Low-frequency noise spectra of a superconducting magnetic gradiometer

Abstract

We present the first measurements of spectra that characterize noise in the frequency range 5×10−4 to 20 Hz of a superconducting magnetic gradiometer operating in a magnetically quiet environment. Spectra are effectively white at frequencies above about 0.1 Hz with a spectral power density of 0.03 (pT/m)2 Hz−1(1 pT/m=10−12 T/m=10−10 G/cm =10−3 γ/m) and are of the form S (f) =S (fo)(fo/f)γ at frequencies below 0.1 Hz. A value of 1.3 for γ with S (fo) =0.002 (pT/m)2 Hz−1 at fo=1Hz is representative of values observed during the quietest observational periods. Intrinsic noise of the superconducting quantum interference device (SQUID) then dominates observed spectra at frequencies below 0.1 Hz.

The Magnetic Field Gradiometer

“…= 7.0 X 10 -'° pT/m. This spatial variation of magnetic field is much lower than the noise level of 0.1-0.3 pT/m (1 Hz bandwidth) that has been measured at 1 lz with a sensitive single-axis superconducting gradiometer [Gillespie et al, 1977]. This calculation suggests that the displacement current source term can certainly be ignored in the air.…”

“…As pointed out by Gillespie et al [1977], the sensitivites reported by Zimmerman and Frederick [1971] and by Wynn et al [1975] do not take adequate account of thc variation of the gradiometer noise with frequency. Gillespie et al [1977] carried out a careful investigation of the noise characteristics of their gradiometer, which was ultimately used to measure the magnetic field gradients associated with internal waves in the sea [Podney and Sager, 1979], and they found that intrinsic noise of its SQUID dominated the measured noise spectrum on quiet days for frequencies less than 0. Superconducting gradiometers differ from all other gradiometers through their combination of high sensitivity and short baseline.…”

“…The former of these spatial variations would be easily measurable with a superconducting gradiometer, and the latter probably also could be measured at 1 Hz [Gillespie et al, 1977]. The sea water spatial variation is quite iarge, and if the computation were to include the electric fields generated by motions of the water in the presence of the earth's steady magnetic field it could well be considerably larger.…”