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Abstract Discovery of the Earth's Van Allen radiation belts by instruments flown on Explorer 1 in 1958 was the first major discovery of the Space Age. The observation of distinct inner and outer zones of trapped megaelectron volt (MeV) particles, primarily protons at low altitude and electrons at high altitude, led to early models for source and loss mechanisms including Cosmic Ray Albedo Neutron Decay for inner zone protons, radial diffusion for outer zone electrons and loss to the atmosphere due to pitch angle scattering. This scattering lowers the mirror altitude for particles in their bounce motion parallel to the Earth's magnetic field until they suffer collisional loss. A view of the belts as quasi‐static inner and outer zones of energetic particles with different sources was modified by observations made during the Solar Cycle 22 maximum in solar activity over 1989–1991. The dynamic variability of outer zone electrons was measured by the Combined Radiation Release and Effects Satellite launched in July 1990. This variability is caused by distinct types of heliospheric structure that vary with the solar cycle. The launch of the twin Van Allen Probes in August 2012 has provided much longer and more comprehensive measurements during the declining phase of Solar Cycle 24. Roughly half of moderate geomagnetic storms, determined by intensity of the ring current carried mostly by protons at hundreds of kiloelectron volts, produce an increase in trapped relativistic electron flux in the outer zone. Mechanisms for accelerating electrons of hundreds of electron volts stored in the tail region of the magnetosphere to MeVenergies in the trapping region are described in this review: prompt and diffusive radial transport and local acceleration driven by magnetospheric waves. Such waves also produce pitch angle scattering loss, as does outward radial transport, enhanced when the magnetosphere is compressed. While quasilinear simulations have been used to successfully reproduce many essential features of the radiation belt particle dynamics, nonlinear wave‐particle interactions are found to be potentially important for causing more rapid particle acceleration or precipitation. The findings on the fundamental physics of the Van Allen radiation belts potentially provide insights into understanding energetic particle dynamics at other magnetized planets in the solar system, exoplanets throughout the universe, and in astrophysical and laboratory plasmas. Computational radiation belt models have improved dramatically, particularly in the Van Allen Probes era, and assimilative forecasting of the state of the radiation belts has become more feasible. Moreover, machine learning techniques have been developed to specify and predict the state of the Van Allen radiation belts. Given the potential Space Weather impact of radiation belt variability on technological systems, these new radiation belt models are expected to play a critical role in our technological society in the future as much as meteorological models do today.
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Atmospheric scattering of energetic electrons from near-Earth space
In near-Earth space, the magnetosphere, energetic electrons (tens to thousands of kiloelectron volts) orbit around Earth, forming the radiation belts. When scattered by magnetospheric processes, these electrons precipitate to the upper atmosphere, where they deplete ozone, a radiatively active gas, modifying global atmospheric circulation. Relativistic electrons (those above a few hundred kiloelectron volts), can reach the lowest altitudes and have the strongest effects on the upper atmosphere; their loss from the magnetosphere is also important for space weather. Previous models have only considered magnetospheric scattering and precipitation of energetic electrons; atmospheric scattering of such electrons has not been adequately considered, principally due to lack of observations. Here we report the first observations of this process. We find that atmospherically-scattered energetic (relativistic) electrons form a low-intensity, persistent “drizzle”, whose integrated energy flux is comparable to (greater than) that of the more intense but ephemeral precipitation by magnetospheric scattering. Thus, atmospheric scattering of energetic electrons is important for global atmospheric circulation, radiation belt flux evolution, and the repopulation of the magnetosphere with lower-energy, secondary electrons.
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- Award ID(s):
- 2019914
- PAR ID:
- 10248942
- Date Published:
- Journal Name:
- Research square
- ISSN:
- 2693-5015
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.21203/rs.3.rs-319558/v1
