Collisionless shock nonstationarity arising from microscale physics influences shock structure and particle acceleration mechanisms. Nonstationarity has been difficult to quantify due to the small spatial and temporal scales. We use the closely spaced (subgyroscale), high-time-resolution measurements from one rapid crossing of Earth's quasiperpendicular bow shock by the Magnetospheric Multiscale (MMS) spacecraft to compare competing nonstationarity processes. Using MMS's high-cadence kinetic plasma measurements, we show that the shock exhibits nonstationarity in the form of ripples.

[1] This paper reports a new global multispecies single-fluid MHD model that was recently developed for Venus. This model is similar to the numerical model that has been successfully applied to Mars. Mass densities of proton and three important ionospheric ion species (O + , O 2 + , and CO 2 + ) are self-consistently calculated in the model by including related chemical reactions and ion-neutral collision processes. The simulation domain covers the region from 100 km altitude above the surface up to 24 R V in the tail. An adaptive spherical grid structure is constructed with radial resolution of about 5 km in the lower ionosphere. Bow shock locations are well reproduced for both solar-maximum and solar-minimum conditions using appropriate solar wind parameters for each case. It is shown that the shock locations are farther from the planet during the solar maximum condition, because of both the enhanced solar radiation strength and the relatively small Mach number. The simulation results also agree well with Venus Express observations, as shown by comparisons between model results with magnetic fields observed by the spacecraft.

Simulations and observations of collisionless shocks have shown that deviations of the nominal local shock normal orientation, that is, surface waves or ripples, are expected to propagate in the ramp and overshoot of quasi‐perpendicular shocks. Here we identify signatures of a surface ripple propagating during a crossing of Earth's marginally quasi‐parallel (θBn∼45∘) or quasi‐parallel bow shock on 27 November 2015 06:01:44 UTC by the Magnetospheric Multiscale (MMS) mission and determine the ripple's properties using multispacecraft methods. Using two‐dimensional hybrid simulations, we confirm that surface ripples are a feature of marginally quasi‐parallel and quasi‐parallel shocks under the observed solar wind conditions. In addition, since these marginally quasi‐parallel and quasi‐parallel shocks are expected to undergo a cyclic reformation of the shock front, we discuss the impact of multiple sources of nonstationarity on shock structure. Importantly, ripples are shown to be transient phenomena, developing faster than an ion gyroperiod and only during the period of the reformation cycle when a newly developed shock ramp is unaffected by turbulence in the foot. We conclude that the change in properties of the ripple observed by MMS is consistent with the reformation of the shock front over a time scale of an ion gyroperiod.

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