The plasma sheet source groove

Abstract

A test particle study of the ionospheric source of plasma in the Earth’s plasma sheet has been performed, in an effort to understand an apparent inconsistency between the results of forward and backward (in time) test particle calculations. Most, if not all, forward calculations of polar wind ion outflows result in energetic plasma sheet ion populations; yet most, if not all, backward trajectory calculations from typical plasma sheet ion populations lead elsewhere than to low energy polar cap outflows. Using a trajectory discovered through forward calculation to connect these two regions, we found that the trajectory was only accurately reversible within an extremely narrow range of energy, pitch angle and gyrophase angle in the plasma sheet, referred to herein as ‘the source groove’. This implies that ionospheric plasma tends to appear in the plasma sheet within narrow regions of velocity 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 supported by forward modeling of plasma flows up to the boundaries of such regions.