Revisiting the structure of low-Mach number, low-beta, quasi-perpendicular shocks

A study of the structure of 145 low-Mach number (M≤3), low-beta (β≤1), quasi-perpendicular interplanetary collisionless shock waves observed by the Wind spacecraft has provided strong evidence that these shocks have large-amplitude whistler precursors. The common occurrence and large amplitudes of the precursors raise doubts about the standard assumption that such shocks can be classified as laminar structures. This directly contradicts standard models. In 113 of the 145 shocks (∼78%), we observe clear evidence of magnetosonic-whistler precursor fluctuations with frequencies ∼0.1–7 Hz. We find no dependence on the upstream plasma beta, or any other shock parameter, for the presence or absence of precursors. The majority (∼66%) of the precursors propagate at ≤45° with respect to the upstream average magnetic field and most (∼87%) propagate ≥30° from the shock normal vector. Further, most (∼79%) of the waves propagate at least 20° from the coplanarity plane. The peak-to-peak wave amplitudes (δB_pk-pk) are large with a range of maximum values for the 113 precursors of ∼0.2–13 nT with an average of ∼3 nT. When we normalize the wave amplitudes to the upstream averaged magnetic field and the shock ramp amplitude, we find average values of ∼50% and ∼80%, respectively.