Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa

Intensive correlative studies of magnetic fields and plasmas within the Earth's magnetotail at geocentric radial distances of ~23-46 R_E during March-October 1974 have revealed striking new features. The instruments employed in this survey were the University of Iowa Lepedea and the Goddard Space Flight Center magnetometer. The hot tenuous plasmas within the plasma sheet were found to be in a state of almost continual flow and were threaded with northward, or closed, geomagnetic field lines. The magnetic field is referred to as 'northward' or 'southward' if the component B_Z is positive or negative, respectively, regardless of the magnitudes of the other components. Proton bulk speeds were in the range 50-500 km s^-1. The magnetic fields are directed northward, irrespective of whether the plasma flows are tailward or earthward. These observations show the demand for a strong persistent source of magnetic flux and hot plasmas for the plasma sheet. No characteristic proton bulk flows, e.g., strong dawn-to-dusk motions, were evident during crossings of the magnetic neutral sheet. Occasionally, the satellite encountered the region of acceleration in the magnetotail, the 'fireball.' This spectacular phenomenon exhibits strong jetting of plasmas in excess of 1000 s^-1, proton temperatures of ~ 10^7° K (kT ~ 1 keV), disordered magnetic fields, southward magnetic fields during tailward jetting of plasmas, and northward magnetic fields for fast plasma flows toward Earth. Observations of electrons are used to demonstrate that earthward plasma flows within the fireball are threaded with closed geomagnetic field lines, and open magnetic field lines are embedded in the tailward jetting plasmas. The magnetosheathlike plasmas within the boundary layers which are positioned contiguously to the plasma sheet display striking evidences of plasma heating, great changes in bulk flow velocities and acceleration of energetic electrons with E >45 keV. Typical temperatures and bulk flow speeds within the boundary layer plasmas are ~10^6° K and ~400 km s^-1, respectively. Persistent zones of southward magnetic fields are detected, which are often positioned adjacent to the plasma sheet and within the boundary layer plasmas. Rotations of the magnetic fields from southward to northward, or vice versa, in these boundary layers are accompanied by large enhancements of energetic electron intensities, substantial heating of the low-energy electron distributions, and strong perturbations of the proton velocity distribution functions. During periods for northward directed fields the proton bulk flows decelerate to speeds of £ 200 km s^-1, densities decrease, and the proton velocity distributions can be fitted with two Maxwellians with temperatures of ~2 X 10^6° K and ~10⁷ to 5 X 10^7° K for specific examples. The boundary layer plasmas appear to be the primal source of hot plasmas and closed geomagnetic field lines for the plasma sheet. Qualitatively, all of the major results of the present study are consistent with macroscopic features which are expected from merging of magnetic field lines in the Earth's magnetotail or, more specifically, in the magnetosheathlike boundary layer plasmas.

J. Geophys. Res., 81, No. 34, 5859-5881, Dec. 1976