Tohoku University has provided search coil magnetometers for each PENGUIn AGO. Variations in magnetic fields are measured along three orthogonal components (N-S, E-W, and vertical) by individual coils which are buried at a depth of 1 m from the snow surface with a distance of at least 5 m between sensors. Each search coil has a linear frequency response from ~0.001 Hz to 2 Hz, and is equipped with a low pass filter with a cutoff frequency of 2 Hz. The output signals of each coil are sampled simultaneously at 0.5-s intervals and are digitized using a 12-bit A/D converter, providing a dynamic range of +/- 1.6 pT to +/- 3.2 nT at 1 Hz and +/- 160 pT to +/- 320 nT at 0.01 Hz.