Spectral composition of geomagnetic field variations in the period range of 5 min to 2 hr as observed at the earth's surface

From geomagnetic records of sixteen observatories distributed about the earth, a spectral analysis of the geomagnetic field was prepared for each day of 1965, a year of relatively quiet solar‐terrestrial activity. Field amplitudes were found to increase almost linearly with periods in the 5 min to 2 hr range of study. The northward and eastward components of field are about equal at all latitudes with slightly larger vaues of H in the auroral and equatorial electrojet region. The vertical component of field is less than the others at all but the auroral zones. Seasonal changes in the amplitude appear at all locations but are more pronounced in the polar cap. Summer solstitial field strengths are about twice those of the winter period. On most quiet days of a month the amplitudes vary from about 0.5 to 0.1 times that of the monthly average levels. On most active days of the month the fields are between 4 to 10 times the size of the average monthly level amplitudes. The greatest variations in these levels occur at the equatorial and auroral zones. The lowest field values occur between 10° and 35° geomagnetic latitude with a minimum near 20° to 25°. Maximum values of the field are found at 65° to 75° geomagnetic latitude. Allowing for induced currents within the earth and for the global area of the distribution, about 91% of the magnetic energy is found between 60° and 80° geomagnetic latitude, about 4% in the higher latitude polar cap, and only 1% between the equator and 15°.