Natural terrestrial plasmas

Instructions
Auroras
St Elmo's fire
Ionosphere
Lightning
More than meets the eye
Why research lightning?
Red sprites
Why research red sprites?

Instructions

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Auroras

When highly energetic charged particles from the Sun enter the Earth’s upper atmosphere, they collide with oxygen and nitrogen atoms. This stimulates the atoms into a temporary excited state, and on returning to their normal resting state, the atoms emit visible light. Oxygen emissions are green or brownish-red, whereas nitrogen emissions are blue or red.

As a result, a natural light display – an aurora – can be seen during the hours of darkness in the sky at high latitudes. During daylight hours, the aurora cannot be readily seen.

Normally, these spectacular light displays are restricted to Arctic and Antarctic regions, but during a geomagnetic storm, the auroral zone reaches lower latitudes.

In the southern hemisphere, auroral displays are referred to as the Aurora Australis, whereas in the northern hemisphere, it is called the Aurora Borealis.

The word ‘aurora’ is named after the Roman goddess of dawn Aurora.

Acknowledgements:
Photo courtesy of Fraser Gunn

St Elmo’s fire

In the region between a thundercloud and the ground, a very strong electric field can be set up. There is a huge potential difference (voltage) established between the negative base of the cloud and the positive ground. When this potential difference reaches a certain value, sharply pointed ground-based objects are seen to glow, often with a hissing sound.

Because this weather-related occurrence sometimes appeared on ships at sea during thunderstorms, it was given the name ‘St Elmo’s fire’. Saint Elmo is the patron saint of sailors, and in the past, sailors regarded such an event as an omen of bad luck and stormy weather.

St Elmo’s fire is a bright blue or violet glow due to the formation of luminous plasma. It appears like fire in some circumstances coming from sharply pointed objects such as ships’ masts, spires, lightning rods and even on aircraft wings.

Acknowledgements:
Etching by Dr G Hartwig/Treasures of the NOAA Library Collection/National Oceanic and Atmospheric Administration /Department of Commerce

Ionosphere

Starting at about 80 km above the Earth’s surface, the atmosphere contains an ionised particle component called the ionosphere. The upper ionosphere extends to a height of about 1000 km. It is hard electromagnetic radiation from the Sun in the UV and EUV range that provides the energy needed to ionise gaseous molecules and atoms present within this zone of the atmosphere.

The degree of ionisation increases with altitude. For example, at a height of 100 km, it is estimated that only one in 10 million atoms and molecules are ionised, whereas at 800 km, all particles are ionised. The ionosphere is a plasma layer that blankets the outer reaches of the Earth’s atmosphere.

Several distinct layers of the ionosphere have been identified based upon their ion and electron densities. The outermost region – the F region – has the highest concentration of free electrons and ions. During daylight hours, this region splits in two – F₂ being the outer one and F₁ the inner one. The F₂ region is the principal reflecting layer for high-frequency (HF) radio communications during both day and night.

At a height of between 90–150 km lies the E region. Here, the degree of ionisation is lower than in the F region, with about one electron present for every 10⁸ neutral particles.

Below E is D at 80–100 km with an even smaller electron content. During the night-time, regions E and D disappear, leaving only the F region as a fully ionised layer.

Acknowledgements:
Diagram © University of Waikato, 2014

Lightning

Craig Rodger

Lightning is a really nice example of a terrestrial plasma, very short lived but beautiful to see, a vast blast of energy. What it is really is just an electrical spark, and you can make an electrical spark fairly easily, but the electrical sparks that most people make are sort of like this long, or if you’re really extreme, that long.

A lightning discharge is normally measured in kilometres, so it’s a gigantic electrical spark carrying a gigantic current zapping the atmosphere – heating it up and causing this tube of plasma to run from the top of the thundercloud down to the ground.

In order to form lightning, you need to have a large electric field. So what happens in practice is you have a thundercloud that’s full of charge, and eventually, that charge needs to go to ground – it needs to dissipate itself.

If the electric field gets strong enough, a current will flow through that space. That current is the lightning flash, so if you just set up too much charge difference, you’ll get a spark. If you set up too much charge difference over some kilometres, you get a spark that is a lightning flash.

So you’ve got this huge pulse of plasma being formed with a big current on it, and that radiates radio waves out into space, producing the sferic.

So a sferic is a radio wave signature, and it sounds like [clicks his fingers]. It’s a click. We call it a sferic as a contraction of the word atmospheric.

And the thunder is a consequence of the production of the plasma. If you take a big chunk of atmosphere and heat it up really, really fast, that expels a whole lot of atmosphere quickly – boof – and that shock wave is the thunder.

Acknowledgements:
Associate Professor Craig Rodger, University of Otago, Department of Physics
Image of lightning strike over Nelson, courtesy of Rick Kiessig
Stephen Witherden
Michael Schollum
Richard Mayston
Helgi Arnar Alfreðsson
Footage of night-time storm with lightning strikes, courtesy of James Insogna Creative Commons 3.0 license
Phillip Bloom

More than meets the eye

Craig Rodger

One of the things that’s really exciting in the modern lightning field is that we’re starting to realise that there are things happening in the lightning discharge that we don’t understand or that we’re only just starting to understand.

So there’s the big electric field, you get the spark forming, you get the lightning discharge, the sound waves, the radio wave. Relatively recently, we’ve started to realise that, coming out of the top of thunderstorms at the same time as the lightning discharge, there are beams of X-rays and gamma rays – really, really, really high-energy particles.

And there’s evidence for the production of anti-matter inside or possibly just above thunderstorms – we’re still arguing about that –and that the satellites that are flying through space above the atmosphere every now and again are being zapped by high-energy particles coming out of the top of a thunderstorm. It’s complicated stuff, but it’s all really recently discovered and really fascinating.

That’s one of the things that’s surprising that, in a lightning discharge, there is anti-matter being produced. We can make anti-matter on Earth, but it requires an awful lot of energy and an awful lot of effort. And so it’s really quite fascinating to think that it’s being naturally produced in very small quantities in a lightning discharge, then that anti-matter moves a small distance, bumps into some of the atmosphere [claps his hands], explodes and destroys itself, and that’s where the beam of X-rays and gamma rays comes out – part of that explosion and energy.

Acknowledgements:
Associate Professor Craig Rodger, University of Otago, Department of Physics
NASA Goddard Space Flight Center

Why research lightning?

Craig Rodger

People want to know about lightning for so many reasons, and that’s one of the reasons why we were able to find friends all over the world who were willing to host radio receivers for the WWLLN network.

People want to know about lightning for lots of different reasons. One reason is because people are worried about being struck by lightning or they’re worried about their houses being struck by lightning or they’re worried about lightning occurring somewhere near their house, which might zap their stereo or their TV or whatever, and that’s not common, but it happens, and people worry about it.

In some parts of the world, lightning triggers forest fires. In Australia, it’s very common. On the west coast of the United States, it’s very common, and if you’re worried about big fires and what they do both to the forest and to people and animals, then you’d really like to know where it started. Knowing where the lightning was, you can do that.

In New Zealand, the New Zealand MetService runs a little lightning location network just for New Zealand, and that was set up because Transpower was worried about their electrical grid being struck by lightning. Doesn’t happen very often, but it does happen.

And airlines are interested in lightning. They’re not interested in lightning so much as thunderstorms, and thunderstorms – there’s very intense winds, which are not good to fly through. If you’re flying along and you get into very intense winds, you might go up or down very suddenly, which would be unpleasant for everybody on the plane, so you’d like to know where the thunderstorms are and you’d like to know where they are a long way in advance. So instead of doing that around the thunderstorm, you do that – you just do a really gradual turn.

There’s lots of reasons, and then there’s scientific reasons because we want to understand the thunderstorms, we want to understand the lightning, we want to understand what the radio waves do, we want to understand the sprites. Oh, lightning’s a cool thing to do.

Acknowledgements:
Associate Professor Craig Rodger, University of Otago, Department of Physics
Daytime thunder and lightning storm footage courtesy MrRegShoe Creative Commons 3.0 license
Scion
MetConnect StrikeCast images courtesy of MetService

Red sprites

Craig Rodger

We used to think we understood lightning, and to some extent, we do understand lightning, but relatively recently, we’ve started to realise that there are a whole lot of things going on in the lightning discharge process that we don’t know about, and one example is anti-matter production in lightning.

But another example is that we think of lightning as being something that happens in the thunderstorm and goes down to the ground or possibly is a flash inside the thunderstorm itself. We’ve started to realise that there’s also upward lightning – lightning that’s occurring at very high altitudes – and there’s a whole lot of different variants of this upward lightning, and they have nice exotic names.

The first one that we discovered was called a red sprite. You could call it upward-going lightning, but when we first discovered red sprites, these pulses – big flashes of pink in the sky high above thunderstorms – they weren’t sure what they were. And rather than calling them upward lightning, which says you understand what they are immediately, the suggestion was made by a researcher from Alaska that we should give it a sort of meaningless name while we went away and tried to work out what they actually were.

And he had just recently watched a Shakespearean play where there were sprites as characters, and he thought this was a lovely name. And the sprites are sort of pinkish-reddish colour, and so we call them red sprites.

And they now have – let’s call them friends – there’s other varieties of upward lightning like jets, elves. Somebody has put forward the idea of a troll, but that name hasn’t taken off yet. Essentially, they’re varieties of high-altitude lightning. They tend to be bigger than the low-altitude lightning that we’re used to, but that’s really just a consequence of the fact that the density of the atmosphere up there is much lower.

There is essentially a one-to-one relationship between lightning and red sprites. A powerful lightning discharge can produce a red sprite.

And as far as we can tell, you always need lightning to produce a red sprite.

Acknowledgements:
Associate Professor Craig Rodger, University of Otago, Department of Physics
NASA Goddard Space Flight Center
Night-time thunderstorm footage courtesy Nathan Boor, Aimed Research, Creative Commons 3.0 license
Lightning over rainy streets courtesy of Sterling Coffey Creative Commons 3.0 license
A Midsummer Night’s Dreampaintings by Arthur Rackham (1867–1939)
Diagram of blue jets and ELVE, courtesy of Abestrobi Creative Commons 3.0 license

Why research red sprites?

Craig Rodger

The interest in trying to understand red sprites partially comes from the unknown. When they were first discovered, it was because somebody was testing a camera, pointed it at the horizon, pressed record and went home for the evening. And when they came back and looked at their footage, they found these flashes of light in the sky where there shouldn’t have been any. That was in 1990. That, all by itself, triggered interest in red sprites.

Now, why are we actually interested in studying red sprites? It’s a good question because they start at maybe 40 kilometres altitude, which is above aeroplane heights, and they got to about 85 kilometres altitude, which is above most things’ heights.

Some of the interest has come from space travel, so most of the time when you’re travelling in space, you’re at 350 kilometres altitude or above if you’re orbiting the Earth. But to get there, you have to pass through the rest of the atmosphere. And if you’re launching from many parts of the world where we launch spacecraft, which are close to the equator, there’s also lots of lightning, thunderstorms there. So there’s a real possibility that you might fly through a sprite on your way to space or on your way back from space.

The probabilities aren’t high, but one of the questions was would this be bad for you? And if you go back not so long ago, the space shuttle Columbia was lost on re-entry as it was coming back from space. It broke up into millions of pieces, the astronauts died and one of the first questions that was asked was did they fly through a sprite? They didn’t, but one of the first questions that was asked was could a sprite have been responsible?

Acknowledgements:
Associate Professor Craig Rodger, University of Otago, Department of Physics
NASA
Diagram of blue jets and ELVE, courtesy of Abestrobi Creative Commons 3.0 license