1. On the Source of Steep TEC Gradients That Impact GPS E.A. Araujo-Pradere 1, T.J. Fuller-Rowell 1, M. Fedrizzi 2, M.V. Codrescu 1 1 CIRES-University of Colorado, NOAA-Space Environment Center 2 Instituto Nacional de Pesquisas Espaciais, Brazil. Now a NRC Research Associate at NOAA-SEC

2. SUMMARY Steep ionospheric gradients: Spatial gradients Temporal gradients. Some sources of ionospheric gradients (diverse, mixed and complex): Development of the neutral composition bulge Equatorial electrodynamics Trough and high latitude processes.

3. STORM Time Empirical Ionospheric Correction Model http://sec.noaa.gov/storm/ Gravity Waves Composition Bulge Electric Field

4. CTIM mean molecular mass NmF2 depletion

5. mean molecular mass latitudinal structure quiet conditions (kp = 2 + ) 40 o perturbed conditions (kp = 7) 40 o equatorsummer polewinter pole solar driven circulation, quiet conditions (kp = 0)

6. NOAA CORS (good spatial resolution and geographic coverage) 21 storms with a p ≥ 200, 10 days period for each one, average of 250 receivers per storm Good seasonal coverage. Data sources 191 receivers 137 receivers

7. 2000-07 summer 13 14 15 16 17 18 12 00:00 UT 06:00 UT 12:00 UT 18:00 UT 2000 – 07 (GAIM TEC)

8. 2000 – 07 (GAIM TEC) ratios quiet conditions perturbed conditions ratio summer Combination of downwelling and the electrodynamics Upwelling and equatorward transport of the neutral composition Possible interpretations:

9. March 2001 – TEC vs. Lat. TEC (TECU)

10. 00:00 UT 06:00 UT 12:00 UT 18:00 UT 24 25 26 27 28 29 23 equinox 1998 - 08 (GAIM TEC)

11. quiet conditions perturbed conditions ratio equinox Combination of the electrodynamics and downwelling Upwelling and equatorward transport of the neutral composition Possible interpretations: “Auroral” processes 1998 - 08 (GAIM TEC) ratios

12. CTIP (physics based model) Trough: transport of the electron density by the high latitude convection E

13. animation window quiet reference November 1997 18 19 20 21 22 23 24 25 26 27

14. VTEC ratio vs. time

15. CONCLUSIONS Steep ionospheric gradients can be spatial and temporal gradients. The sources of such gradients are diverse, mixed and complex, including, but not limited to, development of the neutral composition bulge, the equatorial electrodynamics, the trough and high latitude processes.