This post has been a long time coming. I've thought about it a lot over the last 7 years. This paper has been through hell and back - not so much from the reviewer's side, just that it has been with me through some of the darkest days of my life. Somehow the trauma of my personal life has been associated with the progress of this paper. Not somehow; I am human, as we all are. We may say we can separate work and life, but we can't - not entirely. We do not live in a world where our brains are divided.
Back in late 2014 or early 2015, this paper was conceived. BARREL was just coming off of our final Antarctic campaign, and we were all on a high from the massive amounts (small compared to larger missions) of data. But then, we found this fantastic event. You've heard me talk about it before in other posts. BARREL observed the solar flare, measured impacts from the solar energetic particle event, and inferred the loss of particles from the radiation belts into the magnetosphere. And there's more - at least one paper more after this one.
With this event, we could look at the chorus waves themselves and infer their impact across the whole energy range of electrons in the magnetosphere. Okay - who cares. Well, people looking at how much energy is going from the magnetosphere into the atmosphere care. Often we have to make assumptions - even if we don't think we're making assumptions because we have actual data, we still have to make assumptions about other parts of the problem or the instrument response... We make many (mostly) well-understood assumptions about the problem. This is a consequence of working in a natural laboratory. Like in life, you won't know everything, like did they know how much I loved them...
Anyway - we're getting sidetracked - time to focus.
So often, we assume that the number of particles at any given energy follows different types of shapes. On average, this is probably true and seems to work well enough for much of the work we do - but it is a base assumption that we use, especially as it can be challenging to get those measurements - and even when we can, we run into the "space is big" problem again. Are those shapes and distributions the same everywhere inside the magnetosphere, and if not, how do they change?
What was so engaging with this event was that we had observations across a wide range of energies and observations of all the likely types of phenomena that like to push particles around. This event was unique because we also didn't have a geomagnetic storm, so we could study one phenomenon at a time. Now even with this, we had to make assumptions. As one of the reviewers pointed out - fairly - is that we were still looking at all these particles in space at the geomagnetic equator, we didn't have a satellite closer to Earth or a way to see where those particles went; we had to infer from what we saw where our observations were. But still - we had more observations than in previous studies. So we could put it together with the theory to see if it could explain what happened to those particles we could see.
And that's when my mother passed away.
Grief hits everyone differently. I tried to keep going and did, for the most part. I spent time at home with my father and family finding a new normal. I dived in and out of my work, being hyper-focused one day and totally adrift the next. We were slow with reviews, but we, I mean I, tried to press on. Finally, things seemed back on track, and then my grandmother passed... I changed jobs... then my aunt passed... changed jobs again... and then COVID and lockdowns happened, and my Dad passed away (Not because of COVID, just at the same time as the lockdowns). Needless to say, it's been difficult.
But life moves on, and we find a new normal. But that doesn't mean we scrap what we've worked on, especially when it's so cool. Instead, we might get there more slowly... like 7 years later.
I was proud of the work when we started and am proud of it today. And today, I get to share the finalized, fully published article.
Chorus waves are the smaller, faster, relatively hyperactive sibling of my beloved EMIC waves from my Ph.D. Chorus waves are cyclotron resonance waves like the EMIC waves I often drone on about. Still, instead of resonating with ions, they resonate with the much smaller electrons. And because the electrons have a much tighter gyroradius, the chorus waves tend to have a much higher frequency.
Like the EMIC waves, they also have some nifty features that help us identify them through their interactions with particles in the magnetosphere. For instance, they have a lower energy cut-off or, in other words, a minimum energy of the electron they will interact with (assuming only the cyclotron resonance).
Chorus waves have an additional somewhat baffling trait. Like EMIC waves, they are found below the cyclotron frequency of the particle (electron) they interact with. However, they also have two bands, one that occurs below half the electron cyclotron frequency and one between one-half and the cyclotron frequency. Why is there this break? As far as I am aware, it's still debated, granted I have fallen a bit behind in my reading.
In this instance, we were looking at an upper band chorus wave, which will interact with electrons down to much lower energies than a lower band chorus wave. And that's part of the reason why this is interesting. So often, people only consider the chorus wave impact on higher energy (10s keV and above) electrons - the energy of the electrons which can create the aurora and those that populate the radiation belts. However, these lower energies can be essential for other processes like driving ion outflow, conductance, and other magnetosphere-ionosphere couplings.
The minimum resonant energy we found with the observed chorus wave was about 25 eV. To some physicists, this is quite warm, fast, and would never be called low energy - however, we are not working in a vacuum or close to 0 Kelvin, so this is cold for us.
Grief hits you at odd times. You might be getting lost in your work, thinking things might be back to normal - but let's be honest, you're just not thinking. You are distracting yourself by falling into your work. Then something snaps, and you are back into your grief. Someone mentions their father, someone laughs just a bit like he did, you forget yourself and think, Oh, I need to call him and tell him about this new hiking trail I found... and now you are pulled back into grief and drowning again. So you go deeper into physics and your work as it doesn't ask how you are doing; it doesn't care. It will gladly take all that you give it.
With many of the particle distributions we use to approximate what energy of electrons and how many of them are coming into the ionosphere/atmosphere from the magnetosphere, we assume that the lower the energy, the more there are - to a point. Then it rolls back off. Often getting observations to confirm these theories can take a lot of work as it's just a complex measurement to make. But that was yet a unique part of this event we're studying. This lower energy cut-off is in the observable range of the HOPE instrument on the Van Allen Probes.
Hope is an interesting thing. My mother had congestive heart failure when I was 4. The doctors gave her 6 months to live, and I got her for another 30 years. Hope was strong when I was little that she would always be there, and after about 10 years, I think hope turned to acceptance that she would always be thereβ¦
Being in the magnetic equator, the HOPE instrument can not resolve the loss cone and thus is not actually capable of seeing the minimum resonant energy impact on the particle pitch angle distribution. However, the same theory that gives us the minimum resonant energy will also tell us what energies we should expect to start seeing a loss.
Loss is hard, and each loss is different. Sometimes you just feel numb and plow through, sometimes you feel catatonic, and other times you grieve the way they do in movies with lots of ugly crying and people supporting you with food. One might think I am now a grief expert, but all I know is that I'm not, and I'm not any good at it.
We can't see the loss at the minimum resonant energy because at the minimum resonant energy, the wave only affects particles for that minimum energy that are moving perfectly parallel to the magnetic field. The energy of the electrons that the wave will interact with that aren't moving only perfectly parallel to the magnetic field will be higher. We can find the value where we expect to see an impact in the HOPE data set's different pitch angle and energy bins. That value is about 10 eV higher than the minimum resonant energy, ~ 35 eV. We know that impact should increase as you move to higher energies as the wave will interact with more of the particles across their pitch angle distribution as the waves and the particles travel along the magnetic field lines (so to higher values of the magnetic field).
In addition to the chorus wave-particle interactions, though, we have the impact of the compression of the magnetopause. In fact, we should have started with that as that happened first - but the chorus waves are so much fun to get lost in, and I needed to get lost in something.
Time doesn't always pass the same either when you are in a flow state of thinking or in grief. Often when completing your research, especially as an observationalist, you don't necessarily follow the temporal path. Instead, you find something that makes you say, "Oh, that's odd." Then you move forward and backward in time, across space, trying to figure out what happened and the order of causality of events. That process may be non-linear, but when we convey it, the story is often put into a temporal order so we can more clearly describe our understanding of what happened to others so that they can better understand and so that we can better understand too.
When the solar storm hit, the magnetopause was compressed. This set off a series of connected events, but their starting points were slightly separated in time.
When you compress the magnetosphere, on the day side, you push particles Earthward by creating a traveling impulse and/or setting off ULF waves. These ULF waves don't care what energy or type of particle you are. They treat everyone the same. They change the local magnetic field such that particles may be pushed farther down or not as far down into the ionosphere/atmosphere. It's not a massive impact but a dependable and important one. Moving the particles closer to the Earth pushes them into a region of a larger loss cone. Still, their pitch angle will also become more trapped. It sounds like everything should cancel out, and nothing new will really happen. But that's not true. These two processes don't change the particles' fate the same amount. So the loss cone is opening up faster than the particles are moving toward a trapped population, and we will have some that will be lost.
Grief takes some. For some, the movement and change in life are too much. So many of us know someone for whom's life's unfairness and pressures just got the better of them. That is why we need community, that is why we need distractions, and that is why we cannot let ourselves become too detached.
As the bulk of the particles become more trapped, they also change their temperature anisotropy, effectively by definition. The temperature anisotropy (or you can think of it in terms of pressure anisotropy if you prefer) measures how much of the total particle propagation of a given temperature (pressure) is perpendicular vs. parallel to the field. As you remove particles parallel or nearly parallel to the field line, you have a more significant percentage now perpendicular. Thus you have increased the local temperature (pressure) anisotropy.
Okay, that's all well and good, but what does that mean? That means you are more likely to grow cyclotron waves, like the chorus wave. We've already gone into the chorus wave impact enough here, so let's put it all together in a nice little set of graphs.
We can see what the particles were doing before this nasty set of events occurred. They were happy as a clam just sitting there and actually pretty isotropic or equally distributed across pitch angles. (Notice all the flat solid lines. Similar numbers of particles are at all pitch angles. The dotted lines represent the pitch angles that will be affected by the wave when it turns on, and the height of the dotted line is how strongly the wave will interact with the particle pushing some to the edges and others toward the middle. )
Then the compression of the magnetosphere from this interplanetary coronal mass injection occurred. It shrunk the size of the magnetosphere and pushed those particles Earthward. In the particle data, you can see how the particles all increase as more are observed by the spacecraft, and they start to peak at 90 degrees. But the response is still relatively small.
The wave turns on, and things get ramped up to 11 - okay, we don't know if they are ramped up to 11. I don't think nature cares about a 10 or 11-point scale, but in grief, sometimes even small impacts can feel oversized. Still, we do see that, as expected, the peaks become more pronounced, and the impact is different at different energies. We don't see much change below 30 eV, and we see a more significant change above 200ish eV.
Sure, we don't see what was lost as no observations in low Earth orbit could look at the lower energies precipitated, just as we donβt always see the loss or impact of the loss on our colleagues, friends, or even family. Still, we do infer loss as expected for the higher energies. So there is more work to do and a need for more and more connections between observations. However, I hope we have pushed our understanding of this world further.
Within science and life, we never understand everything. We can't always explain why things happen or happen when they do, but we can try to get to a point where we can have closure, the end of one story, even if incomplete. There is a lot left to learn and observe regarding wave-particle interactions, but this was sufficient for a paper - to help us move forward and grow. And now, it is time to continue down that path toward new understandings, perhaps no longer dwelling on this event and finding the next one to become excited and fascinated with.
I really like the interwoven writing. Hugs from me as well.
This is a moving and well told story of parallel loss processes. Writing it all down helps with healing because it brings closure. Thanks for sharing.