Relationship between wave-like auroral arcs and Pi2 pulsations in plasma sheet during substorms

Abstract:

The observations of substorm onset phenomena in the magnetosphere and ionosphere are examined to investigate their correlation and to understand the substorm onset mechanism. In particular, we examine the Pi2 wave structure, propagation, frequency in the magnetosphere observed by the THEMIS satellites in the near-Earth plasma sheet and the structure and propagation of the substorm auroral onset arcs. The azimuthal mode number values of the wave-like substorm arcs are found to be in the range of ~ 100 – 260 and decrease with increasing geomagnetic latitude of the substorm auroral arc location. 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 pulsations in the near-Earth plasma sheet region. The result suggests that the Pi2 transverse δBy disturbances are related to the intensifying wave-like substorm onset arcs. One plausible explanation of the observations is the kinetic ballooning instability, which has high azimuthal mode number due to the ion gyro-radius effect and finite parallel electric field that accelerates electrons into the ionosphere to produce the wave-like arc structure.