Weakening of polar magnetic fields during cycle 23 Eva Robbrecht Royal Observatory of Belgium* (*) work performed while at the Naval Research Laboratory Ref: Y.-M. Wang, E. Robbrecht & N. Sheeley, ApJ (2009) Remarkable deep minimum ~4- 5G ~7- 8G Polar fields and IMF are ~40% weaker than during previous minima MWO Polar field evolution 20 21 22 23 Polar fields are formed by transporting magnetic flux of AR to poles veff = 20 m/s Mechanism of polar field reversal Understanding the weak polar field during cycle 23 Simulate evolution of Bphot by using a Surface flux transport model Simulated photospheric field 17 m/s 15.5 m/s 14.5 m/s 17 m/s v(L)=vm sin0.1 |L| cos1.8 |L| Simulated photospheric field 17 m/s 15.5 m/s X 14.5 m/s 17 m/s Effect of meridional flow speed A reduction of only 15% in flow speed results in doubling the polar field IMF is weaker because of weaker polar field ➡ more cosmic rays! Note on discrepancy in flow profiles Meridional flow Above AR: diffusion + = Net rate: Below AR: diffusion: Net rate: + = Final Polar field depends on: Initial polar field strength Total flux that emerges (cycle amplitude) Tilt and locations of ARs Supergranular diffusion rate Meridional Flow speed Some details on the model number of BMR = ƒ(SSN) BMR sources have same B strength polarity: Hale-Joy law tilt-angle = ƒ(Latitude) Latitude migrates equatorward during cycle deposited at random longitude (stochastic fluctuations) κ = 500 km2/s (diffusion rate) Signatures of weak polar field Smaller polar coronal holes Less flattening of heliospheric current sheet > 750 - 650 - 550 < 450 (km/s) low latitude coronal holes Signatures of weak polar field > 750 - 650 - 550 < 450 (km/s) Added B field