Case studies of mirror-mode structures observed by THEMIS in the near-Earth tail during substorms

An examination of the magnetic field and plasma observed by the inner THEMIS-D spacecraft (P3) close to the equatorial plane at similar to 11R(E) at local midnight reveals the occurrence of mirror-mode structures. These structures have the same characteristic waveform seen in other regions. The examination of the mirror-mode instability shows that inside these structures the threshold of mirror instability is marginally reached, while the surrounding plasma is mirror stable. The observed mirror structures occur in the dipolarized magnetic field following a substorm-related dipolarization. It is found that after the dipolarization front, the local ions become more anisotropic and initial magnetic holes form inside this anisotropic plasma before the fully-fledged mirror structures are observed. The ions become less anisotropic afterward, but the strong field depression in the magnetic holes enhances the effective plasma beta so that the mirror instability threshold is marginally reached. Thus, the dipolarization process provides the large-amplitude magnetic field fluctuations and the anisotropic plasma environment for mirror structures to grow. The isolated large-amplitude mirror-mode structures survive in the mirror-stable plasma even through the plasma becomes less anisotropic. It is also found that the width of magnetotail mirror-structures is smaller than one gyroradius of a plasma sheet proton, which is different from the width of mirror structures in other regions. These mirror structures appear to have a strong correlation with electron anisotropy changes. These observations suggest that electron kinetics may also play a role during the growth and saturation of mirror instability in the near-Earth tail.