Solar activity lowest in nearly 100 years: so what?

U.S. solar physicists say the sun is experiencing the least sunspot activity since 1913 and activity is becoming less frequent.

National Aeronautics and Space Administration (NASA) solar physicist Dean Pesnell at the Goddard Space Flight Center in Greenbelt, Md., said during 2008 there were no sunspots observed on 266 of the year’s 366 days — 73 percent of the year. To find a year with more blank suns, you have to go to 1913, which had 311 spotless days.

That has led some observers to suggest the solar cycle hit bottom last year. But Pesnell says that might not be the case, since, there were no sunspots on 78 of this year’s first 90 days — 87 percent of that period.

In addition, measurements by the Ulysses spacecraft reveal a 20 percent drop in solar wind since the mid-1990s — the lowest since such measurements began in the 1960s. And NASA says the sun’s brightness has dimmed 0.02 percent at visible wavelengths and 6 percent at extreme UV wavelengths since the solar minimum of 1996.

Competing models created by solar physicists disagree on when the solar minimum will end and NASA says that great uncertainty stems from one simple fact: No one fully understands the underlying physics of the sunspot cycle.

Sunspot activity may be subtly linked to the earth’s weather. Suggestive correlations between solar activity, global temperature, and rainfall have been observed, and analysis of tree-ring data spanning centuries seems to show the presence of an 11-13 year cycle. There is also geological evidence that the solar cycle may have been affecting terrestrial weather since Precambrian times. However, all these data have been disputed on statistical grounds, and there presently no consensus among scientists as to whether sunspots actually affect the earth’s weather or not, or if so, how. The energy output of the Sun varies very little over the solar cycle (i.e., by about 0.1%), and some scientists doubt whether such slight changes can really affect the troposphere (lower atmosphere) of the earth, where precipitation occurs. A possible mechanism for amplifying the effects of the solar cycle on tropospheric weather is its influence on the stratosphere (the region of the atmosphere from an altitude of about 10 mi [16 km] to about 30 mi [50 km]). The stratosphere is home to the ozone layer, a diffuse shield of triatomic oxygen (O₃) that is an efficient absorber of ultraviolet radiation. Since the Sun’s ultraviolet output varies 10 times more over the solar cycle than its overall radiation

output, it is plausible-and has been confirmed by observation-that the temperature (and thus volume) of the stratosphere will vary significantly with the solar cycle. (Those involved in the launching and maintenance of Earth satellites are acutely aware that the upper layers of the earth’s atmosphere respond to solar activity by expanding and thereby inflicting increased drag on satellites in low orbits.) However, the troposphere is many thousands of times more massive than the stratosphere, and scientists continue to investigate the question of whether temperatures in the frail film of the stratosphere can measurably affect surface weather.

Some scientists contend there is a relationship between low solar activity and the mini-ice age in the 19th century. Other are more skeptical. Most agree it is unclear.