Geography of ion acceleration in the magnetotail: X-line versus current sheet effects

Field-aligned ion beams are often observed in the Plasma Sheet Boundary Layer (PSBL) of Earth's magnetotail. We studied two types of ion beams observed in the magnetotail at different times. Both types of ion beams are pitch angle-collimated along the magnetic field direction but they differ significantly with the width of ion velocity distribution functions in parallel energies. The first type represents energy-collimated ion beams with observed durations of 3-23 min. The majority of such beams have energies <= 20 keV. Together with such ion beams, isotropic electron distributions are observed. We suggest that such ion beams are accelerated at spatially localized regions located rather far from the distant X-line where magnetic field lines are already closed. Another type of PSBL ion distributions represents wide in parallel energy ion beams with typical energies >= tens of keV. The registered durations of such beams are 1-6 min. They are observed together with anisotropic electron velocity distributions formed by the cold and hot counter-streaming components. This feature is peculiar for the magnetic separatrix and indicates that the spacecraft crosses still open or very recently closed magnetic field lines. We have analyzed 987 crossings of the high-latitude edge of the PSBL by Geotail at -220 R-E < X < -20 R-E. The majority of Type-I ion beams are moving earthward at X >= -110 R-E and so have their sources located earthward from the distant X-line (at X < -110 R-E). These ion beams were observed during quiet geomagnetic periods. More than 50% of Type-II ion beams move tailward at X <= -50 R-E. Thus their acceleration sources are located closer to Earth. Type-II ion beams were observed during both quiet and disturbed periods. The energy distribution of the first type of ion beams along the dawn-dusk direction approximately conforms to the assumption about their non-adiabatic acceleration by the quasi-steady dawn-dusk electric field. On the other hand, we suggest that ion beams of the second type are generated in close proximity to the X-line. The energy distribution of such ion beams along the Y direction indicates ion energization by a strong (presumably inductive) electric field, especially in cases of ion acceleration tailward from the X-line.