ECSS E 10-04 v0.10 - European Cooperation on Space Standardization ...

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<strong>ECSS</strong>-E-<strong>10</strong>-<strong>04</strong>B (draft v0.<strong>10</strong>) 13/03/2008
Space Environment
by the South Atlantic anomaly. Also important is the asymmetry in energetic proton fluxes
from East and West. The low-altitude environment is characterized by high-energy
radiation-belt trapped protons. The deflection of charged particles from outside the
magnetosphere by the Earth magnetic field (geomagnetic shielding) reduces the fluxes of
cosmic rays and solar energetic particles, but the shielding is not total. Like polar orbits, LEO
orbits also encounter outer-belt trapped electrons at high-latitudes.
I.1.2.4. Polar
Polar orbits are generally of less than 1500 km altitude with inclinations above 80º. They
encounter the inner proton and electron belts in the form of the South Atlantic anomaly and
also the outer electron belt where the geomagnetic field lines bring it to low altitudes at
“auroral” latitudes above about 50º. On the high-latitude parts of the orbit a spacecraft is
exposed to almost unattenuated fluxes of cosmic rays and solar energetic particles. At low
latitudes, geomagnetic shielding considerably reduces these fluxes.
I.1.3. Existing trapped radiation models
For trapped radiation and all earth orbits, the best known and most widely used models of
radiation belt energetic particles are the AE-8 and AP-8 models for electrons [RN.11] and
protons [RN.12], respectively. They were developed at Aerospace Corporation for the NSSDC
at NASA/GSFC based on data from satellites flown in the 1960s and early 1970s. The models
give omni-directional fluxes as functions of idealized geomagnetic dipole coordinates B/B0 and
L. The user defines an orbit, generates a trajectory, transforms it to geomagnetic coordinates
and access the radiation belt models to compute flux spectra, using the same geomagnetic field
as used to develop the model. 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 describe these. In addition, the
models do not contain any explicit flux directionality.
At low altitudes, on the inner edge of the radiation belts, particle fluxes rise very steeply with
altitude and small errors in computing locations can give rise to large errors in particle fluxes.
This is a problem since the geomagnetic field is shifting and decaying so that the situation is no
longer the same as when the model data were acquired. Use of a geomagnetic field model other
than the one used in generating the model can result in large flux errors at low altitude.
Although use of an old field model and epoch can reduce errors in the magnitudes of fluxes, it
does not model the spatial locations of radiation-belt features (e.g. the position of the South
Atlantic anomaly), or particle fluxes, as they are today.
The AP-8 model for protons gives proton fluxes from 0,1 to 400 MeV while the AE-8 model
for electrons covers electrons from 0,<strong>04</strong> to 7 MeV. Figure I.1 shows contour plots of AE-8 and
AP-8 model omnidirectional, integral fluxes for energies above 1 MeV and <strong>10</strong> MeV,
respectively, in idealized dipole space.
Figure I.2 shows values of energetic electron and proton particle fluxes as stored in these
models, for positions on the geomagnetic equator (B=B0), as functions of L for both solar
maximum and solar minimum. This shows that as far as the models are concerned, the solar
activity only affects electron fluxes in the mid-L range and protons at low altitude where the
higher neutral atmospheric density at solar maximum leads to reduced proton fluxes because of
enhanced loss. Solar cycle effects on electrons appear to differ from this behaviour in reality
[RD.63].
I.1.3.1. Uncertainties in trapped particle models
The accuracy of the predicted fluxes is within a factor of 2 for AP8 and within a factor
depending on the location and incident electron energy for AE8. In [RN.11], a reasonable limit
of the error on AE8 is a factor of 2, however, in some regions (L=3) this can increase to a
factor of 4.5 and is energy dependent, the error is higher for the higher energies. In other
regions, such as geostationary orbits, the AE8 models are pessimistic.