Magnetotelluric source effect due to 3D ionospheric current systems using the complex image method for 1D conductivity structures

The complex image method (CIM) is an efficient tool to calculate the electr omagnetic field at the earth's surface produced by 3D ionospheric current s ystems when the earth has a layered conductivity structure. The calculation s are applicable to the estimation of source effects on magnetotelluric dat a. In this paper CIM is used in connection with some typical high-latitude ionospheric events: a westward travelling surge, a Harang discontinuity, an omega band, and a giant pulsation. The complicated ionospheric current sys tems are constructed of short horizontal current filaments with vertical cu rrents at both ends. The currents are given numerically on a 50 km x 50 km grid covering a region of even 1000 km x 2000 km. The investigations indica te that the source distortion very much depends on the event, and may be si gnificant in a wide period range, especially for a resistive earth structur e. The source effect seems quite unpredictable. Sometimes the apparent resi stivity is larger and sometimes smaller than the plane wave value. At times the source effect is very small even if the ionospheric current is strongl y inhomogeneous.