Global Structure of the Nightside Proton Precipitation during Substorms using Simulations and Observations.

摘要

In regions of thin strong current sheets, the first adiabatic invariant of protons can be violated leading to pitch angle diffusion into the loss cone and ultimately auroral precipitation. The central plasma sheet typically provides a stretched enough magnetic field configuration to account for the nightside proton precipitation. During substorms, the outflow from the near earth reconnection line at approximately 20 RE brings magnetic flux from the highly stretched magnetotail into the near earth magnetosphere. Once there, the flux piles up forming an azimuthally localized region where the magnetic field is more dipolar. Current flows into and out of the ionospere at the edges of this dipolarized region forming the substorm current wedge (SCW). As the substorm continues, the SCW typically grows azimuthally and radially as the result of the continued flux pileup. Using the OpenGGCM global MHD simulation, we show that the proton precipitation can be split azimuthally due to the arrested scattering in the strongly dipolarized region at the center of the SCW. However, at the edges of the SCW where the dipolarization is not as complete (and certainly outside the SCW), the mean gyroradii increase due to the energization of the near earth magnetotail may he sufficient to facilitate continued scattering. The simulation predictions of auroral splitting are compared to a statistical study using data from the IMAGE SI-12 instrument. The IMAGE SI-12 frequently shows localized azimuthal splitting of the proton aurora similar to the simulations. Additionally, the splitting of the proton aurora is much more common for stronger substorms (lower AL and onset latitude) winch is also argued to be consistent with the simulations.