A quiet, cool Sun

Scientists probe sunspot cycle, climate

Our nearest star, the Sun, has been awfully quiet lately. In fact, no living scientist has seen it behave this way.

In 2008, scientists observed no sunspots on 73% of the year’s 366 days, making 1913 the last year with more spotless days. The count for 2009 is even lower; as of early May, scientists reported no sunspots on 85% of the year’s days.

“It’s an exciting time,” says Mark Miesch (ESSL/HAO). “This is the deepest solar minimum since the space age began—the first deep minimum we’re getting a really good look at. We have telescopes scrutinizing it like never before.”

Sunspots are dark, planet-sized regions of magnetism on the Sun’s surface that spit out flares and explosions of hot gas called coronal mass ejections, and cause intense ultraviolet radiation. They follow an 11-year cycle of activity discovered by German astronomer Samuel Heinrich Schwabe in 1843. A peak in the sunspot count during the cycle is called the solar maximum, as opposed to the calmer solar minimum, when the least number of sunspots appears. Although the cycle averages 11 years, since 1700 it has varied in length from nine to 14 years.

Images of the Sun on October 30, 2003 at the peak of the last solar cycle and on May 4, 2009 when no sunspots were observed

These images from NCAR’s Mauna Loa Solar Observatory show helium emissions from the upper chromosphere on October 30, 2003, near the peak of the last solar cycle (left), and on May 4, 2009, when no sunspots were observed (right). (Images courtesy ESSL/HAO.)

In 1848, Swiss astronomer Rudolf Wolf came up with a method for counting sunspots that is still in use today. Wolf reconstructed cycles as far back as the 1755–66 cycle, which he dubbed “cycle 1.” Since then, subsequent cycles have been numbered consecutively, so the current cycle, which began with a solar minimum in 1996, is cycle 23.

Cycle 23 is confounding scientists as its solar minimum persists far longer than almost anyone expected. NOAA’s Space Weather Prediction Center called for the next cycle, 24, to heat up between September 2007 and September 2008. Instead, the Sun continues to scrape the bottom of cycle 23. In addition to its absence of sunspots, it set several other records last year, hitting a 50-year low in solar wind pressure (streams of particles poured out from the Sun), a 12-year low in solar irradiance (brightness on visible and extreme ultraviolet wavelengths), and a 55-year low in radio emissions (brightness on radio wavelengths).

Scientists in HAO are observing the quiet Sun with a coronagraph known as the Mark IV operated at the Mauna Loa Solar Observatory, along with other instruments. They’re also involved with Hinode, a satellite-borne solar observatory launched by Japan in 2006 that carries a solar telescope that NCAR helped build. The telescope is taking high-resolution images of the Sun’s surface.

NCAR team predicted late start to cycle 24

Group of sunspots on the surface of the Sun

This image shows a group of sunspots on the surface of the Sun. The direction of the Sun’s magnetic field is indicated by the spines in the penumbrae—the shaded fringes around the center of each spot. The photograph was taken through a Swedish telescope in La Palma, Canary Islands, Spain.

In March 2006, Mausumi Dikpati, Giuliana de Toma, and Peter Gilman in HAO predicted that cycle 24 would begin as much as a year late and be 30–50% stronger than cycle 23. They used a forecasting approach based on a dynamical modeling approach, as opposed to statistical patterns used by other forecasting groups. The group is sticking by its prediction of a strong cycle, even though slow-starting cycles are often weak.

“In our model, the seed magnetic field for a new cycle comes from the recycled magnetic fields of the Sun from the past three cycles; therefore, it does not depend on what is happening at the present,” Mausumi explains.

Mausumi predicted in 2004 that cycle 23 would be a long one, delaying the start ofcycle 24. This prediction has certainly turned out to be true—cycle 24 is now delayed even longer than she predicted. (The HAO model is the only sunspot model that predicted the delayed onset, with all others predicting that cycle 24 would commence as early as 2006.) In the HAO model, the length of a cycle comes from the speed of the Sun’s meridional (north-to-south) flow. Observations indicate a slow-down in meridional flow during the major part of cycle 23, Mausumi says, providing a rationale for a delay of cycle 24.

The HAO group is also standing by its prediction of a strong cycle despite a different forecast from the Solar Cycle 24 Prediction Panel, a multiagency group coordinated by NOAA, released on May 8. The majority (but not a consensus) of the panel reached a prediction that cycle 24 will be below average in intensity, with a maximum sunspot number of 90.

Sunspots and climate

Scientists have long tried to detect climate impacts on Earth from the Sun’s 11-year cycles, with debatable results. Most solar researchers believe that the cycle is too short to have much immediate effect on Earth’s climate, particularly since the variation in solar output during these cycles is only about 0.1%.

Recently, some observers, including a few climate change skeptics, have drawn connections between the current solar minimum and the fact that average temperatures in 2008 were the coolest since 2000 (though 2008 still ranked among the 10 warmest years on record). According to Mark, however, “the climate system takes longer to respond to solar activity than a few years.”

Apart from its 11-year cycles, the Sun also experiences longer-lasting active and quiet phases that can persist for decades. Scientists have observed and closely documented these active phases (grand maxima) and quiet phases (grand minima). During periods with higher solar activity, surface temperatures on Earth appear warmer than average, while periods with markedly less solar activity might be related to cooler conditions on Earth.

In the 17th century, the Sun went into a quiet spell during which no sunspots were observed for 70 years. This period, known as the Maunder Minimum, is often used as a synonym for the Little Ice Age, but the two aren’t interchangeable. “One has to be careful,” cautions Caspar Ammann (ESSL/CGD). “Lower solar activity could have contributed to the Little Ice Age, but other factors such as volcanism are at least as important. The coldest years in the Maunder Minimum were right after strong volcanic eruptions.”

Another quiet spell between 1790 and 1830, the Dalton Minimum, is believed to have cooled Earth slightly, though the early 19th century also saw a number of exceptionally large volcanic eruptions. The Sun was highly active in the 20th century, peaking in the 1950s and 1980s. Although it became active at the same time Earth began to warm, most scientists believe that the Sun’s effect on climate is overwhelmed by human-caused climate change brought about by the release of greenhouse gases.

“One indication for such a conclusion can be gained from the sunspots themselves. Although there were large-amplitude sunspot cycles over the past few decades, there is no marked increase in sunspots that would mimic the recent temperature trends,” Caspar says. He adds that cooling in the upper atmosphere suggests the lack of a solar driver for the changes, pointing to greenhouse gases instead.

The upper atmosphere responds

In HAO, Stan Solomon is studying Earth’s upper atmosphere, which responds to both solar activity and climate change. During the active phase of the 11-year cycle, ultraviolet light and energetic particles from the Sun increase, producing a warming and expansion of the upper atmosphere; when solar activity wanes, the thermosphere settles and cools. The build-up of carbon dioxide in the atmosphere also cools the thermosphere even though it acts to warm the atmosphere near the Earth’s surface, a paradox related to the fact that the atmosphere thins with height.

Stan’s research is aimed at sorting out the effects of both the solar cycle and carbon dioxide on the thermosphere’s cooling trend. “There’s enormous interest in this among the solar-terrestrial community,” says Stan, who’s getting involved with a new NASA research program on the causes and consequences of the current solar minimum. “We need to understand the entire atmosphere, including outer space, and how it responds to a combination of solar change and anthropogenic [human-caused] change.”

Chart showing years with most spotless days In 2008, scientists observed no sunspots on 73% of the year’s 366 days, making 1913 the last year with more spotless days. (Image courtesy spaceweather.com.)

The research can also help NASA and other agencies plan the fuel needs and timing of satellite launches more precisely, potentially saving millions of dollars.

When the thermosphere cools, it contracts, becoming less dense. This reduces the drag on satellites in low Earth orbit, allowing them to stay airborne longer.

Stan emphasizes that, although scientists have demonstrated that climate change is a real threat and the general public increasingly agrees, researchers still have plenty of work cut out for them trying to understand the complex interplay between the Sun, Earth’s atmosphere, and climate change.

“Climate research isn’t over, because there is a huge amount that we don’t understand,” he says. “And we need to be able to quantify the things we think we understand.”