[en] The daily values of the quiet time Dsub(st) field were subjected to power spectrum analysis. The analysis has brought out the submultiples of solar rotation period of the 27 day cycle and of the lunar semi-monthly wave of 14.7 days. The results are discussed in light of auto-correlation values, sector structure pattern and variation with the phases of the Moon. It is inferred that the Dsub(st) field is strongly coupled with the interplanetary medium. It is further proposed that the solar wind particles modulated by Moon's phases enter the magnetosphere on its night side along the geomagnetic tail and form the quiet time Dsub(st) field. The counter electrojet phenomena observed in H is attributed to Dsub(st) field. (author)