Large-scale current systems and ground magnetic disturbance during deep substorm injections
We present a detailed analysis of the large-scale current systems and their effects on the ground magnetic field disturbance for an idealized substorm event simulated with the equilibrium version of the Rice Convection Model. The objective of this study is to evaluate how well the bubble-injection picture can account for some classic features of the substorm expansion phase. The entropy depletion inside the bubble is intentionally designed to be so severe that it can penetrate deep into geosynchronous orbit. The results are summarized as follows: (1) Both the region-1-sense and region-2-sense field-aligned currents (FACs) intensify substantially. The former resembles the substorm current wedge and flows along the eastern and western edges of the bubble. The latter is connected to the enhanced partial ring current in the magnetosphere associated with a dipolarization front earthward of the bubble. In the ionosphere, these two pairs of FACs are mostly interconnected via Pedersen currents. (2) The horizontal ionospheric currents show a significant westward electrojet peaked at the equatorward edge of the footprint of the bubble. The estimated ground magnetic disturbance is consistent with the typical features at various locations relative to the center of the westward electrojet. (3) A prominent Harang-reversal-like boundary is seen in both ground DH disturbance and plasma flow pattern, appearing in the westward portion of the equatorward edge of the bubble footprint, with a latitudinal extent of 5 and a longitudinal extent of the half width of the bubble. (4) The dramatic dipolarization inside the bubble causes the ionospheric map of the inner plasma sheet to exhibit a bulge-like structure, which may be related to auroral poleward expansion. (5) The remarkable appearance of the westward electrojet, Harang-reversal-like boundary and poleward expansion starts when the bubble reaches the magnetic transition region from tail-like to dipole-like configuration. We also estimate the horizontal and vertical currents using magnetograms at tens of ground stations for a deep injection substorm event occurred on April 9, 2008, resulting in a picture that is qualitatively consistent with the simulation. Based on the simulations and the observations, an overall picture of the ionospheric dynamics and its magnetospheric drivers during deep bubble injections is obtained.