Standard radiation environment monitor: simulation and inner belt flux anisotropy investigation

Abstract: The Standard Radiation Environment Monitor (SREM) is a standardised particle
detector for mapping highly-energetic protons and electrons of the radiation
field. It is employed on several ESA spacecraft (Integral, Rosetta, PROBA-1,
Giove-B, Herschel, Planck) to provide radiation level information and to
issue dose warnings to other instruments.
A geometric model of the SREM instrument is simulated using GRAS/Geant4 to
determine its directional response function. The instrument response to both
protons and electrons is obtained for a wide range of discrete energy levels
and directions of particle incidence. Analysis of the simulation output
shows the directional characteristics of the SREM response and the
resulting sensitivity to the pitch angle distribution of the flux. The
directional, spherical and integrated response functions of the SREM are
presented and discussed.
The SREM on PROBA-1 (Project for On-Board Autonomy) gathers data of
geomagnetically trapped protons, particularly in the South Atlantic Anomaly
(SAA). The proton flux in the PROBA-1 orbit is investigated using the
omnidirectional AP-8 model. Combining the SREM response function with the
proton flux yields predictions of the SREM countrates which are then
compared to data measured by PROBA-1.
The influence of flux anisotropies on the SREM countrates is demonstrated
and proves the necessity of including a model for the distribution of
particle pitch angles: the Badhwar-Konradi model of pitch angle
distribution is implemented and combined with the omnidirectional AP-8
model to yield an anisotropic unidirectional flux model.
As a consequence, significant improvements to the AP-8 model are realised.
The importance of considering flux anisotropies is shown both for short-term
SREM countrate features and long-term integrated counts. Data analysis and
comparison to simulated data is performed with respect to different values
of McIlwain's L-coordinates and varying particle pitch angles. To simulate
countrates, the attitude of the SREM on PROBA-1 relative to the magnetic
field vector is determined using the magnetometer on-board PROBA-1.
Radiation due to geomagnetically trapped protons contributes substantially
to the overall radiation levels on the International Space Station (ISS).
Based on the importance of the pitch angle distribution, the relevance of
proton anisotropy for ISS dose levels is motivated.