Drift mirror waves have been detected 207 Re downstream in the distant magnetosheath by instrumentation onboard ISEE-3. The MHD structures have estimated scale sizes of 4.6¿104 to 2.3¿105 km or 130 to 700 proton gyro-radii, assuming 1 keV protons. Simultaneous measurements on IMP-8 indicate that at the time of the events, the earth's magnetosphere was enveloped by a solar flare-associated interplanetary shock and driver gas. The (shocked) driver gas was characterized by an unusually low plasma density with a &Bgr; (plasma pressure/magnetic pressure) of ~0.2. From the instability criterion, &bgr;⊥/&bgr;∥>1/&bgr;⊥, the magnetosheath plasma should not go mirror-mode unstable unless the anisotropy, &bgr;⊥/ &bgr;11 is large, >6. These unusual conditions are met by the demonstrated presence of highly anisotropic, energetic 35--384 keV ions with P⊥/P11≈30. It is calculated that most of the plasma energy density can be due to >1 keV ions. The source of the ions is not well understood, but an appealing possibility is magnetic annihilation of the northward magnetosheath fields with southern hemisphere polar cusp magnetic fields.