The Astrophysics of Plasmas

In our second installment, we connect the physics of electrically charged gases to cosmic rays we observe on Earth.

The second is probably Dalton’s Law of Multiple Proportions, otherwise interpreted as the modern theory of atom

These ideas run counter to much of our direct, daily experience [1]. At least that kind of experience we’ve had in common with our ancestors for thousands of years. Putting those ideas together — which involves a lot of mathematical work — physicists arrived at the modern, kinetic theory of gases.

There are many details and lots of implications from this theory, but one way to understand it goes like this:

In other words, all around you there are gazillions of tiny molecules. At room temperature, they’re moving at 1000 miles per hour, on average. Some move slowly, some more quickly. A tiny fraction of those molecules are moving, really, really quickly, more than twice as fast as the average.

But we can’t see any of it because they’re just too small.

When a gas get really hot, its individual atoms begin to break down. Their collisions have too much energy and break the atoms to pieces. More precisely, the electrons and nuclei split and form separate components of the gas. This often — but not always — coincides with a very low density of atoms.

From the kinetic theory of gases, it’s easy to see how a few electrically charged particles near the surface of the sun occasionally bounce too far from the Sun and stream out in all directions. That stream impacts all the planets, including the Earth.

Magnetic fields contain that solar wind by bending the trajectories of the individual particles. It curves their motion. That’s just what magnetic fields do! The strength of the magnetic field means that those particles can — at best — move in circles. Roughly speaking, the faster the charged particle, the bigger the circle it makes in the magnetic field.

Like any gas of particles, the van Allen belt plasma has particles moving at very low speeds and very high speeds. Very small circles and very large circles. The average speed — in part — determines the approximate size of those van Allen radiation belts.

Particles moving stupidly fast through a magnetic field — like cosmic rays from space — will also bend, but not enough to get trapped. Instead they fly through the magnetosphere and into the upper atmosphere. Breaking apart by spreading their energy around, leaving us to content with that debris of particles.

You might wonder where those high energy particles from space — those cosmic rays — come from.

The aftermath of such explosions often includes clouds of left over gas. These amorphous, interstellar gas clouds are often called nebulae. Nebulae can often find themselves — in part at least — in a plasma state.

These astrophysical plasmas give us beautiful photographs to look at here on Earth. But don’t be fooled. The density of those gorgeous gas clouds — even in star forming reasons like the Horsehead Nebula — aren’t really that visible to the naked eye. Even if you were right up on it, you’d probably have to leave the camera shutter open for a bit to capture all that light.

That is to say, astrophysical plasmas are pretty sparse. By comparison our atmosphere feels like a thick, pea soup. The particles inside those astrophysical plasmas don’t really smash into each other like they do down here on Earth. Rather, the particles interact via the longer range, electromagnetic force.

Astrophysicists will sometimes call them collisionless plasmas to emphasize that fact. The gas behaves less like a game of billiards and more like traffic or a flock of birds.

That’s right. The magnetic field pinches off. It heads outward into space. Sometimes towards us. And a large chunk of the sun’s outer plasma sometimes goes with it.

These are called coronal mass ejections, and they’re a big concern for satellites and astronauts alike. Like a tsunami of plasma, it can overwhelm Earth’s natural defenses. A coronal mass ejection can wreck havoc on our satellites and other electronics.

On Earth, these kinds of events are experienced as a shockwave in the solar wind. Supernovae can generate even larger shockwaves in astrophysical plasmas. So far as we can tell anyway, shockwaves are the things responsible for accelerating cosmic rays out of astrophysical plasmas.

Cornoal mass ejections can generate shockwaves near us, in the solar wind. Outside our solar system, in the gas and dust between stars, even larger shockwaves loom. What could generate those?

Supernovae for sure! Those exploding stars can be brighter than entire galaxies, so it’s probably no surprise they’re sending out a lot of sudden shocks during their expansion. Let’s discuss another example.

That wild, sweeping motion together with those ginormous magnetic fields surely has a massive impact on any nearby plasmas. Neutron stars, in other words, can generate shockwaves in astrophysical plasmas. Shockwaves that can accelerate cosmic rays.

Shockwaves accelerate particles in a plasma into cosmic rays, but not all in one go. It’s not like a baseball bat. It’s more convoluted than that.

Some particles pass through the shockwave, back and forth, picking up an an enormous amount of energy as they go. That’s the standard explanation — filed under technical phrases like diffuse shock acceleration or a first order Fermi acceleration.

Passing back and forth through a shockwave is a funny thing to think about. Initiatively, it’s like a surfer passing through a wave, going faster each time. Which of course makes no logical sense. Such an intuitive image of a shockwave is wrong [2].

The thing to keep in mind is that the gas of charged particles — the plasma — is very far from the normal, equilibrium thermodynamics we experience down here on Earth. The shockwave is by definition moving faster than the speed of sound in a gas. A shockwave will also very likely be spread out broadly with multiple fronts. Some a little ahead, some a little behind. This texture in the electromagnetic field can give electrically charged particles of the plasma a lot of shock fronts to bounce off of, picking up energy each time. Eventually they can get boosted to such a high energy that they shoot off into space — and eventually — towards us.

We don’t normally see muons hanging around in other contexts, like chemistry. Muons are unstable particles. They only live for about 2.2. microseconds. While extremely long lived by particle standards, the muon’s life is still a mere blip by either human or atmospheric standards.

Cosmogenic muons are born at altitudes of nearly 30 miles above sea level, far above where we live or even fly. Created from molecular collisions with extremely fast moving particles, those muons have a lot of energy! They often move close to the speed of light.

These cosmogenic muons are something a curiosity: Light can’t even travel half a mile [3] in 2.2 microseconds. So how it these muons can travel well over 10 times that distance without decaying? We’ll resolve this puzzle next time, with a little help from Einstein.

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