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