  It is shown that the Magnetotelluric (MT) impedance tensor admits an
anti-symmetric generalized eigenvalue - eigenstate decomposition consistent
with the anti-symmetry of electric and magnetic fields referred to the same
coordinate frame: this is achieved by anti-diagonalization through rotation by
2x2 complex operators of the SU(2) rotation group. The eigenstates comprise
simple proportional relationships between linearly polarized eigenvalues of the
input magnetic and output electric field along the locally resistive and
conductive propagation path into the Earth, respectively mediated by the
maximum and minimum characteristic values of the tensor (eigen-impedances). It
is shown from first principles that the eigen-impedances are expected to be
positive real (passive) functions, analytic in the entire lower-half complex
frequency plane and with singularities confined on the positive imaginary
frequency axis. Insofar as the impedance tensor is generated by isometric
transformation of the eigen-impedances, it is also passive. The expected
passivity is an effective means of appraising measured tensors for compliance
with the basic tenets of the MT method: it can be violated only in the presence
of sources in the Earth. In addition to extrinsic effects (e.g. noise), it is
demonstrated with examples, that such sources may be secondary large or small
scale inductive phenomena generated by realistic conductivity configurations.
However, they may not be time-independent effects taking place in a passive
induction context, such as steady-state current channelling, galvanic
distortion and electric field reversals. In general, to assert whether
violation of passivity has occurred, it is necessary to decompose the impedance
tensor, refer it to its intrinsic coordinate frame and evaluate the compliance
of the eigen-impedances with their expected analytic properties
