1National Solar Observatory, Tucson 2University of Aarhus

Convection-Zone Dynamics from GONG and MDI
1995-2003
R.
1National
1
Howe ,
R. W.
1
Komm ,
1
Hill ,
2
Christensen-Dalsgaard ,
F.
J.
3
4
D. Haber , J. Schou
Solar Observatory, Tucson 2University of Aarhus, Denmark 3JILA, University of Colorado 4Stanford University
INTRODUCTION
The combined GONG and MDI data for mediumdegree helioseismology now span nearly nine
years, from just before the end of the previous
solar cycle to the early declining phase of the
current one. This allows us to make a detailed
study of the dynamics of the Convection Zone.
The data consist of 83 overlapping 108-day sets
from GONG, with starting times spaced by 36
days, and 37 non-overlapping 72d sets from MDI.
As in the work of Howe et al (2000), but now with
twice as much data, we use two different 2dimensional inversion methods, Regularized Least
Squares (RLS) and Optimally Localized Averaging
(OLA) to estimate the global rotation profile.
The figure at right shows the residuals after
subtraction of a temporal mean at each location,
for GONG (black) and MDI (green) from RLS
inversions and for OLA inversions of MDI data
(orange). There is good agreement among the
different sets of results. The results suggest that
the variation at each location is more complicated
than a single sinusoid.
Radial and latitudinal slices
The plots show the evolution of the rotation
residuals in selected time-latitude (left) and
time-depth (right) planes. The pattern of flows
migrating towards the equator is visible even
at the 0.84R depth. A new high-latitude branch
can be seen beginning to move towards the
equator around the beginning of 2003; this will
presumably become the main branch in the
next cycle. As in the plots above, there are
hints that the pattern deeper inside the
convection zone leads that at the surface, at
least at lower latitudes. It seems that the
pattern is roughly constant in phase along
surfaces of constant rotation rate, which are
not quite radial.
Joining up the solar cycle
Together, the GONG and MDI data cover nearly nine
years. To project the configuration of the flows for the
remaining two years of the cycle, the combined MDI and
GONG RLS rotation residuals were fitted with sine
waves of period 11 years and 11/3 years (Vorontsov et
al 2002). The sine waves were then extrapolated to fill in
the missing data. This combination of periods was found
to give a fairly stable prediction when fitting 8 years or
more of data.
Acknowledgments
This work utilizes data obtained by the Global Oscillation Network Group (GONG)
project, managed by the National Solar Observatory, which is operated by AURA, Inc.
under a cooperative agreement with the National Science Foundation. The data were
acquired by instruments operated by the Big Bear Solar Observatory, High Altitude
Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de
Astrofísico de Canarias, and Cerro Tololo Interamerican Observatory. We also used
data from the Solar Oscillations Investigation (SOI), obtained using the MDI instrument
aboard SOHO. SOHO is a joint project of NASA and ESA. This work was supported in
part by NASA grants S-92698-F and NAG5-11703 to the National Solar Observatory,
and NAG5-10917, NAG5-11920 and NAG-121491, and by NSF grant ATM-0219581 to
the University of Colorado.
Comparison with results from local
helioseismology
The plot at right shows the estimated zonal
flows at 0.99R from ring-diagram analysis of
MDI Dynamics data (Hill 1988, Haber et al
2002). The ring-diagram results are calculated
for fifteen-degree squares spaced by 7.5
degrees in latitude, and averaged over each
Carrington rotation. The color scale is the
same as for the other plots in the poster. The
overlaid contours show the results at the same
depth from global RLS inversions of the MDI
medium-l data, with black indicating faster and
green slower than average rotation. For the
sake of the comparison, the ring-diagram
results have been symmetrized about the
equator, and have had both a temporal mean
at each location, and a latitudinal mean for
each Carrington rotation, subtracted. The
agreement is quite good both in position and
scale.
Conclusions
•Observations of the zonal flows from global inversions show good
agreement between the GONG and MDI observations and between
two different inversion techniques.
•The migrating zonal flow bands penetrate deep in the convection
zone, but do not lie along radial lines.
•The evolution of the flows can be represented by an eleven-year
sinusoidal oscillation and its third harmonic.
•The high-latitude branch associated with the next solar cycle can be
seen beginning to migrate equatorwards around 2002/3.
•The near-surface results from the global inversions show reasonable
agreement with the results from ring-diagram local analysis.
References
Haber, D. A., Hindman, B. W., Toomre, J., Bogart,
R. S., Larsen, R. M., & Hill, F.
2002, Astrophysical Journal, 570, 855
Hill, F.
1988, Astrophysical Journal, 333, 996
Howe, R., Christensen-Dalsgaard, J., Hill, F.,
Komm, R. W., Larsen, R. M., Schou J., Thompson,
M. J., and Toomre, J.
2000, Astrophysical Journal 533 L163-L166
Vorontsov, S. V., Christensen-Dalsgaard, J., Schou,
J., Strakhov, V. N., & Thompson, M. J.
2002, Science 296, 10