Ballooning-interchange Instability at the Inner Edge of the Plasma Sheet
as a Driver of Auroral Beads: High-resolution Global MHD Simulations
Abstract
Near the inner edge of the plasma sheet, where the geomagnetic field
transitions from dipolar to tail-like, very low values of the northward
component of the field (Bz) are known to be occasionally exhibited,
particularly in the substorm growth phase. It has been suggested that
this may be a signature of a localized magnetic field dip, which are
notoriously difficult to observe in situ. The existence of these
localized minima is significant as they would be ballooning-interchange
(BI) unstable. Previous work has investigated BI instability using
localized particle-in-cell simulations with an imposed Bz minimum as an
initial condition. However, evidence of the existence of localized Bz
minima and BI instability at their tailward edges has been very limited
in self-consistent global magnetosphere simulations. In this
presentation, we demonstrate that the elusive nature of the instability
has been due to the insufficient resolution of previous simulations. We
present a highly-resolved global magnetosphere simulation, using our
newly developed code Gamera. In a synthetic substorm simulation we
demonstrate the formation of a Bz minimum localized in radius, 8-10 Re
from Earth. The region becomes BI unstable in the substorm growth phase,
leading to the formation of earthward and azimuthally propagating
bubbles, distinct from those that form further downtail and become
bursty bulk flows. These bubbles generate field-aligned currents and
optical auroral signatures, similar to those observed on the ground and
from space. We discuss the physical mechanisms for the formation of the
localized Bz minimum by magnetic flux depletion, analyze the nature of
the instability, characterize both magnetospheric and ionospheric
signatures of the unstable region, and compare them with those observed.