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A three-layer (Earth–air–ionosphere) physical model, as well asa two-layer (Earth–air) model, is employed in this paper to investigatethe ionospheric effect on the wave fields for a finite length dipole currentsource co-located at a hypocenter depth and along the main fault of anearthquake when the distance between the epicenter and an observing stationis up to 1000 km or even more. The results show that allelectrical fields are free of ionospheric effects for different frequenciesin a relative short range, e.g.,  ∼  300 km for f =  1 Hz,implying the ionospheric influence on electromagnetic fields can beneglected within this range, which becomes smaller as the frequencyincreases. However, the ionosphere can give a constructive interference tothe waves passing through and make them decay slowly when an observation isout of this range; moreover, the ionospheric effect can be up to 1–2 orders of magnitude ofthe electrical fields. For a ground-based observable 1.3 mV m−1 electric signal at f =  1 Hz 1440 km away from the WenchuanMS 8.0 earthquake, the expected seismo-telluric current magnitude forthe Earth–air–ionosphere model is of 5.0  ×  107A, 1magnitude smaller than the current value of 3.7  ×  108Aobtained by the Earth–air model free of ionospheric effects. This indicatesthat the ionosphere facilitates the electromagnetic wave propagation, as ifthe detectability of the system were improved effectively and it is easierto record a signal even for stations located at distances beyond theirdetectability thresholds. Furthermore, the radiating patterns of theelectrical field components |Ex| and |Ey| arecomplementary to each other, although any two-dimensional (2-D) power distribution ofthese componentsshows strong power areas as well as weak ones, which is advantageous toregister a signal if the observing system is designed to measure both ofthem instead of only one

solid earth

Ionospheric influence on the seismo-telluric current related to electromagnetic signals observed before the Wenchuan &amp;lt;i&amp;gt;M&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;S&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; 8.0 earthquake