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Radiation during the 1st Rosetta Earth flyby


Anders Eriksson, Magnus Billvik and Lennart Åhlén

Swedish Institute of Space Physics, Uppsala
and
Department of Astronomy and Space Physics, Uppsala University

  13 December 2004


Addendum 8 June 2007:
see also predictions for the 2nd Earth swing-by


Contents

Issues

Is the radiation environment encountered in the first Earth flyby so severe that we should turn off our instruments? One should note that the radiation belts almost coincides with the plasmasphere of the Earth, which is the most important region to cover during the Earth flyby for the RPC instruments LAP, MIP and MAG, because of maximum plasma density and magnetic field providing unique opportunities to verify instrument capabilities.

There are two issues to consider:
  1. Long term degradation of electronics (mainly total dose effects)
  2. Perturbation on instrument operations (often dose rate effects)
The total dose acquired in the radiation belt passages is taken into account in the requirements in the EID-A, so all instruments are designed for this. Nevertheless, in this note we calculate the total modelled radiation dose expected, and find it to be rather small.

For issue 2, which is of more interest for operations planning, we should consider the probability of different kinds of single event effects (SEEs). The major source of SEEs use to be heavy ions and protons, while the electrons encountered in the outer radiation belts are usually less efficient. Heavy ions at high energy are mostly due to galactic cosmic rays, and hence less common inside the shielding magnetosphere than outside. However, SEEs, particularly so-called single event upsets (SEUs), are not unknown on spacecraft in the radiation belts. Modelling SEE rates require detailed modelling of components, so we instead compare the expected fluencies (time-integrated fluxes) of particles in the tens of MeV range during the flyby to what we can expect in interplanetary space, and to known effects on other s/c.

Models

We use the SPace ENVironment Information System (SPENVIS, http://www.spenvis.oma.be/spenvis/), a web-based unified interface to a set of models for the space environment and its effect on spacecraft. For the Rosetta trajectory, we use the AUX files provided by ESOC on the DDS. Presently, the SPENVIS web interface only includes the possibility to define the orbit in terms of orbital elements of closed orbits, but thanks to the cooperation of Daniel Heynderickx of the SPENVIS team at the Belgian Institute for Space Aeronomy, we could feed the Rosetta trajectory file, converted to geographic coordinates, into the system.

SPENVIS includes the NSSDC models AP-8 and AE-8 for trapped protons and electrons in the terrestrial radiation belts. We present the results from runs using the AP-8 and AE-8 for solar max conditions here, but the difference when running for solar min conditions was very small in this case, certainly much smaller than the prediction uncertainties.

For the estimate of total radiation dose (energy deposited in the target), we used the SPENVIS implementation of the SHIELDDOSE-2 model (v 2.10), assuming a silicon target inside a sphere of 1, 2 or 3 mm aluminium.

Predictions of SEE phenomena, like SEU rates, are much more dependant on specifics of individual electronic components. Though SPENVIS provides facilities for such modelling, we here only draw some general conclusions, avoiding detailed modelling of selected components.

Results

Calculated fluxes of particles during the Rosetta Earth flyby are shown in Figure 1. The time-integrated flux for the full flyby, i.e. the total number encountered or the particle fluence, is illustrated as a cumulative plot in Figure 2 and tabulated in Table 1. The estimated radiation doses are listed in Table 2.

Figure 1. Predicted fluxes of protons above 10 MeV and above 30 MeV, and of electrons above 1 MeV and 5 MeV, from the SPENVIS implementation of the AP-8 and AE-8 models for solar max conditions.

Figure 2. Predicted fluence of protons above 10 MeV and 30 MeV, and of electrons above 1 MeV and 3 MeV, from the SPENVIS implementation of the AP-8 and AE-8 models.

Particles
 Fluence
(cm-2)
Protons > 10 MeV
 1.5e8
Protons > 30 MeV
 2.1e7
Electrons > 1 MeV
 1.1e10
Electrons > 3 MeV
 2.2e6
Table 1. Predicted fluencies of high energy particles.

Al thickness [mm]
 Total dose [Rad]
 From trapped protons [Rad]
 From trapped electrons [Rad]
1 (sphere)
 660
 72
 579
2 (sphere)
 186
 20
 162
3 (sphere)
 66
 10
 54
1 (slab)
 80
 13
 65
Table 2. Radiation dose expected for the 1st Rosetta Earth flyby, for a silicon target within an aluminium sphere of given thickness , or behind a semi-infinite 1 mm Al plate. The "total dose" column also includes small contributions from bremsstrahlung and solar protons, though the radiation belt particles clearly dominate.

Conclusion

The exact fluencies can of course vary a lot with the actual magnetospheric conditions, but the results above nevertheless provide a baseline for estimating the possible impact of the radiation belts. To put them into perspective, we can compare to what we normally expect to find in interplanetary space. Feynman et al. (1990) suggest typical yearly averaged fluxes of solar proton event particles varying between 10^7 and 10^10  protons/year above 30 MeV, with 10^9 a reasonable number a few years after solar max. This would suggest that for the protons, the radiation belt passage gives a dose equivalent to what we may expect to get in about a week of operations in interplanetary space. There is no reason to worry about total dose effects.

For the RPC electronics box, a relevant model may be a 0.5 mm thick Al sphere, representing the RPC-0 electronics box, behind a 1 mm semi-infinite Al slab, representing the spacecraft. The last row of Table 2 may thus be taken as an upper limit to what may be expected. The total dose on RPC main electronics should thus be small, below 100 Rad. As noted above, the effects of this dose are largely independent on whether we are on or off, so this has little impact on operations.

What about SEUs? According to the above, we may expect below 100 Rad in total, most of it within some 20 minutes, which would indicate dose rates of order 0.1 Rad/s, with perhaps up to 1 -- 10 Rad/s peak values, most of it due to electrons. We should here be safe for latchups, and SEUs appears unlikely but cannot be ruled out.

For a high-voltage instrument, which can actually be damaged by the effects of a bit-flip at the wrong place, turning off in the rad belts may be considered. For RPC-LAP, which has no high voltages and which cannot be killed by any conceivable bit-flip, the worst effect of an SEU would be that the operational mode runs wild and we lose data. Though the SEU risk appears low, it therefore seems prudent for us to include a few resets in the operations timeline.

Acknowledgement

Thanks to Ali Mohammadzadeh at ESA for good comments.

References

http://space.irfu.se/rosetta/sci/radbelts.html
last modified onFriday, 08-Jun-2007 15:30:01 CEST