Electric current neutralization by Wheatland (ApJ 532, 616-621, 2000).
From the abstract we learn the following:
Wheatland (abstract) said:
For each active region, the current over the positive polarity of the field, I+, is estimated, as well as the current over the negative polarity, I-, and the total current over both polarities, In no case is the total , Itot. In no case is the total current Itot significantly different from zero. The currents I+ and I- are found to be significantly different from zero (at the 3 σ level) in more than half of the active regions studied, implying that large-scale currents in active regions are typically unneutralized.
So what we learn here is that (using magnetograms) Wheatfield studies the currents derived from the magnetograms over active regions of negative and positive polarity separately, giving rise to the I- and I+. These two currents I- and I+ are not neutralized over that specific region of polarity (which should not be surprising, actually), however taking the whole active region together (i.e. summation over all polarity region) one gets a Itot, which is not found to be significantly different from zero.
Wheatfield (Introduction) said:
Modern vector magnetographs and Stokes polarimeters permit the determination of the vector magnetic field at the photosphere in active regions and hence allow estimates of the vertical electric current entering the corona at the photosphere. These measurements provide the most direct evidence available on electric currents in flare-producing regions.
This is the technique used to measure the currents. And the definition of “neutralized” is given a little further down:
Wheatfield (Introduction) said:
A neutralized current pattern is one for which there is no net current over one polarity of the magnetic field at the photosphere.
Now depending on what is looked at, one can find different results for the neutralization, like is discussed with two examples that give different results:
Wheatfield (Introduction) said:
Wilkinson, Emslie, & Gary (1992) studied a single active region (Active Region 2372, observed with the Marshall Space Flight Center vector magnetograph on 1980 April 6) and, using the integral form of Ampere’s law (rather than the more usual differential form of the law), they found evidence for a neutralized current pattern in a large leader spot within the region. They concluded that either the current associated with the spot was neutralized or the observed fields are potential, with the apparent current being an artifact produced by Faraday rotation.
Leka et al. (1996) examined current patterns associated with emerging flux in a single active region (AR 7260, observed by the Haleakala Stokes polarimeter and the Imaging Vector Magnetograph at Mees Observatory during 1992 August) and found that new flux appeared contemporaneously with unneutralized currents. They concluded that the new magnetic flux that emerged carried currents that were generated below the photosphere, before the flux emerged.
These conflicting results based on individual active regions suggest the need for a statistical, quantitative study of whether currents in active regions are typically neutralized.
So then there is a discussion of the technique, which I will not go into, anyone can read it as the paper is freely available from ADS and ApJ. So let’s move to the discussion. The first thing mentioned is:
Wheatfield (Discussion) said:
These [stations, 1005] were used to determine the total current over the positive and negative polarities of the vertical magnetic field within the field of view of the magnetogram, I+ and I-,respectively, as well as the total current over both polarities, Itot. Clear evidence was found for unneutralized current patterns in the majority of the active regions examined - e.g.,I+ is different from zero by more than 3 σ in 13 cases. In all 21 cases, Itot is consistent with being zero. In at least three cases, I` is consistent with being zero, indicating that in some active regions there are neutralized current patterns. or that the fields in these regions are potential and that the inferred currents are the products of noise.
Then Wheatfield goes into a discussion of how this statistical sample relates to what other people have proposed about the currents coming out of active regions, as there are opposing views. Melrose, e.g. states that
A clear example is provided by a coronal magnetic loop subject to a twisting flow at one footpoint. The twist generates a current system that flows along the axis and back along the surface of the loop in the corona. The current follows field lines in the corona but closes across field lines at the footpoint of the loop where the stress is applied. The observed current pattern at the photosphere at each footpoint is a patch of current of one sign at the center of the footpoint, surrounded by a concentric region of current of opposite sign. The pattern is neutralized.
However, Melrose argued that
Melrose (1991, 1995) argued that measurements of the vector magnetic Ðeld do not support neutralized current patterns and hence disfavor the in situ storage models. His assessment of the data was based on qualitative descriptions of observed current systems and on published diagrams of currents derived from magnetograms.
And so we come to the conclusions by Wheatfield:
Wheatfield (Discussion) said:
The results presented here are consistent with the qualitative statements of Melrose (1991, 1995) and with the quantitative Ðndings of Leka et al. (1996), but are contrary to the results presented by Wilkinson et al. (1992). Neutralized current patterns appear in some active regions (see § 3), and it is likely that Wilkinson et al. found such an exception to the general rule of unneutralized current patterns.
So, this has an interesting result, namely that:
Wheatfield (Discussion) said:
The observation that current patterns in active regions are typically unneutralized has important consequences for flare physics and more generally for our understanding of magnetic fields on the Sun, as outlined by Melrose (1991, 1995) and Leka et al. (1996). First, it indicates that twisting and shearing of the footpoints of coronal magnetic fields are not responsible for the large-scale currents that are observed and, hence, that the in situ storage model for flares is invalid. Magnetic flux emerges at the photosphere with large-scale electric currents already flowing in it and with free energy already present.
Now, this IS interesting, because footpoint shear is considered to be an important part of driving currents in magnetic loops. However, it should be understood too, that this study deals with the total current above a positive (I+) or negative (I-) polarization active region. It is proposed that the dynamo action in the Sun should be moved to the base of the convection zone in order to be able to get these large scale currents. This also has an effect on solar flares as noted by Melrose:
Wheatfield (Discussion) said:
As argued by Melrose (1991), the long inductive time associated with such an extended current system precludes change on the short timescale of a flare, and so current will be conserved during a flare, an important consideration that is missing from most flare models. Melrose (1997) has presented a model for flaring due to reconnection between current carrying loops subject to conservation of both magnetic flux and total current.
This whole discussion of the Wheatfield paper is because of the answer that Tim Thompson gave on the question “Are stars charged” and his answer was “Yes” and indeed there was a paper in 2001 by Neslusan that discusses this (the link was nicely taken out by MM).
MM told TT that: Ooops? You're OOMS off Tim. Now what?, but does MM actually know what he is talking about? Apparently not, because TT was discussion the charged Sun and the voltage that could be created by that, whereas Wheatfield goes into a totally different calculation. Let’s take a look:
Wheatfield (Discussion) said:
The size of the observed currents also raises interesting questions concerning the appearance of large voltages if the current changes or if the current path changes.
Ah, this is just Maxwell theory, changes in currents or current directions will lead to voltages. This has
nothing to do with the voltages that are related to a charged Sun.
Then Wheatfield goes into a mathematical discussion, so I guess MM will have skipped that, but here it is anyway:
Wheatfield (Discussion) said:
Assuming the change occurs on a timescale τ, there is an associated electromotive force (EMF) V ~L I/τ, where L is the inductance of the circuit. The inductance may be estimated by L ~ μ0 ~ 100 H for a circuit of length l ~ 108 m. If the current flowing through the corona (I ~ 1012 A) or the inductance changes substantially on the timescale for flux emergence (τ ~ 105 s), enormous voltages (V ~ 109 V) are implied.
So there can be enormous voltages created by changes in the current loops on the Sun, which is no surprise. However, it is a surprise that MM wants to say that these voltages (that have nothing to do with net charge) are related to a charged Sun.
I thank MM, though, for pointing out this interesting paper, however, I would appreciate when MM would actually put some effort in
discussing the papers that he links to and show the (non)relevance of said papers.
Denial seems to be the single biggest "tool" in the PC opponent's arsenal.