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In the evening equatorial magnetosphere at about 4 RE geocentric\uddistance and 19 MLT, the four Cluster spacecraft observed a solitary\udstructure with a width of about 1000~2000 km in the propagation\uddirection. The solitary structure propagates sunward with about 5~10 km/s\udcarrying sunward electric field (in the propagation direction) of up to\udabout 10 mV/m (total potential drop of about 5~10 kV), depletion of\udmagnetic field of about 25%, and a duskward E×B convection up to 50 km/s\udof He+ rich cold plasma without O+. At the same time, auroral\udimages from the IMAGE satellite together with ground based geomagnetic field\uddata showed a westward (sunward at this location) propagating auroral bulge\udat the magnetically conjugate ionosphere with the solitary structure. The\udsolitary structure is maintained by flux enhancement of selectively 3000 km/s\udions (about 50 keV for H+, 200 keV for He+, and 750 keV for O+).\udThese ions are the main carrier of the diamagnetic current causing the\udmagnetic depletion, whereas the polarization is maintained by different\udbehavior of energetic ions and electrons. Corresponding to aurora,\udfield-aligned accelerated ionospheric plasma of several keV appeared at\udCluster from both hemispheres simultaneously. Together with good\udcorrespondence in location and propagation velocity between the auroral bulge\udand the solitary structure, this indicates that the sunward moving auroral\udbulge is caused by the sunward propagation of the solitary structure which is\udmaintained by energetic ions. The solitary structure might also be the cause\udof Pi2-like magnetic variation that started simultaneously at Cluster\udlocation

Magnetospheric solitary structure maintained by 3000 km/s ions as a cause of westward moving auroral bulge at 19 MLT