2022 THEMIS SCIENCE NUGGETS
Electron energization by high-amplitude turbulent electric fields: a source of the outer radiation belt
by Maria Usanova
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA
Introduction
Fast earthward flows, observed in Earth's magnetotail are believed to be generated by magnetic reconnection in the tail at distances of ~20-30 Earth radii. The braking of fast earthward flows, as they travel towards the inner magnetosphere, can generate turbulent plasma fluctuations and instabilities. In particular, fast flow braking is often associated with high-amplitude electric fields (> 50 mV/v ). These large-amplitude electric field events share a number of properties such as enhanced magnetic field fluctuations, magnetic field dipolarization, fluctuating ion flow velocities, ion and electron energization, and strong, field-aligned Poynting flux, which might be responsible for the generation of dynamic aurora. An example of an event is shown in Figure 1. The event is observed on THEMIS A at 08:26-08:28 and 08:45-08:57 UT on March 5, 2018.
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| Figure 1. Summary plot of an event example from THEMIS A at 08:26-08:28 and 08:45-08:57 UT on March 5, 2018. a) SST omnidirectional electron differential energy flux in the 41, 52, 65.5, 93, 139 and 203.5 keV energy channels; b) ESA omnidirectional electron differential energy flux in the 9, 12, 15.5, 20, 27 and 32 keV energy channels; c) SST electron differential energy flux variations in the 139 keV energy channel; d) ESA electron temperature (black) and temperature fluctuations (red); e) electric field spectrogram; f) on-board computed FBK electric field RMS amplitude averaged between the 144 and 572 Hz channels (black) and selected peaks (red); g) EFI Ex, Ey, Ez electric field components in burst mode; h) FGM Bx, By, Bz , |B| magnetic field; i) ESA ion Vx, Vy, Vz velocity components. The electric and magnetic time series and ion velocities are plotted in GSM coordinates (x: blue, y: green, and z: red curves). The magnetic field magnitude is plotted in black. |
Results
Electron energization by turbulent electric fields in the fast flow braking region is investigated using both single event analysis from a region adjacent to the outer radiation belt (1) and statistical survey (2). For the case study, data from four MMS spacecraft in tetrahedron configuration is utilized to determine the turbulent electric field correlation length and to further use it as input to numerical simulations. The correlation length is a critical parameter in electron energization; it constrains the minimum electron energy at which it starts experiencing energization from the turbulent electric field.
Figure 2 illustrates the underlying process of particle acceleration by turbulent, uncorrelated, electrostatic E. In the plane of the gyration, a low-energy electron (2 keV in the drawing) experiences a nearly constant E whereas a higher-energy electron (20 keV in the drawing) transits regions of changing E during its gyration. Even though E is primarily electrostatic, the particle does not necessarily return to the same location in the perpendicular plane (Figure 2a) or to the same location along B (Figure 2b) and therefore can experience energy change. The time dependence of E, albeit slow, is crucial in that an electron's energy gain or loss is not limited to the largest variation in ϕ. A finite ∇ * E can enhance acceleration.
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| Figure 2. A drawing of electron orbits in an uncorrelated, electrostatic E illustrating how turbulent acceleration favors higher-energy electrons. (a) A view of the orbital plane. The higher-energy (20 keV) electron's orbit transits several uncorrelated regions of E as it gyrates and therefore does not follow a closed path. It can gain or lose energy. A lower-energy electron (2 keV) sees little change in E over an orbit. (b) A 3D view of an electron's helical path along B. A high-energy electron can experience changes in E faster than its gyration period. |
Over a 1000 days of THEMIS data are used to study the relationship between turbulent electric fields and electron energization statistically. Large-amplitude turbulent electric fields in the fast flow breaking regions are found associated with increases of electron temperature and variations in energetic electron energy fluxes and by three times compared to the intervals when they are not observed; stronger fields are related to larger variations. The turbulence from fast flows is generally outside of the radiation belts, but can penetrate inside the outer radiation belt. The pre-energized electrons, however, may be injected into stronger magnetic field of the inner magnetosphere and, if they have already developed a substantial non-thermal tail, they may make a substantial contribution to the highest-energy electron populations in the outer radiation belt. The schematic in Figure 3 outlines the scenario where energy is initially being transferred from fast flows to electrons via the large-amplitude turbulent electric fields.
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| Figure 3. Schematic showing the cascade of energy transfer from the fast earthward flows to the outer radiation belt. |
Conclusion
The primary finding of this research is that the electron energization favors electrons that already have high energy, and therefore results in particle acceleration in which a relatively few particles receive a disproportionate share of the energy. Furthermore, turbulent regions of fast earthward flows are shown to penetrate to the edge of the outer radiation belt. Statistically, high-amplitude electric fields in the fast flow braking regions are associated with increases of electron temperature by three times compared to the intervals when they are not observed and cause ten-fold electron temperature fluctuations. They are also related to three-fold variations in energetic energy fluxes. There is a clear correlation between the field amplitude and electron temperature and energetic flux variations: stronger fields are related to larger variations, indicative of a local acceleration process. Though these events are transient and their occurrence is less than 1%, their impact on the magnetospheric dynamics may be rather significant. As the locally pre-energized electrons travel toward the inner magnetosphere and get further accelerated by the increasing magnetic field, they may supply the high-energy tail of the outer radiation belt.
Reference
Ergun, R. E., Usanova, M. E., Turner, D. L., & Stawarz, J. E. (2022). Bursty bulk flow turbulence as a source of energetic particles to the outer radiation belt. Geophysical Research Letters, 49, e2022GL098113.Usanova, M. E., & Ergun, R. E. (2022). Electron energization by high-amplitude turbulent electric fields: A possible source of the outer radiation belt. Journal of Geophysical Research: Space Physics, 127, e2022JA030336.
Biographical Note
Maria Usanova is a research scientist at the Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder. Her research interests include the dynamics of energetic particles in the Earth’s radiation belts and ring current, mechanisms for particle acceleration and loss in the magnetosphere, inner magnetosphere coupling, wave-particle interactions and energetic particle precipitation. Bob Ergun is a professor in the Department of Astrophysical and Planetary Sciences, University of Colorado at Boulder. His research involves numerical modeling, spacecraft data and instrumentation to investigate the space and astrophysical plasmas.
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