The effect of non-radial magnetic field on measuring helicity transfer rate
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The effect of non-radial magnetic field on measuring helicity transfer rate. Yongliang Song & Mei Zhang (National Astronomical Observatory of China). Motivation. Our previous studies (Zhang & Low 2005; Zhang et al. 2006; Zhang & Flyer 2008; Zhang et al. 2012) have suggested that:

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The effect of non-radial magnetic field on measuring helicity transfer rate

Yongliang Song & Mei Zhang

(National Astronomical Observatory of China)


Motivation helicity transfer rate

  • Our previous studies (Zhang & Low 2005; Zhang et al. 2006; Zhang & Flyer 2008; Zhang et al. 2012) have suggested that:

  • Magnetic helicity is accumulating in the corona;

  • There is an upper bound on the total magnetic helicity that force-free magnetic field can contain;

  • Once the accumulated helicity has reached the upper bound, an expulsion such as CMEs become unavoidable.


Based on this model, helicity transfer rate

We can monitor the evolution of magnetic helicity and use it to predict the eruption of CMEs.

--- We can calculate the helicity transfer rate on the photosphere to monitor the helicity accumulation in the corona.

--- We can estimate the helicity upper bound corresponding to current boundary flux distribution.

 This requests an accurate measurement of magnetic helicity transfer rate.


Helicity transfer rate helicity transfer rate

  • First term: emergence term; Second term: shearing term

  • Before SDO, previous studies have used MDI data and have put into the calculation two assumptions.

  • if U is from tracking footpoints.


Liu & helicity transfer rate Schuck (2012) checked the first assumption.

  • They calculated the helicity transfer rate in two active regions (AR11072 & AR11158) using HMI/SDO vector magnetograms and DAVE4VM code, and found that

  • apparent tangential velocity derived by tracking field-line footpoints is more consistent with tangential plasma velocity than with the flux transport velocity;

  • relative magnetic helicity in the active-region corona is mainly contributed by the shearing term (88%, 66%) .


However, there is another assumption. helicity transfer rate

In previous studies, they have also assumed that the magnetic field being no-radial will NOT influence the calculation of helicity transfer rate.

Our purpose is to check this assumption.


Histogram of Bt/Bz in a magnetogram of a sunspot helicity transfer rate

There are a good fraction of the sunspot area where the horizontal magnetic field is larger than the vertical field.


We study two active regions observed by HMI/SDO helicity transfer rate

as in Liu & Schuck (2012)

NOAA 11072: 2010.5.20 emerge at S15E48

bipolar AR

144 hours ob. 720 magnetograms

NOAA 11158: 2011.1.10 emerge at S20E60

multipolar AR

produce an X-class flare

120 hours ob. 600 magnetograms


NOAA11072 helicity transfer rate as an example: from the disk center to the limb

2010-05-26 08:00:00 UT

2010-05-23 07:00:00 UT


Bn helicity transfer rate

Near disk center: MDI-like

Near disk center: HMI

Limb: HMI

Limb: MDI-like


dH/dt helicity transfer rate

MDI-like:

LCT

HMI:

DAVE4VM

Near disk center:MDI-like

Near disk center :HMI

Limb:MDI-like

Limb :HMI


dH/dt (shearing term, after a recalibration) helicity transfer rate

NOAA 11158

Blue: MDI-like

Red: MDI-like, DAVE

Black: HMI, DAVE4VM

NOAA 11072


H helicity transfer rate m (accumulated helicity, shearing term, after a recalibration)

NOAA 11158

Blue: MDI-like

Red: MDI-like, DAVE

Black: HMI, DAVE4VM

NOAA 11072


Summary helicity transfer rate

  • We checked the effect of non-radial magnetic field on measuring helicity transfer rate by comparing the results using vector magnetograms taken by HMI/SDO with those using only line-of-sight magnetograms. We find that:

  • The effect of the non-radial magnetic field on the calculation of magnetic helicity transfer rate is strong when the active region is observed near the limb and is relatively small when the active region is close to the disk center;

  • If only considering the accumulation of magnetic helicity from the shearing term, the effect of non-radial magnetic field then becomes minor.


Implication helicity transfer rate

Since the shearing term is the dominant component in the helicity transport and the effect of non-radial field is minor for the calculation of accumulated magnetic helicity, we could still use MDI data tostudy the solar-cycle variation of helicity transport and accumulation, and investigate their possible relation with the solar-cycle variation of CME production.


Thank you for your attention! helicity transfer rate

Huairou Solar Observing Station, NAOC


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