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I am a scientist. Every day I wake up a scientist. I eat breakfast (which may be just coffee) and then go do science. I come home and talk about science with my scientist partner. I live for the most part in a bubble of science. But even given this, I still at times think of science as this complex entity that is filled with mind numbing equations, jargon, and systems. Bit I know that this is not all true! Yes we often study complex systems or ideas, but science can be simple. You can ask the simple questions and use simple approaches and get the answer.
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The Role of Localized Compressional Ultra-low Frequency Waves in Energetic Electron Precipitation by Jonny Rae et al. takes a simple idea and shows how it can have a large impact. It follows on some previous work which I wrote about back in 2016. One of the big questions in inner magnetospheric physics is what drives particle dynamics, why are the radiation belts sometimes full of particles and sometimes not? Another question that then stems from this is when particles are lost from the magnetosphere, how much of that ends up in the ionosphere and upper atmosphere? Now that's a very magnetospheric centric way to put it. If you happen to be more interested in the atmosphere and ionosphere you may rephrase it to say, what drives ionization or particle loss from the magnetosphere? This is a very subtle difference, but we'll talk about that a bit later.
So what did Jonny and us all look at? Well, Jonny is really interested in ultra low frequency or ULF waves. These are really quite long waves with a period of more than 1 second and often something closer to minutes long. Below are two metronomes, one with 1 beat per a second (60 beats per a minute - the upper end of the ULF waves) and another with one beat per a minute ( still not that long of a ULF wave).
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Let's start with reminding our selves about how particles move in the magnetosphere. If you want to play a few games instead reading a quick summary I suggest Magnetospheric Mini Golf or Magneto Bowling. Charged particles when in the presence of a magnetic field will move in a circle around it, this is called gyromotion.
Now some particles when they are bouncing might get lost to the ionosphere and upper atmosphere. They will "hit" or interact with neutral particles and no longer be able to bounce back into the magnetosphere. Why would this happen? Well something has to happen to change their typical motion. For example, the particles move into a region where the magnetic field at the surface of the Earth. In this case the particles would move further along the magnetic field and thus down into the ionosphere and upper atmosphere. Earth's magnetic field isn't completely symmetric, and in fact does have a region where the magnetic field is weaker. This region is called the South Atlantic Anomaly (SAA).
Okay, but that's something that the particles see every time they go around the Earth. Waves can also change the particle's path and we've talked about that in this blog previously. But unlike the cyclotron, surfing, waves discussed in the other post, these ULF waves effectively create a bigger sink hole for the particles. The ULF waves move the particles onto a path that will push them further down the field line and more likely to interact with the atmosphere and ionosphere. If you are a plasma physics or space physics student, you can see the effect by conserving the first and second adiabatic invariants.
So a bit more information before we look at how big of an effect this is and why one may or may not care. First we should look at how we measure this. A particle will have a pitch angle. This is the angle between the velocity vector of the particle and the magnetic field.
Now as the particle moves down the field line it moves towards a stronger magnetic field. This stronger magnetic field changes the velocity vector of the magnetic field, pushing it towards 90 degrees.
If the particle would reach a 90 degree pitch angle in the atmosphere or ionosphere where it may collide with other particles (and what height it is lost at is dependent on energy, density, etc.), then it will become lost.
Thus there is a set of pitch angles at the magnetic equator where we know that those particles will be lost completely from the magnetosphere - they have no hope of being trapped. These sets of pitch angles are defined as the loss cone. They are dependent on the height that the particle will be lost at (and thus energy of the particle). It is also dependent on where you are starting the the magnetosphere. If you notice in the figure above, a particle at an L of 3 will hit the atmosphere earlier along the field line than a particle at an L of 8 (L is the number of Earth radii from the center of the Earth at the magnetic equator- so L of 1 is the surface of the Earth, L of 8 is 8 Earth radii or 4 total Earths away from the center).
Now let's look at some data from the magnetic equator. Below is particle data from the Van Allen Probes mission. The particle flux is plotted in colors, the Y(vertical) axis is pitch angle and the X(horizontal) axis is time. On the left I've tried to draw a cartoon of the pitch angles that the satellite sees and the loss cone. As you can see, the loss cone at the magnetic equator is really quire small. This makes looking at what particles may be lost difficult if you are sitting in the magnetic equator.
This means that the ULF wave will push and pull on the particles. At times it will push the particle closer to the Earth (in to a region with a larger loss cone) or pull them out away from the Earth (into a region of a smaller loss cone). Thus some of the particles will see a larger loss cone and be lost from the magnetosphere.
It's all about perspective.
Okay, so now you know what the paper was about, what the key take away was, and why perspective matters... Why am I saying this was simple? There are a ton of figures and ideas and more jargon than is probably useful in all of the above text.
When we start learning about plasma physics and magnetospheric physics one of the first things you learn about are the three adiabatic invariants. We learn about how they control much of how the the particles move through the magnetosphere. Often we look at times when all three are broken and the more that break the more complicated the math, the interactions, and the impacts. Often when 2 or all three adiabatic invariants are broken, there are also many different types of phenomena going on all at once, so it's difficult to determine exactly what is causing what affect.
When we started looking at this study, we found that it was beautifully simple - it was clear what different phenomena were happening, and this straight forward mechanism could explain the observations. It's something that most first year graduate students could solve for and the math was some simple algebra. There was no need for more complicated mechanisms or theories to explain the observations. You don't always get a chance to work on a paper like this, and it's so nice when it does happen.
So why would you care?
Well if you are a person who studies magnetospheric radiation belts or precipitation of radiation into the ionosphere or atmosphere you may care because it helps explain different types of dynamics. It's also of interest to you because ULF waves occur really frequently, like almost always. So while mechanisms which perhaps affect a wider range of energies happen less frequently, this happens much more often.
Why do you care if you aren't a magnetospheric or ionospheric or atmospheric person? Well, we're making progress towards better understanding the environment in which we live (if you spend a lot of time in a plane maybe) and work (definitely where all the satellites live). It seems like with this mechanism we're better understanding why we have days with drizzle, you know, those days where you can't quite tell if you need an umbrella or not. If you are spending only a few minutes in it, probably leave the umbrella at home. If you are spending a whole day out in it, maybe you want to make sure to bring a rain jacket.