Thanks to Henson Shaving for sponsoring today’s video.
The Earth’s atmosphere may look empty, but it’s really a complex and dynamic place,
swirling with gaseous matter and thermal energy.
Some of its most spectacular activity is best viewed at night.
If you’ve ever watched a lightning storm or meteor shower, the breathtaking aurora
borealis in the North or the shimmering aurora australis in the South, then you know what
an incredible light show the sky can put on.
But in recent years, with the help of highly sensitive cameras, researchers have been able
to document a number of unusual, previously unrecorded light-producing phenomena.
These occurrences, which happen high in the Earth’s atmosphere, are known as transient
luminous events.
What are these strange phenomena?
Why are they so elusive?
And what can they teach us about the hidden workings of our atmosphere?
I’m Alex McColgan, and you are watching Astrum.
Join me today as we look at incredible images of transient luminous events, as we explore,
and unravel, some of the most mysterious and elusive phenomena in the night sky.
Let’s begin with a remarkable image.
It looks like a cross between a lightning storm and a jellyfish, doesn’t it?
This is a red sprite, and the formation you’re looking at is fittingly called a jellyfish.
It’s incredibly big, spanning up to 50 kilometres, and originates at an altitude of 70 to 80
kilometres above the Earth.
Sprites are short-lived events, lasting 3 to 5 milliseconds, and they travel downward
at blazing speeds, reaching 10% the speed of light!
For years, sprites were only rumoured to exist.
Reports can be found as far back as the 18th century, but a theoretical basis wasn’t
published until 1925, when the physicist C.T.R.
Wilson speculated that electrical breakdown could occur in the upper atmosphere.
However, despite years of unverified sightings, it would take more than 6 decades for their
existence to be confirmed.
So, what are sprites?
Unlike lightning, which is extremely hot, sprites are cold plasma events, much like
the reaction inside a fluorescent tube.
Let’s think about that fluorescent light for a moment.
It requires a power source to ionize the gasses trapped inside in order to emit light.
As it happens, sprites also require an electrical discharge to trigger their fluorescent reaction.
You see, inside a storm cloud, there is friction between rising ice crystals, which become
positively charged, and sinking soft hail particles, which become negative.
These positively charged ice crystals, in turn, cause a negative shield layer to form
in the air above them.
When a positive discharge happens in the form of a lightning strike, the cloud becomes neutralized,
but the negative shield layer remains.
We now believe it is this unstable, negatively charged shield layer that causes an electrical
breakdown in the upper atmosphere, producing sprites.
It is even possible that sprites aren’t especially rare.
What makes them incredibly difficult to observe is that they occur high in the ionosphere,
where they are often hidden by the storm systems that produce them.
So, to see a sprite, you need a clear sightline over a thunderstorm – or, perhaps, a camera
positioned above it.
As you can see here, the International Space Station was lucky enough to get one in action!
Sprites glow red because, under the low-pressure conditions where they originate, nitrogen
emits low-frequency red light when its molecules get excited.
Compare this to auroras, which are usually green.
That’s because, at the even higher elevation where auroras occur, Earth’s atmosphere
has greater levels of oxygen, whose molecules fluoresce green when they get excited.
But sprites don’t always produce red light.
Sometimes, a sprite will set off a secondary event at a lower elevation.
These secondary events, or tendrils, often appear blue.
While their light is also produced by nitrogen, the higher pressure causes them to glow blue
and near-ultraviolet.
As a result, some larger sprites, like the jellyfish we saw earlier, have a remarkable
appearance: glowing red at the top and blue at the “tentacles”! Sprite tendrils aren’t
the only TLEs with a blueish tint.
This event is known as a blue jet.
Discovered only recently, blue jets are a distinct phenomenon also initiated by storm
systems.
In these events, the positive charge at the top of a storm cloud forms a leader with the
negative shield layer above the cloud, producing a discharge that propagates upward.
This excites the nitrogen, making a spectacular cone-shaped jet that glows blue.
Why blue?
Well, remember what we said about how atmospheric pressure affects excited nitrogen gasses.
Blue jets occur much lower in the atmosphere than sprites, which is why they fluoresce
a different colour.
There are also smaller TLEs called blue starters, but scientists believe they are simply failed
blue jets.
These diminutive cousins only reach 20 kilometres above the Earth.
At the other end of the family is a separate phenomenon known as the gigantic jet, and
it occurs much higher in the atmosphere, which is why the upper portion changes colour from
blue to red.
We’ll leave today with one of the rarest and least understood TLEs.
These green or red phenomena are known as ELVES, a quaint acronym that stands for a
mouthful.
Its full name is “Emission of Light and Very Low-Frequency Perturbations Due to Electromagnetic
Pulse Sources”.
Try saying that 5 times fast!
Unlike sprites and jets, ELVES are diffuse, ring-shaped phenomena that occur even higher
in the atmosphere.
And unlike what the name suggests, they are huge.
ELVES can grow up to 400 kilometres in diameter and occur 100 kilometres above the Earth.
But despite their size, they are extremely short-lived events.
They only last a millisecond, so brief that they can’t be seen by the naked eye.
While little is known about ELVES, we believe they are caused by an electromagnetic pulse
produced by the discharge from an underlying thunderstorm.
So there, we have it.
An introduction to some of the most spectacular transient luminous events that have been confirmed
in recent years.
What other luminous phenomena may be lurking in the Earth’s atmosphere?
It’s tantalizing to think about.
And, because this is still a fairly new field of research, we can only imagine what new
surprises may be in store for us.
Want more videos about weird atmospheric phenomena on Earth?
Let me know in the comments.
These phenomena, in part, have only been observed recently thanks to the advances in sensitive
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Thanks for watching!
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All the best, and see you next time.