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Spotting the Minimum

Who would have thought that an absence of spots on the Sun would enthrall parts of the Internet.  I keep running across articles and blog posts about a new “Maunder Minimum” starting this year.  The catalyst for those posts is the current sunspot minimum.

I suppose it is all part of the human desire to see doom everywhere.  In the 1960s and 1970s, when I was young, nuclear war and overpopulation were the two contradictory dooms hanging over humanity—the Earth would either be depopulated, or so overpopulated that the whole ecosystem would collapse.  With the gasoline shortages of the 1970s, the exhaustion of finite resources, particularly of  oil, became a third doom.  In the 1980s, worries about finite oil and about overpopulation were superseded by worries about overfull landfills—creating a Wall-E like world of trash-choked streets—and of general economic collapse.  At the end of the 1990s, worries about global warming and the collapse of the world's computer systems because of the “millennium bug”—the inability of old computers to handle years beginning with 20 rather than 19—became the new dooms.

Today the global warming alarmists claim that the Earth's temperature will soon run away, melting the ice caps, flooding the coastlands, and making Canada the hot spring-break destination.  In contradiction to this, a second doom makes a reappearance after a 2 decade absence: we will out of oil, which will cause the collapse of the economy.  So as in the 1960s, we have contradictory dooms over us.  Either we will burn too much oil, causing the ecosystem to collapse, or we will burn too little oil, causing the world economy to collapse.

Of course astronomers are not to be left out of this fun.  Nothing on Earth can cause doom like an astronomical body.  A nearby supernova, a solar superflare, and an asteroid strike each has its charms as a life-altering event.  The asteroid strike, of course, has caused tremendous damage in the past, so it is certainly worth worrying about.  One of the more peculiar claims that an astronomical event is about to affect humanity, however, is the claim I have encountered on the web that the Earth is about to cool rapidly because the Sun is entering a Maunder Minimum, which is a decades-long absence of sunspots on the Sun.

Soho raw image of the Sun, August 19, 2008.  Soho is a cooperative project of ESA and NASA.

The Sun often has cool regions that appear in photographs as dark spots.  These spots are regions of strong magnetic field.  The number of spots on the Sun changes over an eleven year cycle, which reflects the reversal in polarity of the Sun's magnetic field every eleven years.  At the beginning of this cycle, the magnetic field within the Sun is weak and uniform, and there are few or no sunspots.  Over time, convection within the Sun tangles and concentrates the magnetic fields, creating buoyant loops of magnetic field that float out of the Sun's photosphere.  The feet of these magnetic loops at the photosphere appear as sunspots.  Over time, more and more loops of magnetic field are expelled from the Sun, creating numerous sunspots.  Eventually, these tangles fields decay away, leaving behind a weak and uniform magnetic field of reversed polarity, and returning the Sun to a quiet, sunspot-free state.

Magnetic fields are responsible for the hot corona that surrounds the Sun and the x-rays generated by the Sun.  These x-ray interact with Earth's upper atmosphere, and the flares and prominences from the Sun interact with Earth's magnetic field to produce aurora at the magnetic poles.  These interactions accelerate the decay of the orbits of satellites above Earth.  Some researchers claim this solar activity affects the climate, although this is a controversial claim.

The 11 year cycle we see today has not lasted throughout human history.  The time between sunspot minima over the past 5 hundred years has varied dramatically from about 7 years to 14 years.  Between the years 1640 and 1715, a period of 75 years, there were no sunspots on the Sun.  This period of time is called the Maunder Minimum, and it coincides with the middle of the Little Ice Age.  Whether this is simple coincidence, or evidence of a link between sunspot activity and Earth's climate, is unsettled.

Today we are in a time of no sunspots, because we are at the minimum of the 11 year sunspot cycle.  The figure below shows the last six cycles. 

The diagram shows the observed average number of sunspots per month for the period of December 1945 through June 2008.  The data is the American Relative Sunspot Numbers from the National Geophysical Data Center, which is available from their Solar Data Services page.

The data in the figure shows the average number of sunspots each month.  These cycles appear to be slightly less than the 11 year average found from a longer period of data.  The current minimum resembles all the previous minima in this figure; the sunspots should reappear within the next year. 

The current data doesn't show that we are at the start of a Maunder minimum; those who claim that we are clearly reading too much into this plot.  If the Sun has no sunspots three years from now, then we can worry about a new Maunder Minimum.   Mother nature has a wonderful sense of irony, continually demonstrating to us theorists that we generally don't have as much insight into how she works as we would like to think, and I love the idea of the Earth's climate cooling when so many fear mongers are screaming about global warming, but clearly this irony is not now on display.

Jim Brainerd

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