A plasma flow vortex in the magnetotail and its related ionospheric signatures

Abstract. We presented a large-scale plasma flow vortex event that occurred on 1 March 2009 observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. During the interval, THEMIS satellites were located in the premidnight region between 11 and 16 R_E downtail. Dawnward-earthward plasma flows were seen initially in the magnetotail, followed by duskward-tailward flows. This suggests that a clockwise plasma flow vortex (seen from above the equatorial plane) was observed on the dawn side of the plasma sheet. Furthermore, high energy (>1 keV) electrons were observed. Auroral images at 427.8 nm and THEMIS white light all-sky imager (ASI) at Fort Smith showed a discrete auroral patch formed at the poleward of the auroral oval, it then intensified. It extended eastward and equatorward first and followed by westward motion to form the clockwise auroral vortex. The auroral feature corresponded to the ionospheric signatures of the plasma flow vortex in the magnetotail when the Alfvén transit time between the magnetotail and the ionosphere was taken into account. We suggest that the large-scale clockwise plasma flow vortex in association with the high energy (>1 keV) electrons on the dawn side of the plasma sheet generated a downward field-aligned current (FAC) that caused the related ionospheric signatures. The plasma flow vortex had rotational flow speeds of up to 300 km s⁻¹. The current density associated with the plasma flow vortex was estimated at 2.0 μA m⁻², mapped to the ionosphere.