How does the plasma sheet respond to the complex pattern of waves coming over the poles from bursty magnetopause reconnection events, or to the vortices and other irregular perturbations coming around the flanks of the magnetosphere in the low-latitude boundary layer? It is probably too much to expect that the complex input from the dayside will sort itself out into a steady flow on the nightside, but there has been a seductive hope that, on a statistical basis, the observations of the plasma sheet could be rationalized using steady convection thinking. This hope depends on the belief that the average magnetic field configuration in the plasma sheet actually is compatible with steady convection. The first doubts on this score were raised by Erickson and Wolf (1980), and were subsequently elaborated by Tsyganenko (1982), Birn and Schindler (1983), and Liu and Hill (1985); the“plasma sheet pressure paradox” they posed is the subject of Section 9.2. Theoretical arguments are one thing, measurements are another; the truly important issue is whether the real plasma sheet manifests steady flow. Several groups have searched large data sets to see whether the statistically averaged flow in the central plasma sheet resembles the flow predicted by the steady convection model. This effort has led to a growing but still incomplete comprehension of the statistical properties of plasma sheet transport. Results obtained using ensembles of data acquired by ISEE 1 and AMPTE/IRM will be reviewed in Section 9.3. The unusual distribution of bulk flow velocities suggests that the plasma sheet flow is bimodal, alternating between a predominant irregular low-speed state and an infrequently occurring state of high-speed earthward flow. In search of steady plasma sheet flow, one could also look into substormfree periods of stable solar wind properties. One of the best such studies, in which great care was taken to find periods of exceptionally stable solar wind and geomagnetic conditions, is reviewed in Section 9.4. Even this study found highly irregular and bursty flow.
MESSENGER Observations of Mercury's Nightside Magnetosphere Under Extreme Solar Wind Conditions: Reconnection‐Generated Structures and Steady Convection
