2020 ARTEMIS SCIENCE NUGGETS
Solar wind and interplanetary magnetic field influence on ultralow frequency waves and reflected ions near the Moon
by Stephanie Howard
University of Iowa
Introduction
Instead of a global magnetic field, the Moon possesses localized crustal magnetic fields that can reflect a significant percentage of incoming solar wind ions. Once they are reflected back into the surrounding plasma environment, these ions can interact with plasma waves through various mechanisms. One such mechanism is a resonant interaction: when the waves have frequencies near the ion cyclotron frequency, in the frame of reference of the ion, energy transferred from the ions will drive the waves. It can be difficult to determine the intrinsic properties of low frequency waves such as these because they can be heavily Doppler shifted by the solar wind. Here, we use ARTEMIS data to look for simultaneous observations of reflected ions and ultra low frequency waves to study the characteristics of these waves and the conditions under which they occur.
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| Figure 1. Parameter distributions for (a) solar wind speed, (b) magnetic field magnitude, (c) IMF cone angle, and (d) IMF clock angle. Black distributions are of accumulated ARTEMIS data, red distributions are of wave and ion events, blue distributions are of wave-only events, and green distributions are of ion-only events. Distributions are normalized by number of minutes of event/overall data. (e) a 2D distribution showing IMF cone and clock angle of the wave and ion events. |
Results
Using over five years of the ARTEMIS data set, we compile a set of wave and ion events where there are simultaneous observations of polarized 0.01 Hz waves and non-solar wind ions. We also compile a set of wave-only events, where we observe waves but no ions, and a set of ion-only events, where we observe ions but no waves. We then compare the solar wind and interplanetary magnetic field (IMF) conditions for each set of events to the accumulated ARTEMIS data set. The resulting plots are shown in Figure 1. From this comparison, it appears that wave events (either wave and ion events or wave-only events) are more likely to be observed during times when the solar wind speed is slow and when the IMF is in a Parker spiral direction.
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| Figure 2. Combined distribution of component on ion velocity along wave propagation direction for (a) wave and ion events and for (b) wave-only events for all ions in the equatorial region (grey) scaled by a factor of 3.5 to more easily compare with the other distributions, all ions in the region observable by ARTEMIS during events (black), and ions with velocities that can generate right/left-hand polarized waves during events (red/blue) |
Using an ion tracing simulation, we generate reflected ion distributions for the entire equatorial region ARTEMIS orbits based on the ambient plasma conditions during the events. We then narrow these distributions to include just the regions upstream of ARTEMIS at the times of the events. The resulting ion distributions are shown in Figure 2. Based on the velocities of the reflected ions in these regions, we conclude that most of the waves in these events are intrinsically right-hand polarized.
Since the crustal magnetic fields that are exposed to the solar wind varies based on where the Moon is in its orbit, we also check the wave events for a dependence on lunar phase. To do this we plot the location of the Moon in its orbit during the events, the results of which are shown in Figure 3. Both sets of events are spread throughout the lunar orbit and show no clear dependence on lunar phase. The concentration of events on the dawnside in the set of wave-only events is believed to be caused by effects from the Earth’s ion foreshock. We also run simulations of reflected ions for varying lunar phases, and from these we conclude that any effect lunar phase has on the reflected ions is overshadowed by effects due to changing solar wind and IMF conditions.
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| Figure 3. Distribution of lunar locations around Earth at the times of observed (a) wave and ion events and (b) wave-only events in GSE coordinates. Earth is indicated by black circle. |
Conclusions
Based on a comparison of the solar wind and IMF conditions during wave events to the accumulated ARTEMIS data set, we conclude that there are certain conditions during which these types of waves are more likely to be observed. However, we do not find any dependence on the lunar phase. Using simulated reflected ion distributions, we also conclude that most of the waves we observed are intrinsically right-hand polarized.
References
Howard, S. K., Halekas, J. S., Farrell, W. M., McFadden, J. P., & Glassmeier, K.‐H. (2020). Solar wind and interplanetary magnetic field influence on ultralow frequency waves and reflected ions near the Moon. Journal of Geophysical Research: Space Physics, 125, e2019JA027209. https://doi.org/10.1029/2019JA027209Biographical Note
Stephanie Howard is currently a graduate student at the University of Iowa. Her research focuses on ultra low frequency waves and reflected ions in the lunar plasma environment.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu