The quasiperpendicular environment of large magnetic pulses in Earth's quasiparallel foreshock: ISEE 1&2 observations

ULF waves in Earth;aposs foreshock cause the instantaneous angle ϑ Bn between the upstream magnetic field and the shock normal to deviate from its average value. Close to the quasiparallel ( Q ∥ ) shock the transverse components of the waves become so large that the orientation of the field to the normal becomes quasiperpendicular ( Q ⟂ ) during applicable phases of each wave cycle. Large upstream pulses of B were observed completely enclosed in excursions of ϑ Bn into the Q ⟂ range. A recent numerical simulation included ϑ Bn among the parameters examined in Q ∥ runs, and described a similar coincidence as intrinsic to a stage in development of the reformation process of such shocks. Thus, the natural environment of the Q ∥ section of Earth's bow shock seems to include an identifiable class of enlarged magnetic pulses for which local Q ⟂ geometry is a necessary association.