Comparison of MMS Observations of Foreshock Bubbles With a Global Hybrid Simulation

We present multi‐point observations of foreshock bubbles (FBs) for comparison with the predictions of hybrid simulations. The four Magnetospheric Multiscale (MMS) spacecraft observed a series of discontinuities in the region upstream from the bow shock on December 18, 2017. Two solar wind discontinuities were associated with fully developed FBs with core regions exhibiting greatly decelerated and deflected antisunward flows, significant increases in temperature, and depressed plasma densities and magnetic field strengths. A single shock lies upstream (sunward) from each of the two bubbles. The connection to the foreshock before and/or after the discontinuities can play an important role in the foreshock transient formation. We present the predictions of a 2.5‐D global hybrid simulation for the presence of the FBs for two different interplanetary magnetic field (IMF) cone angles (66° and 25°). The topology of the simulated FB is nearly hemispherical for small angles, but becomes linear for large angles. We show that the characteristics of the two simulated FBs for the two different discontinuity normals and IMF cone angles are in good agreement with the observational results.