2019 THEMIS SCIENCE NUGGETS

The Evolution of a Pitch-Angle "Bite-Out" Scattering Signature Caused by EMIC Wave Activity: A Case Study

by Lydia Bingley
UCLA Earth, Planetary, and Space Sciences

Introduction

The near-Earth space environment, while seemingly quiescent to a casual observer's eye, is in fact composed of a variety of dynamic plasma phenomena. One such phenomenon is a toroidal structure of high energy plasma known as the Van Allen Radiation Belts. These belts, existing between altitudes of approximately 0.5-10 Earth radii, contain high energy protons and electrons originating primarily from the Sun that become trapped by the Earth's magnetic field. This "radiation" is harmful to spacecraft and astronauts that pass though them.

One way that high energy electrons can naturally escape from the radiation belts, is by interacting with a type of plasma wave, called an Electromagnetic Ion Cyclotron (EMIC) wave, that causes the particle to "scatter" into a trajectory that will take it into Earth's atmosphere where it becomes "lost". This loss process is somewhat unpredictable, one reason being that the precise interaction between relativistic electrons has been historically difficult to observe. Theoretically, evidence of this loss can take the form of a "bite-out" in radiation belt electron data, so-called because of the shape of the scattering signature.

In this study, we present the first direct observation of a "bite-out" scattering signature co-located with the responsible wave activity, proving that EMIC waves can have a direct and verifiable impact on radiation belt electrons.

Figure 1. (a-h) Radiation belt electron distribution observed with Relativistic Electron Proton Telescope (REPT) and Magnetic Electron Ion Spectrometer (MagEIS) onboard RBSP-A, normalized to the flux perpendicular to the background magnetic field. The region of interest is confined by the black dashed lines. Electromagnetic ion cyclotron (EMIC) wave (i) observed by RBSP-A. Particle scattering "bite-out" feature begins forming near 1.5 MeV (b) and deepens with increasing energy. Curves in c–g show the estimated "bite-out" shape based off observed wave and plasma parameters. MLT = magnetic latitude; REPT = Relativistic Electron Proton Telescope.

Results

This study uses data from THEMIS, Van Allen Probes (RBSP-A, RBSP-B), and LANL/GEO missions to characterize an electron "bite-out" scattering signature in association with a strong EMIC wave event captured on 16 February 2017. The EMIC wave was observed by RBSP-A around 06:30 UT on an inbound pass through the outer radiation belt.

The co-located "bite-out" scattering signature was uncovered by considering the flux of electrons travelling parallel to Earth's magnetic field in comparison with the flux travelling perpendicular to Earth's magnetic field. As EMIC waves more efficiently interact with electrons travelling parallel, normalizing the distribution to the electron population that remains unaffected by the wave activity (those travelling perpendicular) reveals the impact of the wave. This "bite-out" scattering signature, shown in Figure 1, is present in electrons with energies above 1.80 mega-electronvolts (MeV), representing core radiation belt populations. The co-location of the "bite-out" signature along with a strong EMIC wave strongly suggested a causal relationship.

Figure 2. Spacecraft orbits for a period of 24 hr around the scattering event. Spacecraft position at the beginning of the interval is marked by "x." Highlighted ovals along the trajectory represent times when electromagnetic ion cyclotron (EMIC) activity was observed. EMIC waves were persistent in the region of interest until around 16 February 2017 07:00 UT. Panels (a), (b), (d) and (e) represent spacecraft orbits while wave activity is present but not simultaneously accompanied by a scattering signature. The simultaneous EMIC wave and scattering signature took place in panel (c). Panel (f) shows spacecraft orbits once wave activity had diminished.

To support this hypothesis, we considered the local wave and plasma parameters for this event (using measurements from LANL/GEO and RBSP), and used the relevant wave-particle interaction theory equations, to estimate the shape that a "bite-out" signature would take in response to the observed EMIC wave. This estimation is plotted on top of the observation in Figure 1. The visible consistency between the estimation from theory and the "bite-out" observation provide further evidence that the observed EMIC wave was responsible for the "bite-out" scattering signature.

Finally, we explored how this "bite-out" scattering signature might have evolved temporally by scanning THEMIS and RBSP data for additional EMIC wave activity (Figure 2). We found that the scattering signature evolved temporally during a period of continuous EMIC activity (Figure 3). This suggests that while EMIC waves can have an immediate impact on radiation belt electrons, that impact can be sustained over at least a few hours while wave activity continues.

Together, these observations provide evidence that EMIC waves can play a role in rapid changes in Earth's radiation belts.

Figure 3. Temporal evolution of the "bite-out" scattering signature using a single electron energy (3.40 MeV). The "bite-out" is first seen to develop on Pass 2 and continues developing through Pass 4. By Pass 5, the "bite-out" is no longer visible.

Conclusion

The major findings of this study are summarized as follows:

Reference

Bingley, L., Angelopoulos, V., Sibeck, D., Zhang, X., & Halford, A. (2019). The evolution of a pitch-angle "bite-out" scattering signature caused by EMIC wave activity: A case study. Journal of Geophysical Research: Space Physics, 124. https://doi.org/10.1029/2018JA026292

Biographical Note

Lydia Bingley is a Space Physics PhD student at the University of California, Los Angeles. Her dissertation research focuses on the effectiveness of EMIC waves at influencing loss of core populations of radiation belt electrons.

Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu