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