THE QUASI-PERPENDICULAR ENVIRONMENT OF LARGE MAGNETIC PULSES IN EARTHS QUASIPARALLEL FORESHOCK - ISEE-1 AND ISEE-2 OBSERVATIONS

ULF waves in Earth's foreshock cause the instantaneous angle theta(Bn) between the upstream magnetic field and the shock normal to deviate from its average value. Close to the quasiparallel (Q (parallel-to)) shock the transverse components of the waves become so large that the orientation of the field to the normal becomes quasiperpendicular (Q (perpendicular-to)) during applicable phases of each wave cycle. Large upstream pulses of B were observed completely enclosed in excursions of theta(Bn) into the Q(perpendicular-to) range. A recent numerical simulation included theta(Bn) among the parameters examined in Q(parallel-to) 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(parallel-to) section of Earth's bow shock seems to include an identifiable class of enlarged magnetic pulses for which local Q(perpendicular-to) geometry is a necessary association.