The Earth was pummeled with record-setting levels of cosmic rays in 2009. Measurements from NASA's Advanced Composition Explorer (ACE) and other spacecraft found that more high-energy particles from galactic space penetrated the inner solar system in the last few years than at any other time since the beginning of the space age.

The spike is almost certainly due to several weird aspects of the most recent solar minimum, and could be the start of a new normal for cosmic ray levels.

"It's sort of like everything's working in the same direction right now, to allow cosmic rays greater access to the inner solar system," said space scientist Richard Mewaldt of Caltech. Mewaldt and colleagues published their findings Oct. 7 in Astrophysical Journal Letters.

Cosmic rays, high-energy particles that originate in the galaxy and smack into Earth from all directions at near-light speeds, can pose a danger to spacecraft and astronauts spending long periods of time outside the Earth's protective magnetic field. Most of these particles, especially the less-energetic ones, are deflected by the solar wind, which blows a protective bubble around the solar system called the heliosphere.

This solar system shield fluctuates in effectiveness every 11 years, as the sun goes through its regular cycle from lots of sunspots and solar flares to relatively boring solar weather. When the sun is most active, the solar wind is strongest, and even fewer cosmic rays penetrate the barriers. At solar minimum, more cosmic rays make it through.

"Up until now they had been reaching a constant level each solar minimum," Mewaldt said. "But this one was different. This cycle, they're more intense than they were in the past."

The most recent solar minimum started in 2006 and was expected to end in 2008, but the sun stayed quiet through 2010. Using data from the ACE spacecraft, which has been in orbit around the sun since 1997, and historical data from a series of short-lived spacecraft going back to 1965, Mewaldt and colleagues showed that the cosmic ray levels in 2009 were 20 to 26 percent greater than at any previous solar minimum.

There are three main reasons for the upswing in cosmic rays, Mewaldt said. The solar magnetic field has been weaker than usual, which means the magnetic field that permeates the solar system is weaker too, and less efficient at knocking cosmic rays aside.

The long years of low solar activity also contribute to the high cosmic ray numbers. The sun occasionally lets off enormous bursts of plasma called coronal mass ejections, which can block cosmic rays as they explode out into interplanetary space. But there were fewer of these bursts during the most recent solar minimum, and those that happened were smaller than usual. "That's another thing that let down the barriers and let the cosmic rays come in easier," Mewaldt said.

Finally, the constant stream of charged particles that makes up the solar wind is weaker, making the protective bubble of the heliosphere smaller and more permeable. Incoming cosmic rays have a shorter distance to go to reach the Earth, so wimpier particles that would normally never get here can now make the journey.

Astronomers have already seen the impact of these extra cosmic rays on spacecraft, which have shown a 25 percent increase in certain types of errors that result from cosmic ray strikes, Mewaldt says.

The increased cosmic rays could pose a bigger problem for astronauts heading to Mars or building a base on the moon.

"They'd feel the brunt of this radiation for a longer period," Mewaldt said "It's already a problem, this would just make it worse."

Although cosmic ray levels started going back down in early 2010, Mewaldt thinks the new high could be part of the long-term pattern of the sun. Measurements of radioactive elements embedded in ice cores at the poles show that over the past 500 years, cosmic ray levels were 40 to 80 percent higher than in the early 1970s. That means the sun was quieter in the past than it has been in the last few decades.

"It could well be that we are going to one of these longer-term grand minima," Mewaldt said. "We don't know yet for sure if we're starting into one of those periods, but it certainly looks possible. We'll have to wait a little longer to say."

"I believe that this paper is the first paper that really shows us how the heliosphere works as a big global system," commented NASA astronomer William D. Pesnell. "I think it will become an important paper because of that."

Image: NASA

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