2019 THEMIS SCIENCE NUGGETS
MMS Measurements and Modeling of Peculiar Ion Cyclotron Waves
by Justin H. Lee
Aerospace Corporation
Introduction
Electromagnetic ion cyclotron (EMIC) waves have been studied for many years through theoretical work and satellite observations made throughout Earth’s magnetosphere. The past theoretical work has demonstrated the importance of the ion composition for understanding how EMIC waves are generated and how they may interact with charged particles in Earth’s magnetosphere. Many satellite observational studies have also explored EMIC waves. But not all satellites host ion composition instruments that can provide detailed information on the plasma conditions when EMIC waves are observed. In addition, all satellite-based studies have been further limited by a well-known observational problem: satellite charging that causes the lowest energy ions to be unmeasurable. These low-energy ions are thought to be the most abundant population in Earth’s magnetosphere and so it has been difficult to fully understand how these ions participate in EMIC wave generation and in what ways their presence affects wave propagation or wave interactions with other charged particles.
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| Figure 1. (top) Ion energy flux spectra calculated from MMS Fast Plasma Investigation measurements showing fairly stable high-energy (>1000 eV) ion fluxes but significantly more variable low-energy (<~500 eV) ion fluxes. (bottom) Wave power spectra calculated from three components of the magnetic field vector measured by the MMS Fluxgate Magnetometer with local H+, He++, He+ and O+ cyclotron frequencies over-plotted in magenta. |
Results
Previous investigations on the THEMIS mission dataset have helped define times when naturally-occurring plasma flows can accelerate the low-energy ions enough so that they can overcome the satellite charging problem. Examples of times identified in MMS mission data appear in Figure 1’s top panel that shows data from the Fast Plasma Investigation on MMS, resembling upside down letter v’s. Seen in Figure 1’s bottom panel are EMIC wave emissions identified in MMS Fluxgate Magnetometer data that display peak wave power between the He+ and He++ cyclotron frequencies (labeled fHe+ and fHe++), which is among one of the peculiar properties noted previously in THEMIS data as well as in data from other missions. We combined knowledge of these special plasma flow times with plasma composition measurement capabilities of the MMS mission to directly measure plasma composition in unprecedented detail—including those low-energy ions that usually cannot be measured—at the same time as EMIC wave activity displaying properties similar to those previously described as peculiar. The MMS composition data we analyzed was significantly more complex than previous composition data analyzed for EMIC wave studies.
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| Figure 2. Linear wave modeling results calculated using the detailed MMS composition data showing the relation between wave frequency (ω/ΩH+)and parallel (k||) or perpendicular (kperp) wave number to demonstrate how growth rate (γ/ΩH+indicated with the color bar) varies for possible wave modes existing at parallel (kperp = 0) to perpendicular (k||= 0) wave angles. |
Through applications of this detailed dataset to modeling, we were able to investigate why signatures identified in measured wave data were consistent with the measured composition and in the process demonstrate the value of detailed plasma composition measurements for characterizing EMIC waves. In the process we also found that the wave properties previously identified and thought to be peculiar were likely consistent with theory. Figure 2 is an excerpt of modeling performed with linear theory using the detailed plasma composition measured by MMS that shows peak wave growth rate occurring for EMIC waves parallel to the magnetic field (kperp = 0) in a wave frequency range consistent with the frequency range having peak wave power identified in MMS data.
Conclusion
Although the MMS satellites carry the latest state-of-the-art instruments, it was naturally-occurring plasma flow activity in Earth’s magnetosphere that helped the satellites measure the lowest-energy ions needed to more accurately explain the EMIC waves. This could have implications for EMIC wave studies conducted throughout Earth’s magnetosphere, as the ion composition may vary significantly depending on magnetospheric location but plasma flow activity possibly enabling measurement of the lowest energy ions could occur more (or less) frequently. Future studies could use similar methods to improve characterization of plasma composition or for applications to investigating other EMIC wave-related topics, such as wave-induced scattering and loss of radiation belt electrons.
Reference
Lee, J. H., Turner, D. L., Toledo‐Redondo, S., Vines, S. K., Allen, R. C., Fuselier, S. A., et al. (2019). MMS measurements and modeling of peculiar electromagnetic ion cyclotron waves. Geophysical Research Letters, 46, 11622–11631. https://doi.org/10.1029/2019GL085182Biographical Note
Justin Lee is a member of the technical staff in the Space Sciences Department at The Aerospace Corporation in El Segundo, California.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu