Saturday, September 12, 2015

What Gives Rise To Sunspot Pairs?

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Nearly all the sunspots observed occur in pairs, a fact that has perplexed solar astronomers and physicists for generations. The configuration is such that generally one member displays  (+ or N) magnetic  polarity and the other (-). Such a pair is visible in the image of the Sun shown, near the center.   An interesting finding made in 1981, was that sunspots appear preferentially at the boundaries of unipolar magnetic regions on the surface of the Sun, i.e. which display rather magnetic networks of one polarity.

Now, Syun-Ichi Akasofu has offered evidence in support of this claim by reference to solar magnetograms or scanned images of the Sun's magnetic fields taken with an instrument called a vector magnetograph, i.e.
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In the image shown above, for example, the (+) polarities are visible as bright regions, contrasting with the (-) polarities in dark.  Prof. Akasofu, in using such magnetograms,  has noted (Geophysical Research Letters, 2014) that many single spots occur which are not in pairs which cannot be easily explained using present ideas or theories on the formation of spot pairs, which involve magnetic flux tubes rising from below the photosphere.

In classical solar dynamo theory we are looking at the process by which the solar magnetic field is generated through a combination of rotation and convection. We refer to the two parts of the dynamo process the a-effect and w- effect.  In the latter, the shear flows are able to stretch magnetic field lines in the direction of the shear while in the former, helical flows are able to lift and twist field lines into orthogonal planes.

What one gets is a rising "flux rope" that rises to the solar surface and displays opposing polarities at each end.  Thus one would end up with a rope configuration such as depicted below:

Where the spot S1 has (-) polarity and spot S2 has (+). In the line of sight - as through a telescope - we observe them as a pair.

But Akasofu emphasizes there are many spots that can't be explained in this simple way.  He found instead that single spots of positive polarity tended to appear in a positive unipolar magnetic region while single spots of negative polarity tended to appear in a negative unipolar magnetic region.

Thus, using the above observed facts, a sunspot pair can be explained based on a positive single spot forming at the boundary of a positive unipolar magnetic region and then inducing a negative single spot in an adjacent negative unipolar magnetic region across the unipolar magnetic region boundary.

Rather than the classic dynamo, Akasofu's theory shows that unipolar magnetic regions and their boundaries are essential in forming sunspot pairs and also help to better predict where on the solar disk the pairs will occur. Akasofu also notes that because the incidence of sunspots is directly related so solar activity these unipolar magnetic regions may influence space weather and the Sun-Earth connection.

It is interesting to compare Akasofu's model with one proposed by Donald H. Menzel in his wonderful monograph, ''Mathematical Physics' (1961, pp. 274-75) where the typical spot is depicted as a disk with n electrons per cm2  and which rotates with uniform angular velocity. Then:

n =   2 H/   e v o

where v  is the tangential velocity at the periphery. From these basics, Menzel computes:

n =  10 15 electrons / cm2

Menzel  concludes that given the above, "an excess of protons would produce forces 6 x  10 15  as great"  and "a sunspot with such an excess would break up with explosive violence".

He adds:

"An excess of one proton per sixty square centimeters on the solar surface would produce sufficient positive potential just to overcome the solar attraction by electrostatic repulsion of an electron"

Thereby concluding "the Sun is practically neutral electrically"

What about the magnetic aspect?

"No alignment of the individual atomic magnets could possibly be maintained in the presence of the turbulent motion and high temperatures existing on and in the Sun, The effect is undoubtedly electrical..."

So it seems like Akasofu may have more work to do in order to confirm the reality of these unipolar magnetic regions and their role in pair formation!

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