Ballooning Instability in the Plasma Sheet TransitionRegion in Conjunction With Nonsubstorm Auroral Wave Structures

As an important dynamic process in the plasma sheet, ballooning instability was suggested to be responsible for unstable plasma involving azimuthal wave structures. The upward and downward alternately flowing field‐aligned currents developed in association with unstable waves may cause the finger/beading‐like auroral wave structures along the longitudinal direction, which is a common auroral feature preceding the auroral substorm poleward expansions. In our previous study, we directly evaluated the ballooning instability criterion using in situ observations in conjunction with auroral images and found that the ballooning unstable condition is satisfied in the near‐Earth plasma sheet prior to the auroral wave structures preceding substorm onset. In this study, we adopted the same method and examine the plasma sheet conditions within the regions in conjunction with weak transient auroral wave structures that fade out without further intensification and poleward expansion. The results from analyzing eight events show that no abrupt changes were found in plasma moments and field data. However, the magnetohydrodynamic ballooning instability criterion estimated from observations was satisfied shortly before the initiation of the auroral wave structures due to the changes in plasma spatial profiles. Instability develops against waves with shorter wavelength than those in ambient plasma. Weak magnetic oscillations within the Pi2 band were associated with the instability. These results suggest that the ballooning instability in the plasma sheet transition region is a plausible candidate for causing auroral wave structures during non‐substorm time. The unstable state of the plasma may not necessarily lead to favorable conditions for the substorm onset to occur.