Hemispheric Distribution of Subsurface Kinetic Helicity and Its Variation with Magnetic Activity
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We study the hemispheric distribution of the kinetic helicity of subsurface flows in the near-surface layers of the solar convection zone and its variation with magnetic activity. We determine subsurface flows with a ring-diagram analysis applied to Global Oscillation Network Group (GONG) Dopplergrams and Dynamics Program data from the Michelson Doppler Imager (MDI) instrument onboard the Solar and Heliospheric Observatory (SOHO). We determine the average kinetic helicity density as a function of Carrington rotation and latitude. The average kinetic helicity density at all depths and the kinetic helicity, integrated over 2 – 7 Mm, follow the same hemispheric rule as the current/magnetic helicity proxies with predominantly positive values in the southern and negative ones in the northern hemisphere. This holds true for all levels of magnetic activity from quiet to active regions. However, this is a statistical result; only about 55 % of all locations follow the hemispheric rule. But these locations have larger helicity values than those that do not follow the rule. The average values of helicity density increase with depth for all levels of activity, which might reflect an increase of the characteristic size of convective motions with greater depth. The average helicity of subsets of high magnetic activity is about five times larger than that of subsets of low activity. The solar-cycle variation of helicity is thus mainly due to the presence or absence of active regions. During the rising phase of cycle 24, locations of high magnetic activity at low latitudes show a weaker hemispheric behavior compared to the rising phase of cycle 23.
KeywordsActive regions, velocity fields Helicity Helioseismology, observations Velocity fields, interior
This work utilizes GONG and SOLIS data obtained by the NSO Integrated Synoptic Program (NISP), managed by the National Solar Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under a cooperative agreement with the National Science Foundation. GONG data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofísica de Canarias, and Cerro Tololo Interamerican Observatory. The ring-fitting analysis is based on algorithms developed by Haber, Hindman, and Larsen with support from NASA and Stanford University. SOHO is a mission of international cooperation between ESA and NASA. This work was supported by NSF/SHINE Award No. 1062054 to the National Solar Observatory. RK was partially supported by NASA grant NNX10AQ69G to Alysha Reinard. We thank the reviewer for making this a better paper.
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