The Carrington Event

In the middle of a winter night in central Alaska, a family of four gazes up at the dazzling, dancing ribbons of green and purple light in the sky. These Northern lights, also called aurorae, are one of the most benign effects of a major event in space weather known as a geomagnetic storm.

In New York City, dozens of transformers have blown, plunging the city into darkness. The aurorae are now visible, even at latitudes so far from the poles. The power lines, such long lengths of metal wire, have overloaded with current as the protective magnetic shell around the Earth, called the magnetosphere, ripples and wobbles in response to a massive attack by our normally friendly Sun.

Although most people check the weather forecasts at least once a week (“Is it going to rain tomorrow?” “Do I need a sweater today?”), few have ever checked or even heard of Space Weather. Space weather forecasting can tell us if and when the harmful effects of events on the Sun’s surface or in the solar atmosphere will reach the Earth, causing the events described above. Coronal mass ejections (CMEs) are tracked from the Sun to the Earth, where they can be very destructive when they reach the Earth’s magnetosphere.

The Sun-Earth Connection: NOT TO SCALE!

Flares are given categories based on the peak X-ray flux measured by the GOES satellite. X-class flares are the most energetic (and therefore dangerous). M-class flares are sort of in the “middle” (that’s how I remember the order!) energy range, and C-class flares are a hundred times less energetic than X-class flares. Scientists also assign numbers: an M6 flare has six times the peak X-ray flux of an M1 flare and twice the peak flux of an M3 flare. An X2 flare has ten times the peak flux of an M2 and one hundred times the peak flux of a C2 flare. Here are example observations from GOES.

Typically, harmful effects from solar flares and CMEs go unnoticed by the everyday person. Sure, satellite electronics are often interrupted by these solar storms, but since most communications satellites operate as part of a network, there are seldom dropped calls that can be blamed directly on our star. However, there have been massive space weather events in the past that, were they to happen today, could seriously mess with our comfortable, technology-rich lives.

Now before there’s any panicking, I want to clarify that the images in your head from Hollywood disaster movies are not what I’m talking about (Note: spoilers in the following two sentences). Neutrinos from the Sun will not start mutating and cause Bible-level floods (2012). Nor will a massive solar flare actually reach the Earth and kill off everyone (Knowing).

However, let us consider a real, historical example of the actual havoc the Sun can wreak here on Earth. The Carrington Event was a massive space weather event from 1859, caused by a series of flares and coronal mass ejections (CMEs) from a region of intense magnetic fields on the Sun’s surface called an active region. Current solar observatories typically look at these regions in extreme ultra-violet radiation where the heated plasma in the Sun’s atmosphere can be measured. Back in the mid-1800s, however, we had no way of observing the Sun in wavelengths other than visible light. This meant that Richard Carrington was looking at sunspots, darker parts of the Sun’s surface that suggest the presence of strong magnetic fields (a topic for a future post). Here’s what he saw:

Carrington observed bright flashes at the points labelled A-D on the diagram above on September 1st, 1859. The following day, the effects on the Earth started to become apparent. Aurorae are caused when energetic particles from the Sun are able to flow along the magnetic field of the Earth’s magnetosphere and interact with electrons in the Earth’s atmosphere. Typically, aurorae are seen only at the poles of the Earth, where the outermost magnetic field lines of the magnetosphere connect back to the Earth’s surface. The Carrington Event, however, produced large quantities of energetic particles traveling at uncommonly high speeds and caused aurorae to be seen as far south as the Caribbean.

Aurorae themselves are not harmful, but they are a warning that more dangerous effects of a geomagnetic storm are in store. On September 2nd, 1859, telegraph lines around the world failed, shooting off sparks and starting fires. When the CME hit the Earth’s magnetosphere, it deformed it; any change to a magnetic field will produce a current, and these currents overloaded the long wires of the telegraph system. As the storm subsided, these currents diminished, but remained strong enough to run the telegraph system without any other power source.

Consider, then, what would happen if this same storm occurred now. Satellite electronics would be knocked out, taking down our network of communication and GPS. The currents that overloaded the telegraph lines of the 1800s would completely blow out the transformers that our power grids rely on. The storm itself could take out hundreds of giant transformers and cause a chain reaction of failures. Replacing this expensive equipment would take a long time, leaving cities without power for weeks or months. It certainly gives one something to think about every time the media reports on a space weather event.

For more information, check out:

[Edit note: the first two paragraphs are the result of a writing exercise led by Tom Levenson during the Communicating Science Conference (2014 local). The third paragraph was thus amended to fit together with these new pieces.]

“If you want a place in the sun, you have to expect a few blisters.”

Loretta Young


Sphere of Influence

It’s an exciting time right now, when humans have finally made it to interstellar space. If you haven’t heard, Voyager 1 has exited the heliosphere. And this time we mean it!

Seriously, though, the news has been all over the internet lauding this most recent confirmation that Voyager 1 has made it out (see JPL Press Release). The real question, however, is out of what? Put simply, the discovery being presented is that the plasma densities that Voyager 1 is measuring have increased significantly, suggesting it has moved out of the sphere of influence created by the constant outflow from the Sun we call the Solar Wind. This outflow creates a cozy little bubble called the heliosphere, but this is only one way to measure the extent of the Sun’s influence.

Artist’s conception of the location of Voyager 1 and 2 from December 2012

When the Solar wind slows down because of the increased pressure of the interstellar medium, it sets up a termination shock as it decelerates past the speed of sound (think of this as a sonic boom, backwards). There is then a region of the outer heliosphere called the heliosheath, and the idea right now is that Voyager 1 has made it out of this region.

“But wait!” you exclaim. “That’s just a boundary based on pressure and density! What about gravity?” That’s a very astute observation. To put everything into perspective, I’ll call on the graphic design skills of NASA/JPL-Caltech professionals…

A schematic of the key components of the solar system. Please note: the scale on this graphic is logarithmic, not linear

You’re likely familiar with the eight (not nine!) planets of the Solar System and the fact that they orbit the Sun because they are gravitationally bound (if this second point is troublesome, perhaps review this wikipedia article) What you may be less aware of are the other main sources of material in our little neighborhood.

  • The asteroid belt, containing dwarf planet Ceres, lies between Mars and Jupiter.
  • The Kuiper belt, containing among other things dwarf planets Pluto and Eris, lies just beyond the orbit of Neptune.
  • The Oort Cloud best represents the extent of the Sun’s gravitational influence, as it is a spherical cloud extending from a few thousand AU from the Sun to a few hundred thousand AU.

The Oort Cloud is the source of most of the long-period comets we see coming through the inner solar system. Voyager 1 and 2 will be long dead before their trajectories bring them anywhere close to this “edge of the Solar System,” so it isn’t surprising that as a species we’d prefer to take the smaller victory of making it out of the heliosphere.

Finally, if we consider the Sun’s furthest possible influence, we would have to travel five billion light-years away, where the first rays of light emitted by our newborn star are currently streaming outward at the speed of light. Five billion light-years is equivalent to 2.9×10^22 miles i.e. 29,000,000,000,000,000,000,000 miles (for the curious, this would be 29 sextillion miles in America or 47 trilliard kilometers in France). This certainly puts our accomplishments with the Voyagers into perspective. To track these satellites real-time, check out

…and knowing what it had to tell of the night passing swiftly on
toward further darknesses, but moving also toward a new sun.
Fahrenheit 451, Ray Bradbury