Sunspots—temporary dark spots that appear on the outer layer of the Sun—have been observed and recorded by humans for more than two millennia, but exactly what causes them and what determines the cycles of their appearance is still being explored by scientists. A recent article in Reviews of Geophysics presented a synthesis of what we know about cycles of sunspot activity and their drivers. Here, the authors give an overview of scientific understanding to date and suggest where additional research is needed.
What powers the Sun? Is its energy output constant?
The Sun is powered by a nuclear furnace at its center. Under great pressure, four hydrogen nuclei are “burned” to produce one helium atom. This is called nuclear fusion.
Since one helium atom has less mass than the four hydrogen atoms, energy is produced associated with the loss of mass.
While we believe that the production of energy is constant, its delivery to the solar surface, from where it is radiated outward toward Earth and other objects in the solar system, may vary over time because it depends on the convection through the last third of the solar radius to the solar surface.
This convection is in many ways like the motion of boiling water in a pot on a stovetop.
What are sunspots and how often do they appear?
When the hydrogen and helium in the Sun are heated, the heat conducts and radiates its heat upward until it reaches about two-thirds of the solar radius where it becomes more efficient to convect (as in the pot of hot water).
The turbulent motion of the solar matter that is electrically conducting and spinning around the solar rotation axis produces a magnetic field that eventually is carried off into space. Some regions on the “surface” of the Sun (the “photosphere” that we see) can become strongly magnetized and the concentrations of magnetic field flux inhibit convection causing such regions to be cooler and appear darker than the region around them.
The pot on our stove has a much simpler convection pattern than the spherical spinning Sun. The Sun’s convective motions result in about an 11-year variation in the number of sunspots and their location. The direction of the magnetic field changes every sunspot number cycle. The number and distribution of sunspots varies greatly during a “solar cycle” and from solar cycle to solar cycle.
What does the number of sunspots, their location and frequency of appearance tell us?
The spinning, convecting, electrically conducting outer layer of the Sun makes the magnetic fields that lead to sunspots, but we need to “see” deeply into the Sun to understand these processes.
Fortunately, solar physicists have learned how to use the vibrations of the solar surface to deduce the large-scale motion of the solar convection zone. Now we are beginning to be able to track the onset of a new solar cycle from its origin near where convection takes over from radiation and watch the solar material rise to the surface.
We do not understand fully how these phenomena take place, but we do have a qualitative picture of the dynamo that generates the solar magnetic field. The upward thermal convection carries much magnetism to the visible surface. If it rises quickly, we enjoy a very active Sun with many sunspots. If convection is slow, the longer it will take to reach the sunspot maximum and more dissipation occurs in the rising phase. The resultant solar activity is weaker.
Is it possible to predict the long-term cycles of sunspots?
This is the question that keeps solar physicists up at night even though they have a “day” job. In our review, we emphasize that longer and weaker solar cycles often follow the shorter and stronger cycles so that, in the long-term, solar sunspot activity is synchronized to an internal clock. However, determining when the synchronism will take place is still not well understood.
What are some of the unsolved questions where additional research, data or modeling is needed?
Solar physicists have been working hard using better and better solar telescopes, and launching space missions close to the Sun, trying to understand the entire the Sun from all angles and at all depths.
However, scientists are stymied in part by where telescopes are located. The Sun is a three-dimensional object but from Earth we see mainly a low latitude region on one side of the Sun. We need to see the farside of the Sun, the east and west limbs, and the north and south poles.
There are ways to do this, but it will require an investment in solar exploration greater than we have today. However, such a program would be possible if an international effort were coordinated among nations who are interested in the Sun and its effects on the Earth.
—Christopher T. Russell ([email protected]; 0000-0003-1639-8298), Earth Planetary and Space Sciences, University of California, Los Angeles; Lan K. Jian ( 0000-0002-6849-5527), NASA Goddard Space Flight Center; and Janet G. Luhmann, Space Sciences Laboratory, University of California, Berkeley