Solar Interior


Why Study Solar Interior?

There are manifold advantages of studying the solar interior; the Sun is the only star that can be observed in great detail, it thus provides an important input to our understanding of stellar structure and evolution. It also provides a unique laboratory for studying some fundamental physical processes in conditions that cannot be easily achieved on Earth. In addition, the Sun has substantial influence on Earth and near-Earth environment, particularly through its outputs of radiation and particles, thus studying its dynamic nature is important for understanding solar–terrestrial relations. Also, the propagating waves sample the conditions of the invisible layers, this provides important information before the appearance of solar activity on the surface and serves as a tool to forecast space weather that may have significant impact on human life.

The Mean Rotation Rate

GONG observations of medium-degree p-mode oscillations can be used to probe the rotation profile deep inside the Sun.

(Left) rotation rate as a function of fractional solar radius, at selected latitudes. The data are averaged over the period 1995-2009, but the thickness of the curves represents the error bars based on a single 108-day sample. (Right) Contour map of the rotation rate in and below the convection zone. The dashed lines are at a 25-degree angle to the rotation axis. (Click the images for high-resolution versions.)

The main features to note are the shear layers near the surface and at the base of the convection zone (the so-called tachocline), and the approximately radial (but really closer to lying on parallel 25-degree cones) pattern of the rotation in the bulk of the convection zone.

The Torsional Oscillation

Over the course of the solar cycle, bands of slightly faster and slower rotation appear at mid-latitudes and propagate towards the poles and the equator. This pattern was discovered in surface Doppler observations by Howard and Labonte (1980). Helioseismology reveals that the pattern penetrates through much of the convection zone. (Howe et al. 2000a, 2005, 2006, Vorontsov 2002).





Variation of rotation rate with latitude and time from which a temporal average has been subtracted, revealing the banded zonal flows migrating toward the equator. The plots were made by combining the residuals from both GONG and MDI observations.

The new band of faster-rotating flow that would be associated with Solar Cycle 24 can be seen starting around 2003. During the extended solar minimum following Cycle 23, this band migrated noticeably more slowly than the corresponding band in the previous cycle. Howe et al. (2009) pointed out that the position of the flow belts by mid-2009 corresponded to that at the epoch in the previous cycle when solar activity was just about to start rising — and indeed, by the end of 2009 the new cycle was finally underway.

Latitude-time plot of torsional oscillation close to the surface, showing contours of the unsigned longitudinal magnetic field strength. The vertical blue lines correspond to the times where the flows best match the configuration at the beginning of the GONG observations in 1995 (right) and to the situation at the most recent observations (left).

Press Release, 17 June 2009:

Meridional Circulation during the last Solar Cycle

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.

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.

North-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)


Tachocline OscillationsThe 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.(González Hernández et al., ApJ, 2010)

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

The solar cycle variation of the Meridional Circulation


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)(González Hernández et al., ApJ, 2010)


Dr. Kiran Jain
National Solar Observatory

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