Unveiling Latitudinal Structure Evolution of the Topside Ionosphere From Quiet Periods to Geomagnetic Storm Conditions: Insights From In‐Situ Electron Density and Up‐Looking TEC Observations.

In this study, the typical latitudinal structure of the topside ionosphere and its response to the geomagnetic storm were investigated, based on the in‐situ electron density detection around ∼860 km and upward‐looking total electron content (TEC) above ∼830 km on dawn and dusk sides. It was found that the topside TEC had an arc‐shaped latitudinal structure on both dawn and dusk sectors, regardless of longitude, local time, and season. The dusk side in‐situ electron density displayed a narrow, Mexican hat‐shaped structure with the peak at the magnetic equator, while this structure disappeared on the dawn side. The magnitude of this hat‐shaped structure was highly dependent on longitude. In addition, the hat‐shaped structure was partly masked by strong hemispheric asymmetry during the solstice, while the topside TEC remained largely symmetric between the northern and southern hemispheres. During the main phase of geomagnetic storms, topside ionospheric electron density was enhanced at middle latitudes. However, the arc‐shaped structure in the topside TEC appeared less sensitive to geomagnetic disturbances than that of in‐situ electron density. The discrepancies between in‐situ electron density and up‐looking TEC suggested that topside TEC were more representative of plasmasphere than the in‐situ electron density at ∼860 km, which thus highlighted the complex altitude‐dependent behavior of the topside ionosphere. Key Points: Latitudinal structure of electron density at 860 km, as compared with up‐looking total electron content (TEC), is more extensively dependent on longitude and LTIn‐situ electron density at 860 km has a hat‐shaped structure, while topside TEC above 830 km displays an arc‐shaped structure in equinoxThe response of the latitudinal structure of electron density in the topside ionosphere to geomagnetic storms varies with seasons [ABSTRACT FROM AUTHOR]