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The vorticity of photospheric granulation-scale flows and the surface dynamo

The vorticity of photospheric granulation-scale flows and the surface dynamo. Alexei A. Pevtsov. Two Types of Dynamo. 1 – e.g. mean-field, overshoot region, interface dynamo 2 – AKA local, surface, photospheric, turbulent dynamo. Distinguishing Between Two Dynamos.

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The vorticity of photospheric granulation-scale flows and the surface dynamo

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  1. The vorticity of photospheric granulation-scale flows and the surface dynamo Alexei A. Pevtsov

  2. Two Types of Dynamo 1 – e.g. mean-field, overshoot region, interface dynamo 2 – AKA local, surface, photospheric, turbulent dynamo

  3. Distinguishing Between Two Dynamos Mag. Flux of ARs varies with sunspot cycle • AR and QS fields behave differently with • solar cycle • -QS fields have very short lifetime. AR and QS magnetic fields may be generated by separate dynamos. Mag. Flux of QS does not • Em (active region) ~ 1-10 x 1034 erg • Em (network field at granule scale) ~ 1027 erg • Ek (granulation) ~ 3-6 x 1027 erg • -Helical dynamo is necessary to generate strong magnetic fields of ARs. • -Turbulent dynamo can amplify magnetic field only to 10-20% of kinetic energy. • -Numerical simulations: e.g. Cattaneo 1999; Schekochihin et al 2004. • -Pm>1 (in numerical simulations), Pm~ 10-6 <<1 (on the Sun)

  4. Helical or not ? Chaotic (turbulent) Helical • Overshoot region (DeLuca & Gilman 1991) • Mean-field dynamo (Krause & Radler 1980) • Surface dynamo (Emonet & Cattaneo 2001) Pevtsov & Longcope 2007 Tornado: Hurricanes: • Helical dynamo should result in hemispheric preference for sign of helicity • Chaotic (turbulent) dynamo should show no hemispheric sign-preference for helicity.

  5. * Sign of vorticity tends to be random in intergranular spaces * In some cases, vorticity reverses its sign.

  6. Vorticity of Granular Flows. Questions we Address • Does vorticity/kinetic helicty of granular flows show hemispheric preference? • Do we see the hemispheric helicity rule in quiet Sun magnetic fields created by surface dynamo?

  7. QS?? QS??

  8. 25-26 Nov. 2008; HINODE 21 July 2009; NST + 50, 30, 0 degrees along central meridian; ~ 50” x 200”; 50 images, 30 sec cadence; G band 430.5 nm, 0.2 arc sec per pixel + 17, 0 degrees along central meridian ~ 180” x 180”; 50 images, 30 sec cadence; TiO (705.7 nm), 0.1 arc sec per pixel

  9. Vorticities FLCT – Fisher & Welsch, 2008 Hinode NST at BBSO

  10. Current Helicity Proxy from SOLIS

  11. N40W00 ASP - Dependence of scatter on latitude In agreement with recent modeling by Abbet, et al

  12. Conclusions • Vorticity of granualar flows is opposite in sign for granules and intergranular lanes; combined with expected Vz, it implies Hk<0 (Northern hemisphere) and Hk>0 (Southern hemisphere). • Averaged vorticity shows a slight hemispheric preference; however, significant st. deviation of averages makes this hemispheric dependency statistically insignificant. • Average current helicity proxy derived from vector magnetograms shows no hemispheric preference in sign. • Turbulent dynamo operating on spatial scale of granular flows is non-helical in its nature. • Surface magnetic field shows no hemispheric sign-preference because: • It was created by a non-helical surface dynamo • It was “recycled” by the surface dynamo numerous times and as the result, it lost its original helicity from subphotospheric dynamo.

  13. THANK YOU

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