The plasma sheet source groove

A test particle study of the ionospheric source of plasma in the Earth's pl asma sheet has been performed, in an effort to understand an apparent incon sistency between the results of forward and backward (in time) test particl e calculations. Most, if not all, forward calculations of polar wind ion ou tflows result in energetic plasma sheet ion populations; yet most, if not a ll, backward trajectory calculations from typical plasma sheet ion populati ons lead elsewhere than to low energy polar cap outflows. Using a trajector y discovered through forward calculation to connect these two regions, we f ound that the trajectory was only accurately reversible within an extremely narrow range of energy, pitch angle and gyrophase angle in the plasma shee t, referred to herein as 'the source groove'. This implies that ionospheric plasma tends to appear in the plasma sheet within narrow regions of veloci ty space, but is effectively diffused by fluctuations to form the observed more isotropic plasma sheet populations. The implications for backtracking test particle studies are discussed, and it is concluded that test particle backtracking from highly chaotic regions is impractical and should be supp orted by forward modeling of plasma flows up to the boundaries of such regi ons. (C) 2000 Elsevier Science Ltd. All rights reserved.