive grid to study the generation and evolution ofinterplanetary slow shocks (ISS) in the solar wind. Results show that anegative, nearly square-wave perturbation will generate a pair of slow shocks (aforward and a reverse slow shock). In addition, the forward and the reverse slowshocks can pass through each other without destroying their characteristics, butthe propagating speeds for both shocks are decreased. A positive, square-waveperturbation will generate both slow and fast shocks. When a forward slow shock(FSS) propagates behind a forward fast shock (FFS), the former experiences adecreasing Mach number. In addition, the FSS always disappears within a distanceof 150R_⊙ (where R_⊙ is onesolar radius) from the Sun when there is a forward fast shock (with Mach number≥1.7) propagating in front of the FSS. In all tests that we have performed,we have not discovered that the FSS (or reverse slow shock) evolves into a FFS(or reverse fast shock). Thus, we do not confirm the FSS-FFS evolution assuggested by Whang (1987)