Earth’s shield from alien particle invasion (Science Advent 16)

(Every day until December 25, I’m posting a science-related image or video and description.)

Day 16

The Aurora borealis — the Northern Lights — over Alaska. The green color is emission from oxygen atoms high in the atmosphere, when charged particles from the Sun strike them. [Credit: Well_Lucio on Flickr]
The Aurora borealis — the Northern Lights — over Alaska. The green color is emission from oxygen atoms high in the atmosphere, when charged particles from the Sun strike them. [Credit: Well_Lucio on Flickr]
I’ve never seen an aurora except in pictures, though I really hope to before I kick the cosmic bucket.[1] While they are occasionally visible in the more southern regions where I’ve lived, they’re more common the farther north (or if you’re in the southern hemisphere) the farther south you go. From photos alone, they are awe-inspiring sights: shimmering ripples of green, blue, or red hanging in the sky.

While auroras are beautiful, they are actually the result of one our planet’s natural defenses against the Sun. Not that Earth is conscious of the needs of life, but a lot of the Sun’s output is very damaging. X-rays and ultraviolet radiation are bad for living tissues, and the solar wind – charged particles like electrons and protons accelerated by the Sun’s magnetic field to high speeds – is similarly unconducive to health. However, Earth’s atmosphere blocks a lot of the harmful light, while its magnetic field deflects and steers the solar wind.

Auroras — the Northern and Southern Lights — are the result of the shape of the magnetic field. Earth is like a bar magnet, with the ends of the bar close to the North and South Poles. That’s why you can use a magnetic compass to find north — it’s roughly the same place where magnetic north is.[2] The magnetic field exerts forces on the moving solar wind particles, channeling them along paths that concentrate them in vast regions outside Earth’s atmosphere called the Van Allen radiation belts.

The magnetic field doesn’t prevent all of these particles from reaching the atmosphere, but they are channeled preferentially toward the arctic and antarctic regions. When they strike molecules in the atmosphere, they can ionize them — stripping electrons off — or cause electronic transitions, moving the electrons between energy states. The recombination of the electron to the molecule and the return to ground state both produce light: green and red for oxygen, blue and red for nitrogen. The image above is predominantly green, but there are many colorful displays depending on the energy of the solar wind particles. (Auroras are also prominent on the giant planets.)

So if you’re fortunate enough to see auroras, think of their beauty, but also of how they’re a visible sign of the invisible magnetic shield protecting the planet. Ours is a wonderful universe.


  1. Shuffle off the mortal magnetic coil? Increase the earthly entropy? Collapse my personal wavefunction?
  2. Confusingly, the poles of magnets are also labeled north and south, but these are opposite to those on Earth. Our planet’s geographical north pole corresponds to the south pole of Earth’s magnetic field, and vice versa.

One response to “Earth’s shield from alien particle invasion (Science Advent 16)”

  1. Found your blog via your Slate article (“…Hologram…”). Excellent explanation. Some years ago a Cal Berkeley prof published a paper using the same analogy, and the media descriptions were unclear at best. But his use of the H-word in his particular context suggested the implication that information about every region of spacetime could be found in any other region of spacetime. This, by analogy with the property of holograms whereby their images are encoded in their media such that even a small fragment of the original can recover a fuzzy but none the less recognizable image.

    So I decided to write to him to ask about that, as concisely as possible. He wrote back (yay!) to say that my interpretation was not correct, and that his actual work pointed more in the direction of what you just wrote about. That answered the question, and I hadn’t seen anything else on the subject until your article today, which fills in a lot of detail.

    I’m pretty well conversant with QM, but this stuff is still a bit beyond me, so rather than try to repeat what I think I understood from one article (and thereby make a public fool of myself), I’ll keep my proverbial moose shut until I’ve read quite a bit more on the subject.

    Other: re your footnote 1 above: some of my friends & I use the phrase “collapse the state vector” to refer to making a decision or coming to a conclusion in the face of a multitude of uncertainties. If you’re looking for more ways to say “die,” how’bout “falsify naive dualism” or “test the interactionist theory of mind”…?;-)

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