Multi-instrument observations of SED during 24-25 October 2011 storm: Implications for SED formation processes

We present multiple instrument observations of a storm-enhanced density (SED) during the 24-25 October 2011 intense geomagnetic storm. Formation and the subsequent evolution of the SED and the midlatitude trough are revealed by global GPS vertical total electron content maps. In addition, we present high time resolution Poker Flat Incoherent Scatter Radar (PFISR) observations of ionospheric profiles within the SED. We divided the SED observed by PFISR into two parts. Both parts are characterized by elevated ionospheric peak height (h(m)F(2)) and total electron content, compared to quiet time values. However, the two parts of the SED have different characteristics in the electron temperature (T-e), the F region peak density (NmF2), and convection flows. The first part of the SED is associated with enhanced T-e in the lower F region and reduced T-e in the upper F region and is collocated with northward convection flows. The NmF2 was lower than quiet time values. The second part of the SED is associated with significantly increased NmF2, elevated T-e at all altitudes and is located near the equatorward boundary of large northwestward flows. Based on these observations, we suggest that the mechanisms responsible for the formation of the two parts of the SED may be different. The first part is due to equatorward expansion of the convection pattern and the projection of northward convection flows in the vertical direction, which lifts the ionospheric plasma to higher altitudes and thus reduces the loss rate of plasma recombination. The second part is more complicated. Besides equatorward expansion of the convection pattern and large upward flows, evidences of other mechanisms, including horizontal advection due to fast flows, energetic particle precipitation, and enhanced thermospheric wind in the topside ionosphere, are also present. Estimates show that contribution from precipitating energetic protons is at most similar to 10% of the total F region density. The thermospheric wind also plays a minor role in this case.