Strip-like plasma bulges at lower-middle latitudes over a wide range of longitude during 8~9 September 2017 geomagnetic storm

During the geomagnetic storm on 8~9 September 2017, a new kind of ionospheric irregularity is persistently captured in lower-middle latitudes at multiple local times, based on Swarm and DMSP satellites observations. This irregularity is observed as the conjugate strip-like bulge, which extends larger than 150° in longitude but only 1°~5° 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 is short-lived and appears earlier than the sharp type in the afternoon-sunset sector. The sharp type is long-lived and appears at all the observed local times. Both two types of strip-like bulges are dominated by the ion composition of the H⁺ /He⁺. This is the first evidence that the plasmaspheric particles are involved in forming the ionospheric structure at such low latitude. Moreover, the latitude/L-shell of the bulges decreased synchronously with the plasmaspheric compression. Also, these two types of strip-like bulges show different longitudinal dependencies controlled by the magnetic declination. We suggest that the combined effect from the plasmaspheric downwelling and disturbance neutral wind is responsible for the appearance of the strip-like bulges.