Statistical Investigation of the Storm Time Plasma Density Strip-like Bulges at Lower-Mid Latitudes

The strip-like bulge is a storm-time conjugate ionospheric plasma density enhancement that extends widely (over 150° in longitude) in the zonal dimension but occupies only 1°~5° in latitude. Based on in-situ measurements of 11 low earth orbit (LEO) satellites, this study statistically investigates the bulge structures of geomagnetic storms driven by 136 interplanetary coronal mass ejections (ICMEs) during 2000~2021. The statistical results show that the strip-like bulges are initiated at the end of the storm main phase and can persist for more than 60 hours. The spatial and temporal coverage of the strip-like bulge varies from storm to storm. However, the bulges do exhibit occurrence preferences: stronger storms, the Asian-Pacific sector (with eastward magnetic declination), the nightside of the dawn-dusk terminator, and solar minimum periods. A quiet time density enhancement called midlatitude extra peaks could be recognized as a precursor of the strip-like bulge. The plasmaspheric compression shares some similar occurrence features with the strip-like bulge, indicating a field-aligned downward and cross-L inward intrusion of the plasmaspheric ions. The local net ion drifts partly support this scenario with downward/inward being the most dominant but not unique pattern, other diverse net ion drift configurations exist but their impact on the strip-like bulges remains unclear.