Radiation analysis

Radiation can readily penetrate typical spacecraft walls and deposit hundreds of kilorads during missions in certain orbits. Additional shielding can be equated with mass and cost. Therefore, it is important to have reliable computational tools to assist in orbit selection, component selection and shield optimisation. From the foregoing, it is apparent of this page that analysis of radiation environments and effects is highly complex.

Trapped Radiation

For trapped radiation, models of radiation belt energetic particle fluxes exist. These are the AE8 and AP8 models for electrons and protons respectively. They were developed by Vette and co-workers at the NSSDC at NASA/GSFC based on data from satellites flown in the '60s and early '70s. The models give omni-directional fluxes as functions of idealised geomagnetic dipole coordinates B/B0 and L. This means that they must be used together with an orbit generator and geomagnetic field computation to give instantaneous or orbit-averaged fluxes. Such a system is UNIRAD. The user defines an orbit, whereupon a trajectory is generated, transformed to geomagnetic coordinates and the models are accessed to compute flux spectra (cf also SPENVIS).

Apart from separate versions for solar maximum and solar minimum, there is no description of the temporal behaviour of fluxes. At high altitudes in particular (e.g. around geostationary orbit) fluxes vary by orders of magnitude over short times and exhibit significant diurnal variations; the models do not yet describe these adequately. In addition, the models do not contain any explicit flux directionality.

New Trapped Radiation Environment Model Developments:

The standard models described above are outdated and poorly suited to many present-day applications. At high altitudes the main problem is the great variability in the environment over many timescales. Data from spacecraft including Meteosat, CRRES and ISEE have been used to construct databases and subsequently new models of energetic electron flux temporal and spatial characteristics. At low altitude, the standard model takes no account of strong directionality, has no explicit link to atmospheric density and is based on a very short dataset from an atypical solar maximum. This dataset has been re-analysed, an anisotropy model constructed and models derived from analysis of more modern datasets.

Solar Event Protons

It is not possible to predict the exact occurrence, intensity or duration of solar protons events, and consequently mission planning on both a short-term and a long-term basis can be problematic.

Short-term forecasts are necessary for any tasks requiring extra-vehicular activity (EVA) and the operation of radiation-sensitive detectors. Real-time observation of the Sun can provide useful warning of solar event activity, as large proton events are usually associated with the strong emission of electromagnetic radiation, such as visible light, radio waves and soft X-rays.

Long-term predictions of the radiation levels resulting from events are derived from statistical models, as with any form of long term forecasting based on past observations. Two such solar event proton models are the King model, and the JPL model (also referred to as the Feynman model). The former was for a long time the standard model used by spacecraft engineers to predict mission-integrated solar proton fluences. The latter has been recently recommended for use for future mission planning.

Cosmic Rays

Cosmic-Ray environment and effects models have been created by Adams and co-workers at NRL. These bear the name CREME. They provide a comprehensive set of cosmic-ray and event ion LET and energy spectra, including treatment of geomagnetic shielding and material shielding. CREME also includes upset/hit rate computation based on the path length distribution in a sensitive volume and can also treat in a simple manner trapped proton-induced Single Event Upsets (SEU).

Last update: 6 May 2014

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