Yi-Ming Wang/US Naval Research
Laboratory
The Sun's
Polar Fields and the
Dynamo
OBJECTIVES
AND APPROACH: We will derive constraints on the solar dynamo as
follows:
- Using polar faculae counts since 1906,
Greenwich sunspot data, and magnetograph measurements since 1967 as
observational constraints, we will carry out surface flux-transport
simulations of the photospheric field evolution during cycles 15--23.
We will then explore the relationship between cycle amplitude and
meridional flow speed (a time-varying parameter in our model), cycle
amplitude and the strength of the polar fields, and cycle length and
flow speed. In addition, we will determine whether the observed giant
poleward surges of flux, and the accompanying fluctuations in the polar
fields and faculae numbers near sunspot maximum, require higher flow
speeds.
- As a by-product of these long-term
simulations, we will quantify the relationship between total
photospheric flux (facular brightness) and total sunspot area, in order
to verify our hypothesis that increased diffusive annihilation during
high-amplitude cycles causes TSI to saturate.
- Using MWO, WSO, NSO/Kitt Peak, and MDI
photospheric field measurements, MDI 676.7 nm images of polar faculae,
and flux transport simulations, we will address the question of why the
polar fields ended up so weak in 2008. The flow speeds derived at low
latitudes from our simulations will be compared with helioseismic
measurements, keeping in mind that the latter may not refer to the same
depths that control the magnetic field transport. We will also measure
the axial tilts of cycle 23 active regions to see if they are
systematically smaller than the cycle 21 tilts analyzed in Wang &
Sheeley (1989).
- We will determine how much
leading-polarity flux diffuses across the equator during cycles 20--23
by measuring the steepness of the observed photospheric-field gradients
at the equator, and check if the resulting values are consistent with
the observed polar field strengths at the end of each cycle. We will
ascertain whether these latitudinal gradients were smaller during cycle
23 than during the previous three cycles, which would account for the
weak polar fields during the current activity minimum.
- We will provide a physical explanation
for the success of geomagnetic activity precursors in predicting the
amplitude of the following solar cycle, by separating the aa index into
its constituent parts (mainly solar wind speed and IMF strength). At
the same time, we will test our hypothesis that the IMF strength near
sunspot minimum provides a better cycle predictor than the aa index
itself.
RELEVANCE: The proposed research directly
addresses LWS TR&T Focused Science Topic (a): "Measure the
properties of the solar dynamo that affect solar irradiance and active
region generation." More specifically, it answers the call for analyses
revealing how the magnetic characteristics of the solar poles affect
the dynamo and the solar activity cycle, and for the "use of
observations that discriminate between models that forecast the
properties of cycle 24." Our flux transport model will also allow us to
investigate how sunspot and plage areas interact to determine the solar
irradiance variation.