Temporal structure of the fast convective flow in the plasma sheet: Comparison between observations and two-fluid simulations



[1] The present study examines the temporal structure of the fast flow in the plasma sheet using both observations and simulations. The data analysis part adopts the strictest criterion ever for the satellite location so that selected flows are mostly convective. From Geotail measurements at X > −31 RE, 818 earthward-flow and 290 tailward-flow events are selected. Superposed epoch analyses are conducted with two different reference times: the start of the fast flow and the time of a sharp change in the Bz component. The results are summarized as follows: (1) The magnetic field becomes dipolar in the course of the fast earthward flow; (2) Sharp dipolarization tends to be preceded by a transient decrease in BZ, which starts along with the fast flow and is accompanied by an increase in the plasma density; (3) The corresponding signatures, albeit less clear, can also be found for the tailward flow; (4) Whereas the plasma density decreases in association with the fast flow irrespective of the flow direction (though, more gradually for the tailward flow), the ion temperature increases for the earthward flow and decreases for the tailward flow; (5) The plasma and total pressures decrease in the course of the fast flow, suggesting the reduction of the lobe field strength; (6) In general, magnetic field and plasma parameters change more gradually in time for the tailward flow than for the earthward flow. Those characteristics of the fast flow can be found irrespective of the X distance, even though the ambient magnetic field and plasma vary significantly between X = −5 and −31 RE. The near-Earth reconnection is inferred to be the responsible mechanism for most, if not all, flow events, and the difference between the earthward and tailward flows presumably reflects difference in downstream conditions. On the earthward side of the reconnection site, the flow needs to proceed against the rigid terrestrial magnetic field, whereas on the tailward side the flow does not have any obstruction once reconnection reaches the lobe magnetic field. This idea is consistent with the change of the magnetic inclination, which suggests that the plasma sheet becomes thicker and thinner in the course of the earthward and tailward flows, respectively. These observational results are compared with fast plasma flows modeled by two-fluid simulations of magnetic reconnection. A focus is placed on the reduction of BZ prior to dipolarization for the earthward flow (the precursory BZ increase for the tailward flow) since this is the new finding owing to our strict condition for the convective flow. It is found that the fragmentation of the current sheet and the formation of multiple neutral lines create signatures similar to the satellite observations. After multiple X lines form, one of them dominates and establishes the overall flow pattern associated with reconnection. Magnetic islands formed between the X lines are swept downstream by the reconnection process. The signature of this earthward convection of a magnetic island past a satellite at rest in the magnetotail is a strongly bipolar signature in Bz with a sudden enhancement in the density: Bz spikes negative and then positive in rapid succession, with a maximum in the density between these two spikes. It is therefore suggested that the temporal structure of the observed fast plasma flows contains information directly linked to their genesis.