Characteristics Of Reconnection Sites And Fast Flow Channels In An Mhd Simulation

We investigate an interval of moderate magnetic activity from 0-8 UT on 14 March 2008 by using a global magnetohydrodynamics (MHD) simulation with high spatial and temporal resolution. Observations show several distinct substorms during this interval. One of these with onset at 04:44 UT occurs at 04:42 UT in the simulation. The simulation shows reconnection is continuously present at multiple sites. During the growth phase, the number of X-lines decreases as their total length increase and their locations approach the Earth. The X-lines create multiple fast flow channels with dipolarization fronts. The total length and area of these channels increase during the growth phase as they penetrate closer to the Earth carrying more magnetic flux. The 04:42 UT onset in the simulation was preceded by the growth of an X-line that eventually extended 55 R-E from 12 R-E premidnight to 50 on the dawn side. It produced a narrow flow channel parallel to the X-line that eventually penetrated inside 10 R-E rapidly depositing magnetic flux as the expansion phase developed. Despite this good agreement in expansion phase onset time, ground and satellite observations suggest a quiet growth phase with a sudden onset of reconnection. It may be possible to explain the difference between observations and simulations if all growth phase activity in the simulation maps to the ionosphere at very high latitudes. An auroral streamer at onset maps in the simulation very close to Earth. A recovery phase streamer maps to the middle tail.Plain Language Summary The solar wind confines the Earth's magnetic field to a volume called the magnetosphere. When the solar wind magnetic field is antiparallel to the Earth's field they connect in a process called reconnection. This links dipole field lines to the solar wind field that then drags ant sunward creating a long tail. When forced together in the tail, they reconnect and return to the dayside. These processes are studied using numerical simulations that solve the basic equations of magnetized fluids called magnetohydrodynamics (MHD). Spacecraft and ground-based observations suggest that there is a growth phase of quiet thinning before the onset of a major reconnection event. The simulations described here behave differently. They indicate that reconnection is continuously present producing bursts of earthward flowing plasma. As the growth phase progresses, the reconnection becomes more intense and moves closer to the Earth, and the flow channels lengthen, carry more magnetic flux, and penetrate closer to Earth. At some point, flux accumulates near local midnight, and auroral and ionospheric currents expand poleward. We have too few satellites in the tail to determine if this is true observationally. The interaction of a narrow flow channel with the ionosphere creates a narrow streak of aurora.