Solar Sunspots Hibernation?

June 24, 2011

Is the cycle of sunspots going dormant for an extended period?

That’s what astronomers suggested at the June 14 annual meeting of the American Astronomical Society’s solar physics division, held at New Mexico State University. Frank Hill, associate director of the National Solar Observatory’s Solar Synoptic Network announced, “The solar cycle may be going into a hiatus.”

First, let’s review what a solar cycle is.

Check out my previous blog on the topic.

Like all fluid bodies in our solar system, the sun has a magnetic field.  Where these field lines intersect the sun’s surface, convection from inside the sun is blocked, resulting in a cooler region on the sun’s surface.  The cooler region is darker because it emits more infrared light, which is invisible to our eyes.  The number of sunspots on the sun is not constant but varies over a period of about eleven years.  Since we began keeping systematic track of sunspots, scientists have observed 23 such cycles.

02 Sun Structure
Creative Commons License photo credit: Image Editor

However, the most recent solar minimum lasted much longer than we expected.

We had hoped to begin seeing sunspots in 2008 or 2009, leading to a 2012 peak.  Instead, solar minimum persisted until 2010.  Scientists now expect the current cycle (#24) to peak in May 2013.

According to Frank Hill, several lines of evidence point to a larger trend, in which solar maxima become delayed as well as less and less pronounced, possibly resulting in an extended period largely without sunspots.  One involves the solar ‘jet stream,’ a stream of plasma inside the sun which is analogous to jet streams in Earth’s atmosphere.  About every 11 years, such streams of plasma form near the poles of the sun and then migrate towards the sun’s equator.  When they reach a latitude of about 22 degrees, more sunspot formation is allowed.

Although cycle 24 is well underway, Hill attempted to detect the solar jet stream that will start cycle 25, which in theory should already be forming in the polar regions.  He was unable to do so, leading him to believe the solar cycle 25 may be delayed and its maximum smaller than for cycle 24.

Also, astronomers Matt Penn and William Livingston, upon analyzing 13 years of sunspot date taken at Kitt Peak in Arizona, determined that magnetic fields associated with sunspots now are weaker than during cycle 23.  If the trend continues, these magnetic fields could become too weak to inhibit convection at the sun’s surface, thus preventing sunspot formation.

This may mean that future solar cycles (25, 26, etc.) will have only very small maxima, resulting in a decades-long period of few if any sunspots.

A sunspot viewed close-up in ultraviolet light, taken by the TRACE spacecraft

The last time this happened was the Maunder Minimum, which occurred roughly from 1645-1715.

Astronomers of the day, such as Giovanni Cassini and Johannes Hevelius, were making systematic observations of the sun, and they noted very few sunspots – only about 50 over one 30-year period.  A less severe drop in sunspot activity, called the Dalton Minimum, occurred in the early ninteenth century.  Each of these extended minima were associated with below average temperatures on Earth.  For example, the Great Frost of 1708-09 was among the worst winters in recorded history.

However, not all solar scientists agree that another Maunder Minimum is on the way.

Douglas Biesecker of the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center points out that cycle 24’s polar jet stream formed about eight months after solar minimum and remained patchy for up to 30 months after that. It may still be too soon after the last solar minimum (December 2008) to draw conclusions about that jet.

Also, Biesecker points out that the raw data on the graph showing the weakening of the magnetic fields in sunspots is scattered and indeterminate enough to allow other analyses.

Of course, only the real sun will determine who’s correct on this issue, and you can observe the real sun right here at the Museum.

Our sundial has three sets of holes aligned with the sun’s midday position at each solstice and at the equinoxes.  As we are  now just past the summer solstice (which occurred at 12:17 p.m. June 21), anyone willing to brave the heat can come to our sundial near local noon (1 p.m. during Daylight Saving Time) and project an image of the sun onto a sheet of paper.  Any sunspots present will be revealed.

Authored By James Wooten

James is the Planetarium Astronomer at the Houston Museum of Natural Science. He teaches students every school morning in the planetarium, and also answers astronomy questions from the public.

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