NSO/GONG: Solar Meridional Circulation

Meridional circulation is a major focus of solar studies as it has become a key component of solar-dynamo models. Long-term, high-resolution observations from GONG give us the unprecedented opportunity to study the meridional flow using local helioseismology with continuity.

Meridional Circulation during the last Solar Cycle

Time-distance helioseismology, introduced by (Duvall, 1993), has proven to be a powerful tool to study the dynamics and local inhomogeneities in the solar convection zone. In particular, the meridional flow can be measured by analyzing the wave travel times in opposite directions along the same ray path that lies between a pair of points at constant longitude.

Nort-South travel time differences obtained from GONG spherical harmonic time series. Blue color corresponds to the flow propagating to the south and red one to the north. At high latitudes some B-angle related artifacts are visible. Cross-correlations were computed from velocity images reconstructed using zonal (low m) coefficients. In this particular set of measurements separation distance for cross-correlations is about 14 degree. Approximately lower turning point of this waves is about 0.91R (Kholikov et al., in preparation)

The solar cycle variation of the Meridional Circulation

Meridional circulation can also be inferred using the ring-diagram analysis technique (Hill, 1988). Since the upgrade of the GONG instrument in 2001, the higher resolution of the Dopplergrams has allowed for the application of this technique. GONG calculates horizontal subsurface flows on a daily basis.

Temporal variation of the fitted polynomial to the meridional circulation observations at a depth of 5.8\,Mm. A symmetrical plot averaging both hemispheres is shown in the bottom panel. Positive velocities are taken towards each respective pole.(Gonzalez Hernandez et al., ApJ, 2010)

Overall, the amplitude of the meridional flow is anticorrelated with the magnetic activity, that is, increases toward solar minimum.

The Effect of Surface Activity on the Meridional Circulation

In 2001, Chou and Dai found overimposed "bumps" in the meridional circulation located at the active latitudes. Since then, several local helioseismology studies have confirmed the existence of this extra circulation.


Yearly averages of the meridional flow obtained by ring-diagram analysis of GONG continuous set of data at four different depths. The variation with the solar cycle clearly observed at the superficial layers is less pronounced at deeper layers. The extra circulation (bumps) is also clearly visible in the shallow layers.

The first explanation given to this extra circulation was related to the existence of large, converging flows towards the active regions. However, after aggressive removal of the surface activity contribution to the inferred meridional circulation, the extra circulation at the active latitudes remain present (Gonzalez Hernandez et al., 2008).

A new component of the Torsional Oscillation?

The recent extended minimum of solar cycle 23 has provided us with the opportunity of studying the subsurface meridional flows in the absence of surface magnetic activity. Similarly to the torsional oscillation of the zonal flow, we observe the extra circulation of the meridional circulation in the active belts appear at medium-high latitudes before the onset of the new cycle. This confirmed our previous results of this extra component being present at the active latitudes even when all the surface activity is removed from the analysis.

Temporal variation of the meridional circulation residuals at a depth of 5.8\,Mm (central panel). Positive velocities are directed towards the center of activity. A symmetrical plot averaging both hemispheres is shown in the bottom panel. The top panel shows the location and magnetic strength of the activity during the same period (calculated from MDI synoptic magnetograms)(Gonzalez Hernandez et al., ApJ, 2010)