OBSERVATIONS OF NONADIABATIC ACCELERATION OF IONS IN EARTHS MAGNETOTAIL

We present observations of the three-dimensional velocity distribution s of protons in the energy range 20 eV to 52 keV at locations within a nd near the current sheet of Earth's magnetotail at geocentric radial distances 35 to 87 R(E). These measurements were acquired on December 8, 1990, with a set of electrostatic analyzers on board the Galileo sp acecraft during its approach to Earth in order to obtain one of its gr avitational assists to Jupiter. It is found that the velocity distribu tions are inadequately described as quasi-Maxwellian distributions suc h as those found in the central plasma sheet at positions nearer to Ea rth. Instead the proton velocity distributions can be categorized into two major types. The first type is the ''lima bean' shaped distributi on with high-speed bulk flows and high temperatures that are similar t o those found nearer to Earth in the plasma sheet boundary layer. The second type consists of colder protons with considerably lesser bulk f low speeds. Examples of velocity distributions are given for the plasm a mantle, a region near the magnetic neutral line, positions earthward and tailward of the neutral line, and the plasma sheet boundary layer . At positions near the neutral line, only complex velocity distributi ons consisting of the colder protons are found, whereas both of the ab ove types of distributions are found in and near the current sheet at earthward and tailward locations. Bulk flows are directed generally ea rthward and tailward at positions earthward and tailward of the neutra l line, respectively. Only the high-speed, hot distribution is present in the plasma sheet boundary layer. The observations are interpreted in terms of the nonadiabatic acceleration of protons that flow into th e current sheet from the plasma mantle. For this interpretation the ho t, ''lima bean' shaped distributions are associated with meandering, o r Speiser, orbits in the current sheet. It is suggested that the colde r, lower-speed proton velocity distributions are the result of fractio nal or few gyromotions before ejection out of the current sheet, but t his speculation must be further investigated with appropriate kinetic simulation of trajectories.