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
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list too, check the links in the description below. All the best, and see you next time.
