Importance of the Height Distribution of Joule Heating for Thermospheric Density

Arthur D. Richmond and Astrid MauteNCAR High Altitude Observatory

Field-Aligned Poynting Flux, IMF Bz = -5 nT, By = 0

Summer

Winter

.b .b Sp||.b

104 GW

56 GW

177 GW

120 GW64 GW

73 GW

10

mW

/m2

mW

/m2

mW

/m2

mW

/m2

mW

/m2

mW

/m2

10 10

10

-10 -10

-10 -10

-10

-10

20

20

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 vi

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 (e5 = 148)

Height-Integrated Additional Heating

mW

/m2

X

700 W

0.56

0.28

0.06

6 UT 12 UT 18 UTTemperature Difference at -700 Longitude

High Heat, 3 K contour interval

Low Heat, 1K contour interval

6 UT 12 UT 18 UTDensity Difference (%) at -700 Longitude

High Heat, 3% contour interval

Low Heat, 1% contour interval

High Heat

Low Heat

Global Mean

Global Mean

KK

KKTemperature Difference at 400 km

700 W

700 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.