Evolution of relative helicity in active regions |
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Authors: | Ma Zhenguo |
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Institution: | (1) National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012, China |
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Abstract: | The evolution dH
R/dt of relative helicity H
R provides a gauge-invariant measure of the helicity flow across the open surface S
o of an active region. With the incompressible approximation, reformulation of the evolution reveals that it is determined not only by the widely used cross-helicity h
mvp=A
p v contributed by the vector potential A
p of a reference potential field B
p, where v is the fluid velocity, but by another cross-helicity h
mvo=A
o v contributed by the vector potential A
o of the open field B
o in the region as well. Only under two conditions, (1) A
p=A
(A
is the transverse component of A
o), (2) v
z=0 (v
z is the longitudinal component of v) or A
z=0 (A
z is the longitudinal component of A
o), can h
mvo be merged into h
mvp to give the pioneering dH
R/dt equation shown in Equation (4) of Berger (1984). Results show that h
mvo originates from v![sdot](/content/g76t510146063144/xxlarge8901.gif) h
o (h
o=A
o B
o is the helicity density of the open field) and should also be considered in dealing with the development of relative helicity in active regions. Finally, the equation to calculate dH
R/dt in active regions is synthesized and presented. |
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Keywords: | |
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