As noted below Solar cycle 24 appears to be an interruption to the solar magnetic cycle as opposed to a slow down in the solar magnetic cycle. An interruption indicates something fundamental has changed in the sun. A slow down is just a reduction in the number of sunspots that are produced by the sun. Specifically what will happen next is not known. I will up date this thread when there is new observational evidence to validate or invalidate the competing solar mechanism theories.
As noted below, Livingston and Penn have found that over the last 10 years each new sunspot that is produced by the sun has less and less magnetic energy. (This is a change from previous solar observations.) The sunspots require a magnetic field strength of 2000 gauss to survive their trip through the solar turbulent convection zone. (One solar theory has the magnetic ropes that rise to the surface of the sun to form sunspots being created at the interface of the solar convection zone and the solar radiative zone which is called the solar tachocline.)
In the paleo record on the earth there is evidence of significant cyclic changes to the sun. The changes are not just more or less sunspots per cycle. In the mass media there is often discussion of the number of sunspots per cycle noting past evidence of changes in climate on the earth that correlate with weak solar magnetic cycles. As noted below weak solar magnetic cycles results in a weak solar heliosphere which results in high GCR striking the earth’s atmosphere.
The solar heliosphere is the name for a bubble of solar material that this ejected by the sun. The extent of the solar heliosphere is dependent on the density and speed of the solar wind. The solar heliosphere contains pieces of magnetic flux that are ejected by the sun. When the solar magnetic cycle is active (high speed solar wind and high density solar wind), the solar heliosphere extends further into the solar system.
The pieces of magnetic flux in the solar heliosphere deflected high speed cosmically generated ions “galactic cosmic rays” GCR. When the solar magnetic cycle is weak more and higher energy GCR strikes the earth’s atmosphere.
It appears based on an analysis of the paleo record on the earth that when the sun spot mechanism has been interrupted there are very strong coronal mass ejections (CME) that occur when the sun spot mechanism restarts. It appears the sunspot mechanism stabilizes the sun. Counter intuitively, in the past very large coronal mass ejections correlate with times when the solar magnetic cycle has been interrupted. Specifically what causes CME is not known. There are competing mechanisms and there are observations which the mechanisms cannot explain.
This is a link to another NASA reduction to the predicted number of sunspots for solar cycle 24. As this is the seventh reduction to the predicted number of sunspots it is evident that what is happening is not understood. The initial solar cycle 24 prediction was that solar cycle 24 would be stronger than past cycles.
Solar Cycle Prediction (Updated 2010/12/06)
Current prediction for the next sunspot cycle maximum gives a smoothed sunspot number maximum of about 64 in June of 2013.
This is a link to Livingston and Penn’s paper that discuss the observation that newly formed sunspots have a weaker magnetic field and the observation that there are no surface sunspots that have a magnetic field strength less than 1500 gauss.
http://science.nasa.gov/headlines/y2..._jetstream.htmLong-term Evolution of Sunspot Magnetic Fields By Matthew J. Penn And William Livingston
Independent of the normal solar cycle, a decrease in the sunspot magnetic field strength has been observed using the Zeeman-split 1564.8nm Fe I spectral line at the NSO Kitt Peak McMath-Pierce telescope. Corresponding changes in sunspot brightness and the strength of molecular absorption lines were also seen. This trend was seen to continue in observations of the first sunspots of the new solar Cycle 24, and extrapolating a linear fit to this trend would lead to only half the number of spots in Cycle 24 compared to Cycle 23, and imply virtually no sunspots in Cycle 25. ...
...We reported in Penn & Livingston (2006) that a time series of this magnetic field data showed a decrease in the umbral magnetic field strength which was independent of the normal sunspot cycle. Also, the measurements revealed a threshold magnetic field strength of about 1500 Gauss, below which no dark pores formed. A linear extrapolation of the magnetic field trend suggested that the mean field strength would reach this threshold 1500 Gauss value in the year 2017. Furthermore, analysis of the umbral continuum brightness showed another linear trend, and extrapolation showed the umbral brightness would be equal to the quiet Sun brightness at about the same year. Finally, the molecular line depths showed a decreasing strength with time, and again the trend suggested that molecular absorption lines would disappear from the average sunspot umbra near 2017.
http://science.nasa.gov/headlines/y2...p_sunspots.htmSeptember 3, 2009: “Are Sunspots Disappearing? – The sun is in the pits of the deepest solar minimum in nearly a century. Weeks and sometimes whole months go by without even a single tiny sunspot. The quiet has dragged out for more than two years, prompting some observers to wonder, are sunspots disappearing?
http://science.nasa.gov/headlines/y2...cosmicrays.htmSeptember 29, 2009 “Cosmic Rays Hit Space Age High – In 2009, cosmic ray intensities have increased 19% beyond anything we’ve seen in the past 50 years,” says Richard Mewaldt of Caltech. “The increase is significant, and it could mean we need to re-think how much radiation shielding astronauts take with them on deep-space missions.”
http://science.nasa.gov/science-news..._conveyorbelt/March 12, 2010 “NASA – Solar ‘Current of Fire’ Speeds Up – the top of the sun’s Great Conveyor Belt has been running at record-high speeds for the past five years. ”
http://science.nasa.gov/science-news..._thermosphere/July 15, 2010 “A Puzzling Collapse of Earth’s Upper Atmosphere – This is the biggest contraction of the thermosphere in at least 43 years”
http://science.nasa.gov/science-news.../21sep_zigzag/Sept. 21, 2010: “Solar Storms can Change Directions, Surprising Forecasters”
http://science.nasa.gov/science-news...lobaleruption/Dec. 13, 2010: “Global Eruption Rocks the Sun – A global eruption on the sun has shattered old ideas about solar activity.”