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Radiation during the 2nd Rosetta Earth swing-by


Anders Eriksson

Swedish Institute of Space Physics, Uppsala

  8 June 2007
13 June 2007

Updated 13 June with longer orbit section (c/a ±6 hrs). This increased e- fluxes by about 10%, but changed no conclusions.


Contents

Issues

This modelling is completely similar to the runs we did for the 1st Earth flyby.

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. The Rosetta trajectory file for 13 Nov 2007 18:00 - 24:00 UT, converted to geographic coordinates, was loaded into the system. Figure 1 shows trajectory data, starting at 18:00.

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 minimum conditions: the difference to solar max is small for the trapped radiation fluxes, according to our experience for ESB1.

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 Figures 2-5. The time-integrated flux for the full flyby, i.e. the total number encountered or the particle fluence, is tabulated in Table 1. The estimated radiation doses are listed in Table 2.

Figure 2. Predicted proton flux above 10 MeV.
Figure 3. Predicted proton flux above 20 MeV.
Figure 4. Predicted electron flux above 1 MeV.
Figure 5. Predicted electron flux above 3 MeV.
(For all plots, t = 0 corresponds to 2007-11-13 18:00 UT)

Particles
 ESB2 fluence (cm-2) ESB1 fluence
(cm-2)
Protons > 10 MeV
 1.18e5
 1.5e8
Protons > 30 MeV
 0.00 (below SPENVIS threshold)
 2.1e7
Electrons > 1 MeV
 8.4e9
 1.1e10
Electrons > 3 MeV
 1.9e8
 2.2e8
Table 1. Predicted fluencies of high energy particles, with values for the 1st Earth swing-by for comparison.

Al thickness [mm]
 Total dose [Rad]
 From trapped protons [Rad]
 From trapped electrons [Rad]
1 (sphere)
 590 (660)
 0.074 (72)
 588 (579)
2 (sphere)
 167 (186)
 0.006 (20)
 166 (163)
3 (sphere)
 54 (66)
 0.001 (10)
 53 (76)
1 (slab)
 67 (80)
 0.008 (13)
 66 (65)
Table 2. Radiation dose expected for the 2nd Rosetta Earth flyby (values for the 1st Earth swing-by in parenthesis), 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. Compared to the 1st Earth flyby, the total dose is about equivalent, though the ion contribution has dropped considerably to become negligible. The predicted doses are not very high: see the discussion for the 1st Earth fly-by.

References

Pat Mokashi, SwRI, provided trajectory data. All calculations were done by use of the SPENVIS system on the web.

http://www.space.irfu.se/rosetta/sci/radbelts2.html
last modified on Friday, 8-June-2007 14:39:01 CEST