I last wrote about the lack of sunspots in September 2008. Now, seven months later here in April 2009, even NASA is raising a quizzical eyebrow as it releases a press release discussing what it describes as a very deep solar minimum.
Despite the press release being dated 1st April, this is no April Fool. Likening solar activity to the stock markets, it describes 2008 as a “bear” year, with 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 31st, there were no sunspots on 78 of the year’s 90 days (87%).
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Solar physicist Dean Pesnell of the Goddard Space Center said: “We’re experiencing a very deep solar minimum.” The Marshall Space Flight Center’s sunspot expert David Hathaway agrees: “This is the quietest sun we’ve seen in almost a century.”
It’s natural for the sun to undergo a quiet period every 11 years or so. But this quiet?
The sun set some impressive records during 2008:
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.
It might also mean increased cloud cover, causing a cooling of the planet — as I discussed in There Goes The Sun.
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. (Click image for larger version)
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.
Whether these lows are “weird”, “extreme” or just an overdue “market correction” following a string of unusually intense solar maxima is currently being hotly debated amongst the experts.
Competing models by dozens of top solar physicists disagree, sometimes sharply, on when this solar minimum will end and how big the next solar maximum will be … The great uncertainty stems from one simple fact: No one fully understands the underlying physics of the sunspot cycle.
Pesnell believes sunspot counts will pick up again soon, “possibly by the end of the year,” to be followed by a solar maximum of below-average intensity in 2012 or 2013. But like other forecasters, he knows he could be wrong.
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