Science Seminars

Earth’s Van Allen Radiation Belts: From Discovery to the Van Allen Probes Era

Speaker: Mary Hudson (HAO)
Date: Thursday, Oct 31, 2019
Time: 4:00 PM
Location: SPSC W120

Seminar 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 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 (CRAND) 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 in 1989–1991. The dynamic variability of outer zone electrons was measured by the Combined Radiation Release and Effects Satellite (CRRES) launched in July 1990. This variability is caused by distinct types of heliospheric structure which 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 100s of keV, produce an increase in trapped relativistic electron flux in the outer zone. Mechanisms for accelerating electrons of 100s of eV stored in the tail region of the magnetosphere to MeV energies 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, as well as in astrophysical plasmas.