New Insights From Long‐Term Measurements of Inner Belt Protons (10s of MeV) by SAMPEX, POES, Van Allen Probes, and Simulation Results

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 at L > 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 below L = 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.

Long‐Term Monitoring of Energetic Protons at the Bottom of Earth’s Radiation Belt

“…Moreover, the radiation flux at higher L ‐values ( L > 1.7) is controlled both by the atmospheric density (loss process) and the GCR flux variations (source process). They are feasible because the drift particles at L < 2 (whole Hisaki orbit), as they fly at very low altitudes at the SAA, are strongly affected by atmospheric density variations, and the sensitivity to the GCR flux is higher at higher L ‐values (Li et al., 2020).…”

Relativistic Electron Model in the Outer Radiation Belt Using a Neural Network Approach

“…The uncertainty in the electron fluxes caused by the uncertainties in the input parameters is outside the scope of the current study and will be quantitatively analyzed in a future study (e.g., the comprehensive analysis in Licata et al, 2020). (Baker et al, 1993;Cook et al, 1993;Li et al, 2020;Selesnick, 2015). Since SAMPEX is located at LEO orbit, it measures electrons with a very small range of equatorial pitch angles very close to the loss cone, in contrast to the Van Allen Probes which observed the full equatorial pitch angle distribution, and thus the two datasets are closely correlated but not identical to each other, so the comparison is only qualitative (Kanekal et al, 2001;Li et al, 2013Li et al, , 2017Zhang, Li, et al, 2020).…”

“…We emphasize that the errors are contributed from both the error of the ORIENT-R model and the error in the predicted AL index. Nevertheless, the good correlation coefficient shows the out-of-sample predictive capability of the ORI-ENT-R model on the Van Allen Probes data set.The trapped electron fluxes reconstructed by the ORIENT-R model can be further validated in a completely out-of-sample fashion using the ∼2 MeV electron fluxes measured by the Proton-Electron Telescope on the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) satellite, which is located in a polar-orbiting LEO (launched into a 690 × 510 km altitude and 82° inclination orbit in July of 1992, and decayed to ∼490 × 410 km by 2009)(Baker et al, 1993;Cook et al, 1993;Li et al, 2020;Selesnick, 2015). Since SAMPEX is located at LEO orbit, it measures electrons with a very small range of equatorial pitch angles very close to the loss cone, in contrast to the Van Allen Probes which observed the full equatorial pitch angle distribution, and thus the two datasets are closely correlated but not identical to each other, so the comparison is only qualitative(Kanekal et al, 2001;Li et al, 2013Li et al, , 2017Zhang, Li, et al, 2020).…”

Solar-Cycle Variations of South-Atlantic Anomaly Proton Intensities Measured With The PAMELA Mission

“…Our current knowledge of the solar-cycle variations of the low-altitude proton environment essentially relies on the long-term measurements below 250 MeV made by a few LEO missions, including the NOAA Polar Orbiting Environmental Satellite (since 1978) series at an ∼800-850 km altitude (e.g., Huston et al (1996Huston et al ( , 1998; Qin et al (2014)), and the Solar, Anomalous, andMagnetospheric Particle Explorer (1992-2012), launched into a ∼510-690 km altitude orbit (e.g., Li et al (2020)).…”

“…The proton flux was found to rapidly decrease in the solar-maximum phases and slowly increase approaching solar minima. In particular, it was shown that the intensity variations are anticorrelated with those of the solar radio flux at 10.7 cm (also called the F 10.7 index), commonly used as a proxy of the corona activity and as an input to atmospheric models, with an energy-and L-dependent phase lag (Huston et al 1996(Huston et al , 1998Li et al 2020).…”