Importance of the Height Distribution of Joule Heating for Thermospheric Density

1. Importance of the Height Distribution of Joule Heating for Thermospheric Density Arthur D. Richmond and Astrid Maute NCAR High Altitude Observatory

2. .b Field-Aligned Poynting Flux, IMF Bz = -5 nT, By = 0 Sp||.b .b Summer 104 GW 73 GW 177 GW 10 10 20 mW/m2 mW/m2 mW/m2 -10 -10 -10 Winter 56 GW 64 GW 120 GW 10 10 20 mW/m2 mW/m2 mW/m2 -10 -10 -10

4. TIE-GCM Experiments • Base case: • Steady-state, Day number = 80 • F10.7 = 150 • Polar-cap Potential = 45 kV • Hemispheric Power of auroral particles = 16 GW • Test cases: • - 9 GW additional neutral heating in each hemisphere • - proportional to vi2 in latitude and longitude • - turned on during UT hours 1-6 on Day 1, then off • (a) High Heat at 270 km ± 1 scale height • (b) Low Heat at 120 km ± 1 scale height • High Heat is centered 5 scale heights above Low Heat (e5 = 148)

5. Height-Integrated Additional Heating 0.56 mW/m2 0.28 X 700 W 0.06

7. Temperature Difference at -700 Longitude 6 UT 12 UT 18 UT High Heat, 3 K contour interval Low Heat, 1K contour interval

8. Density Difference (%) at -700 Longitude 6 UT 12 UT 18 UT High Heat, 3% contour interval Low Heat, 1% contour interval

9. Temperature Difference at 400 km High Heat Global Mean K K 700 W Low Heat Global Mean K K 700 W

11. F-region Joule heating, though only a small fraction of total Joule heating, produces a strong, fast (hours) temperature and density response at 400 km that decays rapidly. • F-region Joule heating depends on highly variable conductivity. It may be much more important with intense soft particle precipitation and at solar maximum. • E-region heating produces a slow (~1 day) response at 400 km that decays slowly. It is important for long-term variations of temperature and density. • Poynting flux and geomagnetic indices are inadequate for determining high-altitude Joule heating. Observations and modeling of polar F-region electron densities, in relation to electric fields, are needed. Conclusions