Mechanical Magnetic Field Generator for Communication in the ULF Range

We present results of analysis of lightning observations and satellite measurements of ionospheric magnetic field fluctuations in the ultralow frequency (ULF) range. The fluctuations are measured by Swarm satellites, while the lightning observations are provided by the Geostationary Lightning Mapper (GLM) and the World ELF Radiolocation Array (WERA). We identify spatio‐temporal relationships that demonstrate a leakage of electromagnetic fluctuations caused by lightning into the upper ionosphere. Causal association between the two phenomena is evidenced by investigating relations between lightning and fluctuation properties. The presented results suggest that lightning generate ULF fluctuations in the ionosphere that can be detected by satellites, if the lightning‐satellite geographic distance is less than ∼5°. Typical properties of the fluctuations caused by lightning are described in the paper. To our knowledge, this is the first direct experimental confirmation of a link between lightning and magnetic field fluctuations in the upper ionosphere in the ULF range.

The bimodal quarter-wave impedance matching theory, with which an incident longitudinal (transverse) wave can be completely converted to a transmitted transverse (longitudinal) wave, requires that the matching element must exhibit specific anisotropy. Previously, the specific anisotropy was satisfied between components of the stiffness tensor, and the phenomenon was only realized in the ultrasonic frequency range. In this work, we find that such anisotropy can also be satisfied between components of the mass density tensor, which allows an ultralow frequency realization. Meanwhile, the stiffness should also exhibit special anisotropy. To meet such unique anisotropy, we propose to design ternary locally resonant metamaterials. The dipolar local resonance around the lowest bandgap allows us to deal with the effective stiffness and mass density separately. The requirement on stiffness is satisfied by designing the matrix, and the mass anisotropy is realized through design of the coating layer. With the designed metamaterials, the matching elements can convert wave modes, which have a wavelength much larger than the element’s width. Considering that mode conversion is a fundamental phenomenon in the elastic field, our finds and design can be critically useful to extend its application in the ultralow frequency range.

There have been reported several papers on the ionospheric F region perturbations prior to the 2008 Sichuan earthquake (EQ) (magnitude 8.0), but it seems that very few reports have been published on the characteristics of ground-based ULF (ultra low frequency) magnetic field variations for this EQ. This paper deals with two different aspects of ground-based ULF magnetic field variations: 1) ULF radiation from the lithosphere, and 2) depression of ULF horizontal magnetic field as a signature of lower ionospheric perturbations. ULF data from two Chinese stations [Chengdu (epicentral distance, 80 km) and Xichang (about 300 km away from the EQ epicenter)] are analyzed, with paying attention to the local nighttime period (LT = 22 h to 02 h, UTC = 14 h to 18 h) in order to avoid man-made noise. We have analyzed powers of the horizontal component (H2), vertical component (Z2), polarization as their ratio (Z2/H2), depression of the horizontal component (as an inverse of horizontal magnetic field component power, 1/H2) and δDep as a variation of depression at a particular frequency of 0.01 - 0.02 Hz (10 - 20 mHz). It is then found that there seems no clear signature of lithospheric ULF radiation. Whilst, the most evident fact is the finding of depression of ULF horizontal magnetic field at Chengdu a few days before the Sichuan EQ, which suggests that the lower ionosphere was perturbed before the EQ. The characteristics of the lower ionospheric perturbations are compared with those of upper ionospheric perturbations reported before.

Natural time analysis on the ultra-low frequency magnetic field variations prior to the 2016 Kumamoto (Japan) earthquakes

This paper reports on the analysis of dispersion in the imaginary part of impedance often observed at low frequencies in a variety of systems. The experimental data were obtained with an electrolytic cell containing KCl aqueous solution in the frequency range from 0.1 mHz to 10 MHz, where the use of ultra-low frequencies helps clarify the analysis of the imaginary impedance dispersion. It is shown that the low frequency dispersion described in the literature is the tail of a relaxation peak located at f ≅ 20 mHz. This ultra-low frequency dispersion peak is analyzed with a Cole-Cole impedance element, being associated with the electric double layer at the metal-electrolyte interface. Quantitative information can be extracted for the double layer, including its thickness (∼1 nm) and electrical resistivity (∼50 GΩm).

Theoretical and experimental investigations on semi-active quasi-zero-stiffness dynamic vibration absorber

The source characteristics of the Saturn F-1 engine in the infrasonic frequency range of 0.1–20 Hz and the seismic and magnetic fluctuations from 4 to 16 Hz have been ascertained. The acoustic signals have shown a characteristic ignition and cutoff pulse. The ignition signal shows a typical explosion signature (N-type wave), while the cutoff signal produces a bubble pulse-type effect, which might have resulted from an implosion structure. The engine run produces a steady state signal. However, its acoustic energy peak in the ultralow frequency range is a function of the engine-run duration. The power spectrum shows energy distribution in two regions, with a maxima at 4–8 Hz and at approximately 1.4 Hz, with energy extending to about 0.1 Hz. The lower frequency peak becomes more significant with the increase in the engine-run duration. The magnetic signals have shown a narrow-band signal between 8 and 12 Hz during the engine run and a high-intensity cutoff signal of a wider band. The cutoff signal resembles a pulse. The downward exhaust of the static running of a rocket motor is a source for ultralow frequency acoustic and magnetic waves. Similar conclusions are drawn for the behavior of an Atlas rocket during the first few kilometers of flight.

Ultra-low frequency active vibration isolation in high precision equipment with electromagnetic suspension: Analysis and experiment

It is generally accepted that the magnetic fields of planets and stars are produced by the self-exciting dynamo process (first proposed by Larmor) and that observed near-alignments of magnetic dipole axes with rotation axes are due to the influence of Coriolis forces on underlying fluid motions. The detailed role of rotation in the generation of cosmical magnetic fields has yet to be elucidated but useful insight can be obtained from general considerations of the governing magnetohydrodynamic equations. A magnetic field B cannot be maintained or amplified by fluid motion u against the effects of ohmic decay unless (a) the magnetic Reynolds number R = ULno is sufficiently large, and (b) the patterns of B and u are sufficiently complicated (where U is a characteristic flow speed, a characteristic length and J and o are typical values of the magnetic permeability and electrical conductivity respectively). Axisymmetric magnetic fields will always decay (a result that suggests that palaeomagnetic and archaeomagnetic data might show evidence that departures from axial symmetry in the geomagnetic field are systematically less during the decay phase of a polarity ‘ reversal ’ or * excursion ’ than during the recovery phase). Dynamo action is stimulated by Coriolis forces, which promote departures from axial symmetry in the pattern of uwhen B is weak, and is opposed by Lorentz forces, which increase in influence as B grows in strength. If equilibrium is attained when Coriolis and Lorentz forces are roughly equal in magnitude then the system becomes ‘ magnetostrophic' and the strengths of the internal and external parts of the field, and respectively, satisfy B i < B 8 R 1/2 and B e < B 8 R -1/2 if B 8 = UL~1 *X)/c f)i » (p being the mean density of the fluid and Q the angular speed of rotation). The slow and dispersive ‘magnetohydrodynamic inertial wave’ with a frequency that depends on the square of the Alfven speed [B]/(up) 1/2 and inversely on Q exemplifies magnetostrophic flow. Such waves probably occur in the electrically conducting fluid interiors of planets and stars, where they play an important role in the generation of magnetic fields as well as in other processes, such as the topographic coupling between the Earth’s liquid core and solid mantle.

A new phase in the vortex solid region in Bi2Sr2CaCu2O8

A novel photonic scheme for the simultaneous generation of microwave amplitude shift keying (ASK) and frequency shift keying (FSK) signal based on gain switching in Vertical Cavity Surface Emitting Laser (VCSEL) is proposed, for the first time. In this approach, gain switched VCSEL is utilized to produce optical frequency comb (OFC) and the linearly polarised OFC signal is splitted into two orthogonal polarizations using polarisation controller and polarisation beam splitter. Two electro absorption modulators (EAM) are employed to modulate each polarization by 1Gbps data. Microwave FSK signals at 2/4 GHz and 6/8 GHz with 1 Gbps data are detected at the photodetector. Moreover, microwave ASK signals are also generated from each polarization branch at 2 GHz and 4 GHz. Simulations are performed to substantiate the proposed system. The influence of VCSEL modulating signal power and EAM reverse bias on FSK output power is also discussed. Results predict a maximum sideband suppression ratio of 33 dB, for a modulating signal current and EAM reverse bias voltage of 15 mA and 1.5 V respectively. The receiver sensitivity of the FSK signal at 2/4 GHz for a 20 km fibre transmission is found to be −16.3 dBm with a Q factor of 6, at a BER of 10−9. This scheme offers a simple, cost-effective approach and requires a low modulating current and its applications include radar, wireless and optical communications.

In Earth's inner magnetosphere, electromagnetic waves in the ultralow frequency (ULF) range play an important role in accelerating and diffusing charged particles via drift resonance. In conventional drift resonance theory, linearization is applied under the assumption of weak wave‐particle energy exchange so particle trajectories are unperturbed. For ULF waves with larger amplitudes and/or durations, however, the conventional theory becomes inaccurate since particle trajectories are strongly perturbed. Here we extend the drift resonance theory into a nonlinear regime, to formulate nonlinear trapping of particles in a wave‐carried potential well, and predict the corresponding observable signatures such as rolled‐up structures in particle energy spectrum. After considering how this manifests in particle data with finite energy resolution, we compare the predicted signatures with Van Allen Probes observations. Their good agreement provides the first observational evidence for the occurrence of nonlinear drift resonance, highlighting the importance of nonlinear effects in magnetospheric particle dynamics under ULF waves.

Excitation of electromagnetic fields at extremely low and lower frequencies in the near-field zone of the Earth–ionosphere waveguide is considered. Variations in the field amplitude in the lower part of the extremely low frequency (ELF) range and lower frequencies are experimentally determined under different geophysical conditions in the absence of variations in the ultralow frequency (ULF) range. The effects related to such variations are analyzed with the aid of theoretical calculations that show significant effect of ionosphere in the near-field zone at relatively low terrestrial conduction.

Low-Frequency (LF) Magneto-Inductive (MI) communication, used in LF Radio-Frequency Identification (RFID) systems, is very well suited for indoor positioning applications since LF magnetic fields are not greatly affected by the environment. However, LF magnetic fields may be distorted by ferromagnetic objects or get affected by LF Electromagnetic Interferences (EMI), which affects the accuracy and reliability of LF-RFID based positioning systems. In this paper, we consider an indoor positioning technique based on the LF (125 kHz) MI communication and study the challenges faced by this technique because of ferromagnetic objects. We analyze the results based on experiments and simulations.

This brief proposes an amplitude shift keying (ASK) demodulator that uses switched-capacitor differentiators to make it compliant with the very high bit rate amendment to the ISO/IEC 14443 standard for contactless smart card applications. These differentiators detect transitions in modulated ASK signals with a carrier frequency of 13.56 MHz at data rates up to 6.78 Mb/s. The demodulator has been implemented in 0.18 μm CMOS technology. The total power consumption is under 350 μW. Measured results confirm correct operation, and it is further shown that this differentiating scheme allows the modulation index to be reduced to 2.56%.