With the launch of the Defense Meteorological Satellite Program F‐15 spacecraft in late 1999, data for calculating Earth‐directed, magnetospheric Poynting flux became available for the 09–21 solar local time sectors. We have assembled a data base for this key element of the upper atmosphere energy budget, for the interval 2000–2005. Here we briefly introduce the data set and show a subset that reveals a pattern of extreme Poynting flux deposition associated with a large east‐west interplanetary magnetic field component. At such times the dayside high‐latitude Poynting flux may exceed 170 mW/m2—an order of magnitude above typical values. The likely source of these events is merging at the magnetopause flank and lobe. A significant fraction of these events occur with high speed solar wind. This pattern of extreme Poynting flux deposition has, to date, eluded detection. Energy deposition at these high rates is a likely source of previously reported, but poorly understood, near‐cusp neutral density enhancements.

Tri‐axial accelerometer data from the Challenging Minisatellite Payload (CHAMP) satellite have revealed the thermospheric density and its variability in unprecedented detail. The data often contain regions of high density located in the cusp sector at high latitudes. In this paper we provide the first detailed explanation of a high latitude density enhancement observed by CHAMP, focusing on the August 24, 2005 interval. The Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM) was driven by high‐fidelity high‐latitude inputs specified by the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) algorithm, and reproduced the main features of the density enhancements. The TIMEGCM and AMIE provide a global framework for interpretation of the CHAMP densities. Our simulations reveal that the observed density enhancement in the dayside cusp region resulted from unexpectedly large amounts of energy entering the Ionosphere‐Thermosphere system at cusp latitudes during an interval of strong (+20 nT) BY.

[1] The question of whether the cross polar cap potential drop in the Earth's ionosphere saturates under conditions of extreme electric field in the solar wind has been tested observationally during the past several years. The challenge to proving the existence of this phenomenon is that periods of such extreme electric fields in the solar wind are relatively rare. The three superstorms of October and November 2003 provided ideal cases for testing this idea. We first review the earlier evidence of the saturation seen by the DMSP-F13 spacecraft during the 31 March 2001 superstorm and other storm events during the 1998-2002 time period. Then we present observations from the DMSP-F13 spacecraft during the October and November 2003 superstorms that show definite evidence of this saturation. In addition, some of the electric fields during these superstorms were almost twice as large as the largest fields previously studied, thus increasing the range of our sample set and further increasing our confidence in the existence of the saturation phenomenon. The data are compared with the saturated potentials predicted by the Hill-Siscoe model to test its validity. The DMSP measurements indicate that the saturation limit of the cross polar cap is about 260 kV.

[1] We show evidence that solar wind density enhancements and pressure pulses can lead to intense low-energy particle precipitation and an associated, but unexpected, damping of thermospheric density response. Ground-based indices, used as proxies for thermospheric energy deposition, fail to capture these interactions in forecasting algorithms. Superposed epoch comparison of a group of poorly specified neutral density storms suggests an event-chain of (1) multihour, pre-storm solar wind density enhancement, followed by solar wind dynamic pressure pulses that trigger excess low-energy particle flux to the upper atmosphere; (2) enhanced production of thermospheric Nitric Oxide (NO) by precipitating particles and storm heating; (3) NO infrared cooling and damping of the thermosphere; and (4) misforecast of neutral density. In the control storms, these features are absent or muted. We discuss the roles of solar wind pre-conditioning and solar cycle dependency in the problem storms. These problem neutral-density storms reveal an element of "geo-effectiveness" that highlights competition between hydrodynamic aspects of the solar wind and other interplanetary drivers.