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

Abstract

[1] 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 (N m F 2 ), 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 N m F 2 was lower than quiet time values. The second part of the SED is associated with significantly increased N m F 2 , 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~10% of the total F region density. The thermospheric wind also plays a minor role in this case.

On the generation/decay of the storm‐enhanced density plumes: Role of the convection flow and field‐aligned ion flow

“…The equatorward thermospheric wind can induce such an upward ion drift along the magnetic field line. This mechanism is found to be very important during the storm recovery phase and at midlatitudes [Lu et al, 2012] and contributes to the vertical flows near the poleward edge of the SED base region [Zou et al, 2013].…”

“…A superfountain effect has been invoked to explain the SED formation during superstorms when the Dst index drops to lower than À250 nT [Tsurutani et al, 2004;Mannucci et al, 2005], and as a result, the peak of the low-latitude ionization anomaly can be pushed from the nominal ±10°to~± 25°to 40°magnetic latitudes [Tsurutani et al, 2013;Lu et al, 2013]. For geomagnetic storms with lower intensity, the superfountain effect is not believed to be able to account for the density increase at higher latitudes [Huang et al, 2005;Zou et al, 2013]. Huang et al [2005] reported a 2-4 times F region peak density enhancement at~53°MLAT over Millstone Hill and suggested that penetrating eastward electric fields and the associated northward flows lead to the density increase.…”

“…Huang et al [2005] reported a 2-4 times F region peak density enhancement at~53°MLAT over Millstone Hill and suggested that penetrating eastward electric fields and the associated northward flows lead to the density increase. In a more recent study, Zou et al [2013] reported observations from multiple instruments, including Poker Flat Incoherent Scatter Radar (PFISR) and SuperDARN, and they also suggested that large vertical flows associated with northwestward convection flows play the dominant role for the density increase over Alaska (>65°MLAT). Equatorward thermospheric wind and particle precipitation also contribute but to a lesser degree.…”

“…Observations using ground-based incoherent scatter radar have revealed elevated F region electron densities, increased ionospheric F 2 layer peak density height (h m F 2 ), low electron temperature (T e ), and large convection flows associated with SEDs [Foster et al, , 2007Huang et al, 2005;Zou et al, 2013]. The low-density midlatitude trough is usually poleward of the SED and eastward of the plume, which results in large density and total electron content (TEC) gradients there.…”

“…The SED plume is suggested to be the ionospheric projection of a plasmaspheric plume in the magnetospheric equatorial plane . Within the magnetosphere, plasmaspheric plumes can play a crucial role in regulating the dayside reconnection at the magnetopause [Borovsky et al, 2008;Walsh et al, 2014].Observations using ground-based incoherent scatter radar have revealed elevated F region electron densities, increased ionospheric F 2 layer peak density height (h m F 2 ), low electron temperature (T e ), and large convection flows associated with SEDs [Foster et al, , 2007Huang et al, 2005;Zou et al, 2013]. The low-density midlatitude trough is usually poleward of the SED and eastward of the plume, which results in large density and total electron content (TEC) gradients there.…”