Kinetic-Scale Magnetic Holes Inside Foreshock Transients

In Earth's foreshock, there are many foreshock transients that have core regions with low field strength, low density, high temperature, and bulk velocity variation. Through dynamic pressure perturbations, they can disturb the magnetosphere-ionosphere system. They can also accelerate particles contributing to particle acceleration at the bow shock. Recent Magnetospheric Multiscale (MMS) mission observations showed that inside the low field strength core region, there are usually kinetic-scale magnetic holes with even lower field strength (<1 nT). However, their nature and effects are unknown. In this study, we used MMS observations to conduct case studies on these magnetic holes. We found that they could be subion-scale current sheets without a magnetic normal component and guide field, driven by the motion of demagnetized electrons. These magnetic holes can also be subion-scale flux ropes or magnetic helical structures with weak axial field. The low field strength inside them can be either driven by external expansion or electron mirror mode. Electrons inside them show flux depletion at 90 degrees pitch angle resulting in an "electron hole" distribution. These magnetic holes can play a role in electron dynamics, wave excitation, and shaping the foreshock transient structures. Our detailed study of such features sheds light on the turbulent nature of foreshock transient cores.