Continuous Subsurface Flow Measurements for Solar Cycle 23 from MDI Spherical Harmonic Time Series

Abstract:

Subsurface flows play an important role in the flux-transport dynamo models. In
particular, the transport of surface poloidal magnetic field to the bottom of the
convection zone is governed by the meridional circulation, where the poloidal field
is converted into a toroidal field by rotational shear. The past two decades have
witnessed rapid advancements in measuring the subsurface flow patterns due to the
availability of high-resolution observations from space and ground-based
observatories. However, continuous measurement of meridional flow for the entire
solar cycle 23 is not available due to the limited high-resolution observations from
Michelson Doppler Imager (MDI) instrument on board Solar and Heliospheric
Observatory (SoHO). Here we report on a novel method that combines two
methods; the image reconstruction from spherical harmonic time series, and the
local helioseismic technique of ring diagrams, to produce zonal and meridional
flow measurements from May 1996 to December 2010 up to a depth of 40 Mm
below the surface. In addition to the temporal variation of the meridional flow,
where the flow is faster at solar cycle minimum and slower at maximum, we find
several interesting features notably that the meridional flows have a strong depth
dependence that differs during the solar minimum and maximum phases. We also
find that the meridional flows in the activity belt differs from the flows at higher
latitude around 20 Mm below the surface.

This work is partially funded by NASA grants NNH18ZDA0001N-HDEE and
NNH18ZDA001N-LWS.