About the world-wide magnetic-background noise in the millihertz frequency range

Abstract

In the millihertz range, a single magnetometer can detect magnetic waves in the near-field regime. For such long wavelengths, it can measure the world-wide magnetic-background noise due to any charge displacement on Earth and within its environment. In this frequency band, the normal modes of the Earth's free oscillations exist and when excited, they shake the air column above them, up to the ionosphere where the moving charges emit a magnetic fluctuation, via Ampère's law. We show the magnetic-background noise spectrum obtained by an FFT analysis of 72 consecutive hours of magnetic-seismic calm. It is mostly due to vertical charge oscillations. Even in the absence of a quake larger than Mw= 5.2, spherical and toroidal modes are deected. Instrumental and analytical perspectives are discussed.

Floquet maser

“…However, the demonstrations of masers have remained unexplored in periodically driven (Floquet) systems ( 17 ), limiting the broad applications in sensing, spectroscopy, and fundamental physics. The generalization of masers to periodically driven systems would pave the way for many new applications, such as ultralow-frequency magnetic field sensing ( 18 , 19 ) and searching for oscillating electric dipole moments (EDMs) [see review in ( 20 , 21 )].…”

“…We note that the combination of maser techniques and Floquet systems can overcome the decoherence effect and thus permits a fresh look at many phenomena. For example, successful realization of maser using Floquet systems may open new opportunities for observing long-range temporal dynamics ( 22 , 23 ) and spectroscopy with submillihertz resolution, with important implications in quantum metrology ( 26 – 28 ), for example, in searches for gravitational waves in evolved Laser Interferometer Space Antenna (eLISA) with the bandwidth of 1 to 100 mHz ( 18 ), measurement of the worldwide magnetic-background noise (including attempts at earthquake prediction) ( 19 ), and axion dark matter searches ( 29 – 31 ). Despite these appealing features, a demonstration of masers based on Floquet systems was heretofore lacking.…”

Earth–ionosphere couplings, magnetic storms, seismic precursors and TLEs: Results and prospects of the [SQUID]2 system in the low-noise underground laboratory of Rustrel-Pays dʼApt

“…12. All the indexed peaks correspond, with a precision better than 1%, to a normal mode of the Earth from the PREM model [9]. In other terms the Earth-ionosphere interaction in this case preserves these excitation frequencies.…”

Suspending test masses in terrestrial millihertz gravitational-wave detectors: a case study with a magnetic assisted torsion pendulum

“…Ambient magnetic fields from geophysical and anthropogenic sources create fluctuations at the level of σ B ≈ 1 pT/Hz 1/2 in our frequency band [19]. Shielding can reduce magnetic noise to σ B 1 fT/Hz 1/2 [20].…”