A simulated magnetic induction investigation of Europa
Abstract
We will present results of a simulated magnetic induction experiment designed to measure the electrical conductivity structure within Europa, as seen from a series of spacecraft flyby encounters. As Europa moves in its orbit about Jupiter, it experiences a time varying magnetic field, which induces electrical currents in any conductive layers. These currents produce induced magnetic fields, whose spatio-temporal pattern is diagnostic of Europa's internal conductivity structure. The main frequencies at which the imposed magnetic field changes at Europa are low order integer linear combinations of Jupiter's rotation (9.925 hour period) and Europa's orbit (85.228 hour period). Longer period oscillations penetrate more deeply, as they have more time to diffuse into the interior. At each forcing period, we define the admittance as the amplitude ratio (induced/imposed) of the associated magnetic fields. Each measurement of the admittance constrains the conductance (product of conductivity and thickness) of the main internal conductor, at the appropriate period. With measurements at multiple periods, a low frequency magnetic tomography inversion becomes possible. A spacecraft orbiting Europa could almost certainly constrain ice shell thickness, and both the depth extent and salinity of the ocean. The main challenge of a flyby version of that experiment is to have a sampling cadence which adequately separates the main signal frequencies. The magnetometer on Galileo has already detected an induction signal which is consistent with the presence of a salty ocean. However, the low number of flybys restricts the ability of that data-set to say more. We have simulated a time series of magnetometer measurements which could be acquired by the Europa Clipper mission concept, along a hypothetical planning trajectory, which would involve a total of 45 encounters with Europa. This trajectory has been designed to satisfy numerous instrument requirements, including that it encounter Europa at a variety of phases of the two main magnetic forcing frequencies. In our simulation, the orbit of the spacecraft, the orbit of Europa, the rotation of Jupiter, and the magnetic field of Jupiter are all assumed known. The main unknowns are the conductivity profile of Europa, and the essentially stochastic magnetic field fluctuations associated with plasma waves. The former is our main point of interest, and the latter is our main impediment. Our simulation starts with the optimistic assumption that the instrumental measurement error is the main error source. We thus ignore the plasma waves. In that scenario, the errors are uncorrelated. We then add various amounts of correlated error, with a range of correlation times, and examine how the recovered results degrade. Within the expected range of plasma noise errors, we expect to be able to recover admittance values at the 2 largest amplitude forcing periods.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.P53A1837B
- Keywords:
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- 6030 PLANETARY SCIENCES: COMETS AND SMALL BODIES Magnetic fields and magnetism;
- 6024 PLANETARY SCIENCES: COMETS AND SMALL BODIES Interiors