Hydromagnetic vortices. I - The 11 December 1977 event

Through a synthesis of magnetometer, plasma, energetic particle and electric field data from the ISEE satellite pair, the characteristics of the initial (11 December 1977) magnetotail plasma vortex event reported by Hones et al. (1978), are described. The event is associated with a hot (beta is approximately unity) compressional hydromagnetic wave and apparent vortical motion is seen because at two points in the flow cycle the flow is field-aligned. The behavior of the energetic ions receives special study: when combined with the thermal flow measurements energy dispersion is evident in the field-aligned flow, while the large pitch angle energetic ions reveal the presence of gradients. It is argued that these gradients are wave-induced, and the data are used to determine the perpendicular wave wavelength together with the speed and direction of transverse wave propagation.