Deformation of plasma bubbles and the associated field aligned current system during substorm recovery phase

Abstract

Multiple plasma bubbles have been detected by CLUSTER satellites on August 15th 2001 in midtail (∼-18 Re). Those bubbles can be classified into two types. Type-I bubbles are similar to those bubbles in previous studies [ Sergeev et al., 1996; Walsh et al., 2009]. Unlike type-I bubbles, the trailing parts/tails of type-II bubbles are much more dynamic than their leading parts/heads and have larger the flux transfer rate. The leading parts of type-II bubbles were suffering a deceleration process and the interaction between the leading and trailing parts will lead to the intensification of |B| and B_z and also a flow shear layer at the trailing parts. Those shear flows cause the twist of magnetic field line, the enhancement of x and y components magnetic field and the generation of field aligned current system. Enhancement of electric field fluctuations also can be found at the trailing parts of type-II bubbles. The corresponding ionospheric signatures were also detected by ground geomagnetic stations. We suggested that the type-II bubbles are bubbles in their late evolution stage and our results are important in understanding the evolution of plasma bubble or fast flow and the transportation of energy from magnetotail to ionosphere.