Local time dependence of the prompt ionospheric response for the 7, 9, and 10 November 2004 superstorms

Abstract

In this paper, we study the local time response of the low‐latitude ionosphere during the sequence of geomagnetic storms occurring on 7, 9, and 10 November 2004. Three distinct storm main phases leading to peak negative Dst index of at least −250 nT occur during this period. We analyze total electron content (TEC) measurements from the global network of GPS receivers available from the International Global Navigation Satellite Systems (GNSS) Service. Average TEC within the geomagnetic latitude band ±25° is computed as a function of local time for several hours following storm commencement. We find that significant TEC increases are observed on 7 November (increase of 100%) and 9 November (50%). During 7 November, the increases tend to occur during solar local times 1200–1600 LT. During 9 November, the largest TEC increases tend to occur at earlier local times (∼1000–1200 LT). No daytime TEC increase is observed on 10 November, although the Kp index suggests that this storm is comparable in intensity to the 7 November storm. Vertical drift measurements from the Jicamarca radar (Peruvian sector) for the 7 and 9 November storm periods suggest large (2–3 mV/m) low‐latitude “prompt penetration” eastward electric fields (PPEF), consistent with observed increases in TEC. For the 10 November storm, published estimates of electric fields using the dual‐magnetometer technique near the Japanese sector, which is near ∼1400 LT at storm onset, suggest the absence of eastward directed electric fields during daytime. A time series analysis of TEC in the Japanese sector throughout the 7–11 November period reveals significant daytime TEC reduction of 33% relative to quiet conditions. Reduced TEC occurs coincident with the onset of the 10 November storm period, persisting into the 11th. Global Ultraviolet Imager (GUVI) retrievals of thermospheric atomic oxygen to nitrogen ratio suggest depleted oxygen in the southern hemisphere that may contribute to the TEC reduction on 10 November. Solar rotation reduces solar X‐ray and EUV fluxes from 10 through 12 November, which may be a contributing factor to reduced TEC on 10 and 11 November. In conclusion, we postulate that an additional source of electric field is present on 10 November during daytime that counteracts the PPEF of magnetospheric origin.