Correlation between ionospheric and magnetospheric features of substorm events

Abstract:

Wave-like substorm arc features in the aurora and Pi2 magnetic disturbances
observed in the near-Earth plasma sheet are frequently, and sometimes simultaneously,
observed around the substorm onset time. In this study, we investigate the major
substorm events during 2008-2009 which are listed in the substorm timing table
provided by the UCLA to perform the correlation analysis of these two phenomena.
We perform the statistical analysis of the THEMIS ASI auroral observations that show
wave-like bright spot structure along the arc prior to substorm onset. The azimuthal
mode number values of the wave-like substorm arcs are found to be in the range of ~
100 – 240 and decrease with increasing geomagnetic latitude of the substorm auroral
arc location. We suggest that the azimuthal mode number is likely related to the ion
gyroradius and azimuthal wave number. We also perform correlation study of the
pre-onset wave-like substorm arc features and Pi2 magnetic disturbances for substorm
dipolarization events observed by THEMIS satellites during 2008-2009. The
wave-like arc brightness structures on the substorm auroral arcs tend to move
azimuthally westward, but with a few exceptions of eastward movement, during tens
of seconds prior to the substorm onset. The movement of the wave-like arc brightness
structure is linearly correlated with the phase velocity of the Pi2 δBy disturbances in
the near-Earth plasma sheet region. Moreover, the mean azimuthal mode number of
the wave-like auroral arc structure is also consistent with the estimated azimuthal
mode number of the Pi2 δBy disturbances observed in the near-earth plasma sheet.
These results suggest that the Pi2 transverse δBy disturbances are related to the
intensifying wave-like substorm onset arcs.