Radiation Belts
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The Earth's radiation belts - sometimes referred to as the Van Allen belts, after their discoverer, James Van Allen - are rings of energetic protons and electrons that surround the Earth. These electrons can have energies from tens of keV (kilo-electron-Volts) to tens of MeV, and the protons extend into the hundreds of MeV. (Electrons a few hundred keV are "relativistic"; electrons with energies of 511 keV have a mass that is double their rest-mass, thanks to relativity.) The radiation belts extend from altitudes of 1000 km all the way to 40,000 km, beyond geosynchronous altitude. As such, many of our valuable and costly spacecraft orbit within the radiation belts, and these energetic particles can be extremely damaging to spacecraft - not to mention humans in spaceflight! In fact, spacecraft hardware is designed - at great cost - to withstand this radiation environment through "radiation tolerance" or "radiation hardening" design techniques. Still, exposure to the radiation belts can dramatically affect the lifetime and operability of these spacecraft.
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Radiation belt particles are trapped in the Earth's magnetic field thanks to the "magnetic bottle" effect. However, the physics is complicated significantly by the presence of electromagnetic and electrostatic waves, by the reconfiguration of the Earth's magnetic field during geomagnetic storms and substorms, and by input particle fluxes from the solar wind. Radiation belt fluxes can be increased by orders of magnitude during storms, significantly increasing the danger to spacecraft and astronauts. Radiation belt particles are then "lost" through the outer edge of the magnetosphere - the magnetopause - and by loss into the Earth's upper atmosphere, a process we call "energetic particle precipitation" or EPP.
Our Work
In the LAIR, we study the effects of radiation belt precipitation - EPP - on the upper atmosphere and on the radiation belt populations. An accurate, quantitative understanding of the precipitated flux is critical to understanding the effects on the atmosphere and the lifetimes of radiation belt populations (i.e., how long they last after a storm before they go back to normal). We use Monte Carlo modeling to understand the precipitation process, including ionization in the atmosphere, optical emissions - aurorae! - and the production of x-rays and gamma rays in the upper atmosphere. Our modeling work is the first to predict optical emissions, similar to the aurora, created by radiation belt EPP. (Typical aurora is caused by precipitation of electrons from hundreds of eV to tens of keV - low energies compared to the radiation belts.) We predict a very small, but detectable, optical signal due to this precipitation. We have built optical instruments to try to detect this optical signal from the ground, and these instruments are soon to be deployed and tested in Alaska.
We are also utilizing multipoint observations of the radiation belts from GPS satellites, POES, and the Van Allen Probes to quantify electron losses and enhancements. During geomagnetic storms, we are quantifying the amount of POES-measured EPP that can be attributed to dropouts measured by GPS and the Van Allen Probes.
Furthermore, we are working on instrument designs to measure energetic particle populations from CubeSats and other space platforms. Current particle detectors use very wide fields-of-view, and are not able to resolve the directions of incoming particles; those directions are critical to determining which particles will be lost in the atmosphere. This effort is integrated with our work in Small Satellites. |
Right: [Xu et al., 2018] (a) Illustration of EEP interaction with the Earth's atmosphere, including processes of bremsstrahlung radiation, photoelectric absorption, and Compton scattering. Also shown in this figure is the illustration of balloon‐, ground‐, and space‐based measurements. (b) Comparison of ionization rate profiles produced by beams of monoenergetic electrons between present modeling results and those reported in Fang et al. (2010, Figure 2). The simulations are performed using the MSIS atmosphere with F10.7=300 and Ap=65. The total incident energy of precipitating electrons used in each simulation is 1 erg/cm2/s. (c) Comparison of ionization rate profile between present modeling results and those presented in Frahm et al. (1997, Figure 1). The dashed curve shows the bremsstrahlung‐induced ionization rate. The energy distribution and fluxes of precipitating electrons used in this simulation are obtained from Frahm et al. (1997, Plate 1) |
References
- Thorne, R. M. (2010), Radiation belt dynamics: The importance of wave-particle interactions, Geophys. Res. Lett., 37, L22107, doi:10.1029/2010GL044990.
- Millan, R. M. and R. M. Thorne (2007), Review of radiation belt relativistic electron losses, J. Atmos. Solar Terr. Phys., 69, pp. 362-367, doi:10.1016/j.jastp.2006.06.019.
- Marshall, R. A., M. Nicolls, E. Sanchez, N. G. Lehtinen, and J. Neilson (2014), Diagnostics of an artificial relativistic electron beam interacting with the atmosphere, J. Geophys. Res. Space Physics, 119, doi:10.1002/2014JA020427.
- Dahlgren, H., J. L. Semeter, R. A. Marshall, and M. Zettergren (2013), The optical manifestation of dispersive field-aligned bursts in auroral breakup arcs, J. Geophys. Res. Space Physics, 118, 4572-4582, doi:10.1002/jgra.50415.
- Marshall, R. A., J. Bortnik, N. Lehtinen, and S. Chakrabarti (2011), Optical signatures of lightning-induced electron precipitation, J. Geophys. Res., 116, A08214, doi:10.1029/2011JA016728.
- Marshall, R. A., R. T. Newsome, N. G. Lehtinen, N. Lavassar, and U. S. Inan (2010), Optical signatures of radiation-belt electron precipitation induced by ground-based VLF transmitters, J. Geophys. Res., 115, A08206, doi:10.1029/2010JA015394.
- Inan, U. S., S. A. Cummer, and R. A. Marshall (2010), A survey of ELF and VLF research on lightning-ionosphere interactions and causative discharges, J. Geophys. Res., 115, A00E36, doi:10.1029/2009JA014775.
- Marshall, R. A., U. S. Inan, T. Neubert, A. Hughes, G. Satori, J. Bor, A. Collier, and T. H. Allin (2005), Optical observations geomagnetically conjugate to sprite-producing lightning discharges, Ann. Geophys., 23(6), 2231, doi:10.5194/angeo-23-2231-2005.
- Xu, W., R. A. Marshall, X. Fang, E. Turunen, and A. Kero (2018), On the effects of bremsstrahlung radiation during energetic electron precipitation, Geophysical Research Letters, 45, 1167-1176, doi:10.1002/2017GL076510.
- Xu, W., and R. A. Marshall (2019), Characteristics of energetic electron precipitation estimated from simulated bremsstrahlung X‐ray distributions. Journal of Geophysical Research: Space Physics, 124, 2831– 2843, doi:10.1029/2018JA026273.