Accurate Field Measurements Research Articles

Spatiotemporal heterogeneity of street thermal environments and development of an optimised method to improve field measurement accuracy

Quantification of thermal environments in cities is of significance for revealing heat-related impacts and the performance of urban cooling measures, in which field measurement has been a prevailing method. However, the accuracy of field measurement is a critical issue, especially considering the spatial heterogeneity of urban spaces and its interactions with urban heat sources. Accordingly, the selection of fixed measurement points is of importance to accurately capture urban thermal environments, which has not been well considered in previous studies. Therefore, this study examines the heterogeneity of street thermal environments and explores an approach to improving field measurement accuracy. The study was empirically conducted in two typical streets in Chongqing, a hot-humid city in China. The results indicate that both microclimatic and thermal comfort indices could exhibit strong spatiotemporal heterogeneity, whilst they were measured in a same street. At a specific time, within a street, the largest air temperature (Ta), mean radiant temperature (MRT), physiological equivalent temperature (PET) and universal thermal climate index (UTCI) difference within a same street could reach 9.4 °C, 45.9 °C, 29.6 °C and 16.3 °C. Moreover, the measured Ta, MRT, PET and UTCI deviated from the average values which are used to represent street thermal environments and thermal comfort. Ta and UTCI converged towards the averaged values stronger than MRT and PET, while only 53.4% and 67.1% of the measured Ta and only 34.3% and 40.7% of measured UTCI fell into 95% accuracy zones. A framework of optimised multi-objective mobile measurement path was proposed, based on which the optimised measurement paths were obtained, and the measurement accuracy could be improved to above 98.0%. Overall, this study provides a reference for understanding spatiotemporal heterogeneity of street thermal environments and offers an approach to overcoming this issue for high accuracy.

우주과학임무를 위한 큐브위성 자기장 청결도 분석

CubeSat is a satellite platform that is widely used not only for earth observation but also for space exploration. CubeSat is also used in magnetic field investigation missions to observe space physics phenomena with various shape configurations of magnetometer instrument unit. In case of magnetic field measurement, the magnetometer instrument should be far away from the satellite body to minimize the magnetic disturbances from satellites. But the accommodation setting of the magnetometer instrument is limited due to the volume constraint of small satellites like a CubeSat. In this paper, we investigated that the magnetic field interference generated by the cube satellite was analyzed how much it can affect the reliability of magnetic field measurement. For this analysis, we used a reaction wheel and Torque rods which have relatively high-power consumption as major noise sources. The magnetic dipole moment of these parts was derived by the data sheet of the manufacturer. We have been confirmed that the effect of the residual moment of the magnetic torque located in the middle of the 3U cube satellite can reach 36,000 nT from the outermost end of the body of the CubeSat in a space without an external magnetic field. In the case of accurate magnetic field measurements of less than 1 nT, we found that the magnetometer should be at least 0.6 m away from the CubeSat body. We expect that this analysis method will be an important role of a magnetic cleanliness analysis when designing a CubeSat to carry out a magnetic field measurement.

Maximizing the Accuracy of Double Probe Electric Field Measurements Near Perigee: The Case of the Van Allen Probes Instruments

AbstractThe Van Allen Probes spacecraft flew through the inner radiation belt and the plasmasphere, probing the extremely dynamic coupling between the thermosphere, the ionosphere and the subauroral magnetosphere. Examining the electrodynamics of this region using Van Allen Probes data required that the ∼1 mV/m electric field (E‐field) of interest be measured in a region where the E‐field due to spacecraft motion was ∼100 mV/m. This means that the E‐field, the magnetic field, and the spacecraft velocity had to be measured to better than 1%. The instruments on board the Van Allen Probes have achieved this accuracy, delivering reliable near equatorial E‐field measurements even below three Earth radii. The objective of this commentary is to summarize the methodology developed over the years to maximize the accuracy and scientific return of these double probe E‐field measurements below L = 3.

Accurate Strain Field Measurement During Strip Rolling by Exploiting Recurring Material Motion with Time-Integrated Digital Image Correlation

Background95% Of all metals and alloys are processed using strip rolling, explaining the great number of existing strip rolling optimization models. Yet, an accurate in-situ full-field experimental measurement method of the deformation, velocity and strain fields of the strip in the deformation zone is lacking.ObjectiveHere, a novel time-Integrated Digital Image Correlation (t-IDIC) framework is proposed and validated that fully exploits the notion of continuous, recurring material motion during strip rolling.MethodsHigh strain accuracy and robustness against unavoidable light reflections and missing speckles is achieved by simultaneously correlating many (e.g. 200) image pairs in a single optimization step, i.e. each image pair is correlated with the same average global displacement field but is multiplied by a unique velocity corrector to account for differences in material velocity between image pairs.ResultsDemonstration on two different strip rolling experiments revealed previously inaccessible subtle changes in the deformation and strain fields due to minor variations in pre-deformation, elastic recovery, and geometrical irregularities. The influence of the work roll force and entry/exit strip tension has been investigated for strip rolling with an industrial pilot mill, which revealed unexpected non-horizontal material feed. This asymmetry was reduced by increasing the entry strip tension and rolling force, resulting in a more symmetric strain distribution, while increased distance between the neutral and entry point was found for a larger rolling force.ConclusionsThe proposed t-IDIC method allows for robust and accurate characterization of the strip’s full-field behavior of the deformation zone during rolling, revealing novel insights in the material behavior.

Forecasting bean yield losses under weed and Fusarium impacts from field plot statistical modeling

A better understanding of bean dynamic threats and pathogens is needed to improve accuracy of field measurement for breeding and mitigation programs. Thus, a two-year field plot study examined associations of bean production, Fusarium root rot, and weed in two cultivars sown under different dates and weed control treatments. We examined six standard curves to model bean leaf area, disease incidence, and weed biomass development across 256 plots. Gaussian, linear-by-linear and exponential models were fitted to bean-disease-weed development data, respectively, and then model parameters besides maximum values were used in the remainder of analyses. There were significant correlations between bean-disease-weed development descriptors. Principal component analysis showed bean-disease-weed development descriptors accounted for 84% of total data variance. Using six principal components as variables, a multivariate regression model showed the descriptors leaf area, Fusarium root rot, and weed biomass development explained 31% of variations in bean yield.

Development of low-cost inclination sensor based on MEMS accelerometers

Rapid development of Micro Electro Mechanical Systems (MEMS) and the minimization of sensor cost, size and energy consumption in the last two decades leads to an effort to replace traditional sensors with their MEMS alternatives. The power consumption is one of the key problems, due to necessity to provide long term device power supply. Therefore a newly developed device was designed with the accent to low power consumption, to be able to operate with one small internal battery at range several months to years. The main goal was to develop a wireless monitoring system capable of continuous stability monitoring of various building structures. The sensor is designed to measure slow inclination variations or changes and in combination with variant designed for high frequency monitoring should represent complete solution of real time structure health monitoring. The STATOTEST compact measurement system is mainly composed of triaxial MEMS accelerometer as a sensing unit, motherboard containing IOT modules and battery, all placed in single waterproof box. The raw signal measured by MEMS accelerometer is preprocessed inside the unit and the data are sent to the cloud via LoRaWan, NBIoT or satellite. The results can be displayed, managed and exported through the web application. This paper presents current state of sensor development, refer to number of problems, which were solved during the process and deals with estimation of its accuracy characteristics in the laboratory conditions. During the laboratory experiment, small defined changes of inclination were performed and compared with values registered by the inclination sensor. The testing was performed before and after calibration procedures. After eliminating of accelerometers production errors, the accuracy of the unit measurement RMSE is less than 0.002° for the step change of 0.09°, tested in six different orientations of the sens or. One measurement is mean of 1000 measurements and its residual random error for one measurement is 2°x10e-5. Series of laboratory tests proved high short-term device accuracy in stable conditions. It is well known, that MEMS accelerometers strongly depend on the sensor temperature. To perform temperature compensation, we built own climate chamber, which is able to change automatically temperature of the several devices at once in specified ranges. Temperature compensation was then performed by using of polynomial approximation to obtain the field measurement accuracy close to laboratory conditions. This task is challenging because it is necessary to improve the proper material composition between the MEMS and the monitored structure and the device fixing methods.

Experimental investigation of relative velocity field based on image rotation method in pump impeller

The continuous instantaneous flow field in pump impeller is vital for studying the mechanism of some complex flow phenomenon. In this paper, a high frequency PIV system with maximum sampling frequency 10 k Hz is built and used to measure the continuous flow field in impeller. In order to reduce the minimum sampling frequency required for PIV while ensuring the accuracy of flow field measurement, an image rotation method was proposed for calculating the relative velocity flow field in impeller. This method removed the particle displacement caused by impeller rotation in circumferential direction before the FFT calculation was performed. The relative velocity value calculated by velocity triangle method and image rotation method are compared and analyzed at different sampling line positions for different experimental conditions. The results show the image rotation method can not only obtain a more accurate velocity field near the blade area, but also has a lower requirement for the sampling frequency of the PIV system. The relative velocity error between two methods shows a trend of decreasing and then increasing when the sampling frequency decreases from 10 k Hz to 1 k Hz. Meanwhile, the relationship between the impeller speed and optimal sampling frequency range is established, which provides guidance for the selection of the appropriate PIV sampling frequency used for the measurement of the flow field in impeller.

Experimental and Numerical Study for Improved Understanding of Mixed-Convection Type of Flows in Turbine Casing Cavities During Shut-Down Regimes

Abstract Due to an increase in the power generation from renewable sources, steam and gas turbines will be required to adapt for more flexible operations with frequent start-ups and shut-downs to provide load leveling capacity. During shut-down regimes, mixed convection takes place with natural convection dominance depending on the operating conditions in turbine cavities. Buoyant flows inside the turbine that are responsible for nonuniform cooling leading to thermal stresses and compromise clearances directly limits the operational flexibility. Computational fluid dynamics (CFD) tools are required to predict the flow field during these regimes since direct measurements are extremely difficult to conduct due to the harsh operating conditions. Natural convection with the presence of cross-flow -mixed convection has not been extensively studied to provide detailed measurements. Since the literature lacks of research on such flows with real engine representative operating conditions for CFD validation, the confidence in numerical predictions is rather inadequate. This paper presents a novel experimental facility that has been designed and commissioned to perform very accurate unsteady temperature and flow field measurements in a simplified turbine casing geometry. The facility is capable of reproducing a wide range of Richardson, Grashof, and Reynolds numbers which are representative of engine realistic operating conditions. In addition, high fidelity, wall resolved large eddy simulation (LES) with dynamic Smagorinsky subgrid scale model has been performed. The flow field as well as heat transfer characteristics have been accurately captured with LES. Finally, the inadequacy of Reynolds-averaged Navier–Stokes (RANS) for the mixed type of flows has been highlighted.

Distortion correction of two-component two-dimensional PIV using a large imaging sensor with application to measurements of a turbulent boundary layer flow at $$\hbox {Re}_{\tau } = 2386$$

In the past decade, advances in electronics technology have made larger imaging sensors available to the experimental fluid mechanics community. These advancements have enabled the measurement of 2-component 2-dimensional (2C-2D) velocity fields using particle image velocimetry (PIV) with much higher spatial resolution than previously possible. However, due to the large size of the sensor, the lens distortion needs to be taken into account as it will now have a more significant effect on the measurement quality that must be corrected to ensure accurate high-fidelity 2C-2D velocity field measurements. In this paper, two dewarping models, a second-order rational function (R2) and a bicubic polynomial (P3) are investigated with regards to 2C-2D PIV measurements of a turbulent boundary layer (TBL) using a large imaging sensor. Two approaches are considered and compared: (i) dewarping the images prior to the PIV cross-correlation analysis and (ii) undertaking the PIV cross-correlation analysis using the original recorded distorted images then followed by using the mapping functions derived for image dewarping to provide the correct spatial location of the velocity measurement point. The results demonstrate that the use of P3 dewarping model to correct lens distortion yields better results than the R2 dewarping model. Furthermore, both approaches for the P3 dewarping model yield results which are statistically indistinguishable.

A handheld device for measuring the diameter at breast height of individual trees using laser ranging and deep-learning based image recognition

BackgroundAccurate and efficient measurement of the diameter at breast height (DBH) of individual trees is essential for forest inventories, ecological management, and carbon budget estimation. However, traditional diameter tapes are still the most widely used dendrometers in forest surveys, which makes DBH measurement time-consuming and labor-intensive. Automatic and easy-to-use devices for measuring DBH are highly anticipated in forest surveys. In this study, we present a handheld device for measuring the DBH of individual trees that uses digital cameras and laser ranging, allowing for an instant, automated, and contactless measurement of DBH.ResultsThe base hardware of this device is a digital camera and a laser rangefinder, which are used to take a picture of the targeted tree trunk and record the horizontal distance between the digital camera and the targeted tree, respectively. The core software is composed of lightweight convolutional neural networks (CNNs), which includes an attention-focused mechanism for detecting the tree trunk to log the number of pixels between the edges. We also calibrated the digital camera to correct the distortion introduced by the lens system, and obtained the normalized focal length. Parameters including the horizontal distance between the digital camera and the targeted tree, number of pixels between the edges of the tree trunk, and normalized focal length were used to calculate the DBH based on the principles of geometrical optics. The measured diameter values, and the longitudes and latitudes of the measurement sites, were recorded in a text file, which is convenient to export to external flash disks. The field measurement accuracy test showed that the BIAS of the newly developed device was − 1.78 mm, and no significant differences were found between the measured diameter values and the true values (measured by the conventional tape). Furthermore, compared with most other image-based instruments, our device showed higher measurement accuracy.ConclusionsThe newly developed handheld device realized efficient, accurate, instant, and non-contact measurements of DBH, and the CNNs were proven to be successful in the detection of the tree trunk in our research. We believe that the newly developed device can fulfill the precision requirement in forest surveys, and that the application of this device can improve the efficiency of DBH measurements in forest surveys.

Older Eastern White Pine Trees and Stands Accumulate Carbon for Many Decades and Maximize Cumulative Carbon

Pre-settlement New England was heavily forested, with trees exceeding 2 m in diameter. The forests have regrown since farm abandonment, representing what is arguably the most successful regional reforestation on record and identified recently in the “Global Safety Net.” Temperate “old-growth” forest and remnant stands demonstrate that native tree species can live several hundred years and continue to add to forest biomass and structural and ecological complexity. Forests globally are an essential natural climate solution that accumulate carbon and reduce annual increases in atmospheric CO2by approximately 30%. Some studies emphasize young, fast-growing trees and forests while others highlight carbon storage and accumulation in old trees and intact forests. We addressed this directly within New England with long-term, accurate field measurements and volume modeling of individual trees and two stands of eastern white pines (Pinaceae:Pinus strobus) and compared our results to models developed by the U.S. Forest Service. Within this sample and species, our major findings complement and clarify previous findings and are threefold: (1) beyond 80 years, an intact eastern white pine forest can accumulate carbon above-ground in living trees at a high rate and double the carbon stored in this compartment in subsequent years; (2) large trees dominate above-ground carbon and can continue to accumulate carbon; (3) productive stands can continue to accumulate high amounts of carbon in live trees for well over 150 years. Because the next decades are critical in addressing the climate emergency, and most New England forests are less than 100 years old, a major implication of this work is that maintaining and accumulating carbon in some existing forests—proforestation—is a powerful regional climate solution. Furthermore, older and old-growth trees and forests are rare, complex, highly dynamic and biodiverse: dedication of some forests to proforestation will produce large carbon-dense trees and also protect ecosystem integrity, special habitats, and native biodiversity long-term. In sum, strategic policies to grow and protect suitable existing forests in New England will optimize a proven, low cost, natural climate solution that also protects and restores biodiversity across the landscape.

Clinical staging of accidental hypothermia: The Revised Swiss System: Recommendation of the International Commission for Mountain Emergency Medicine (ICAR MedCom)

Clinical staging of accidental hypothermia is used to guide out-of-hospital treatment and transport decisions. Most clinical systems utilize core temperature, by measurement or estimation, to stage hypothermia, despite the challenge of obtaining accurate field measurements. Recent studies have demonstrated that field estimation of core temperature is imprecise.We propose a revision of the original Swiss Staging system. The revised system uses the risk of cardiac arrest, instead of core temperature, to determine the staging level. Our revised system simplifies assessment by using the level of responsiveness, based on the AVPU scale, and by removing shivering as a stage-defining sign.

Two-dimensional dose distribution measurement based on rotational optical fiber array: A Monte Carlo simulation study

Quality assurance is particularly important for modulated radiotherapies which improve dose conformity and involve complex processes of dose delivery. This work proposes a dose measurement method based on a rotational optical fiber array and the Cherenkov effect as an alternative pretreatment quality assurance method. Monte Carlo simulation toolkit Geant4 is used to study the accuracy and influencing factors of this method. Four different shapes of radiation field are designed to explore the influence of different reconstruction algorithms, fiber spacing, rotation angles, and radiation types on the accuracy of dose field measurement. The results show that the difference between the reconstructed dose field and the reference dose field is within 8% under the irradiation of uniform radiation field. To obtain high-precision reconstructed images, the fiber spacing should be within 2 mm, and at least 60 angles of projection data should be acquired when the fiber array is rotated 180°. In conclusion, the proposed new method using a rotating fiber array can reconstruct the radiation field distribution rapidly, and provides accurate information on the shape and intensity distribution of the radiation field. Further studies are needed to improve the accuracy and feasibility in real clinical practice.

Detection of the Inductive Magnetic Field of Submarine with Upper Arc Sensor Array based on Integral Equation Method

In order to effectively solve the problem of the accuracy of the magnetic field measurement of submarine upper vault sensor, an integral equation method based on the magnetic field detection method of submarine upper vault was proposed. By analyzing the simulation data of the dome magnetic sensor, the analysis solution and the integral equation solution of the ellipsoid shell of the ship model are compared, so as to verify the accuracy of the integral equation method and provide a reference for the magnetic field measurement accuracy of the submarine upper part.

Accuracy of polarization field measurements by electron holography in InGaN quantum wells

We thoroughly investigate two methods for estimating electric field strength across semiconductor heterostructure quantum wells from electron holograms. Due to the small width of quantum wells these holograms must often be taken under dynamical diffraction conditions, since projection effects forbid the investigation under larger tilt angles. We especially investigated the robustness of the evaluation methods against dynamical diffraction artifacts due to excitation conditions and strain, artifacts due to the material contrast, as well as limited resolution effects. For this we developed ways to incorporate polarization effects into combined strain and multislice simulations. It turns out that errors estimated from detection noise of single measurements alone mostly underestimate the true error, since variation due to hardly controlled diffraction effects and measurement details often prevails.

Infrared diagnostics of the solar magnetic field with Mg I 12 μm lines: forward-model results

Context. The Mg I 12.32 and 12.22 μm lines are a pair of emission lines that present a great advantage for accurate solar magnetic field measurement. They potentially contribute to the diagnosis of solar atmospheric parameters through their high magnetic sensitivity. Aims. The goal of this study is to understand the radiation transfer process of these lines in detail and explore the ability of magnetic field diagnosis in the infrared. Methods. We calculated the Stokes profiles and response functions of the two Mg I 12 μm lines based on one-dimensional solar atmospheric models using the Rybicki-Hummer (RH) radiative transfer code. The integration of these profiles with respect to the wavelength was used to generate calibration curves related to the longitudinal and transverse fields. The traditional single-wavelength calibration curve based on the weak-field approximation was also tested to determine if it is suitable for the infrared. Results. The 12.32 μm line is more suitable for a magnetic field diagnosis because its relative emission intensity and polarization signal are stronger than that of the 12.22 μm line. The result from the response functions illustrates that the derived magnetic field and velocity with 12.32 μm line mainly originate from the height of 450 km, while that for the temperature is about 490 km. The calibration curves obtained by the wavelength-integrated method show a nonlinear distribution. For the Mg I 12.32 μm line, the longitudinal (transverse) field can be effectively inferred from Stokes V/I (Q/I and U/I) in the linear range below ∼600 G (∼3000 G) in quiet regions and below ∼400 G (∼1200 G) in penumbrae. Within the given linear range, the method is a supplement to the magnetic field calibration when the Zeeman components are incompletely split.

Particle Image Velocimetry (PIV) experiment of the buoyant flow field of a thermal chimney model designed for geothermal power plants

ABSTRACT To enhance the air-cooling process in geothermal power plants for economical utilization of the exhaust steam from expansion, a natural-draft thermal chimney design was proposed and studied here in this paper. In view of the necessity of accurate velocity field measurements which would provide further insight into the physics behind the evolving plumes above heated horizontal cylinders, Particle Image Velocimetry (PIV) was employed to experimentally investigate the buoyant flow in the thermal chimney system. Two configurations have been tested to understand the flow induced by the horizontally heated cylinders inside the thermal chimney. Firstly, flow field above a single row of cylinders was tested while they were isothermally heated to simulate of an air-cooled condenser. After that, a second row of cylindrical heaters (air-heater) was added above the first row to enhance the buoyant flow, aiming at enhancing the air side flow of the air-cooled condenser. Flow characteristics and velocity enhancement were studied for both configurations. The results show that significant flow unsteadiness occur near the cylindrical heaters because of the non-steady crossing flows between adjacent cylinders, and the unsteadiness attenuates in the downstream. The effects of cylinder row distance, surface temperature as well as downstream distance on the flow field were then analyzed. Flow velocity is increased by the air-heater as the buoyancy force is enhanced, proving the idea of flow enhancement of the thermal chimney configuration. It is also observed that the velocity fluctuation, turbulent kinetic energy and vorticity change significantly after adding the second row of heaters. The present study provides further insight into natural convection flow theory of heated cylinders for a Rayleigh number range of 1.3E4 to 2.2E4, which is fundamental for the flow enhancement designing of the proposed natural-convection-driven cooling system.

Using Historical Data to Examine the Accuracy of Sand Transport Field Measurements in Two Nearshore Marine Settings

Amos, C.L.; Kassem, H.; Townend, I.; Umgiesser, G.; Madricardo, F.; Zaggia, L.; Manfe, G.; Lorenzetti, G., and Gomez, E., 2020. Using historical data to examine the accuracy of sand transport field measurements in two nearshore marine settings. Journal of Coastal Research, 36(5), 1013–1028. Coconut Creek (Florida), ISSN 0749-0208.A re-analysis of historical data from two field campaigns was undertaken to examine the accuracy of measurements of bed load (Qb) and total transport (Qtot) of sand in (1) a wave-dominant shoreface off western Newfoundland, Canada, and (2) a tide-dominant inlet of Venice Lagoon, Italy. Video tapes recorded within Sea Carousel (a benthic annular flume) deployed off Newfoundland were used to determine the transport of medium to coarse sand under controlled unidirectional flow conditions. These results were compared with Helley-Smith sand trap measurements of bed load of fine to medium sand in a tidal inlet of Venice Lagoon, Italy. Ripple migration rates in Sea Carousel were similar to those measured in rivers and shallow marine settings at similar flows. Accuracy of sand transport rate (derived from ripple motion) was assessed by comparison to fundamental methods presented in the literature. Some of the scatter in correlations with earlier methods was removed by using a nondimensional form of total sand transport and correlating it to excess stream power (i.e. above a traction threshold). Better correlations were found between immersed (bed load) transport rate and excess stream power by applying a published adjustment to the observations for flow depth and grain diameter. Total immersed (normalized) sand transport () in Sea Carousel correlated with excess stream power in a fashion similar to results reported in the literature: = 0.288(ω – ωcr)1.65 kg m–1 s–1, where the immersed total sand transport is normalized with respect to flow depth and grain diameter. The sand trap data also followed this fit in part (2006 data only) but demonstrated greater scatter. The data herein thus fell in line with those reported in the literature from a wide variety of flume and field settings and for a wide variety of grain sizes. It is concluded that annular benthic flumes offer a reasonable and reliable method of assessing sand transport under controlled conditions of flow. The results from Sea Carousel and the Helley-Smith traps appear to follow the same relationships and so appear compatible. However, benthic sand traps show a higher degree of scatter, perhaps due to the uncertainties in how they sit on the seabed, and due to the arbitrary conditions of flow to which they are subjected when deployed.

Magnetometer in-flight offset accuracy for the BepiColombo spacecraft

Abstract. Recently the two-spacecraft mission BepiColombo launched to explore the plasma and magnetic field environment of Mercury. Both spacecraft, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO, also referred to as Mio), are equipped with fluxgate magnetometers, which have proven to be well-suited to measure the magnetic field in space with high precision. Nevertheless, accurate magnetic field measurements require proper in-flight calibration. In particular the magnetometer offset, which relates relative fluxgate readings into an absolute value, needs to be determined with high accuracy. Usually, the offsets are evaluated from observations of Alfvénic fluctuations in the pristine solar wind, if those are available. An alternative offset determination method, which is based on the observation of highly compressional fluctuations instead of incompressible Alfvénic fluctuations, is the so-called mirror mode technique. To evaluate the method performance in the Hermean environment, we analyze four years of MESSENGER (MErcury Surface, Space ENvironment, GEophysics and Ranging) magnetometer data, which are calibrated by the Alfvénic fluctuation method, and compare it with the accuracy and error of the offsets determined by the mirror mode method in different plasma environments around Mercury. We show that the mirror mode method yields the same offset estimates and thereby confirms its applicability. Furthermore, we evaluate the spacecraft observation time within different regions necessary to obtain reliable offset estimates. Although the lowest percentage of strong compressional fluctuations are observed in the solar wind, this region is most suitable for an accurate offset determination with the mirror mode method. 132 h of solar wind data are sufficient to determine the offset to within 0.5 nT, while thousands of hours are necessary to reach this accuracy in the magnetosheath or within the magnetosphere. We conclude that in the solar wind the mirror mode method might be a good complementary approach to the Alfvénic fluctuation method to determine the (spin-axis) offset of the Mio magnetometer.

Highly sensitive magnetic field microsensor based on direct laser writing of fiber-tip optofluidic Fabry–Pérot cavity

In this paper, a polymer micro-Fabry–Pérot interferometer (FPI) fabricated via direct laser writing using two-photon polymerization techniques on a single mode fiber tip is proposed, designed, simulated, and experimentally demonstrated as a magnetic field probe. The sensor comprises a tapered waveguide with a length of 100 µm and a diameter of 1 µm along the axis connecting the two reflecting surfaces of the micro-FPI open cavity. The cavity is filled with a magnetic fluid (MF) changing the evanescent coupling and consequently leading to a change in the phase of the FPI. To improve the signal reflection, a thin layer of Au is coated on the device before fabricating the probe sensor, which is then sealed in a MF-filled glass tube. The experimental results show that the proposed probe sensor has a low temperature sensitivity and a sensitivity to magnetic field as high as 1.54 nm/mT, and the magnetic field measurement accuracy is ∼649.4 μT within a range from 1 mT to 8 mT. The microsensor, which is very stable and easy to fabricate, can be used as a probe to detect weak magnetic fields.