During my Ph.D. I lived and breathed electromagnetic ion cyclotron waves... as my brother would say, I am a nerd. These waves are often abbreviated as EMIC, and there is a debate as to whether you pronounce it Ee-Mic or spell it out E-M-I-C when you talk about them. Either way, they were the topic of my dissertation. I loved them and still do. They are just so cute and unassuming but can have a significant impact.
So what are these things? Well, it's in the name; they are an electromagnetic ion cyclotron wave... I know it doesn't help. They are a wave we see in electric and magnetic fields. They resonate with ions. When Ions are around in the magnetic field, they start moving in a circle around it. When the conditions are right, the particles moving around the field line can start growing these EMIC waves. As the waves grow, they can scatter the particles, as shown in the youtube video below from NASA. This changes the conditions around the wave, and ultimately the wave is turned off. It's a bit self-defeating.
So why do we care about these emo waves? It's because of what particles they interact with. The ions themselves are essential. They carry a lot of energy and are often found in the region called the ring current. In fact, they carry so much energy that they can change the observed strength of the magnetic field on Earth. These EMIC waves are thought to resonate with those ions and ultimately push many of them into the Earth's atmosphere. Another group of particles they resonate with is the very, very, very energetic electrons in the radiation belts. Like with the ions, the EMIC waves can ultimately push these particles into the Earth's atmosphere, where they become lost - ionizing the upper atmosphere. Below is a youtube video showing how particles move through the radiation belts.
This is all fine and good, but this is a post about a paper... This paper became the fourth chapter of my dissertation. It was focused on observations of these waves from one mission, CRRES. One of the debates at the time was whether EMIC waves were seen during the main phase or only during the recovery from a geomagnetic storm. Many people were using what is called a superposed epoch. A superposed epoch picks a point in an event, and then you line all events up by this time. For EMIC wave studies, most people chose either the start of a geomagnetic storm or when it is at its most intense period. They then looked a day to 6 before the epoch and a day to 6 after. Once everything is lined up like this, you can take the average and/or look at other statistics. Assuming that all events are similar and have similar lengths, you will likely start seeing trends. However, trends may get smoothed out if the events vary a lot. We used a multi-epoch study in our paper, EMIC wave activity during geomagnetic storm and nonstrom periods: CRRES results. We know that different mechanisms are happening in other parts of the storm.
For instance, when a solar storm hits the magnetosphere, it pushes it closer to the Earth. This can affect the ions in the magnetosphere, making them more likely to produce EMIC waves. This only happens at the beginning of the geomagnetic storm. We also know that the storm's main phase is when ions come from the tail into the inner magnetosphere where the satellite sits. This injection of ions can produce EMIC waves. So we picked two epochs, the start of the storm and the point where it was most intense. We also chose the end of the storm to give us the third epoch. This last epoch was necessary because some storms last less than a day, and others can last more than a few. We wanted to avoid having a non-storm condition mixed in with our storm periods.
Now, after all of this, what did we find? First, we discovered that waves were more common during storms than not and that they happen more at noon and dusk than at midnight and dawn. Why does one care? We care because it helps us understand how much area is affected by these waves and, thus, how much they will affect the radiation belts. This is important because it helps us know what types of space weather may impact satellites, communication, and radiation at aviation altitudes. Of course, there are a few steps between this study and these applications, but it is all part of the process.
However, one of my favorite parts of this paper was the method. We picked this multi-epoch approach, which was entirely novel at the time. But, it had one major flaw. This was not the method used by others, so how could we compare our results to theirs. They had used different data sets than us, so our results differed because of the method used or the various data. To answer that question, we did a second small study using a similar approach to the previous papers. Lo and behold, we got the same results as them. We found that the method impacted our interpretation of what was going on. Our new multi-epoch analysis allowed us to better understand what parts of the storm had the conditions where you might find an EMIC wave. This is like saying we see that schools close when there is snow vs. the schools close when there has just been a snowstorm, and the roads are yet to be cleared. It might not seem like a big difference, but it is essential if you are predicting when schools will close in Minnesota. (The first approach might be okay if trying to predict when schools will close in Georgia)
As my career has progressed, I have moved on to looking at a much more comprehensive set of space weather activities. But I have been fortunate enough to continue to work with EMIC waves from time to time. They really are a neat type of wave seen in our magnetosphere... My brother is right. I really am a nerd.