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

Field-Aligned Poynting Flux, IMF B z = -5 nT, B y = 0 Summer Winter.b.b.b.b S p||. b 104 GW 56 GW 177 GW 120 GW64 GW 73 GW 10 mW/m

TIE-GCM Experiments Base case: - Steady-state, Day number = 80 - F 10.7 = Polar-cap Potential = 45 kV - Hemispheric Power of auroral particles = 16 GW Test cases: - 9 GW additional neutral heating in each hemisphere - proportional to v i 2 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 (e 5 = 148)

Height-Integrated Additional Heating mW/m 2 X 70 0 W

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

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

High Heat Low Heat Global Mean K K K K Temperature Difference at 400 km 70 0 W

Conclusions - 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.