Speaker
Atefeh Barekat
(Max-Planck-Institut für Sonnensystemforschung (MPS))
Description
Helioseismology has provided unprecedented information about
the internal rotation of the Sun. One of the important
achievements was the discovery of two radial shear layers:
one near the bottom of the convection zone (the tachocline)
and one near the surface. These shear layers may be
important ingredients for explaining the magnetic cycle of
the Sun. We measure the logarithmic radial gradient of the
rotation rate ($\dd\ln\Omega/\dd\ln r$) near the surface of
the Sun using 15 years of f~mode rotational frequency
splittings from the Michelson Doppler Imager (MDI) and four
years of data from the Helioseismic and Magnetic Imager
(HMI). We model the angular velocity of the Sun in the upper
$\sim 10$~Mm as changing linearly with depth and use a
multiplicative optimally localized averaging inversion to
infer the gradient of the rotation rate as a function of
latitude. Both the MDI and HMI data show that
$\dd\ln\Omega/\dd\ln r$ is close to $-1$ from the equator to
60$^{\circ}$ latitude and stays negative up to 75$^{\circ}$
latitude. However, the value of the gradient is different
for MDI and HMI for latitudes above $60^{\circ}$.
Additionally, there is a significant difference between the
value of $\dd\ln\Omega/\dd\ln r$ using an older and recently
reprocessed MDI data for latitudes above $30^\circ$. We
could reliably infer the value of $\dd\ln\Omega/\dd\ln r$ up
to 60$^{\circ}$, but not above this latitude, which will
hopefully constrain theories of the near-surface shear layer
and dynamo. Furthermore, the recently reprocessed MDI
splitting data are more reliable than the older versions
which contained clear systematic errors in the high degree f
modes.
Primary author
Atefeh Barekat
(Max-Planck-Institut für Sonnensystemforschung (MPS))
