The magnetic field (magnetosphere) at Mercury is highly dynamic. At the planet closest to the Sun, the solar wind can cause drastic changes in the structure and composition of the magnetosphere. We can, however, categorize different regions of the magnetosphere based on certain characteristics. Looking at the diagram, starting left to right, first we see the cusp. The cusp is the one characteristic of the magnetosphere that we do not expect to change drastically in size. At this location, we can see high H+ flux, lots of alpha particles, and a decent ion count. When looking at different regions of the magnetosphere, we try to look at flux, energy, and different species of ions in order to determine where different boundaries are located. After the cusp is the northern lobe. In the northern lobe, we expect to see an empty region with relatively low ion counts. We know we have exited the northern lobe when we see a spike in energy, flux, and ion counts again- this is how we know we have entered the plasma sheet. The magnetosphere has the same characteristics on the bottom half of the magnetosphere, but due to the orbit orientation (red-dotted line) we tend to focus on the northern aspects.
(Courtesy of NASA)
Since the instrument that Michigan made was aboard the MESSENGER spacecraft, and because the magnetosphere is highly variable (even within seconds), we are able to pass through those characteristics three times a day, on average. Meaning, the instrument onboard the spacecraft made three orbits a day throughout the mission, which lasted from early January 2011 until March of 2015. That’s a lot of orbits. This means that if we can define these boundaries for each orbit, then we can dig deeper and try to solve some of Mercury’s puzzles. Such as, why is the magnetosphere so dynamic? Why does the plasma sheet often change in size? Can we see that ions are being energized through the northern lobe? Luckily, that is where I come in. My first task at hand was to assemble a large database of each orbit and where you can find the cusp, northern lobe, and plasma sheet. Once we have thousands of data points to work with, we can delve deeper into the information we have and try and answer those questions.
I knew creating the database from so many orbits would be a task, but I didn’t realize there would be such high variability in the orbits. While we expect some variation in the boundary definitions, often enough the characteristics are so different that my lab mates and I have to ask our advisor what might be going on in the orbit. The boundary definitions have not been as cut and dry as we have hoped, but these findings may lead into new discoveries. They can bring about new questions we did not even know we should be asking. Once we have a solidified database, we can begin to try and answer the questions Mercury has presented us with.