Solar-Cycle Variation of Subsurface Meridional Flow Derived with Ring-Diagram Analysis

• Published in: Solar physics Vol. 290; no. 11; pp. 3113 - 3136
• Main Authors: Komm, R., González Hernández, I., Howe, R., Hill, F.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01.11.2015; Springer Nature B.V
• Subjects: Amplitudes; Analysis; Astrophysics and Astroparticles; Atmospheric Sciences; Bands; Covering; Flow; Gongs; Hemispheres; Latitude; Meridional flow; Physics; Physics and Astronomy; Probing the Sun: Inside and Out; Seismology; Solar physics; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Studies; Sun; Velocity fields, interior; Helioseismology, observations
• ISSN: 0038-0938; 1573-093X
• DOI: 10.1007/s11207-015-0729-5

We study the solar-cycle variation of the meridional flow in the near-surface layers of the solar convection zone from the surface to a depth of 16 Mm. We have analyzed Global Oscillation Network Group (GONG) Dopplergrams with a ring-diagram analysis covering about 13 years (July 2001 – October 2014), from the maximum of Cycle 23 through the rising phase of Cycle 24, and Helioseismic and Magnetic Imager (HMI) Dopplergrams covering more than four years (May 2010 – January 2015). GONG and HMI lead to similar meridional flows during common epochs and latitudes. The meridional flow averaged over a Carrington rotation is poleward up to about 70 ∘ in both hemispheres at all depths after correcting for systematic effects. The flow amplitude peaks at about 40 ∘ latitude with an amplitude of about 16 to 20 m s − 1 depending on depth. The meridional flow varies with the solar cycle; the flow amplitudes are larger during cycle minimum than during maximum at low- and mid-latitudes. The flows are mainly faster or more-poleward-than-average on the equatorward side of the mean latitude of activity and slower or less-poleward-than-average on its poleward side. The residual meridional flow converges near the mean latitude of activity. A comparison with the corresponding zonal flow derived from GONG and HMI data shows that the bands of more-poleward-than-average meridional flow coincide with the bands of faster-than-average zonal flow and that the bands of less-poleward-than-average meridional flow coincide with the bands of slower-than-average zonal flow. This implies that the residual flows are cyclonic. The bands of fast meridional flow appear at mid-latitudes about three years before magnetic activity of Cycle 24 is present in synoptic maps.