科研之友
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摘要
The magnetosphere of Saturn exhibits a wide variety of periodic phenomena in magnetic fields, charged particles, and radio emissions. The periodicities are observed from a moving spacecraft, so an issue arises about the periodicities being influenced by the Doppler effects. Doppler effects can be investigated using models of the periodicities and then flying the spacecraft through the model, effectively measuring any Doppler phenomena with the simulation. Using 200 days of typical elliptical orbits from the Cassini mission at Saturn, three models were tested: an azimuthal wave (or "searchlight") model, a radial wave (or "pond ripple") model, and a model of an outwardly traveling spiral wave. The azimuthal wave model produced virtually no Doppler effects in the periodicities because its wave vector is nearly perpendicular to the spacecraft trajectory. The radial wave model generated strong Doppler effects of an upshifted and a downshifted signal (a dual period) on either side of the true period, because the wave vector is either parallel or antiparallel to the spacecraft trajectory. Being intermediate to the searchlight and radial waves, the spiral wave produced Doppler effects but only for low wave speeds (<10 R-S/h). For higher wave speeds the Doppler effects were not as clear. The Doppler effects can be mitigated by employing only observations beyond similar to 15 R-S where the spacecraft speed is low compared to the wave speed. The observed periodicities over the same 200 day interval do not show evidence of Doppler effects but generally display a single feature at the expected similar to 10.7 h period.
- 出版日期2015-11
