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Volume 580, August 2015
Article Number A128
Number of page(s) 15
Section The Sun
Published online 19 August 2015

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Appendix A: Helicity test without the condition A (ztop) = A (ztop)

In the numerical implementation of both the practical DeVore method and the DeVore-Coulomb methods, we imposed the condition of Eq. (46), of having the same distribution of A and Ap at the top boundary, in addition to the DeVore gauge (Eq. (45)), which is valid in the whole volume. Because of this condition, A and Ap are both different when computed with one method or the other. The gauge of A and the gauge of Ap are linked by Eq. (46). It induces that FAAp is null at the top boundary and reduces its intensity in the whole domain.

It is possible, however, to compute and estimate the helicity without enforcing Eq. (46). We can follow the helicity evolution by mixing the vector potential computed with both methods. Since the gauge invariance of Eq. (10) does not require using the same gauge for A and Ap, we can use the A computed with the practical DeVore method with Ap derived with the DeVore-Coulomb methods. Ap thus still satisfies both the DeVore and the Coulomb gauge. As A, which is only satisfying the DeVore gauge condition, has been computed independently of Ap, there is no boundary surface along which they share any common distribution. In this appendix we refer to this derivation as the “general DeVore-Coulomb” case.

Figure A.1, right panel, presents the different terms entering in the decompositions Eqs. (11), (15) of the relative helicity. As in Sect. 5.1, the gauge invariance of in this computation relative to the other methods is ensured with a high precision (<0.3%). As with the others methods, Hj and Hpj remain constant, while Hm, Hmix and Hp are different in the general DeVore-Coulomb case. This further confirms the gauge-dependance properties of each decomposition.

The time-integrated helicity fluxes (Fig. A.1, right panel) again show that tightly follows the variation of helicity

. The helicity dissipation is also very small in the other two methods and has a precision similar to the practical DeVore case (Sect. 4.3). The repartition of the helicity flux Ftot between the different terms that compose it is significantly different here compared to the other two methods, however.

Since Ap fulfills the Coulomb condition, the term dH/ dtp,var is null to the numerical precision. Since Fφ only involves quantities based on the derivation of the potential field, are equal for both the DeVore-Coulomb and the general DeVore-Coulomb cases. On the other hand, as FBn, FVn only involve A, and in the general DeVore-Coulomb are equal with their respective estimations in the practical DeVore case.

FAAp concentrates the helicity flux contribution, which enables Ftot to be quasi gauge-invariant for the three derivations (Fig. A.1, right panel). While was negligible in both the practical DeVore and the DeVore-Coulomb cases, we observe that this term is now a main contributor of the helicity fluxes. This is expected since results from the existence of large differences between the distribution of A and Ap on the boundaries. The computations in the practical DeVore and the DeVore-Coulomb method both enforced Eq. (46), which induces a very weak value of . We observe that dropping condition (46) creates a strong .

This test again demonstrates that the choice of the gauge strongly influences the distribution of the helicity fluxes that compose the total helicity flux Ftot. Only the total flux Ftot is quasi gauge-invariant. None of the terms that compose the helicity flux Ftot must be neglected a priori. Depending on the gauge, each term can carry a significant contribution. In a numerical estimation, it is thus highly advisable to compute all the terms that form the helicity flux density (Eq. (23)). Explicitly computing each term allows us to verify that the constraints set on the used gauges are effectively enforced numerically.

thumbnail Fig. A.1

Left panel: relative magnetic helicity (, black dashed line) and its decomposition in the general DeVore-Coulomb method. The plotted quantities are the same as in Fig. 2, bottom panel, and Fig. 7, right panels. Right panel: and H∂𝒱,# evolution in the system computed with the general DeVore-Coulomb method. The plotted quantities are the same as in Fig. 9.

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© ESO, 2015

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