Mirror-mode Structures in Coronal Mass Ejection Sheaths

Abstract Coronal mass ejections (CMEs) compress the upstream solar wind as they travel through interplanetary space, forming sheath regions and preceding shocks rich in plasma dynamics. In this study, we identify an abundance of mirror-mode structures within the CME-driven sheath, spanning spatial scales from magnetohydrodynamic to the ion gyroradius. These structures preferentially emerge downstream of quasi-parallel shocks, rather than quasi-perpendicular ones, where elevated ion temperature, anisotropy, and beta provide favorable conditions for their excitation. Unlike planetary magnetosheaths—where mirror modes gradually evolve downstream of bow shocks from wave-like form into nonlinear peak- and hole-like forms—mirror modes in CME-driven sheaths are dominated by magnetic holes, with wave- and peak-like forms rarely present, indicating a rapid instability saturation process. These magnetic holes exhibit larger spatial scales and amplitudes than wave- and peak-like forms and remain stable within the plasma flow, indicating a fully developed state. Multispacecraft observations reveal that they are convected with the sheath plasma flow, suggesting they may constitute the early-stage counterparts of magnetic holes later observed in the solar wind. Our findings highlight the plasma processes triggered by CMEs and help advance understanding of the microphysics of CME-driven disturbances in interplanetary space, including plasma instability excitation, nonlinear evolution, and energy conversion.