Weird sense of calm: The sunspot cycle from 1995 to the present. The jagged curve traces actual sunspot counts. Smooth curves are fits to the data and one forecaster's predictions of future activity.
Credit: David Hathaway, NASA/MSFC
2008 was a bear. There were no sunspots observed on 266 of the year's 366 days (73%). To find a year with more 'blank' Suns, you have to go all the way back to 1913, which had 311 spotless days.
Prompted by these numbers, some observers suggested that the solar cycle had hit bottom in 2008. Maybe not. Sunspot counts for 2009 have dropped even lower. As of March 31, there were no sunspots on 78 of the year's 90 days (87%).
It adds up to one inescapable conclusion: "We're experiencing a very deep solar minimum," says solar physicist Dean Pesnell of NASA's Goddard Space Flight Centre in Maryland, USA.
"This is the quietest Sun we've seen in almost a century," agrees sunspot expert David Hathaway of the Marshall Space Flight Centre in Huntsville, Alabama.
Quiet Suns come along every 11 years or so. It's a natural part of the sunspot cycle, discovered by German astronomer Heinrich Schwabe in the mid-1800s.
Sunspots are planet-sized islands of magnetism on the surface of the Sun; they are sources of solar flares, coronal mass ejections and intense ultraviolet radiation. Plotting sunspot counts, Schwabe saw that peaks of solar activity were always followed by valleys of relative calm – a clockwork pattern that has held true for more than 200 years.
The current solar minimum is part of that pattern. In fact, it's right on time. "We're due for a bit of quiet – and here it is," says Pesnell.
But is it really supposed to be this quiet? In 2008, the Sun set the following records:
A 50-year low in solar wind pressure: Measurements by the Ulysses spacecraft reveal a 20% drop in solar wind pressure since the mid-1990s – the lowest point since such measurements began in the 1960s. The solar wind helps keep galactic cosmic rays out of the inner Solar System. With the solar wind flagging, more cosmic rays are permitted to enter, resulting in increased health hazards for astronauts. Weaker solar wind also means fewer geomagnetic storms and auroras on Earth.
A 12-year low in solar "irradiance": Careful measurements by several NASA spacecraft show that the Sun's brightness has dropped by 0.02% at visible wavelengths and 6% at extreme UV wavelengths since the solar minimum of 1996. The changes so far are not enough to reverse the course of global warming, but there are some other significant side-effects: Earth's upper atmosphere is heated less by the Sun and it is therefore less 'puffed up'. Satellites in low Earth orbit experience less atmospheric drag, extending their operational lifetimes. Unfortunately, space junk also remains longer in Earth orbit, increasing hazards to spacecraft and satellites.
A 55-year low in solar radio emissions: After World War II, astronomers began keeping records of the Sun's brightness at radio wavelengths. Records of 10.7 cm flux extend back all the way to the early 1950s. Radio telescopes are now recording the dimmest 'radio Sun' since 1955. Some researchers believe that the lessening of radio emissions is an indication of weakness in the Sun's global magnetic field. No one is certain, however, because the source of these long-monitored radio emissions is not fully understood.
