Horizontal Field Components Research Articles

The role of the storm-time prompt penetrating electric field on the net distribution of plasma density over the low latitude ionospheric regions

The relative role of the prompt penetration of electric fields and neutral winds in the distribution of ionospheric plasma during geomagnetic storms over the Indian ionosphere region is studied in this paper. Four geomagnetic storms with similar onset times but of varying strengths have been analyzed with the help of data from ground-based magnetometers, and the ACE, GPS, and TIMED satellites. The strength and magnitude of the prompt penetration electric field were inferred from the magnetic field variations of the horizontal component of the magnetic field (ΔH) at Tirunelveli, a dip-equatorial station, using the ground-based magnetometers. The electron density distribution over the Indian ionospheric region was quantified using GPS-derived vertical total electron content (VTEC). Signatures of prompt penetration electric field were present for all the storms considered whose main phase was during the daytime. It is noted that as the strength of the penetration electric field increases, the latitudinal extent to which the electron density gets distributed also increases. The gradient in electron density extends even up to Shimla (a mid-latitude station) during a severe geomagnetic storm. The gradient was less for moderate geomagnetic storms. Since during the geomagnetic storm, a factor that controls electron density modulation is the neutral wind, its contribution is analyzed with the help of the thermospheric O/N2 ratio, measured by the global ultraviolet imager (GUVI) instrument, onboard NASA’s TIMED satellite. The observed increase in the O/N2 ratio does not correspond with the increase in the VTEC. We surmise that during the first day of the storm, the penetrating electric field is an essential factor that controls the electron density distribution in the low-latitude region. Later the plasma density distribution is controlled mainly by the thermospheric wind.

Motion Estimation for Complex Fluid Flows Using Helmholtz Decomposition

In this paper, we proposed a novel motion model with Helmholtz decomposition for complex fluid flows in a filtering-based optical flow framework, where the optimization of the regularization term is treated as a filtering process, and different motion patterns can be captured by finding appropriate filter kernels based on the designed filters. In this framework, we introduce a novel optical flow method with a joint spatial filter, which is based on the Helmholtz decomposition theorem that assumes a local motion field is composed of a curl field and a divergence field. By adjusting the scale of the weights in the filter kernels and combining a curl filter with a divergence filter in a certain ratio, it can simulate different motion patterns. In addition, if the correlation between the horizontal and vertical components of the optical flow field in the filter kernel is eliminated, it will be transformed into a linear motion model. Based on this linear motion model, we also develop a novel optical flow method with an adaptive guided filter. By finding an adaptive filter kernel driven by both the input image and the guided motion field for the designed filter, it can successfully capture different motion patterns, and yield an edge-preserving smoothing optical flow field. Most importantly, the proposed optical flow model provides a new way to design the regularizers for capturing different motion patterns in complex fluid flow. In particular, the designed optical flow method with an adaptive guided filter significantly outperforms the current state-of-the-art optical flow methods in predicting complex fluid flows.

Coastal Effect in the Horizontal Components of the Electric Field

The results of experimental studies of electric field disturbances generated by sources of an ionosphericorigin in the coastal zones of the White and Barents seas are presented. The data recording was performedby geophysical stations GI-MTS-1. The horizontal electric field was recorded by telluric lines ~50 mlong. The Ex component was set orthogonally to the coastline in the direction from the shore, and the Еycomponent was directed along the coastline. In the coastal strip of the sea–land contact, a zone of anomalousincrease in the amplitude of ULF variations (F 1 Hz) of the Ex component adjacent to the water’s edge wasrevealed. An additional narrow zone of anomalous amplification of the ULF variations of the electric field isobserved at a distance of ~30 m from the water’s edge. The coastal effect reaches its maximum near thewater’s edge in the coastal zone and decreases with distance from the seashore

New Concept Ocean-Bottom Multiphysics (OBMP) Nodes for Reservoir Monitoring

Marine-controlled source electromagnetics (CSEM) have been extensively applied to various exploration scenarios worldwide. However, its perceived value and cost relative to seismic and the scarcity of realistic case studies have limited the industry’s interest in time-lapse reservoir-monitoring (4D) applications. A feasible way to make demand for CSEM for 4D-monitoring programs would be to increase the value of information and reduce survey costs by performing joint operations where seismic and CSEM data are acquired during the same survey and at equivalent spatial densities. To this end, we propose a new multiphysics ocean-bottom nodes (OBN) concept and show the industry that CSEM can be a cost efficient and effective integrators to 4D seismic projects. To this end, we conducted a feasibility study demonstrating that horizontal magnetic field components have the required sensitivities and can be used instead of horizontal electric field components in mapping the 3D resistivity distribution and 4D fluid change responses in a given reservoir. This makes engineering a new OBN class simpler and cheaper, as various miniaturized magnetic field sensors are available off-the-shelf or readily working along with packaging and coupling solutions.

Robust global mantle flow trajectories and their validation via dynamic topography histories

SUMMARYThe ability to construct time-trajectories of mantle flow is crucial to move from studies of instantaneous to time-dependent earth models and to exploit geological constraints for mantle convection modelling. However mantle convection is chaotic and subject to the butterfly effect: the trajectories of two identical mantle convection models initialized with slightly different temperature fields diverge exponentially in time until they become uncorrelated. Because one may use seismic inferences about the mantle state as a starting or terminal condition to project mantle flow forward or backward in time, and because the seismic inference is invariably subject to uncertainties, this seemingly would rule out any construction of robust mantle flow trajectories. Here we build upon earlier work which showed that assimilation of the horizontal component of the surface velocity field from a known reference model allows one to overcome the butterfly effect and to construct robust mantle flow trajectories, regardless of the choice of the initial state perturbation. To this end, we use high resolution 3-D spherical mantle convection models in four end-member configurations: an isoviscous purely internally heated model, an isoviscous purely bottom heated model, a model with a radial increase in viscosity along with pure internal heating as well as a model that combines the effects of radial viscosity increase, internal and bottom heating. In order to capture the impact of seismic filtering, we perturb the initial temperature fields of these end-member models through either radial or horizontal smoothing of the temperature field or the application of the tomographic filter of seismic model S20RTS. We assess the quality of the constructed model trajectories via a number of statistical measures as well as comparisons of their dynamic topography histories. The latter is an essential step since mantle flow cannot be directly observed but has to be inferred via its surface manifestations. Importantly, linking mantle flow to surface observations yields patterns representable on a latitude–longitude grid similar to meteorological observables such as precipitation. This invites the application of meteorological quality metrics, such as the power ratio and Taylor diagram, to assess the quality of mantle flow trajectories. We introduce these metrics for the first time in the context of mantle convection and demonstrate their viability based on the compact manner in which they summarize model performance.

Ionospheric irregularities observed during the geomagnetic storm of March 2015 and June 2015 over Ethiopia

In this paper we studied the occurrence of ionospheric irregularities over Ethiopia as the consequence of the intense geomagnetic storm of March 2015 and June 2015. Total Electron Content (TEC) data obtained from six Ethiopian GPS reciever stations and those data were used to derive the ionospheric irregularities proxy indices, e.g.,rate of change of TEC (ROT) and ROT index (ROTI). These indices were used to characterize the intensity of the fluctuation of total electron content. ROTI was used as a criterion of ionospheric irregularities that took place during the storm. These indices were characterized along side with the disturbance storm time (Dst),the Interplanetary Electric Field (IEFy) and the SYM-H index (high resolution version of Dst) from ground-based magnetometers to see the effect of geomagnetic storm on horizontal component of geomagnetic field. Irregularities manifested in the form of fluctuations in TEC. Prompt penetration of electric field (PPEF) modulated the behaviour of irregularities during the initial and recovery phases of the geomagnetic storms. As a result,it was observed that the irregularities in the ionosphere of Ethiopia was in the range from weak to moderate. These irregularities generally occurred during the initial and recovery phases of the storm at all selected stations. The effect of electric field is found to depend on the local time at which the IMF Bz turned into southward. Therefore,east - west fluctuating Ey in March storm main phase is associated with suppression of irregularities in all stations and eastward Ey in the main phase of June 2015 is associated with small scale irregularity formation. The generation (inhibition) of irregularities is related to the effect of eastward (westward) storm time electric field disturbance dynamo electric fields and prompt penetration electric fields that created favourable (unfavourable) conditions for the generation of irregularities by uplifting (lowering) the F region.

Liouville-type results for the time-dependent three-dimensional (inviscid and viscous) water wave problem with an interface

Under consideration here are time-dependent three-dimensional stratified water flows of finite depth with a free surface and an interface (playing the role of an internal wave and separating two layers of constant and different densities). The main outcome of the paper is that, under the assumption that the vorticity vectors in the two layers are constant, non-vanishing and parallel to each other, we prove that bounded solutions to the three-dimensional equations are essentially two-dimensional. More precisely, the free surface, the interface, the pressure and the velocity field present no variations in the direction orthogonal to the direction of motion. In addition, one of the horizontal components of the velocity field is constant throughout the flow and the vorticity also points in the direction orthogonal to the wave propagation direction. The rigid lid case is analyzed from the perspective of relaxed assumptions concerning the vorticity vectors. We also consider the viscous water wave problem case with normal and tangential stress conditions at the free surface and find that under similar conditions on the vorticity vectors, the conclusion about the two-dimensionality remains in place. Along the way, we also provide an explicit solution for the time-dependent three-dimensional inviscid water wave problem with an interface.

An advanced detection method for unfavorable geological boulder based on electrical source in drill holes under shield machine

Abstract In recent years, due to the increasingly complex terrain, geological conditions, and working environments in engineering, the accuracy requirements in explorations have been continuously growing. In this study, with the goal of high-resolution prospecting for unfavorable geological boulder under a complex detection environment (such as subway shield machine tunnel). A tunnel model with an unfavorable geological boulder was constructed, and a time domain finite element method was adopted for 3D transient electromagnetic forward modeling. To realize high resolution in the positioning of small-scaled boulders, a detection method in complex environments was explored. A hole was drilled from the central point of the tunnel face toward the tunnel construction direction. Then, an electrical source was placed into the hole and array data was collected on the tunnel face. Electromagnetic sounding was achieved through the movement of the source, and the plane positions of the geological bodies were determined through the electromagnetic field distribution characteristics on the tunnel face. It was observed that the Ex and Ey (horizontal components of the electric field) had a higher resolution for a high resistance geological boulder. Therefore, the results indicated that in complex environments, collecting the horizontal components of the electric fields on the tunnel face excited by the electrical source within a drill hole could be a feasible method for fine explorations of a small-scale high resistance boulder.

Ionosphere response to geospace storm on 25 September 2016 over Kharkiv (Ukraine)

Instrumental observations of variations in the level of the geomagnetic field and ionospheric parameters were conducted during a weak geospace storm on September 25, 2016. It was found that a moderate magnetic storm was accompanied by variations in the level of the H component of the geomagnetic field up to 20–30 nT, as well as variations in the level of fluctuations of the horizontal components of the geomagnetic field up to ±(2–3) nT. It was revealed that the ionospheric storm was three-phase, mostly moderate. The decrease in the electron density reached a factor of 1.5, and its increase reached a factor of 2.2. The increase in the electron temperature during the negative phase of ionospheric storm did not exceed 200–300 K, while its increase during the positive phase of ionospheric storm reached 400–500 K. The ion temperature variations were within ±100 K. During the negative phases of the ionospheric storm, individual deviations in the vertical plasma drift velocity from those on a geomagnetically quiet day were observed both in the direction of decreasing the downward drift velocity (increasing the upward drift velocity at high altitudes) and vice versa. Significant (17–53 m/s) deviations in velocity variations towards positive values (decrease of downward and increase of upward drift velocity), accompanied by the rise of the F2 layer, after significant changing in the zonal component of the magnetospheric electric field (from –2 to 3.5 mV/m), contributed to the increase in the electron density and to the appearance of the positive phase of the ionospheric storm. During the positive phase, the increase in the velocity of downward plasma drift reached 9–20 m/s at altitudes above 450 km. The negative phases of the ionospheric storm were formed due to an increase in the electron and ion temperatures and a decrease in the O/N2 ratio during the storm. The generation of ionospheric storm positive phase at mid-latitudes apparently was caused by several mechanisms at once: a penetrating electric field, a meridional wind directed toward the equator, and downward plasma flows in the protonosphere. The phases of the ionospheric storm were accompanied by significant variations in the dynamic and thermal processes in the ionosphere.

A Comparative Study of Solar Active Region 12371 with Data-constrained and Data-driven Magnetohydrodynamic Simulations

We performed two data-based magnetohydrodynamic (MHD) simulations for solar active region 12371, which produced an M6.5 flare. The first simulation is a full data-driven simulation where the initial condition is given by a nonlinear force-free field (NLFFF). This NLFFF was extrapolated from photospheric magnetograms approximately 1 hr prior to the flare, and then a time-varying photospheric magnetic field is imposed at the bottom surface. The second simulation is also a data-driven simulation, but it stops driving at the bottom before the time of flare onset and then switches to the data-constrained simulation, where the horizontal component of the magnetic field varies according to an induction equation, while the normal component is fixed with time. Both simulations lead to an eruption, with both simulations producing highly twisted field lines before the eruption, which were not found in the NLFFF alone. After the eruption, the first simulation based on the time-varying photospheric magnetic field continues to produce sheared field lines after the flare without reproducing phenomena such as postflare loops. The second simulation reproduces the phenomena associated with flares well. However, in this case, the evolution of the bottom magnetic field is inconsistent with the evolution of the observed magnetic field. In this Letter, we report potential advantages and disadvantages in data-constrained and data-driven MHD simulations that need to be taken into consideration in future studies.

A Statistical Analysis of Magnetic Field Changes in the Photosphere during Solar Flares Using High-cadence Vector Magnetograms and Their Association with Flare Ribbons

We analyze high-cadence vector magnetograms (135 s) and flare-ribbon observations of 37 flares from the Solar Dynamics Observatory to understand the spatial and temporal properties of changes in the photospheric vector magnetic field and their relationship to footpoints of reconnected fields. Confirming previous studies, we find that the largest permanent changes in the horizontal field component lie near the polarity inversion line, whereas changes in the vertical field are less pronounced and are distributed throughout the active region. We find that pixels swept up by ribbons do not always exhibit permanent changes in the field. However, when they do, ribbon emission typically occurs several minutes before the start time of field changes. The changes in the properties of the field show no relation to the size of active regions, but are strongly related to the flare-ribbon properties such as ribbon magnetic flux and ribbon area. For the first time, we find that the duration of permanent changes in the field is strongly coupled with the duration of the flare, lasting on average 29% of the duration of the GOES flare. Our results suggest that changes in photospheric magnetic fields are caused by a combination of two scenarios: contraction of flare loops driven by magnetic reconnection and coronal implosion.

New Insights into the Anomaly Genesis of the Frequency Selection Method: Supported by Numerical Modeling and Case Studies

The frequency selection method (FSM) is the further development of the audio frequency telluric electricity method (TEFM). However, debate continues regarding the mechanisms leading to anomaly genesis. Therefore, the present study intends to explore this using 2D forward modeling of magnetotelluric (MT) sounding and practical applications of FSM on three Chinese case studies in karst and granitic settings. In the first stage, the profile curves and pseudo-section of apparent resistivity ( $${\rho }_{s}$$ ) and horizontal electric field component $${(E}_{y})$$ in MT mode are obtained by forward calculation. As a result, the static shift in $${\rho }_{s}$$ is observed over the near-surface inhomogeneities, as documented in the literature. Additionally, the profile curves of $${E}_{y}$$ showed an obvious static shift in the rectangular coordinate system (i.e., the curve rises with the increase in frequency) which is a well-known phenomenon. The pseudo-sections of $${E}_{y}$$ also showed static shift characteristics at the horizontal position above the anomaly, referred to as the “noodles phenomenon.” The FSM results were obtained from case studies related to groundwater and low-resistivity clay-filled karst body identification. The $$\Delta V$$ section curves and pseudo-section showed a significant low potential, and a “noodles phenomenon,” respectively, above the low-resistivity anomalous body. These abnormal characteristics of $$\Delta V$$ are the basis for delineating the horizontal position of the groundwater aquifer by applying FSM. It is concluded that the anomaly of FSM is the reflection of the static shift in MT, and hence, the FSM can be categorized as a “static shift method.” Therefore, this inspired us to investigate whether the static shift feature of the surface $${E}_{y}$$ component could be utilized to explore near-surface geological bodies such as clay-filled or water-filled cavities.

Effect of geomagnetic storms on l-band scintillation over Ilorin, Nigeria

A study on the effects of geomagnetic storms on l-band scintillation at Ilorin (latitude = 8.48 °N, longitude = 4.67 °E, Geomagnetic Latitude = 1.89 °S) is presented using newly available data. Three storms with minimum Dst<-100nT(i.e. the storm of April 24, 2012, the St. Patrick’s day storm of March 17, 2015 and the storm of October 13, 2016) were selected for the study based on the availability of data and the season of frequent occurrence of scintillation at Ilorin. The indices used are the ring current index (Dst), the auroral electroject activity index (AE), the horizontal component of the Interplanetary Magnetic Field (IMFBZ) and the ratio of the atomic to molecular neutral species. Results showed that the main phase of the April 24, 2012 caused little enhancement in scintillation during the pre-midnight period while scintillation was suppressed during the post-midnight period. However, scintillation was inhibited on the night of the main phase of the storm of March 17, 2015 and slightly inhibited on the night of the main phase of the storm of October 13, 2016. During the recovery phase, little enhancement was observed for the storm of October 13, 2016 while scintillation was inhibited for one day for the April, 24 2012 and for two successive days for the storm of March 17, 2015. The most interesting results was that of St. Patrick’s day storm of March 17, 2015 in which scintillation was inhibited for three successive days. The Prompt Penetration Electric Field which brings about enhancement of E × B lifting of ionization based on the local time of the minimum Dst is good enough to explain the observation during April 24, 2012 storm. Whereas, northward turning in IMFBZ values and decay in AE values which causes damping of E × B lifting of ionization were used in explaining observations during the storms of March 17, 2015 and October 13, 2016. In addition, significant reduction is seen in the ratio of the atomic to molecular neutral species especially during the storm of March 17, 2015 in which scintillation was inhibited during the main phase and the first two successive days of the recovery phase.

On regularity criteria for MHD system in anisotropic Lebesgue spaces

&lt;abstract&gt;&lt;p&gt;This paper concerns the regularity criteria of the three-dimensional magnetohydrodynamic (MHD) system in anisotropic Lebesgue spaces. Two regularity results were proved under additional assumptions on the horizontal components of the velocity field $ {{\bf{u}}} $ and the magnetic field $ {{\bf{B}}} $, or directions of Elsässer's variables $ {{\bf{u}}}\pm{{\bf{B}}} $.&lt;/p&gt;&lt;/abstract&gt;

Statistics of Unusual Naturally Occurring VLF Radio Emissions Termed Bursty‐Patches Observed at Kannuslehto, Finland

AbstractWe report the results from a statistical study of a new type of naturally occurring, but differently structured very low frequency (VLF) radio waves called “VLF bursty‐patches.” Their main characteristic is that they are detected at frequencies above the electron gyrofrequency (fce) in the equatorial plane for the L‐shell of observation. Here we use the data from the station of Kannuslehto, Finland (KAN, MLAT = 64.4°N, L = 5.5) with fce ∼ 5–6 kHz where VLF bursty‐patches are commonly detected on at least 43% of its campaign days. We analyzed all available data for 2017 (∼8 months) separating the observations by spectral type, periodicity, number of bursts, observed frequency, and accompanying usual VLF emissions at lower frequencies (f &lt; 6 kHz). We found that the overall highest occurrence rate is in the morning sector (05–12 MLT) correlating with general VLF occurrence rate at KAN (07–13 MLT, MLT = UT + 1.5 hr). Grouping usual VLF emissions and bursty‐patches showing similar spectral features, suggests that both groups are being generated by the same mechanisms (e.g., temperature anisotropy magnetospheric plasma distribution). However, unlike the situation for usual VLF emissions, the daily occurrence of bursty‐patches did not show a clear relationship with AE index. Superposed epoch analysis of magnetic components from the Sodankylä magnetometer (∼45 km from KAN) shows that VLF bursty‐patches are detected when the horizontal component of the magnetic field shows minimum variations. This suggests that propagation to the ground of bursty‐patches is conditioned to localized low variations of magnetic activity.

Tsallis’ Analysis of the Horizontal Component of the Earth’s Magnetic Field over India during 2002

A widely used measure of entropy to quantify uncertainity in an open system is the Boltzmann-Gibbs (B-G) entropy. It, however, cannot describe non-equilibrium systems with large variability and multi-fractality. A generalisation of the B - G entropy is the Tsallis’ entropy. The values of the horizontal components of the Earth’s magnetic field, observed at various stations in India in 2002 were used. During the years 2000 – 2002, solar cycle 23 reached its maximum. Data from Ettaiyapuram (ETT, latitude = 90 10’ N, longitude = 780 01’ E, geomagnetic latitude = 0.130 N), Visakhapatnam (VIS, 170 41’ N, 830 19’ E, 8.170 N), Hyderabad (HYD, 170 25’ N, 780 33’ E, 8.170 N), Alibag (ABG, 180 37’ N, 720 52’ E, 10.020 N) and Tirunelveli (TVI, 80 42’ N, 770 48’ E, 0.320 S) were used. Using these values as inputs, we demonstrate that Tsallis’ entropy can be used to detect minor differences in the horizontal components of the geomagnetic field observed between different pairs of stations. The method is a very simple and elegant one to detect minor variations between pairs of similar signals.

Large time behavior of solutions to the 3D anisotropic Navier–Stokes equation

We consider the large time behavior of the solution to the 3D Navier–Stokes equation with horizontal viscosity Δhu=∂12u+∂22u and show that the Lp decay rate of the horizontal components of the velocity field coincides to that of the 2D heat kernel, while the vertical component decays like the 3D heat kernel. Moreover, we consider the asymptotic expansion of the solution and find that a portion of the nonlinear term affect the leading term of the horizontal components of the velocity field, whereas the leading term of the vertical component is given by only the linear solution.

Reconfigurable polarization and pattern microstrip antenna for IRNSS band applications

In this article, polarization and pattern reconfigurable antenna is designed for 2.492 GHz Indian Regional Navigational Satellite System (IRNSS) band. The proposed antenna consists of a square radiating element fed with quad coaxial probes for polarization reconfigurability. It is surrounded by switch loaded octagonal parasitic ring for pattern beamwidth reconfigurability. RF switch loaded Electronically feeding network is used to fed the proposed antenna. The right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) is realized by exciting feed ports in the antenna with 90° phase difference. At the same time switch loaded parasitic ring provides the pattern beamwidth reconfigurability for designed circularly polarized antenna. To verify the proposed design, a prototype is fabricated and tested experimentally. The measured axial ratio is well-below 3 dB in polarization mode. The measured 3 dB beam-width of the elevation gain patterns for the horizontal field component (Gϕ) varies between 107.40°∼113.51° and 74°∼77.55° in pattern reconfigurable modes. The maximum gain of 6.46 dBi is achieved in both polarization and pattern reconfigurable mode. The proposed antenna is intended for the IRNSS navigation receiver system and designed to minimize the effect of interference incident on the antenna by narrowing the pattern beamwidth near the horizon.

Optimization of the Antenna Unit of the Superconducting Magnetometer: Research of Weak Magnetic Signals

Introduction. The article describes the optimization of the superconducting antenna design and balancing method of the superconducting antenna of magnetometer. This magnetometer is designed to study the magnetic properties of materials, as well as non-invasive study of the distribution of magnetic particles in organs of small laboratory animals, when toxic particles or drugs are introduced into the animal's body. The superconducting antenna of the magnetometer is a second-order axial gradiometer and is a part of the flux transformer of the SQUID sensor. For stable operation within the dynamic range of the magnetometer, the antenna must have a balance degree of at least 1/1000. Traditionally, it is balanced in a separate tuning module (Helmholtz coils), which creates a homogeneous magnetic field with a high degree of uniformity. The purpose. The problem is that the subsequent displacement of the SQUID magnetometer unit into the measuring complex, even within the laboratory, reduces the degree of balance of the magnetometer due to objects that create a strong gradient magnetic field, which negatively affects the measurements. Results. As a solution of this problem, the some approaches are described in the article. In order to increase the uniformity of the field inside the solenoid and eliminate field distortions at its edges, the using of solenoid with a special arrangement of turns to balance the antenna directly at the measurement site was proposed. The balancing elements for the horizontal components of the magnetic field were eliminated. Also the antenna balancing element in the form of a ring along the vertical component of the magnetic field, which does not violate the symmetry of the antenna was applied. Conclusions. Using this approach, it is possible to optimize the design of the antenna unit and the balancing process of the magnetometer antenna with a sufficient degree of balance in the vertical component directly in the geometric center of the measuring system, which reduced the time of preparing the system for work and improved the metrological indicators of the system. Keywords: superconductivity, SQUID-magnetometry, axial gradiometer, antenna balance, optimization.

Wideband and metal‐only linear to circular polarization converter using multi‐slot elements

A wideband, metal-only, and low-cost transmission-type polariser is proposed in which a plane wave with linear polarization impinging on the structure is converted to circular polarization wave on the other side and vice versa. The proposed structure is a quad-layer frequency selective surface that its unit cell is composed of two sets of multi-slot elements placed perpendicular to each other to achieve a wideband polariser. In this unit cell, by changing the length of the slots, the amplitude and phase of transmission coefficient can be controlled, which are used to create a 90-degree phase difference between vertical and horizontal components of the incident electric field. A 9 × 9 $9\times 9$ array of the proposed polariser with a quad-layer is simulated and fabricated that shows the bandwidth of 22% (11.7–14.7 GHz) for an axial ratio below −3 dB and a transmission efficiency of more than 85%.