Europa's magnetic field is of interest because of the information it can provide about the composition beneath the surface.
When analyzing Europa, scientists are looking for internally generated fields. Based on what is observed they can narrow the various possibilites for Europa's composition. For instance, an intrinsic dipole moment would require either a large amount of ferromagnetic material or a self-generated dynamo action (such as a deep conducting fluid with convective motion).
Another magnetic field of interest is that caused by the time-varying aspect of Jupiter's tilted dipole moment (the time variance is cause by Jupiter's rotation; like earth, Jupiter's magnetic dipole axis is not the same as its rotational axis). This temporal variation creates electric currents within Europa, which produce magnetic pertubations. These magnetic pertubations are called induced fields. Induced fields would require large global-scale conducting paths near the surface. For icy moons, the most likely agent of this is salty subsurface oceans.
Unfortunately, finding these magnetic fields is not as simple a task as first percieved. Europa is a moon of Jupiter, the most hostile place in the solar system short of the sun. Besides the vast technical difficulties of just getting a spacecraft to survive in the area, Jupiter's magnetosphere is composed of flowing plasma in which its moons are embedded.
This plasma interacts with the planets to create localized external current, which in turn creates magnetic pertubations that mask the internal fields. In order to draw conclusion from any data, scientists must be able to discern the internal fields from the external.
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