Disappearing Sunspots, Minus 50 Gauss/yr

[Saul]

A sunspot is a concentrated clump of magnetic energy on the sun's surface that typically varies from 1800 to 3000 Gauss. The Earth's magnetic field strength is in comparison 0.3 Gauss.

Recently Solar physicists have found the magnetic field strength of newly produced sunspots is declining linearly year by year by 50 gauss per year. Why that is happening is not known.

The magnetic ropes that create sunspots are believed to be created deep within the sun at the tachocline zone which is the boundary between the solar convection zone and the radiative zone.

Theoretical calculations indicate that the magnetic ropes that are created at the tachocline require a field strength of around 2000 gauss to avoid being broken up during their rise to the solar surface through the turbulent convection zone. The magnetic ropes rise up through the convection zone to the surface of the sun where they form sunspots. If the trend continues the sun will therefore not be capable of producing new sunspots.

A consequence of the weakening magnetic field of individual sunspots is that the lifetime of recent sunspots is reduced. The typical lifetime of a new sunspot during a normal cycle is around a month. Many of the recently produced weak sunspots break apart in a few days.


http://science.nasa.gov/science-news/science-at-nasa/2009/03sep_sunspots/

"Sunspot magnetic fields are dropping by about 50 gauss per year," says Penn. "If we extrapolate this trend into the future, sunspots could completely vanish around the year 2015."

This disappearing act is possible because sunspots are made of magnetism. The "firmament" of a sunspot is not matter but rather a strong magnetic field that appears dark because it blocks the upflow of heat from the sun's interior. If Earth lost its magnetic field, the solid planet would remain intact, but if a sunspot loses its magnetism, it ceases to exist.
"According to our measurements, sunspots seem to form only if the magnetic field is stronger than about 1500 gauss," says Livingston. "If the current trend continues, we'll hit that threshold in the near future, and solar magnetic fields would become too weak to form sunspots."

http://iopscience.iop.org/1538-4357/649/1/L45/pdf/1538-4357_649_1_L45.pdf

Temporal Changes in Sunspot Umbral Magnetic Field and Temperatures

We have observed high-resolution intensity spectra near the Fe i 1564.8 nm line at a single umbral point corresponding to the darkest position in over 900 sunspots from 1998 through 2005. From these data we determine that the maximum sunspot magnetic fields have been decreasing at about 52 G per year. The same data set shows a concurrent increase in the normalized umbral intensity from 0.60 to 0.75 corresponding to a blackbody temperature rise from 5137 to 5719 K) and a decrease of more than 50% in the molecular OH line strength. The magnetic field and intensity changes observed over time in the sunspot umbrae from different spots behave in the same way as the magnetic field and intensity changes observed spatially across single sunspots.

 

[Saul]

The authors of the paper that noted the magnetic field strength of individual sunspots is declining have continued their measurements. The trend has continued into 2010.

http://www.leif.org/research/Livingston and Penn.png

 

[Saul]

This is a comparison of solar cycle 24 to solar cycles 21, 22, and 23.

The newly produced solar cycle 24 sunspots continue to diminish in size, magnetic field strength, and in life time.

http://www.solen.info/solar/cyclcomp.html

 

[Saul]

As noted in the above comments sunspots are hypothesized be created at the tachocline deep within the sun. The tachocline is the interface to the solar radiative zone and convection zone.

http://www.ifa.hawaii.edu/~barnes/ast110_06/tsaas/1003a.jpg

Also as noted above, the magnetic field strength of the solar cycle 24 sunspots that are being produced at the tachocline are linearly declining. This indicates that the tachocline has been disturbed.

The following is a possible explanation for what is observed. The sun is move about its barycenter by the large planets. That motion is cyclic. It appears when there is a specific change in direction of the sun's motion about its barycenter the tachocline is disturbed and the sunspot cycle is interrupted.

This link discusses related solar anomalies. The solar conveyor belt on the surface of the sun has suddenly increased in speed while the conveyor belt deep with the sun has slowed down. The two conveyor belts were previously expected to be linked. I supposed that is also indication that the tachocline deep within the surface of the sun has been interrupted which explains the de-coupling of the surface and deep conveyor belts.

http://science.nasa.gov/science-news/science-at-nasa/2010/12mar_conveyorbelt/

The speed-up was surprising on two levels.

First, it coincided with the deepest solar minimum in nearly 100 years, contradicting models that say a fast-moving belt should boost sunspot production. The basic idea is that the belt sweeps up magnetic fields from the sun's surface and drags them down to the sun's inner dynamo. There the fields are amplified to form the underpinnings of new sunspots. A fast-moving belt should accelerate this process.

So where have all the sunspots been? The solar minimum of 2008-2009 was unusually deep and now the sun appears to be on the verge of a weak solar cycle.
Instead of boosting sunspots, Hathaway believes that a fast-moving Conveyor Belt can instead suppress them "by counteracting magnetic diffusion at the sun's equator." He describes the process in detail in Science ("Variations in the Sun's Meridional Flow over a Solar Cycle," 12 March 2010, v327, 1350-1352).

The second surprise has to do with the bottom of the Conveyor Belt.
SOHO can only clock the motions of the visible top layer. The bottom is hidden by ~200,000 kilometers of overlying plasma. Nevertheless, an estimate of its speed can be made by tracking sunspots.

He's done that—plotted sunspot speeds vs. time since 1996—and the results don't make sense. "While the top of the conveyor belt has been moving at record-high speed, the bottom seems to be moving at record-low speed. Another contradiction."