Observations of a positive storm phase on September 10, 2005

In this study, we present multi-instrument observations of a strong positive phase of ionospheric storm, which occurred on September 10, 2005 during a moderate geomagnetic storm with minimum Dst=−60 nT and maximum Kp=6-. The daytime electron density measured by the Millstone Hill incoherent scatter radar (42.6°N, 288.5°E) increased after 13 UT (∼8 LT) compared with that before the storm. This increase is observed throughout the daytime, lasts for about 9 h, and covers F-region altitudes above ∼230 km. At the altitude of 300 km, the maximum increase in Ne reaches a factor of 3 by 19:30-20:00 UT and is accompanied by a ∼1000 K decrease in electron temperature, a ∼100-150 K increase in ion temperature, and a strong upward drift. Observations by Arecibo ISR (18.3°N, 293.3°E) reveal similar features, with the maximum increase in electron density reaching a factor of 2.5 at 21:30 UT, i.e. 1.5-2 h later than over Millstone Hill. The GPS TEC data show that the increase in electron density observed at Millstone Hill and Arecibo is only a part of a global picture reflected in TEC. The increase in TEC reaches a factor of 2 and covers middle and low latitudes at 19 UT. At later times this increase moves to lower latitudes. A combination of mechanisms were involved in generation of positive phase. The penetration electric field resulted in Ne enhancements at subauroral and middle latitudes, the TAD/TID played an important role at middle and lower latitudes, and increase in O/N2 ratio could contribute to the observed positive phase at middle and lower latitudes. The results show the importance of an upward vertical drift at ∼140-250 km altitude, which is observed for sustained period of time and assists in the convergence of ionization into the F-region.