Transcript DeRosa-LoHCo

Large-Scale Inflows Around Active Regions:
Consequences on Surface Magnetic Field Dispersal
Marc DeRosa, Karel Schrijver

Lockheed Martin Solar and Astrophysics Laboratory
Palo Alto, CA

9 November 2006
LoHCo Meeting, Boulder, CO
Solar Surface-Flux Evolution

Appearance and evolution of surface-flux provides many
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All scales are important! For example, active regions
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MDI has enabled detailed studies of the evolution of
surface flux (especially small-scale ephemeral population).
constraints on (and many clues toward understanding)
the solar dynamo.
would evolve differently if magnetic carpet were absent.
Can we build an idealized model based on their
characteristics? What can we learn about the dynamo
from such models?
Evolving Surface-Flux Transport Model
flux
emergence
Research areas:
Dynamo, alpha effect
3D magnetoconvection
Global field-flow coupling
Sub-resolution dynamics
3D flux transport
This model includes:
random
stepping
diff. rot. &
merid. flow
flux fragmentation
collision &
cancellation
Schrijver (2001)
– active to ephemeral region flux, atomic description (no grid)
– bipole strengths, emergence latitudes, tilts chosen from
empirically determined statistical distribution functions
– nonlinear magnetoconvective coupling: nesting, and
flux-dependent dispersal coefficient
Consistency Check
cycle maximum
cycle minimum

(Mx cm-2)
Histograms of model flux match histograms of flux
observed from magnetograms very well.
Schrijver (2001)
model magnetogram
Model Activity Cycles
pure simulated Sun
(from 40° corotating frame)

Formation of polar caps
occurs naturally, arising
from the tilt of emergent
bipoles, combined with the
convective dispersal and
poleward meridional flows.
Schrijver (2001)
Model Activity Cycles
pure simulated Sun
(from 40° corotating frame)
for fun: simulated star that is
30 more active than sun
Schrijver (2001)
Schrijver (2001)
Historical Sunspot Cycles
Schrijver, DeRosa & Title (2002)
(1022 Mx)
Model Surface Flux
Schrijver, DeRosa & Title (2002)
(1022 Mx)
Net Flux Poleward of North-60°
Schrijver, DeRosa & Title (2002)
No polar
polarity
inversion?
(1022 Mx)
What If Flux Decayed with a
Half-Life Set to 5 yrs?
Schrijver, DeRosa & Title (2002)
Possible Solution to the Conundrum:
Active Region Inflows

Helioseismic inferences of subsurface and near-surface
flows indicate that many active regions seem to be
surrounded by horizontal inflows on the order of 20-50 m/s
very near the surface.
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Additionally, there is evidence that the magnitude of the
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What effects do these inflows have on the transport and
evolution of surface magnetic fields? Can these inflows help
to solve the polar-flux paradox?
inflow velocities scales with the amount of flux contained
within the active region.
Measurements of Active Region Inflows
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Below are shown surface flows inferred from f-mode timedistance analysis for part of CR1949 (in 1999), as an
example of this phenomenon.
Gizon (2004)
Measurements of Active Region Inflows
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Inflows surrounding active regions are also found in
ring-diagram analyses of active regions.
Hindman et al. (2004)
What effects can these active-region
inflows have on surface fields?

Inflows affect the appearance and evolution of
active regions.

Inflows affect the amount of flux transported
poleward during each sunspot cycle.

Polar-cap flux is the source of much of the
heliospheric field (especially during solar
minimum).

Polar-cap flux might eventually be recycled into
the convection zone, and appear as emergent
flux during future sunspot cycles.
MDI Assimilation Model
Schrijver & DeRosa (2003)
Adding Inflows to the Model
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Model inflows scale with the gradient in absolute
flux density, after 15 smoothing: v =    .
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Model inflows are time-invariant.
Adding Inflows to the Model
 → |v| = 0 m/s
 → |v| = 30 m/s
 → |v| = 10 m/s
DeRosa & Schrijver (2007)
MDI assimilation model
Concluding Remarks
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Inflows faster than 10 m/s are needed to resolve the polarflux conundrum. However, …
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Inflows faster than 10 m/s markedly affect the evolution of
active-region flux. The flows are either not as fast, not as
persistent, or not uniformly converging around the active
region as modeled here (or some combination of all three).
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We have assumed that active-region inflows and magnetic
fields couple as efficiently all other observed surface flows.
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We have also assumed that the inflows are not dependent
on the evolutionary stage of the active region. (The time
dependence of the measured inflows is not well known.)
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Maybe too there is a selection effect in the helioseismic
analyses? Looking forward to results of forward modeling
efforts…