Do Supergranules Tend to Align in the North-South Direction?

Details Do Supergranules Tend to Align in the North-South Direction? Do Supergranules Tend to Align in the North-South Direction?

May 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusmsp24a..04z&link_type=abstract

American Geophysical Union, Spring Meeting 2005, abstract #SP24A-04

Physics

7522 Helioseismology, 7529 Photosphere

Scientific paper

We investigate the recent claim by Lisle, Rast, and Toomre (2004, ApJ) that supergranules tend to align preferentially in the north-south direction. We repeat their analysis with an extensive set of maps of the horizontal divergence of flow fields derived from time-distance helioseismology. We construct temporal averages of the divergence maps for different east-west tracking velocities (vx) and measure the rms values in the east-west (σx) and north-south (σy) directions from these maps. In agreement with Lisle et al. (2004) we find that, near the equator, the ratio σx/σy is maximum (and greater than one) for a tracking velocity 120 m/s above the Carrington velocity. In addition, we find that σx/σy displays a second local maximum at a tracking rate close to the Carrington velocity. We argue that the variations of the ratio σx/σy as a function of vx is a direct consequence and additional evidence of the wavelike power spectrum of supergranulation observed by Gizon, Duvall, and Schou (2003, Nature). Thus, a north-south alignment of supergranules is not the only explanation nor a necessary condition for a ratio σx/σy greater than one. Additionally, we also study the variations of the ratio σy/σx as a function of north-south velocity shifts (vy), and find that the ratio σy/σx also has two maxima of similar amplitude separated by approximately 120 m/s.

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