Jet Propulsion Laboratory, C. I. T., Pasadena, CA 91109

Drift mirror waves have been detected 207 R_e downstream in the distant magnetosheath by instrumentation onboard ISEE-3. The MHD structures have estimated scale sizes of 4.6 x 10⁴ to 2.3 x 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 b (plasma pressure/magnetic pressure) of ~ 0.2. From the instability criterion, b _^ /b _{ê ê} > 1 + 1/b _^ , the magnetosheath plasma should not go mirror-mode unstable unless the anisotropy, b _^ /b _{ê ê} is large, >6. These unusual conditions are met by the demonstrated presence of highly anisotropic, energetic 35-384 keV ions with P_^ /P_{ê ê} » 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.

Geophys. Res. Lett., 11, No. 10, 1102-1105, Oct. 1984