J/A+A/619/A99 Helioseismic measurements of solar meridional flow (Liang+, 2018)
Solar meridional circulation from twenty-one years of SOHO/MDI and SDO/HMI
observations: Helioseismic travel times and forward modeling in the ray
approximation.
Liang Z.-C., Gizon L., Birch A.C., Duvall T.L.Jr, Rajaguru S.P.
<Astron. Astrophys. 619, A99 (2018)>
=2018A&A...619A..99L 2018A&A...619A..99L (SIMBAD/NED BibCode)
ADC_Keywords: Sun ; Solar system
Keywords: Sun: helioseismology - Sun: oscillations - Sun: interior
Abstract:
The solar meridional flow is an essential ingredient in flux-transport
dynamo models. However, no consensus on its subsurface structure has
been reached.
We merge the data sets from SOHO/MDI and SDO/HMI with the aim of
achieving a greater precision on helioseismic measurements of the
subsurface meridional flow.
The south-north travel-time differences are measured by applying
time-distance helioseismology to the MDI and HMI medium-degree
Dopplergrams covering May 1996-April 2017. Our data analysis corrects
for several sources of systematic effects: P-angle error, surface
magnetic field effects, and center-to-limb variations. For HMI data,
we used the P-angle correction provided by the HMI team based on the
Venus and Mercury transits. For MDI data, we used a P-angle
correction estimated from the correlation of MDI and HMI data during
the period of overlap. The center-to-limb effect is estimated from the
east-west travel-time differences and is different for MDI and HMI
observations. An interpretation of the travel-time measurements is
obtained using a forward-modeling approach in the ray approximation.
In the latitude range 20°-35°, the travel-time differences are
similar in the southern hemisphere for cycles 23 and 24. However, they
differ in the northern hemisphere between cycles 23 and 24. Except for
cycle 24's northern hemisphere, the measurements favor a single-cell
meridional circulation model where the poleward flows persist down to
∼0.8R☉, accompanied by local inflows toward the activity belts
in the near-surface layers. Cycle 24's northern hemisphere is
anomalous: travel-time differences are significantly smaller when
travel distances are greater than 20°. This asymmetry between
northern and southern hemispheres during cycle 24 was not present in
previous measurements, which assumed a different P-angle error
correction where south-north travel-time differences are shifted to
zero at the equator for all travel distances. In our measurements, the
travel-time differences at the equator are zero for travel distances
less than ∼30°, but they do not vanish for larger travel
distances. This equatorial offset for large travel distances need not
be interpreted as a deep cross-equator flow; it could be due to the
presence of asymmetrical local flows at the surface near the end
points of the acoustic ray paths.
The combined MDI and HMI helioseismic measurements presented here
contain a wealth of information about the subsurface structure and the
temporal evolution of the meridional circulation over 21 years. To
infer the deep meridional flow, it will be necessary to model the
contribution from the complex time-varying flows in the near-surface
layers.
Description:
The measured travel-time shifts are averaged over three periods, cycle
23 (May 1996 to April 2008; 3051 days used), cycle 24 (May 2008 to
April 2017; 2833 days used), and both the cycles 23 and 24 (May 1996
to April 2017; 5884 days used), as a function of latitude and travel
distance. Also provided are the standard errors of the temporal means
over the three periods. They are all in units of seconds. The
coordinates of these maps can be obtained from the WCS keywords in the
headers; that is,
latitude = (i - CRPIX1)*CDELT1 + CRVAL1 [deg], and
distance = (j - CRPIX2)*CDELT2 + CRVAL2 [deg], where i=1..200 and
j=1..61.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
both_avg.dat 24 12200 *Measured travel-time shift over three periods
both_sem.dat 24 12200 *Measured travel-time shift over three periods
cy23_avg.dat 24 12200 Measured travel-time shift over cycle 23
cy23_sem.dat 24 12200 Measured travel-time shift over cycle 23
cy24_avg.dat 24 12200 Measured travel-time shift over cycle 24
cy24_sem.dat 24 12200 Measured travel-time shift over cycle 24
fits/* . 6 Indivudal files in fits
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Note on both_avg.dat, both_sem.dat : cycle 23 (May 1996 to April 2008;
3051 days used), cycle 24 (May 2008 to April 2017; 2833 days used),
and both the cycles 23 and 24 (May 1996 to April 2017; 5884 days used).
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Byte-by-byte Description of file: *.dat
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Bytes Format Units Label Explanations
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1- 5 F5.1 deg LAT [] Latitude
7- 11 F5.1 deg Dist [6/42] Distance
13- 24 E12.6 s dTSN ?=- Measured travel-time shift
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Acknowledgements:
Zhi-Chao Liang, zhichao(at)mps.mpg.de
(End) Patricia Vannier [CDS] 27-Aug-2018