Inward diffusion and loss of radiation belt protons

Abstract

Radiation belt protons in the kinetic energy range 24 to 76 MeV are being measured by the Relativistic Electron Proton Telescope on each of the two Van Allen Probes. Data have been processed for the purpose of studying variability in the trapped proton intensity during October 2013 to August 2015. For the lower energies (≲32 MeV), equatorial proton intensity near L = 2 showed a steady increase that is consistent with inward diffusion of trapped solar protons, as shown by positive radial gradients in phase space density at fixed values of the first two adiabatic invariants. It is postulated that these protons were trapped with enhanced efficiency during the 7 March 2012 solar proton event. A model that includes radial diffusion, along with known trapped proton source and loss processes, shows that the observed average rate of increase near L = 2 is predicted by the same model diffusion coefficient that is required to form the entire proton radiation belt, down to low L, over an extended (∼103 year) interval. A slower intensity decrease for lower energies near L = 1.5 may also be caused by inward diffusion, though it is faster than predicted by the model. Higher‐energy (≳40 MeV) protons near the L = 1.5 intensity maximum are from cosmic ray albedo neutron decay. Their observed intensity is lower than expected by a factor ∼2, but the discrepancy is resolved by adding an unspecified loss process to the model with a mean lifetime ∼120 years.

Variability of the Proton Radiation Belt

“…The external source is specified in the model by the boundary condition at L =2.4, but the changing intensity there shows that the source is not steady. In fact, it is likely a slowly depleting reservoir of solar protons that were trapped prior to the initial time, possibly during the March 2012 solar event (Selesnick et al, ). However, over the modeled time interval, changes in the L =2.4 boundary condition do not have a significant influence on model intensity for .…”

Investigation of Solar Proton Access Into the Inner Magnetosphere on 11 September 2017

“…If the probability density is lower than the minimum threshold value 10 −3 , then the particle is considered as a proton incident from outside the field of view or an electron and is excluded (Selesnick et al, ). This tool has been shown to effectively eliminate the contamination from high‐energy background protons and has been applied to study the dynamics of the protons in the inner radiation belt (Selesnick et al, , , ). In this work, this method is adopted to investigate the cutoff rigidity of solar protons for the first time.…”

SAMPEX observations of the South Atlantic anomaly secular drift during solar cycles 22–24

“…More recent observations have shown a more complex structure for the outer belt, at times giving rise to a “third belt” [ Baker et al , ]. The outer belt is made up largely of highly variable and dynamic electron populations, while the inner belt is composed mostly of protons and varies only slowly over time during quiet times [ Selesnick et al , , ], but during large geomagnetic storms, inner belt proton populations can change quickly [ Selesnick et al , ; Zou et al , ; Selesnick et al , ; Zou et al , ]. The weak field region of the SAA allows the inner belt particles to dip close to the surface of Earth, to within hundreds of kilometers.…”