Global helioseismic measurement of the meridional flow in the solar interior

Measured flow in depth range 20 - 126 Mm. Positive velocities correspond to northward flows, negative to southward.

Comparison of our measurements of the small-scale flows (bottom) with those of von Komm et al. (top).

In the solar subsurface layers one can observe a meridional flow which stretches from the equator to the poles. In our group we developed and applied a new helioseismic analysis method to measure the extension of the meridional flow in the deeper solar interior. The method uses the deformation of deeply penetrating solar acoustic waves due to the flow. We have analyzed 6 consecutive years of doppler velocity data with this method. The data were obtained between 2004-2010 by the Michelson Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory (SoHO), a spacecraft of ESA/NASA. Figure 1 depicts a cross-section of the meridional flow measured below the solar surface between 0.82-0.97 solar radii in the radius-latitude-plane. A dashed line marks the solar surface; dashed-dotted lines mark heliographic latitudes of +/-60 degree. Positive (negative) velocities correspond to a northward (southward) directed flow. Our measurement shows a complex flow profile in the solar interior with horizontal velocities less than 50 m/s. It is composed of multiple flow cells of larger and smaller scales.

The meridional flow is present at greater depths than depicted in Figure 1, but the current analysis does not yet allow measurements of the large-scale flow component at depths beyond 126 Mm.

In Figure 2 (bottom), we show the flow component consisting of small flow cells as a function of latitude and depth below the solar surface; positive (negative) values again correspond to a northward (southward) directed flow. For this flow component we obtain significant velocities down to a depth of about 200 Mm. Hence, the result provides evidence for a deeply penetrating meridional flow that permeates the whole solar convection zone. We compared our measurement with a subsurface measurement of the small-scale flow component (Bild 2, top) published by Komm et. al. (2004). This measurement reaches from 0.5-14 Mm depth. Near the surface, both results are in good agreement. The small-scale flow cells observed already near the surface stretch deep down into the interior and change their flow direction at a depth of about 100 Mm.

References:

Schad A., Timmer J., Roth M.: "Global helioseismic evidence for a deeply penetrating meridional flow consisting of multiple flow cells", ApJL, 778, L38-L44 (2013)

Komm, R., Howe, R., Hill, F., González Hernández, I., & Toner, C., ApJ, 631, 636 (2005)