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What is a coronal mass ejection or CME?



During a CME, enormous bubbles of superheated gas—called plasma—are ejected from the sun.  Over the course of several hours, one billion tons of material are lifted off the sun’s surface and accelerated to speeds of one million miles per hour.  This can happen several times a day when the sun is most active. During its quieter periods, CMEs occur only once every five days.

Coronal mass ejection in 2001

In 2001, the Solar and Heliospheric Observatory—a joint space mission between NASA and the ESA—recorded this coronal mass ejection. Credit: SOHO (ESA & NASA)

The underlying cause of CMEs is not well understood.  Astronomers agree, however, that the sun’s magnetic field plays a major role.  Because the sun is a fluid, turbulence tends to twist the magnetic field into complex contortions.  Twist the field too much, and it kinks—much like a phone cord or toy Slinky.  These kinks snap the magnetic field and can potentially drive vast amounts of plasma into space.

The plasma itself is a cloud of protons and electrons carried aloft by the solar wind.  Traveling at a million miles per hour, the ejecta can cross the 93 million mile distance to Earth in just a few days.  A jet moving that fast could get you from Los Angeles to New York in 18 seconds.

CME on August 31, 2012

On August 31, 2012, the Solar Dynamics Observatory caught the sun launching streams of plasma into space at nearly 900 miles/sec. Credit: NASA/GSFC/SDO

Because CMEs get blown off the sun in all directions, most don’t come anywhere near Earth.  But every so often, an eruption is aimed right at us.  When the plasma cloud hits our planet, a geomagnetic storm follows. The shock wave of charged particles compresses the Earth’s dayside magnetic field while the nightside gets stretched out.  Like an elongated rubber band, the terrestrial magnetic field eventually snaps back with the same amount of energy as a bolt of lightening.

The onslaught of charged particles and the temporary restructuring of the Earth’s magnetic field has observable effects.  Auroral lights, usually only seen near the poles, can drift to lower latitudes and become more brilliant.  The disturbance of the magnetic field can also expose Earth to deadly cosmic rays.  The atmosphere still provides enough protection for everyone on the ground.  But astronauts in space may receive lethal doses of radiation.  During a solar storm in 1989, cosmonauts aboard the Mir space station received their maximum yearly radiation dose in just a few hours!

The real long-lasting danger comes from the storm’s effect on technology.  The flurry of magnetic activity and induced electric currents has the potential to severely disrupt power grids, satellites, communication networks—anything that uses electricity.  When the sun aimed a CME at us in 1989, the resulting storm collapsed the Hydro-Québec power grid.  Six million people were without power for nine hours.

A coronal mass ejection in 2000

A CME recorded by the SOHO satellite in 2000 shows a billion tons of plasma launched two million kilometers off the surface of the sun! The dark area in the middle of the image is from a disk used to block out the light of the sun. The white circle outlines the location of the sun’s surface.Credit: SOHO (NASA/ESA)

But the 1989 storm is nothing compared to the geomagnetic storm of 1859.  Known as the Carrington Event, after amateur astronomer Richard Carrington who observed the flares that triggered the storm, it is the most powerful geomagnetic storm ever recorded.  Aurora were observed as far south as Hawai’i and the Caribbean.  Witnesses at higher latitudes reported being able to read newspapers by the light of the aurora alone.  Telegraph networks around globe catastrophically failed; operators received shocks and telegraph paper caught on fire.

A repeat of the Carrington Event in today’s far more interconnected world would be devastating.  Cascading failures could quickly shut power down to millions of people in a matter of minutes.  Communication networks would fail and GPS satellites, upon which the entire air traffic system relies, would shut down.  A repeat of 1859 could be truly catastrophic!

Obviously, we don’t want to be surprised by a powerful Earth-bound CME.  That’s why astronomers study the sun.  Besides the joy of discovering how stars work, a better understanding of solar activity can help us be better prepared.  With even just a few hours warning before an impending CME strike, we could safely shut down and protect essential services.  Disruptions may then only last a few hours, rather than the days, weeks, and months that might otherwise occur.

Coronal mass ejections are powerful eruptions on the sun’s surface.  Caused by instabilities in the sun’s magnetic field, they can launch a billion tons of superheated gas into space at over one million miles per hour.  While most drift harmlessly across the solar system, occasionally one is aimed at Earth.  When that happens, the resulting magnetic storm can severely disrupt electrical systems and produce brilliant auroral displays.  CMEs are just another reminder of how fragile our pale blue dot is as it races around the sun.

Christopher Crockett