Simulating the driven magnetosphere

[摘要] A significant effort is focused on understanding the behavior of the Earth;;s magnetosphere during times of southward interplanetary magnetic field, when the magnetosphere is in a driven state. In situ observations of the space environment provide us with real magnetic and electric field and plasma data with which to study magnetospheric processes, yet we lack the ability to experimentally control the parameters that influence these processes. On the other hand, a niche of computational modeling is the ability to experiment with these parameters in a straightforward effort to understand the magnetospheric response. The simulation model employed in this thesis (the Rice Convection Model-Equilibrium, or RCM-E) is unique in its ability to calculate the energy dependent drifts of plasma particles as well as their feedback on both the electric and magnetic fields. Three different RCM-E simulations are presented. First, the magnetospheric response to a moderate level of external driving is modeled, showing that the model reproduces several of the features of steady magnetospheric convection (SMC) events. The simulation is then repeated with a more rigorous calculation of the magnetic field that generally produces a higher quality result but suffers from excessive numerical noise in the important inner plasma sheet region. This simulation produces a more stressed magnetic field, but encounters numerical breakdown due largely to the numerical noise. The proper response to steady driving in the RCM-E is likely to be more stressed than the first simulation, yet more stable than the second simulation. Somewhat counter to conventional wisdom, these simulations suggest that enhanced convection by itself is insufficient to inject a ring current, since the magnetic field response acts to mitigate the injection. In the third simulation, a method for injecting plasma into the ring current without drastically affecting the near-Earth magnetic field configuration is demonstrated: significantly reduce the specific entropy of the injection source. The steady driving simulations apparently failed to produce a realistic ring current injection because the model equations conserve specific entropy as plasma is transported (adiabatic transport). These simulations suggest that a non-adiabatic plasma process---possibly the substorm---plays an important role in the dynamics of geomagnetic storms.

[发布机构] Rice University