On the nature of precursor flows upstream of advancing dipolarization fronts

Earthward propagating dipolarization fronts, interpreted as thin, vertical current sheets that separate plasmas of different origins in the Earth's magnetotail, are embedded within flow bursts, often near the leading edge of bursty bulk flows. Observations have also shown that bursty bulk flow onset typically precedes dipolarization front arrival by similar to 1 min. Ion distribution functions reveal that earthward flows in advance of front arrival are often caused by the appearance of a new ion population atop a preexisting plasma sheet component. Particle simulations suggest that this second population, which contributes most to the plasma velocity, is composed of ions that have been reflected at and accelerated by the approaching front. We propose that in the presence of a finite upstream B-z field, the reflected ions would be confined in a region with a size comparable to the ion thermal gyroradius over the upstream B-z. THEMIS observations confirm that the measured time difference delta t between the appearance of earthward plasma flows and the dipolarization front arrival is consistent with the predicted size of the ion accessibility region.