Power spectra and coherence of the geoelectric field at Kakioka. Power spectra of the geoelectric field (a, b) and coherence between the geoelectric and geomagnetic fields (c, d) are shown. Spectra at shorter period bands were computed using 1-s values on 7 November 2004 (a, c), while those at longer period bands were computed using hourly values from 22 October 2004 to 24 March 2005 (b, d). The spectra related to E
x (red line) and E
y (blue line) are shown.