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Solar cycle variation of kinetic helicity. Mei Zhang （ National Astronomical Observatory, Chinese Academy of Sciences ）. Collaborators : Junwei Zhao (Stanford, USA) Mark Miesch (HAO/NCAR, USA). Plan of the Talk. Motivation: Solar-cycle variation of current helicity
（National Astronomical Observatory, Chinese Academy of Sciences）
Junwei Zhao (Stanford, USA)
Mark Miesch (HAO/NCAR, USA)
Magnetic fields are observed to emerge into each hemisphere with a preferred helicity sign:
Positive in southern hemisphere;
Negative in northern hemisphere.
(e.g. Seehafer 1990, Pevtsov et al. 1995, Bao & Zhang 1998)
(Image credit: A. Pevtsov)
No argument on the existence of this hemispheric helicity sign rule.
However, there is an argument on Is this rule preserved for the whole solar cycle?
Longcope et al. (1998): YES:solar-cycle independent
--- Helicity is produced in the process of magnetic flux tubes rising through the solar convection zone and being buffeted by turbulence with a non-vanishing kinetic helicity (Σ-effect)
Choudhuri et al. (2004): NO:solar-cycle dependent
--- Poloridal flux getting warped around a toroidal flux tube rising through the convection zone to give rise to the helicity
--- Deviations from the usual sign rule at the beginning of a solar cycle
Bao et al. (2000): NO (of hc) for the ascending phase of cycle 23
Pevtsov et al. (2001): Yes for the first four years of cycle 23
Hagino & Sakurai (2005): Yes for maximums; NO for minimums
(Mitaka data, 1983-2000)
Pevtsov et al. (2008): No agreement on the ‘wrong-sign’ years
(19 years’ data of different instruments)
(Hao & Zhang 2011, ApJ, 733, L27 and Juan Hao’s talk tomorrow for more development)
End of cycle 23:
do not follow
Beginning of cycle 24:
Hint: Choudhuri et al. (2004) may be right in its physical picture but wrong in phase prediction.
Despite with the accurate measurement by SP/Hinode, the scatter is still very large.
Hint: The scatter is an inherent property, not induced by measurement error. Consistent with Longcope et al. (1998).
How to understand this hemispheric helicity sign rule and its solar cycle variation?
Solar cycle variation in a convective dynamo model
A Convective Babcock-Leighton Dynamo Model (Miesch & Brown 2012)
Hemispheric helicity sign rule shows up clearly in magnetic helicity density map.
Current helicity does show cycle variation, with opposite-sign patches presenting.
(More analysis in progress)
(Only vertical component, to compare with observation)
Solar cycle variation of kinetic helicity
Subsurface velocity field obtained
through time-distance helioseismology
using HMI/SDO data
Each line: one-month averaged
(r= 0 - 1 Mm)
in HMI data, time distance method
in Model: Miesch et al. (2006)
(produce solar-like differential rotation)
(r = 21 Mm)
(r = 0 - 1 Mm)
with error bars
Yes, as reported before in Zhao & Kosovichev (2004).
Means that previous non-detection on Hk is not by noise.
These give constrains on the mechanism of helicity production as well as dynamo models.
Huairou Solar Observing Station, NAOC
We calculate two helicity parameters:
1. average local twist
2. normalized average current helicity
With two vector B representatives:
1. flux density map (B1)
2. true field strength (B2)
The same hemispheric helicity sign rule exists, extending to 60 degrees high in latitudes, and is preserved through the whole solar-cycle.
(Wang & Zhang 2010, ApJ, 720, 632)
Left: MDI; (September 1996) Right: KPVT
(Following the approach in Petvsov & Latushko 2000)