Distribution And Temporal Evolution Of High Energy Protons In The Inner Radiation Belt

Trapped proton population in the inner radiation belt is highly dense, posing a potential danger to astronauts and man-made space assets traversing through this region. In this paper, we use data from TRIOS/NOAA and Van Allen Probes to study the distribution, including the spatial distribution and angular distribution of the inner radiation belt proton flux, and their temporal evolution.Through seventeen pitch angle resolution data detected by the ECT-REPT aboard Van Allen Probes, we get direct observation of pitch angle distribution, altitude/L distribution and energy spectrum distribution of proton flux. It shows that:(1) the proton flux is higher near the 90° pitch angle and the distribution function fits well with the empirical model J=J90°sinnα. The anisotropy index gets higher when L gets lower at the equatorial plane. (2) Using these data, we also study the distribution of proton flux with altitude. It shows that the proton flux at a fixed longitude and altitude rises at first and goes down later with the increasing of altitude and L value. The detected proton flux is lower compared with that of AP-8 model. (3) Through eight differential energy channels of proton flux detected by ECT-REPT, we also fit the energy distribution of proton flux with power-law spectrum.Through data aboard NOAA satellite, we found that the proton flux at altitude of 808 km shows two characteristics:(1) A Gaussian function can favorably reconstruct the proton flux spatial distribution in the SAA. (2) There exist apparent east-west anisotropy, i.e., the eastward proton flux is higher than the westward proton flux at an altitude of 808 km.While being significantly stable within timescales up to hundreds of days, inner zone proton fluxes can exhibit considerable solar cycle variations, which have not been investigated comprehensively yet. To analyze the long-term variation of the South Atlantic Anomaly (SAA),Then we use Gaussian fit to study the temporal evolution of the SAA protons for almost three solar cycles (1998-2009)。 To analyze the long-term variation of the inner zone proton flux in the South Atlantic Anomaly (SAA), we have adopted the proton flux data measured by NOAA 6, NOAA 10, NOAA 12, and NOAA 15 from 1980 through 2009 and performed reasonable Gaussian fits for a statistical analysis. Our main conclusions are summarized as follows:(1) There is an anti-correlation and a phase lag between the measurements of SAA proton fluxes and F10.7 fluxes. The local maximum of the proton flux tends to increase over an entire solar cycle. The variation trends are approximately the same with respect to the four energy channels in our research. The phase lag in the anti-correlation relationship differs in different solar cycles; (2) The longitudinal center of the SAA drifts faster during the solar minimum than the solar maximum. This trend is different from the secular variation of the geomagnetic field. The westward drift rate shows a good correlation with the F10.7 flux, which however requires further investigations. (3) The maximal latitudinal position of the SAA proton flux has different direction drifts within a solar cycle. Such an opposite trend of long-term drift profile may be associated with the solar wind dynamic pressure. (4).The area of the SAA and the F10.7 flux is anti-correlated. The SAA area expands within an entire solar cycle. It also shows a rapid decrease during the solar maximum and a slow increase during the solar minimum.Trapped protons in the South Atlantic Anomaly (SAA) have a rather narrow pitch angle distribution and exhibit east±west anisotropy. In low Earth orbits, the E±W effect results in different amounts of radiation dose received by different sections of the spacecraft. This effect is best studied on missions in which the spacecraft flies in a fixed orientation. The magnitude of the effect depends on the particle energy and altitude through the SAA. In this paper, we describe a clear example of this effect from measured proton flux> 70 MeV on board NOAA 15 (98 degree inclination X 808 km altitude). Data from 1999 through 2009 are adopted to perform statistical analyses on the basis of reasonable Gaussian fits. We report that there exists EW effect in the SAA in the whole solar cycle. (1) The ratio of the eastward proton flux to the westward proton flux keeps almost the same (-1.3) at this altitude. (2) The peak longitude positon of the SAA region determined by the eastward proton flux is east to the westward proton flux. The peak latitude position of the SAA region determined by the eastward proton flux is south to the westward proton flux, opposite to the result predicted the IGRF model...