          Astron. Astrophys. 339, 610-614 (1998)

## 5. The magnetic flux tube radius b below -36 Mm

Call z the vertical coordinate normal to the photosphere, with the unit vector pointing up, and the origin at the photosphere. We now calculate the flux tube radius b as a function of the external pressure , and then use the Standard Solar Model (Bahcall & Pinsonneau 1992) to deduce the flux tube radius b as a function of the depth z, where z is negative.

We saw above that, judging by the S&S data, the electric current density J seems to be about uniform. However, let us be more general and set where is the azimuthal current density at the periphery. Assume that the exponent n is independent of z. If , then J is uniform.

We have no information on the conductivity , except for the value of at the surface, as in Sect. 4. If we did know the conductivity at depth, we could estimate the value of from the values of J and of B.

Below -36 Mm, b varies slowly with z and we can use the analysis of Lorrain & Salingaros (1993). At the radius , As with a long solenoid, B outside the flux tube is negligible because the return flux extends over a large region.

The magnetic flux in the tube is If and n are independent of z, then is also independent of z.

Call B on the axis of the flux tube , and set the magnetic pressure on the axis equal to a fraction K of the external pressure , where . We do not assign a value to K for the moment. Then Eliminating from Eqs. (8) and (9), The magnetic flux tube radius is not very sensitive to the value of either n or K: if n increases from 0 to 5, b decreases by a factor of 2/3 while, if K decreases from 1 to 0.1, b increases by a factor of 1.8. In view of the coarseness of our model, setting and is a fair approximation at this stage.

Also, With the z-axis pointing up as above, the last derivative is negative, and the flux tube radius b increases with increasing z, because U, defined in Eq. (10), is positive.

For the Stanchfield et al. (1997) sunspot, if and , then .

From Eq. (9), for any n, and, from Eq. (6), the azimuthal current per meter of length is So, with increasing pressure , or with increasing depth,

• a) increases, , from Eq. (9),

• b) b decreases, , from Eq. (10),

• c) increases, , from Eqs. (8) and (10),

• d) increases, , from Eqs. (9) and (13), and

• e) decreases, from Fig. 1.

To calculate b as a function of z, we now require the external pressure as a function of depth below the photosphere. See Bahcall & Ulrich (1988), Bahcall & Pinsonneau (1992), and Hendry (1993).

The Bahcall & Pinsonneau (1992) table extends from the center of the Sun up to -36 Mm, but the range -100 Mm to -36 Mm is sufficient for our purposes. A third-order regression analysis for this range yields The fit is near-perfect but, clearly, the Bahcall-Pinsonneau table cannot be extrapolated upward to the photosphere, where . The table given in Bahcall & Ulrich (1988) does not extend above -56 Mm.

Fig. 3 shows the magnetic flux tube radius b for the Stanchfield et al. (1997) sunspot as a function of z, using Eq. (10) with Wb, , and , for the range Mm to Mm. Note that the radius scale covers about 50 km, while the depth scale covers 60 Mm. So b varies slowly with z at these depths. Indeed, at -50 Mm, . Fig. 3. Magnetic flux tube radius b as a function of depth for the Stanchfield et al. (1997) sunspot. The z-axis points up. Over this range, the flux tube radius decreases slowly with depth. That sunspot had a radius of 3 Mm at the level of the photosphere.

According to the Bahcall & Pinsonneau (1992) table, the pressure at a depth of 50 Mm is Pa. Then, at that depth, if and , from Fig. 4 and from Eqs. (10), (9), (12), and (13), km, T, kA/m2, and kA/m.    © European Southern Observatory (ESO) 1998

Online publication: October 21, 1998 