Secular variation of the geomagnetic φ-component dY/dt = dBφ/dt at the station Niemegk: the 1991 jerk seen by different model data sets with maximum degrees N = 8, N = 8(13), N = 8(15), N = 13, N = 15 top: on the Earth’s surface (RE) bottom: for the same (θ, φ) position on the CMB (Rc) where both possibilities of the downward continuation method are compared: harmonic (’har’, σ = 0) and non-harmonic (NHDC) (σ(r) ≠ 0). See also section 2.3.
