Density Gradients

The sub-auroral ionosphere, at the magnetic latitudes which characterize the northeastern United States, is subject to severe F-region ionospheric density structuring due to the space weather effects of magnetospheric disturbance electric fields. Communications and navigation systems relying on trans-ionospheric propagation must be able to compensate for the effects of the sharp changes (>10x) in total electron content (TEC) associated with the ionospheric trough and storm-time disturbance effects at mid latitudes. Vo and Foster [2001] used the Millstone Hill incoherent scatter radar database to investigate the spatial extent and temporal evolution of TEC and density altitude/latitude structure at mid and sub-auroral latitudes. More than 11,000 radar elevation scans covering >20 degrees of latitude and altitudes between 150 km and 750 km were used to identify the characteristics of the density gradient near the equatorward edge of the ionospheric trough in a variety of circumstances spanning 20 years and two solar cycles. Pronounced density gradients can be identified in ~35% of the Millstone Hill scans. Larger TEC gradients occur at solar maximum, when the background TEC is higher. The steepest gradients occur in an environment of high TEC (solar max and adjacent to regions of storm-enhanced density, SED), when the processes which generate the trough are strongest (high Kp).

Foster and Rideout [2005] used observations from the array of North American GPS receivers to examine the formation and severity of mid-latitude enhancements and steep gradients in total electron content (TEC) during the October 30-31, 2003 superstorm. A large (~10x) enhancement in dayside TEC was observed over the US mainland during these events as Dst decreased sharply and strong SAPS electric fields eroded the outer reaches of the post-noon plasmasphere boundary layer (PBL) forming poleward-streaming plumes of storm enhanced density. TEC increased to >250 TECu equatorward of the PBL. TEC gradients across the PBL over the central US exceeded 60 TECu per deg latitude. While intense, these features are qualitatively similar to those seen in lesser storms.

Foster, J. C., and W. Rideout, Midlatitude TEC enhancements during the October 2003 superstorm, Geophys. Res. Lett., 32, L12S04, doi:10.1029/2004GL021719, 2005.

Vo, H. B, and J. C. Foster, Quantitative investigation of ionospheric density gradients at mid latitudes, J. Geophys. Res., 106, 21555-21563, 2001.

SEARCH: