On the relationship between polar cap flows and the IMF

W.A. Bristow, R.T. Parris, J. Spaleta, T. TheurerGeophysical Institute, University of Alaska Fairbanks

McMurdo SuperDARN

• McMurdo Station SuperDARN built January 2010

• Station Latitude 77.88° South Geographic (~80° Magnetic)

• Magnetic Pole near center of FOV at ~1200 km range

The Geometry

Radar at MLT midnight ~730 UT ~1800 SLT

Radar at MLT Noon ~1930 UT ~0600 SLT

The Geometry

Terminator

100 km

200 km

300 km

The Geometry

200 km Shadow Height

Convection Maps

•Ruohoniemi and Baker convection mapping routine

•McMurdo data fill in region over magnetic pole

Convection Dependance• Examine

dependance Earth-Sun component of velocity on the IMF

• Use line of sight velocity to eliminate model influence

• Average of all LOS vectors where λ>85° and kr ∙kE-S > 0.9

Convection Dependance

Convection Dependance

Convection Dependance

Convection Dependance

Convection Dependance

Convection Dependance

Convection Dependance

Convection Dependance

Convection Dependance

Average Correlation

Convection Dependance

Convection Dependance

Convection Dependance

Expected Velocity

Ohms Law

In ionosphere B≈50µT

E???

Imposed Potential

Imposed Potential

Bow Shock

Magnetopause

Sheath

Vsw ≈300 km/sVi ≈ 1 km/s

Polar cap ~4000 km(OCB at 70°)

Expected Velocity• About 1-hour of IMF maps into

the polar cap at any given time

• Might expect ionospheric velocity to vary from place to place depending on solar wind mapping

• Except... sound speed in ionosphere

Expected Velocity• About 1-hour of IMF maps into

the polar cap at any given time

• Might expect ionospheric velocity to vary from place to place depending on solar wind mapping

• Except... sound speed in ionosphere

Expected Velocity

• For B≈50 µ T, oxygen n=1011, VA≈860 km/s

• 860 km/s >> 1km/s (flow speed)

• Flow is incompressible

- ∇∙v = 0

Uniformity?

Divergence Free?

Conclusion

• In some intervals the observed polar cap convection is strongly correlated with the IMF.

• Next step - quantify relation to IMF. Determine when good correlation is expected

• Quantify velocity divergence, vorticity, etc.