Bi-directional electron distributions associated with near-tail flux transport

Bi-directional field-aligned electron distributions are sometimes observed in the plasma sheet. We looked through AMPTE-IRM data at high time resoluti on to study their appearance in the high-beta plasma sheet between 10 and 1 5 Re. We found that the occurrence frequency of bidirectional distributions drops sharply at r less than or equal to 15Re. We did not observe such ani sotropic distributions in the undisturbed plasma sheet, but found them in t he locally disturbed central plasma sheet preferentially in the flux tubes with low density and strong magnetic field. No simple association with a sp ecific magnetotail state or substorm phase or rapid flows were observed, bu t opposite to expectations the largest field-aligned anisotropy values have been attained during strong magnetic pulse events characterized by the lar gest magnetic field compression (Bz greater than or equal to 15 - 20 nT) in which the perpendicular temperature decreased instead of being increased. We study such events to show their identity with plasma bubbles and discuss the reasons why the betatron acceleration is not as effective as Fermi acc eleration in these cases. The observations are interpreted in terms of unde rpopulated newly-reconnected plasma tubes of limited cross-tail extent (pla sma bubbles) which are produced by impulsive magnetic reconnection and tran sported to the inner magnetosphere.