The magnitude and distribution of ambient fluctuations in the scalar magnetic field

Total magnetic field measurements were obtained at ocean floor depths averaging about 100 m over a period of several days. Ambient or background fluctuations are deduced from the data by three schemes: high pass filtering in the frequency domain, detrending in the time domain, and synchronous reference subtraction. For frequencies larger than 2(32) mHz, the fluctuations have a vanishing mean, and a standard deviation of 3.3(0.1) nT. Our analysis shows that the magnitude of the background fluctuations, which set the threshold in classical target detection, varies with the high pass frequency and the detrending interval. We discuss the corresponding threshold limitations. We find that the fluctuations, when scaled by their respective standard deviations, have a common probability distribution function. This distribution is neither Gaussian nor exponential but is closely approximated by a stretched exponential (i.e. ∼ e^-|η|γ) with γ < 1 for rms deviations 0.3 < η < 3. The frequency power spectra of the magnetic field measurements have f^δ power-law scaling with δ ∼ -2 for frequency 0.06 ≤ f ≤ 0.6 mHz and δ ∼ -11/3 for 1 < f < 10 mHz.