Thank you to Draper and its Hack the Moon initiative for supporting PBS Digital Studios.
Every winter thousands of tourists head North, past the Wall and risking White Walkers, hoping
to get rewarded with a glimpse of these amazing multicolored lights.
And while many succeed, many leave un-auroraed.
Which begs the question: when, where, and how is the best place to see an aurora?
Auroras start with the sun.
Specifically, when the Sun decides to shoot billions of tons of charged particles mostly
protons and electrons at the Earth, at speeds that would do some serious damage…
IF we were not surrounded by a protective magnetic field.
Our magnetic field redirects most of these protons and electrons around us, but some
of them follow Earth’s magnetic field lines to their source: the magnetic poles.
On their way to the poles, those charged particles crash into atoms and molecules in our upper
atmosphere, producing this.
Because of how our magnetic field is shaped and the height auroras typically happen above
the Earth’s surface, the best place to see an aurora is actually not right at the magnetic
poles, but anywhere in an oval shaped zone about 2,000 km from the poles.
This zone is called the auroral oval and it’s much easier to be in the auroral oval if you
go North… because the southern one is mostly over Antarctica and the Antarctic ocean.
If you’re lucky, green.
If you’re extra lucky, purple, pink, even blue.
If you’re the luckiest person on the planet, red.
But the most common aurora you’ll see is green.
The reason is kinda complicated, but welle take it slow.
First, you need to know a few things: one: it’s the electrons from the Sun, not typically
the protons, that make auroras.
Two: as you go further up in our atmosphere, there’s less and less of that atmosphere;
and the gases that make it up change.
Down here at sea level, air is 78% nitrogen, 21% oxygen, and 1% other stuff.
In our upper atmosphere, about 80 kilometers and up, there’s actually more oxygen than
nitrogen.
Three: at those high altitudes, UV light from the Sun actually splits oxygen molecules into
oxygen atoms.
OK.
So.
When an electron crashes into an oxygen atom, it excites it to what scientists imaginatively
call... “an excited state.”
Eventually, the oxygen will release its extra energy as a photon of light.
That photon can travel down to the surface of the Earth, where you see it as part of
an aurora.
The exact color of that photon depends on how much energy it has.
Higher energy photons are greener, and lower energy photons are redder.
Now, here’s another fun fact about oxygen: it has multiple different excited states,
so it can absorb electrons of different energies.
If it absorbs a high energy electron, then a high energy photon is produced, aka green.
If it absorbs a lower energy electron, then a lower energy photon is produced: aka red.
OK so at this point, you might be asking: if oxygen is so cool that it can produce both
green and red auroras, how come green ones are much more common?
If an oxygen atom absorbs a high energy electrons, it relaxes back down and emits a photon one
second later.
But if it it absorbs a low energy electron, it stays excited much longer -- 110 seconds,
to be exact.
You can think of those time windows as aurora-killing opportunities, because if a nearby molecule
crashes into an excited oxygen atom, it totally throws the oxygen off its game.
The extra energy that would have gone to the photon gets transferred instead to the molecule
that crashed into the oxygen.
And because red photons need 110 seconds to brew, those red-photon-emitting oxygens are
much more likely to get bashed before they release photons than the green-photon-emitting
oxygens.
There are more wrinkles to this story -- for example, nitrogen can also make auroras -- pinks,
blues, reds, and purples… and hydrogen and helium can too, but those are ever rarer.
Now let’s talk about…
You probably already knew this, but… at night.
Auroras are over a million times dimmer than sunlight, so you’re not gonna see one during
the day.
Unless it’s the apocalypse.
What that means, though, is that you’re much less likely to see a Northern aurora
in the summer, because the Sun is up so much longer.
So, at night, up North, in the winter.
And try and avoid the Moon, if you can.
And well, clouds.
Also, the Sun is a mercurial beast.
The number of particles and how fast they’re going changes day to day based on different
types of solar activity, but since it takes a few days for the stuff to reach us, we can
actually predict the best aurora-spotting a few days beforehand.
And yes, there’s apps for that.
PBS is bringing you the universe with Summer of Space which includes six incredible new
science and history shows streaming on PBS dot org and the PBS video app along with lots
of spacey episodes from PBS Digital Studios creators.
Thank you to Draper and its Hack the Moon initiative for supporting PBS Digital Studios.
You know the story of the Astronauts who landed on the Moon.
Now, you can log onto wehackthemoon.com to discover the story of the male and female
engineers who guided them there and back safely.
Hack the Moon chronicles the engineers and technologies behind the Apollo missions.
Brought to you by Draper, the site is full of images, videos and stories about the people
who hacked the moon.
Are you heading out on an auroral journey?
Make sure to bundle up aurorally, tell us your auroral stories in the comments, and
we’ll auroraraorarora next week.
Try saying that ten times fast.
Thanks for watching.