Tuesday, November 15, 2011

Sunspots & the Earth

In previous posts, I introduced sunspots, discussed sunspot cycles, and tried to explain why the Sun has sunspots.  As a professor, I'm used to getting the "Why should we care?" argument from students.  My stock answer, expressed a bit more eloquently, is that it's fucking interesting.  But, in the case of sunspots, there are valid reasons why we, as a society, should be interested in them.  Check out this video.

This is a coronal mass ejection (CME) - a massive release of electromagnetic energy and ionized (charged) particles, mostly electrons and protons.  If the event occurs on the side of the Sun facing the Earth, electromagnetic energy from across the spectrum, long-wavelength radio waves to short-wavelength gamma rays, travel to Earth at the speed of light taking only 8.5 minutes or so to get here.  The stream of charged particles takes a bit longer to reach the Earth traveling, on average, about 500 km/s although sometimes reaching speeds of 2000 km/s.  Since the Sun is 150 million km away, it will take the charged particles anywhere from 1-4 days to arrive (depending on their speed).

These eruptions of energy on the Sun are associated with active regions - in other words sunspots.  They're not well understood but are thought to occur when lines of magnetic force break and reconnect releasing stored energy.  As much energy as a billion hydrogen bombs!

What are the consequences of this here on Earth?

Fortunately, here on Earth, we're shielded from much of the dangerous electromagnetic radiation (gamma rays and x-rays) and high-energy charged particles by the Earth's atmosphere and magnetic field.  Future astronauts on the surface of the Moon, or traveling on a ship to Mars, could get radiation poisoning or even be killed by such events (astronauts aboard the International Space Station are in a low-Earth orbit and still somewhat shielded from such events).

OK, you're thinking, I'm not planning a trip to Mars anytime soon so what's the worry?  The problem is that with a large enough CME, our atmosphere and magnetic field become a bit overwhelmed and there are effects here on Earth - some harmless and some more serious.

Our Earth has a magnetic field generated by the rotation of liquid iron in the outer core.  This field normally deflects away the constant stream of charged particles from the Sun (the solar wind).  This solar wind compresses the Earth's magnetic field on the side facing the Sun and stretches it out on the far side into a tail.

During a CME, so many charged particles (ions) interact with the magnetic field that some are able to leak down toward the Earth in the vicinity of the north and south magnetic poles.  Some get trapped in a doughnut-shaped ring called the Van Allen radiation belt and others spiral into the upper atmosphere (called the ionosphere).  These results in auroras.

More on auroras in a bit while we first take a short digression and talk about satellites and power grids...

Satellites are greatly affected by the charged particles released during a CME.  Most satellites don't orbit in the vicinity of the Van Allen belt, but those that do need to have their electronic components radiation hardened to survive.  Satellites in higher orbits are susceptible to damage from the high-energy particles from CMEs.  High energy electrons can physically damage the electronics and solar cells of satellites and even scramble the data stored in computer chips.

Large CME events can also compress the magnetosphere (the magnetic field "bubble" around the Earth) leaving the satellite outside of the protection of the magnetic field and more vulnerable to damage.  In addition, since satellites often use the Earth's magnetic field for guidance, this can disrupt their attitude control systems.  In 1997 and 1998, during the last sunspot cycle, a number of satellites were damaged from CMEs including the AT&T Telstar 401, PanAmSat Galaxy IV, and several Motorola Iridium satellites.  Almost a billion dollars in insurance claims were paid out in 1998 for satellite failures in orbit.

Low-Earth orbit satellites can also suffer from CMEs.  During a CME, the added energy into the Earth's atmosphere causes it to expand.  This creates increased frictional drag on these satellites reducing their orbital life-span.

There is another effect CMEs can have as well.  Large CMEs can induce currents in electrical lines here on Earth.  In March of 1989, two solar cycles ago, a large CME caused the power grid in Quebec to go down resulting in six million people losing power.  Are we still vulnerable 20+ years later?  More so than ever - check out these images from a recent study of this issue (click on the images to enlarge and read the captions).

It's theoretically possible for a large CME to knock out half of the U.S. power grid for weeks to years!  Think about that when wondering if it's worthwhile funding scientific research of the Sun.

Next time I'll post about auroras but for now I'll leave you with some information about the solar "superstorm" or 1859 (thought to be a once in every 500 years event).

On September 1, British astronomer Richard Carrington observed a large CME erupt from the Sun which took only 18 hours to reach the Earth (a velocity for the particles of over 2,000 km/s).  This triggered a massive geomagnetic storm on Earth resulting in auroras seen around the world (most notably down in the Caribbean!).  There were reports of people here in the Northeast being able to read newspapers by the light of the auroras at night.  Telegraph systems throughout the world failed.  Sparks flew from wires, operators received electrical shocks, and telegraph paper even caught fire.

Such an event today, in our electrified, wired world, would be literally catastrophic.

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