Analysis of Helicity Variation Via Collision of 2 Solar Loops In Relative Proximity (2)

 Another complicating factor is that loops and twists-writhes within them will surely become more intricate than posed in the simple (circuit) analogs shown in Part 1. One ought to expect, say in Delta sunspot groups, multiple loops of multiple twists and conceivably mutual inductances applicable to the system as well as self-inductances.

A more complex solar loop is depicted analogous to a solenoid below: 

                                Coronal loop as a solenoid with different “turns” N1 and N2 

with turns N1 and N2. For our purposes, the two coil turns can be thought of as topological twists that generate two mutual inductances at two different layers or radii.   Visualize the outer layer (for N1) as having a “writhe” number that generates a current channel through which a current I1 flows. If the length is l, the total flux  F1 linking the “writhe” coil may be written:

F1 =  m₀ N1 N2 l p r² I1

If there occurs a change in the (interior or “twist”-generated) channel current I2  an EMF V1 will be induced in the writhe layer such that:

V1 =  - m₀ N1 N2 l p r²  dI2 / dt

This EMF can be re-cast as:

V1 = - M12  dI2 / dt

Similarly, if a change occurs in the writhe channel current I1, an EMF V2 will be induced in the “twist” layer such that:

V2 =  - m₀ N1 N2 l p r²  dI1 / dt  =  - M21  dI1 / dt

where M12 and M21 are the mutual-inductances.

In general, provided the field lines’ writhe and twist are at the same distance r from the center of the loop:

M12 = M21 = M

In classical E-M theory, the total magnetic energy in a circuit incorporating i loops may be expressed^[1].

U(M) =  ½ å L_iI _i ²   + ½   å _i å  _j  i  M_ij  I_i I_j

where L  is the self-inductance for the ith element, and    M_ij = M_ji

for all  j < i

The power P generated is (Jackson, op. cit.):  

P =  ¶U(M)/ ¶t = å ⁿ _i  I _i (dF_i )/dt

where   F_i   is the magnetic flux of the ith loop

and:  F_i     =    { L_iI _0i +  å _{j . i}  M_ij  I_0j} f _t

where  f _t  = (t/ T) denotes the fraction of the total time  T

to establish the currents I _i which increase to final values I_0j.

In general, for the condition of linear force-free fields, we require currents:

I _i  =   a _i (F_i  ) / m₀

where the a _i   are the force free scale factors for the ith elements, whence the magnetic helicity:    H =   å  i_j  L_ij  F_i F_j

where  L_ij  =  (a_i+  a _j) (M_ij  ) / m₀

In actual practice it is unrealistic to treat solar circuits in this global fashion so that one wants to take into account the volume of the current carrying tubes in calculating the total energy, as well as the inductances. So for N-distinct current carrying circuits, we have     (Wheatland and Farvis, 2004)^[2]:

E =  ½ å^N  _i=1   L_iI _i ²   + ½   å _i=1 å _{j > i}  M_ij  I_i I_j

where now^[3] :

L_i  =  m₀ /( 4p Ii Ij )   òd ³x ò_Vi  d³ x    [J(x_i)  J(x_j’)]/ [x_i  - x_j’]

M_ij  =  m₀ /( 4p Ii Ij )   ò d ³x_i ò_Vi  d³ x_j    [J(x_i)  J(x_j’)]/ [x_i  - x_j’]

where V_i  is the volume of the ith circuit.

In order for it to hold that M12 = M21 = M we must have both currents in both coils for N1 and N2) at effectively the same distance r from the center of the loop. However, it has been noted that more realistically – and especially in a flare scenario – there will be different resonant surfaces associated with the “coils” ^[4], which means a radius differential, e.g. Dr = b – a. So, the outer surface may have radius b and the inner surface a. However, this means:  M12 ¹  M21

and the analogy breaks down.

What one really needs, therefore, is a means to track the Gauss winding number topologically, as opposed to the “coil” (inner or outer) for loop resonant surfaces and the currents within them. Mutual helicity is first and foremost about topology and not electro-dynamics.  Strengthening this view, is the finding that – unlike the magnetic helicity – only a part of the current helicity in the photosphere can be inferred from photospheric vector magnetograms, by virtue of the limitations imposed via ^[5] :

H_c  =  B· (Ñ X B)  =  B_⊥ · (Ñ X B) _⊥  +  B_‖ · (Ñ X B) _‖ 

Where B_⊥  and  B_‖  are the perpendicular and parallel magnetic field vectors, respectively. Analogous limitations apply to the linear force-free factor ^[6]

a  =  m J _z / B _z

for which:  H_c  =  aB² = (B/ B_‖)²  B_‖·(Ñ X B) _‖ 

The relevant estimates were given in Table I.  In the equation, all the information is confined to the normal or vertical components, none to the horizontal.

Of course, these limits may account for why it is so difficult to apply a circuit ansatz^[8] such as used in this post, and also why the approach to flares, magnetic fields and currents in solar active regions has bifurcated into two distinct paradigms: the B-v, wherein emphasis is on magnetic fields, and the E-J wherein the emphasis is on the behavior of currents.

One attempt to circumvent the reliability limits can be treated by introduction of a “resonant cavity” model. E.g.

 A Cavity Resonator Model Applied to Solar Loops and Flare Triggers (1)..

And:

• A Cavity Resonator Model Applied to Solar Loops and Flare Triggers (2)...

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[1] J.D. Jackson: 1998, Classical Electrodynamics,  John Wiley, New York.

[2] M.S. Wheatland and F.J. Farvis, Vol. 219, p. 109, Solar Physics, 2004.

[3] See, e.g. M.S. Wheatland and F.J. Farvis, op. cit., 2004.

[4]  See, e.g. P. Demoulin, E. Pariat and M.A. Berger: ‘Basic Properties of Mutual Magnetic Helicity’ in Solar Physics, Vol. 233, p. 3, 2006. The authors obtain an accurate form for the mutual helicity by using the Gauss linking number of the respective curves. This restores the proper topological context and allows an explicit way to compute the mutual helicity. The correct expression for two elementary flux tubes is given by their equation (28). The central difference from the earlier referenced formalism is the inclusion of a mulit-valued angular function, q ^m  which keeps track of how fast one set of footpoints is rotating about the other.

[5] See, D.S. Spicer, Solar Physics, Vol. 53, p. 305, 1977. Spicer alludes to families of resonant surfaces such that the total resistance will vary as: R = (rmr(s)^m-1 ) ^m   and r = ln r - ¼, with r the “mean island radius”. Spicer goes on to note the “novel” behavior in plasmas vis-à-vis conventional electric circuits, especially that the (dL/dt) term is no longer zero, unlike in prosaic circuits – where L = const.

[6] S.D. Bao and H.Q. Zhang, Vol. 496, p. L43, Astronomical Journal, 1998.

[7] In German, ‘der Ansatz’ means’ basic approach,’ or ‘beginning’.  In the case of the reliability circuit approach, the definition fits perfectly. We are using a very primitive ‘basic approach’ to nail down some particulars of the complex flare process in the hope of making useable forecasts.