Persistent Occurrence of Strip-Like Plasma Density Bulges at Conjugate Lower-Mid Latitudes During the September 8-9, 2017 Geomagnetic Storm

During the geomagnetic storm on September 8-9, 2017, shoulder-like structures were persistently captured in the lower-mid latitudes at multiple local times, by Swarm and DMSP satellite observations. A more telling terminology of such shoulder-like structure called strip-like bulge is proposed in this study, due to its global presence that spanning over 150 degrees in longitude but only over 1 degrees-5 degrees in latitude. The strip-like bulges can be categorized into sharp and blunt types depending on the sharpness of the density peaks. The blunt type was short-lived and appeared earlier than the sharp type in the afternoon-sunset sector. The sharp type was long-lived and appeared at all the observed local times. Both two types of strip-like bulges were dominated by the ion composition of the H+/He+, rather than O+. This is the first evidence that the plasmaspheric particles are involved in forming the ionospheric structure at such low latitude. Moreover, the equatorward movement of the bulges shared the same trends with the earthward movement of the plasmapause. These two types of strip-like bulges showed different longitudinal dependencies controlled by the magnetic declination. We suggest that the combined effects from the plasmaspheric downwelling and disturbance neutral wind were responsible for the appearance of the strip-like bulges.