Satellite in-situ electron density observations of the storm enhanced density 2 and the polar Tongue of Ionization on the noon meridional plane in the F 3 region during the  The first report on satellite in-situ electron density measurements of the storm enhanced 15 density at the noon meridian plane 16  The lifecycle of ionospheric storm enhanced densities is mainly controlled by variations 17 of the dayside prompt penetration electric fields 18  The key methodologies include a comparison of TIEGCM modeling with satellite in-situ 19 electron density observations and a correlation analysis 20 21 22 Abstract 23 Ionospheric storm enhanced density (SED) has been extensively investigated using Total 24 Electron Content (TEC) deduced from GPS ground and satellite-borne receivers. However, in-25 situ electron density measurements have not been reported for SEDs yet. We report in-situ 26 electron density measurements of a SED event and its associated polar tongue of ionization 27 (TOI) at the noon meridian plane measured by the CHAMP polar-orbiting satellite at about 390 28 km altitude during the 20 November 2003 magnetic storm. The measurements provided rare 29 evidence about the SED’s life cycle at a fixed magnetic local time. CHAMP detected the SED 30 onset right after the arrival of an interplanetary coronal mass ejection shock front. The SED 31 electron density enhancement extended from the equatorial ionization anomaly to the noon cusp, 32 through which plasmas entered into the polar cap as polar plasma clouds/TOI. For several 33 satellite-ground conjunction passes, CHAMP measured the electron density of plasma clouds 34 comparable to the TOI density measured by the Tromso ISR, establishing that the plasma clouds 35 were related to the TOI. The SED plume in the NH retreated gradually to lower latitudes six 36 hours after the SED onset. We conducted TIEGCM modeling to demonstrate that the SED 37 density enhancement was likely due to the vertical transport of plasmas. The observed mid-38 latitude electron density varied with the cross-polar cap electric fields, suggesting that prompt 39 penetration electric fields (PPEFs) in the zonal direction played a dominant role. The 40 implication is that variations of the dayside PPEFs largely control the SED lifecycle. 41 42 Plain Language Summary 43 Ground radar and GPS stations have frequently detected enhancement of ionospheric electron 44 density at mid-latitudes and in the polar cap during the magnetic storm recovery phase. We 45 report in-situ satellite observations near 400 km at the noon meridian plane during an intense 46 magnetic storm. It provides for the first time clear evidence about the life cycle of ionospheric 47 electron density enhancement, starting from its onset at mid-latitudes, entry into the polar cap, 48 and retreat to lower latitudes. The mid-latitude ionospheric electron density was mainly 49 enhanced in the northern hemisphere, triggered by the passage of a solar wind dynamic pressure 50 shock front. Global circulation modeling suggests that the vertical transport of ionospheric 51 plasmas probably produced the enhancement. The dayside prompt-penetration electric fields in 52 the zonal direction likely drove the vertical plasma uplift. Thus, it appears that the SED lifecycle 53 is mainly controlled by variations of the dayside prompt electric field. 54