By Sean McCloat
Greetings, Aurorasaurans! In our recent #tweetchat about the aurora, we received a tweet requesting more information about what Bz, Kp and solar wind density are, and how they relate to the aurora. Since these are good questions that cannot adequately be answered in tweet form, we will be publishing a series of blog posts that explain what these terms are and how they relate to aurora. We will also include discussions of other related concepts to help flesh out more of the science about aurora.
If you have been keeping up with our Aurorasaurus blog posts (which if you have not been, you are seriously missing out), or have read through our learn section, then you are probably familiar with the general mechanisms that create auroras. But just to recap, here is a broad overview: the Sun’s constant stream of energetic particles, called the solar wind, can sometimes be bolstered by events called coronal mass ejections (CMEs), which are usually cone-shaped explosions of fast moving, electrically charged solar particles.
Once a CME reaches Earth, it interacts with Earth’s magnetic field – known as the magnetosphere. CMEs can compress the magnetosphere, causing changes in the configuration (e.g. shape and direction) of Earth’s magnetic field lines. Some particles, which are trapped along these field lines, are accelerated into Earth’s atmosphere where they collide with atmospheric particles, such as nitrogen and oxygen molecules. The atmospheric particles are then able to emit light, which is what we know as the aurora. Check out our Learn Page to get a more in depth look at this process.
Not all CMEs are created equal and you might say that each one is a unique ten million degree snowflake. Some are hotter, some are ejected with more speed, some are ejected with (generally speaking) more solar matter, and the magnetic fields they carry can be all over the place. As a result of the variation in CME properties, not all interactions between CMEs and Earth are the same. There are certain properties that have more bearing on how strong of an impact they have on the Earth’s magnetosphere.
The next few blog posts that will be coming out will discuss how we measure the properties of CMEs and the solar wind as they fly from the Sun to Earth and which properties in particular Aurorasaurus uses in order to anticipate aurora activity. In this post, we talk about how we measure the properties that can dictate the nature of those interactions and what some of those properties are.
CATCHING THE SOLAR WIND
At the moment, we have no way to measure the properties of a CME as it is first seen erupting off the surface of the Sun. Scientists have to wait until the CME blows past the Advanced Composition Explorer (ACE) satellite in order get this data. It takes a day or two for the CME to reach ACE. Once it reaches ACE, there is only about an hour until the CME reaches Earth! ACE does not orbit Earth, but instead orbits a gravitationally stable point that is always between Earth and the Sun called Lagrangian point 1, or L1 for short (as shown in figure to the right). That way, it is always in the best place to intercept the solar wind before it reaches Earth.
When ACE measures the solar wind, it sends data to Earth that looks like the graph below. These graphs are measuring 5 properties of the solar wind (listed on the left side) over time (specified on the bottom). What we’re going to focus on in this blog post are the density (in orange) and speed (in yellow) components. These, along with the Bz (which we’ll cover next time!), are the most important solar wind quantities to keep tabs on for most aurora hunters.
Now, what about the solar wind density?
The density of the incoming solar particles influences how compressed the CME causes the Earth’s magnetosphere will become. Density, as measured by ACE, is looking at how many particles are in a cubic centimeter (roughly the size of a sugar cube). Typical densities of the solar wind are usually around 1 – 10 microscopic solar particles per sugar cube-sized area. This may not sound like very much, and indeed space is mostly empty, but remember the solar wind is blowing a constant stream of these particles. When the density of these particles is higher (like when a CME plows through and pushes the solar wind particles in front of it), it creates more pressure on Earth’s magnetosphere and the result is a stronger aurora.
A quick word on speed
The speed of the incoming solar wind also plays a major in role in driving the aurora, and may be a bit more intuitive than the role of density. If you imagine that generating the aurora is like getting a windmill to spin, then naturally the wind speed plays a central role: the faster the wind, the faster and more easily the blades of the windmill turn. Similarly, higher solar wind speeds (like when a gusty CME blows through space) can more easily drive the processes that generate aurora. Typical solar wind speed is about 1 million miles per hour, or 400 – 1000 kilometers per second.
Sean McCloat interned with Aurorasaurus in the summer of 2015 while pursuing his masters degree in Space Studies at the University of North Dakota with a focus on the planetary sciences and astrobiology. He helped analyze the project’s data, contributed to scientific papers, presentations, and blog posts, and became good friends with Rory, the Aurorasaurus plush doll mascot.