Vortex‐like plasma flow structures observed by Cluster at the boundary of the outer radiation belt and ring current: A link between the inner and outer magnetosphere

Abstract

Two vortex‐like plasma flow structures have been observed at the outer radiation belt and/or the ring current region on 11 April 2002, from 0415 to 0635 UT, when the Cluster fleet entered (in the Southern Hemisphere) and exited (in the Northern Hemisphere) the boundary layer of the inner magnetosphere near 2130 MLT. On 11 April 2002 during the period of interest, the solar wind speed was high, and the geomagnetic activity was moderate. These two vortices have opposite rotation directions and are characterized by bipolar signatures in the flow Vx components with peak‐to‐peak amplitudes of about 40 km/s. The inflection points of the plasma flow coincide precisely with the local maxima of the duskward core flow Vy (30 km/s) which exceed the surrounding flow by 3–4 times in magnitude for both vortices. A pair of bidirectional current sheets and bipolar electric fields (Ey) are found to be closely associated with these vortices. Whereas magnetic field disturbances are observed only in Bx and By components, the magnetic magnitude stays almost unchanged. Vortices observed both inbound and outbound at the boundary of the radiation belt at nearly the same location (L shell and latitude), suggesting they may last for more than 140 min. The scale sizes of the two vortices are about 810 km and 1138 km, respectively. Interestingly, it is found that Earth's ionospheric singly charged oxygen are precipitating in the vortex dynamic process, having energies less than 1 keV and having a strong field‐aligned pitch angle distribution. These plasma flow vortices are suggested to be formed at the interface between the enhanced ionospheric outflow stream from the polar ionosphere and a sudden braking and/or azimuthal deflection of bursty bulk flows generated by the tail reconnection. These observed flow vortices provide a link among the inner magnetosphere, the tail plasma sheet, and the Earth's ionosphere by coupling magnetic shear stresses and plasma flow momentum.