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- Title
A New Model of Electron Pitch Angle Distributions and Loss Timescales in the Earth's Radiation Belts.
- Authors
Glauert, S. A.; Atkinson, J. W.; Ross, J. P.; Horne, R. B.
- Abstract
As the number of satellites on orbit grows it is increasingly important to understand their operating environment. Physics‐based models can simulate the behavior of the Earth's radiation belts by solving a Fokker‐Planck equation. Three‐dimensional models use diffusion coefficients to represent the interactions between electromagnetic waves and the electrons. One‐dimensional radial diffusion models neglect the effects of energy diffusion and represent the losses due to the waves with a loss timescale. Both approaches may use pitch angle distributions (PADs) to create boundary conditions, to map observations from low to high equatorial pitch angles and to calculate phase‐space density from observations. We present a comprehensive set of consistent PADs and loss timescales for 2 ≤ L* ≤ 7, 100 keV ≤ E ≤ 5 MeV and all levels of geomagnetic activity determined by the Kp index. These are calculated from drift‐averaged diffusion coefficients that represent all the VLF waves that typically interact with radiation belt electrons and show good agreement with data. The contribution of individual waves is demonstrated; magnetosonic waves have little effect on loss timescales when lightning‐generated whistlers are present, and chorus waves contribute to loss even in low levels of geomagnetic activity. The PADs vary in shape depending on the dominant waves. When chorus is dominant the distributions have little activity dependence, unlike the corresponding loss timescales. Distributions peaked near 90° are formed by plasmaspheric hiss for L* ≤ 3 and E < 1 MeV, and by EMIC waves for L* > 3 and E > 1 MeV. When hiss dominates, increasing activity broadens the distribution but when EMIC waves dominate increasing activity narrows the distribution. Plain Language Summary: As the number of satellites on orbit grows it is increasingly important to understand their operating environment. Physics‐based models of the radiation belts can be used to model how the radiation environment varies with time. Three‐dimensional models typically include the effect of interactions between electromagnetic waves and electrons in the belts on both the motion and energy of the electrons. One‐dimensional models simplify the modeling by neglecting the effect of the waves on the energy of the electron and only consider how rapidly they are lost. Both types of model require an understanding of how electrons are distributed in the belts and how they are lost. We present calculations of the timescale for electron loss from the belts and for the shape of the distribution of electrons with equatorial pitch angle, a variable related to latitude, due to the combined effect of all the VLF waves that typically interact with radiation belt electrons. Our results show good agreement with the observations. They vary significantly with location, energy and geomagnetic activity, the influence of individual waves can be identified, and the model provides a useful resource for radiation belt models. Key Points: The combined effect of all the VLF waves that typically interact with radiation belt electrons is included in the calculationsWhen chorus is the dominant wave pitch angle distributions show little activity dependence, unlike the corresponding loss timescalesIncreasing geomagnetic activity tends to broaden the peak of the distributions dominated by hiss and narrow those dominated by EMIC waves
- Subjects
RADIATION belts; TERRESTRIAL radiation; DIELECTRIC loss; ELECTRON distribution; ELECTRONS; ELECTRON energy loss spectroscopy
- Publication
Journal of Geophysical Research. Space Physics, 2024, Vol 129, Issue 6, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2023JA032249