To reacquaint you with this topic, lets do a little review. Livingston and Penn have been measuring the umbral intensity of sunspots and the corresponding magnetic field that spawns them since 1990. In 2006 they submitted a paper to the journal “Science” reporting on their efforts and suggested that if the trend of weaker sunspot magnetic fields continued at its current rate, they would be too weak to produce sunspots. This paper was rejected in peer review. Undeterred, they have continued to study these phenomena and so far, they seem to be on to something.
Sunspots are the product of the enormous magnetic fields created on the Sun. What make them especially interesting is that the Earth’s climate and sunspots have a high degree of correlation. Periods where the climate has cooled off seem to coincide with periods of few sunspots and periods of warmer climate seem to coincide with periods of high sunspot counts.
Sunspots appear as dark spots on the face of the Sun. Very strong magnetic forces (thousands of times stronger than Earths magnetic field) block the hot solar plasma and sunspots are the result. The spots are cooler than the surrounding surface of the Sun. NASA says that the spots are about 3700K versus 5700K for the surrounding photosphere.
©UCAR, image courtesy Matthias Rempel, NCAR
The photo sh0ws the “spot” (the umbra) surrounded by the penumbra that is shaped by the magnetic lines of force.
Livingston and Penn have studied over 1700 spots and they see a trend in which the darkest parts of the sunspot umbra have become warmer (45K per year) and their magnetic field strengths have decreased (77 Gauss per year**), independently of the normal 11-year sunspot cycle.
The latest data is shown in the two charts, UMBRAL INTENSITY AND UMBRAL MAGNETIC FIELD.
Charts courtesy of Leif Svalgaard
The umbral intensity is a measurement of the light from the umbra (the dark center) and compared to a measurement of the light from a calm sun surface. Note that the umbra is getting hotter and brighter as the umbral magnetic field gets weaker. The two scientists believe that if the magnetic field weakens to ≈1500 gauss, the sunspots will not form. If the trend continues linearly, that could happen in this decade.
If there are no visible sunspots in Cycle 25, it could mean that we would be experiencing a solar minimum like the Maunder minimum that heralded in the Little Ice Age. It should be noted that while this is a suggestion, rather than a promise, it certainly is consistent with the observable trend of a less energetic Sun.
** Gauss is measure of the strength of a magnetic field. Its units are Maxwells per square centimeter. A small bar magnet will range from 40 to 100 gauss. The Sun’s average magnetic field strength is 1 and the Earth’s is 0.5.