skip to main content

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

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.

more » « less
Award ID(s):
Author(s) / Creator(s):
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Space Physics
Page Range / eLocation ID:
p. 8319-8351
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Quantification of energetic electron precipitation caused by wave‐particle interactions is fundamentally important to understand the cycle of particle energization and loss of the radiation belts. One important way to determine how well the wave‐particle interaction models predict losses through pitch‐angle scattering into the atmospheric loss cone is the direct comparison between the ionization altitude profiles expected in the atmosphere due to the precipitating fluxes and the ionization profiles actually measured with incoherent scatter radars. This paper reports such a comparison using a forward propagation of loss‐cone electron fluxes, calculated with the electron pitch angle diffusion model applied to Van Allen Probes measurements, coupled with the Boulder Electron Radiation to Ionization model, which propagates the fluxes into the atmosphere. The density profiles measured with the Poker Flat Incoherent Scatter Radar operating in modes especially designed to optimize measurements in the D‐region, show multiple instances of close quantitative agreement with predicted density profiles from precipitation of electrons caused by wave‐particle interactions in the inner magnetosphere, alternated with intervals with large differences between observations and predictions. Several‐minute long intervals of close prediction‐observation approximation in the 65–93 km altitude range indicate that the whistler wave‐electron interactions models are realistic and produce precipitation fluxes of electrons with energies between 10 keV and >100 keV that are consistent with observations. The alternation of close model‐data agreement and poor agreement intervals indicates that the regions causing energetic electron precipitation are highly spatially localized.

    more » « less
  2. Abstract

    The Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) mission provided long‐term measurements of 10s of megaelectron volt (MeV) inner belt (L < 2) protons (1992–2009) as did the Polar‐orbiting Operational Environmental Satellite‐18 (POES‐18, 2005 to present). These long‐term measurements at low‐Earth orbit (LEO) showed clear solar cycle variations which anticorrelate with sunspot number. However, the magnitude of the variation is much greater than the solar cycle variation of galactic cosmic rays (>GeV) that are regarded as a source of these trapped protons. Furthermore, the proton fluxes and their variations sensitively depend on the altitude above the South Atlantic Anomaly (SAA) region. With respect to protons (>36 MeV) mirroring near the magnetic equator, both POES measurements and simulations show no obvious solar cycle variations atL > 1.2. This is also confirmed by recent measurements from the Van Allen Probes (2012–2019), but there are clear solar cycle variations and a strong spatial gradient of the proton flux belowL = 1.2. A direct comparison between measurements and simulations leads to the conclusion that energy loss of trapped protons due to collisions with free and bound electrons in the ionosphere and atmosphere is the dominant mechanism for the strong spatial gradient and solar cycle variation of the inner belt protons. This fact is also key of importance for spacecraft and instrument design and operation in near‐Earth space.

    more » « less
  3. Abstract

    Earth's slot region, lying between the outer and inner radiation belts, has been identified as due to a balance between inward radial diffusion and pitch angle (PA) scattering induced by waves. However, recent satellite observations and modeling studies indicate that cosmic ray albedo neutron decay (CRAND) may also play a significant role in energetic electron dynamics in the slot region. In this study, using a drift‐diffusion‐source model, we investigate the relative contribution of all significant waves and CRAND to the dynamics of energetic electrons in the slot region during July 2014, an extended period of quiet geomagnetic activity. The bounce‐averaged PA diffusion coefficients from three types of waves (hiss, lightning‐generated whistlers [LGW], and very low frequency [VLF] transmitters) are calculated based on quasi‐linear theory, while the CRAND source follows the results in Xiang et al. (2019, The simulation results indicate that both LGW and VLF transmitter waves can enhance loss and weaken the top hat PA distribution induced by hiss waves. For 470 keV electrons atL = 2.5, simulation results without CRAND show a much quicker decrease than observations from the Van Allen Probes. After including CRAND, simulated electron flux variations reproduce satellite observations, suggesting that CRAND is an important source for hundreds of keV electrons in the slot region during quiet times. The balance between the CRAND source and loss due to wave‐particle interactions provides a lower limit to relativistic electron fluxes in the slot region, which can act as an important reference point for instrument calibration when a true background level is warranted.

    more » « less
  4. Abstract The Van Allen Probes Electric Fields and Waves (EFW) instrument provided measurements of electric fields and spacecraft floating potentials over a wide dynamic range from DC to 6.5 kHz near the equatorial plane of the inner magnetosphere between 600 km altitude and 5.8 Re geocentric distance from October 2012 to November 2019. The two identical instruments provided data to investigate the quasi-static and low frequency fields that drive large-scale convection, waves induced by interplanetary shock impacts that result in rapid relativistic particle energization, ultra-low frequency (ULF) MHD waves which can drive radial diffusion, and higher frequency wave fields and time domain structures that provide particle pitch angle scattering and energization. In addition, measurements of the spacecraft potential provided a density estimate in cold plasmas ( $<20~\text{eV}$ < 20 eV ) from 10 to $3000~\text{cm}^{-3}$ 3000 cm − 3 . The EFW instrument provided analog electric field signals to EMFISIS for wave analysis, and it received 3d analog signals from the EMFISIS search coil sensors for inclusion in high time resolution waveform data. The electric fields and potentials were measured by current-biased spherical sensors deployed at the end of four 50 m booms in the spacecraft spin plane (spin period $\sim11~\text{sec}$ ∼ 11 sec ) and a pair of stacer booms with a total tip-tip separation of 15 m along the spin axis. Survey waveform measurements at 16 and/or 32 S/sec (with a nominal uncertainty of 0.3 mV/m over the prime mission) were available continuously while burst waveform captures at up to 16,384 S/sec provided high frequency waveforms. This post-mission paper provides the reader with information useful for accessing, understanding and using EFW data. Selected science results are discussed and used to highlight instrument capabilities. Science quantities, data quality and error sources, and analysis routines are documented. 
    more » « less
  5. Abstract

    The coupling response between solar wind structures and the magnetosphere is highly complex, leading to different effects in the outer radiation belt electron fluxes. Most Coronal Mass Ejections cause strong geomagnetic storms with short recovery phases, often 1–2 days. By contrast, High‐Speed Solar Wind Streams lead to moderate and weak storms often with much longer recovery phases, from several to ∼10 days. The magnetosphere receives energy for a long time under the influence of the HSSs, considerably changing its dynamics. This in turn has an effect on the charged particles trapped in the outer radiation belt. Although the high‐energy electron flux enhancements have received considerable attention, the high‐energy electron flux enhancement pattern (L > 4) has not. This paper identifies 37 events with this enhancement pattern in the high‐energy electron flux during the Van Allen Probes era. We find the enhancements coincident with HSS occurrence. The interplanetary magnetic field (IMF) exhibits north/south Bz fluctuations of Alfvénic nature with moderate amplitudes. The high‐energy electron flux enhancements also correspond to long periods of auroral activity indicating a relationship to magnetotail dynamics. However, the AE index only reaches moderate values. Ultra‐Low Frequency waves were present in all of the events and whistler‐mode chorus waves were present in 89.1% of the events, providing a convenient scenario for wave‐particle interactions. The radial gradient of the ULF wave power related to theL, under the influence of the HSSs, is necessary to trigger the physical processes responsible for this type of high‐energy electron flux enhancement pattern.

    more » « less