Measurement Of Magnetic Field Strength Research Articles

Unveiling complex magnetic field configurations in red giant stars

The recent measurement of magnetic field strength inside the radiative interior of red giant stars has opened the way toward full 3D characterization of the geometry of stable large-scale magnetic fields. However, current measurements, which are limited to dipolar $( mixed modes, do not properly constrain the topology of magnetic fields due to degeneracies on the observed magnetic field signature on such $ mode frequencies. Efforts focused toward unambiguous detections of magnetic field configurations are now key to better understand angular momentum transport in stars. We investigated the detectability of complex magnetic field topologies (such as the ones observed at the surface of stars with a radiative envelope with spectropolarimetry) inside the radiative interior of red giants. We focused on a field composed of a combination of a dipole and a quadrupole (quadrudipole) and on an offset field. We explored the potential of probing such magnetic field topologies from a combined measurement of magnetic signatures on $ and quadrupolar ($ mixed mode oscillation frequencies. We first derived the asymptotic theoretical formalism for computing the asymmetric signature in the frequency pattern for $ modes due to a quadrudipole magnetic field. To access asymmetry parameters for more complex magnetic field topologies, we numerically performed a grid search over the parameter space to map the degeneracy of the signatures of given topologies. We demonstrate the crucial role played by $ mixed modes in accessing internal magnetic fields with a quadrupolar component. The degeneracy of the quadrudipole compared to pure dipolar fields is lifted when considering magnetic asymmetries in both $ and $ mode frequencies . In addition to the analytical derivation for the quadrudipole, we present the prospect for complex magnetic field inversions using magnetic sensitivity kernels from standard perturbation analysis for forward modeling . Using this method, we explored the detectability of offset magnetic fields from $ and $ frequencies and demonstrate that offset fields may be mistaken for weak and centered magnetic fields, resulting in underestimating the magnetic field strength in stellar cores. We emphasize the need to characterize $ mixed-mode frequencies (along with the currently characterized $ mixed modes) to unveil the higher-order components of the geometry of buried magnetic fields and to better constrain angular momentum transport inside stars.

Accuracy of the scalar magnetometer aboard ESA's JUICE mission

Abstract. This paper discusses the accuracy of the scalar Coupled Dark State Magnetometer on board the Jupiter Icy Moons Explorer (JUICE) mission of the European Space Agency (ESA). The scalar magnetometer, referred to as MAGSCA, is part of the J-MAG instrument. MAGSCA is an optical omnidirectional scalar magnetometer based on coherent population trapping, a quantum interference effect, within the hyperfine manifold of the 87Rb D1 line. The measurement principle is only based on natural constants; therefore, it is in principle drift-free, and no calibration is required. However, the technical realisation can influence the measurement accuracy. The most dominating effects are heading characteristics, which are deviations of the magnetic field strength measurements from the ambient magnetic field strength. These deviations are a function of the angle between the sensor axis and the magnetic field vector and are an intrinsic physical property of the measurement principle of the magnetometer. The verification of the accuracy of the instrument is required to ensure its compliance with the performance requirement of 0.2 nT (1σ) with a data rate of 1 Hz. The verification is carried out with four dedicated sensor orientations in a Merritt coil system, which is located in the geomagnetic Conrad Observatory (COBS). The coil system is used to compensate the Earth's magnetic field and to apply appropriate test fields to the sensor. This paper presents a novel method to separate the heading characteristics of the instrument from residual (offset) fields within the coil system by fitting a mathematical model to the measured data and by the successful verification of the MAGSCA performance requirement.

Method for improving accurate measurement of the magnetic field of wireless passive micro-magnetic stimulator micro-coil

In recent years, the wireless passive micro-magnetic stimulator (WP-μMS) has emerged as a focal for implantable neuromagnetism modulation research. However, accurately measuring the magnetic field strength of the micro-coil presents a significant challenge due to its small size and low mutual inductance (M). Firstly, this paper establishes a theoretical model of magnetic coupling with a three-stage structure of transmitting coil-micro-coil-detecting coil. The calculation method of detecting the coil to measure the magnetic field is determined by calculating the M between each pair of coils. Secondly, the magnetic coupling resonance method enhances the measured value M to the nH-scale. This approach also eliminates the influence of the M between the transmitting coil-detecting coil on the measurement of the transmitting coil-micro-coil M, thereby achieving accurate measurement of the mT-scale magnetic field strength of the WP-μMS micro-coil. Finally, the effectiveness of the measured mT-scale magnitude magnetic stimulation is validated biologically.

EURAMET.EM.RF-S46 (EURAMET Project No 1538) EURAMET supplementary comparison on magnetic field strength measurements for frequencies up to 30 MHz

Main textThe results of the supplementary comparison in the area of magnetic field probe calibration of the field strength are presented. This supplementary comparison has been discussed to organize a comparison on magnetic field strength measurements during the EURAMET TC-EM Subcommittee Radiofrequency and Microwave (SC-MW) meeting, including an EMC meeting, organized as a virtual (online) meeting on 20-21 April 2021. The comparison was conducted in accordance with the Technical Protocol of "Comparison of Magnetic Field Strength Measurements for Frequencies up to 30 MHz, EURAMET Project No 1538, EURAMET.EM.RF-S46", which was prepared by the TÜBİTAK UME and approved by the participants. The measurements started in January 2022 and were completed in May 2023. Measurement results reported by nine participants for magnetic field strength values were compared and degrees of equivalence are reported.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Dark halos around solar active regions

Context. Dark areas around active regions (ARs) were first observed in chromospheric lines more than a century ago and are now associated with the Hα fibril vortex around ARs. Nowadays, large areas surrounding ARs with reduced emission relative to the quiet Sun (QS) are also observed in spectral lines emitted in the transition region (TR) and the low corona. For example, they are clearly seen in the SDO/AIA 171 Å images. We name these chromospheric and TR-coronal dark regions “dark halos” (DHs). Coronal DHs are poorly studied and, because their origin is still unknown, to date it is not clear if they are related to the chromospheric fibrillar ones. Furthermore, they are often mistaken for coronal holes (CHs). Aims. Our goal is to characterize the emission properties of a DH by combining, for the first time, chromospheric, TR, and coronal observations in order to provide observational constraints for future studies on the origin of DHs. This study also aims to investigate the different properties of DHs and CHs and provide a quick-look recipe to distinguish between them. Methods. We studied the DH around AR NOAA 12706 and the southern CH that were on the disk on April 22, 2018 by analyzing IRIS full-disk mosaics and SDO/AIA filtergrams to evaluate their average intensities, normalized to the QS. In addition, we used the AIA images to derive the DH and CH emission measure (EM) and the IRIS Si IV 1393.7 Å line to estimate the nonthermal velocities of plasma in the TR. We also employed SDO/HMI magnetograms to study the average magnetic field strength inside the DH and the CH. Results. Fibrils are observed all around the AR core in the chromospheric Mg II h&k IRIS mosaics, most clearly in the h3 and k3 features. The TR emission in the DH is much lower than in the QS area, unlike in the CH. Moreover, the DH is much more extended in the low corona than in the chromospheric Mg II h3 and k3 images. Finally, the intensities, EM, spectral profile, nonthermal velocity, and average magnetic field strength measurements clearly show that DHs and CHs exhibit different characteristics, and therefore should be considered as distinct types of structures on the Sun.

Evolution of coronal magnetic field parameters during X5.4 solar flare

The coronal magnetic field over NOAA Active Region 11,429 during a X5.4 solar flare on 7 March 2012 is modeled using optimization based Non-Linear Force-Free Field extrapolation. Specifically, 3D magnetic fields were modeled for 11 timesteps using the 12-min cadence Solar Dynamics Observatory (SDO) Helioseismic and Magnetic Imager photospheric vector magnetic field data, spanning a time period of 1 hour before through 1 hour after the start of the flare. Using the modeled coronal magnetic field data, seven different magnetic field parameters were calculated for 3 separate regions: areas with surface |Bz|≥ 300 G, areas of flare brightening seen in SDO Atmospheric Imaging Assembly imagery, and areas with surface |B| ≥ 1000 G and high twist. Time series of the magnetic field parameters were analyzed to investigate the evolution of the coronal field during the solar flare event and discern pre-eruptive signatures. The data shows that areas with |B| ≥ 1000 G and |Tw|≥ 1.5 align well with areas of initial flare brightening during the pre-flare phase and at the beginning of the eruptive phase of the flare, suggesting that measurements of the photospheric magnetic field strength and twist can be used to predict the flare location within an active region if triggered. Additionally, the evolution of seven investigated magnetic field parameters indicated a destabilizing magnetic field structure that could likely erupt.

SULIS: A coronal magnetism explorer for ESA’s Voyage 2050

Magnetism dominates the structure and dynamics of the solar corona. To understand the true nature of the solar corona and the long-standing coronal heating problem requires measuring the vector magnetic field of the corona at a sufficiently high resolution (spatially and temporally) across a large Field-of-View (FOV). Despite the importance of the magnetic field in the physics of the corona and despite the tremendous progress made recently in the remote sensing of solar magnetic fields, reliable measurements of the coronal magnetic field strength and orientation do not exist. This is largely due to the weakness of coronal magnetic fields, previously estimated to be on the order of 1-10 G, and the difficulty associated with observing the extremely faint solar corona emission. With the Solar cUbesats for Linked Imaging Spectro-polarimetry (SULIS) mission, we plan to finally observe, in detail and over the long-term, uninterrupted measurements of the coronal magnetic vector field using a new and very affordable instrument design concept. This will be profoundly important in the study of local atmospheric coronal heating processes, as well as in measuring the nature of magnetic clouds, in particular, within geoeffective Earth-bound Coronal Mass Ejections (CMEs) for more accurate forecasting of severe space weather activity.

Evaluation of Electromagnetic Field Levels and Student Exposure at Aydın Adnan Menderes University Central Campus

With the effect of developing technology, humanity is constantly exposed to more electromagnetic fields. Regional studies are gaining importance for determining the possible problems. In this study, extremely low-frequency electromagnetic field (ELF-EMF) measurements were carried out in Aydın Adnan Menderes University Central Campus in areas where university students are frequently found, at 25 randomly selected points, in the region where high voltage lines pass and around transformers, and to evaluate the awareness of university students about the ELF-EMF they are exposed to. For this purpose, the necessary magnetic field and electric field strength measurements were made and a questionnaire was applied to determine the awareness level of the students. Results were evaluated by comparing them with the ICNIRP limit values. Accordingly, it has been concluded that Aydın Adnan Menderes University Central Campus can be accepted as a relatively safe area since electromagnetic field exposure values are within the ICNIRP safe limit values. In addition, as a result of the questionnaire, it was determined that the young people were conscious to a certain extent about ELF-EMF, but they did not pay attention to protect themselves against the possible effects of ELF-EMF. It is important to make the necessary initiatives in order to raise awareness of young people on this issue.

Magnetic field measurement in TMC-1C using 22.3 GHz CCS Zeeman splitting

ABSTRACT Measurement of magnetic fields in dense molecular clouds is essential for understanding the fragmentation process prior to star formation. Radio interferometric observations of CCS 22.3 GHz emission, from the starless core TMC-1C, have been carried out with the Karl G. Jansky Very Large Array to search for Zeeman splitting of the line in order to constrain the magnetic field strength. Toward a region offset from the dust peak, we report a detection of the Zeeman splitting of the CCS 21–10 transition, with an inferred magnetic field of ∼2 mG. If we interpret the dust peak to be the core of TMC-1C, and the region where we have made a detection of the magnetic field to be the envelope, then our observed value for the magnetic field is consistent with a subcritical mass-to-flux ratio envelope around a core with supercritical mass-to-flux ratio. The ambipolar diffusion time-scale for the formation of the core is consistent with the relevant time-scale based on chemical modelling of the TMC-1C core. This work demonstrates the potential of deep CCS observation to carry out future measurements of magnetic field strengths in dense molecular clouds and, in turn, understand the role of the magnetic field in star formation.

Measurement of Electric and Magnetic Field Strengths under 765 kV UHVAC Single Circuit Transmission Line

Transmission line corridors of developing countries are increasingly dense and expensive with construction of new ultrahigh voltage (UHV) transmission lines. Right of way of transmission line in the corridor, construction cost of transmission lines including towers, is directly influenced by electric and magnetic field strength emission (EMF) of that line. In UHV range, emission of electric and magnetic field strength in the vicinity of lines is of great public concern, as are likely to have hazardous effects to human being when their exposure level exceeds specified limits. Higher the transmission voltage and stronger the electric field. Though the results on effect of EMF on human being is inconclusive, to create awareness to common public and working professional, it is mandatory to measure the emission level of EMF in lateral and longitudinal profile. This paper reports experiences of EMF emission measurement of 765 kV UHVAC quadruple ACSR bundled conductor single circuit transmission line in India. Measurement results of EMF with vegetation is reported.

Izloženost učenika srednje elektrotehničke škole "Nikola Tesla" bežičnom elektromagnetnom zračenju

Introduction/Aim: Wireless electromagnetic radiation from routers is an everyday occurrence in the life and work of modern man. They are widely used in many professional, public and domestic facilities, which means that people of all ages can be constantly exposed to electromagnetic radiation from these devices. The aim of this study was to determine the electric field strength, the magnetic field strength, and the electromagnetic exposure of students in different laboratories of the Secondary School "Nikola Tesla", depending on the type of routers installed in them, as well as to check some of the obtained results with the help of the appropriate software program of high-frequency (High-Frequency Simulation Software - HFFSS) for the simulation of electromagnetic radiation through different environments. Methods: Measurements of electric and magnetic field strength, as well as electromagnetic exposure, were carried out using the measuring instrument Spectran HF 60105, on four different types of routers installed in four laboratories of the Secondary School of Electrical Engineering "Nikola Tesla" in Niš. Measurements were realized on each router, through five horizontal and five vertical positions, with a distance of 45o in relation to the horizontal and normal axis of the router, and with spaces of 1 cm, which gives a total number of one thousand measurements. Results: The measured results obtained on four different routers that operated at a frequency of 2.4 GHz for the electric field strength (measured in millivolts per meter), magnetic field strength (measured in microampere per meter) and electromagnetic exposure (in microWatts per square centimeter) were below the limit values defined by standards. The achieved results were verified only for the router TL-WR841HP with the help of HFSS for the simulation of electromagnetic radiation through different environments. The result of the simulation check practically confirms the reliability of the measured results for TL-WR841HP router. Conclusion: Measuring and monitoring electromagnetic radiation is a very important task aimed at preserving and improving the quality of the working and living environment, as well as the quality of life and health of people and children. Software programs for the simulation of electromagnetic radiation can be used when it is not possible to carry out experimental measurements due to technical problems, hindered access to the device, danger of excessive radiation or some other reasons.

A Soft 3-dimensional Force Sensor For Measuring Head Impacts In Football Helmets

Three-dimensional (3D) force data from head impacts in football is important for understanding traumatic brain injuries (TBIs). Football helmets have not yet been instrumented with true 3D force sensors compelling the development of soft 3D force sensors that could be seamlessly integrated into existing helmet padding. PURPOSE: To develop and determine the accuracy and reliability of an in-helmet soft 3D force sensor in comparison to force plate data. METHODS: A force sensor was fabricated by embedding magnetic powder within a cylindrical rubber cushion which was affixed to a magnetometer. Prior to integration in a helmet, known compressive forces were slowly applied to the sensor to assess performance in a controlled setting. The sensor was then inserted and fixed into a cavity created on the forehead pad of a football helmet. The sensor was controlled by a microcontroller, which communicated data wirelessly to a nearby computer at a sampling rate of 29 Hz. Thirteen trials were performed in which the helmet, while being worn, impacted a vertically mounted 3D force plate from different angles. The reliability of the helmet sensor in relation to the force plate was assessed by the interclass correlation coefficient (ICC) of the peak force measured in the primary axis of the impact (i.e. z-axis for a straight impact, x-axis for an angled impact) from the helmet sensor and the force plate. RESULTS: The magnetometer’s measurements of magnetic field strength exhibited a strong second order correlation to the force applied to the sensor (r2 = 0.995). For the helmet-force plate impacts, the force vectors were clearly observable for all trials by analyzing the force-time curves of each of the three axes of the helmet sensor. The helmet sensor displayed excellent agreement with the force plate for peak force (ICC = 0.981). In addition, peak forces for 8 of 13 impacts exhibited moderately low error (1-13%) when compared to the force plate, but the rest of the impacts had larger errors (25-51%). CONCLUSIONS: The sensor performed excellent in controlled studies and had high agreement with the force plate according to the ICC. Most impacts were accurately measured, but a higher sampling rate is needed for consistent and accurate quantification of impacts that contain higher frequency components.

Measurements of Coronal Magnetic Field Strengths in Solar Active Region Loops

Abstract The characteristic electron densities, temperatures, and thermal distributions of 1 MK active region loops are now fairly well established, but their coronal magnetic field strengths remain undetermined. Here we present measurements from a sample of coronal loops observed by the Extreme-ultraviolet Imaging Spectrometer on Hinode. We use a recently developed diagnostic technique that involves atomic radiation modeling of the contribution of a magnetically induced transition to the Fe x 257.262 Å spectral line intensity. We find coronal magnetic field strengths in the range of 60–150 G. We discuss some aspects of these new results in the context of previous measurements using different spectropolarimetric techniques, and their influence on the derived Alfvén speeds and plasma β in coronal loops.

Magnetic Field and Gravitational Waves from the First-Order Phase Transition.

We perform the three-dimensional lattice simulation of the magnetic field and gravitational wave productions from bubble collisions during the first-order electroweak phase transition. Except for the gravitational wave, the power-law spectrum of the magnetic field strength is numerically calculated for the first time, which is of a broken power-law spectrum: B_{ξ}∝f^{0.91} for the low-frequency region of f<f_{⋆} and B_{ξ}∝f^{-1.65} for the high-frequency region of f>f_{⋆} in the thin-wall limit, with the peak frequency being f_{⋆}∼5 Hz at the phase transition temperature 100GeV. When the hydrodynamics is taken into account, the generated magnetic field strength can reach B_{ξ}∼10^{-7} G at a correlation length ξ∼10^{-7} pc, which may seed the large scale magnetic fields. Our study shows that the measurements of cosmic magnetic field strength and gravitational waves are complementary to probe new physics admitting electroweak phase transition.

Magnetic resonance imaging simulation with spin-lock preparations to detect tiny oscillatory magnetic fields

Spin-lock preparation was studied to detect tiny oscillatory magnetic fields such as a neural magnetic field without the blood oxygen level-dependent effect. This approach is a direct measurement and independent of static magnetic field strength. Accordingly, it is anticipated as a feasible functional magnetic resonance imaging (fMRI) in low and ultra-low-field MRI. Several reports have been published on spin-lock preparation but reports on imaging simulation are rare. Research in this area can assist in investigating magnetic resonance signal changes and, accordingly, can help to develop new spin-lock methods. In this study, we propose an imaging simulation method with an analytical solution using the Bloch equation. To demonstrate the feasibility of our proposed method, we compared simulated images with experimental results in which the number of sub-voxels and the amplitude and phase of the target oscillatory magnetic fields varied. In addition, we also applied graphics processing unit parallel computing and investigated the feasibility of avoiding an impracticable calculation time by doing so.

Hinode/EIS Measurements of Active-region Magnetic Fields

Abstract The present work illustrates the potential of a new diagnostic technique that allows the measurement of the coronal magnetic field strength in solar active regions by utilizing a handful of bright Fe x and Fe xi lines commonly observed by the high-resolution Hinode/EUV Imaging Spectrometer (EIS). The importance of this new diagnostic technique is twofold: (1) the coronal magnetic field is probably the most important quantity in coronal physics, being at the heart of the processes regulating space weather and the properties of the solar corona, and (2) this technique can be applied to the existing EIS archive spanning from 2007 to 2020, including more than one full solar cycle and covering a large number of active regions, flares, and even coronal mass ejections. This new diagnostic technique opens the door to a whole new field of studies, complementing the magnetic field measurements from the upcoming DKIST and UCoMP ground-based observatories, and extending our reach to active regions observed on the disk and until now only sampled by radio measurements. In this work, we present a few examples of the application of this technique to EIS observations taken at different times during the EIS mission, and we discuss its current limitations and the steps to improve its accuracy. We also present a list of EIS observing sequences whose data include all of the lines necessary for the application of this diagnostic technique, to help the solar community navigate the immense set of EIS data and to find observations suitable for measuring the coronal magnetic field.

Untangling Magnetic Complexity in Protoplanetary Disks with the Zeeman Effect

Abstract With the recent advent of circular polarization capabilities at the Atacama Large Millimeter/submillimeter Array (ALMA), Zeeman effect measurements of spectral lines are now possible as a means to directly probe line-of-sight magnetic fields in protoplanetary disks (PPDs). We present a modeling study that aims to guide physical interpretation of these anticipated observations. Using a fiducial density structure based on a typical ringed disk, we simulate line emission for the hyperfine components of the CN J = 1−0 transition with the POLARIS radiative transfer code. Since the expected magnetic field and typical CN distribution in PPDs remain largely unconstrained, we produce models with several different configurations. Corresponding integrated Stokes I and V profiles and 0.4 km s−1 resolution, 1″ beam convolved channel maps are presented. We demonstrate that the emission signatures from toroidally dominated magnetic fields are distinguishable from vertically dominated magnetic field based on channel map morphology. Due to line-of-sight and beam cancellation effects, disks with toroidal -field configurations result in significantly diminished Stokes V emission. Complex magnetic fields therefore render the traditionally used method for inferring line-of-sight magnetic field strengths (i.e., fitting the derivative of the Stokes I to the Stokes V profile) ambiguous, since a given intrinsic field strength can yield a variety of Stokes V amplitudes depending on the magnetic field geometry. In addition, gas gaps can create structure in the integrated Stokes V profile that might mimic magnetic substructure. This method should therefore be applied with caution in PPD environments and can only confidently be used as a measure of magnetic field strength if the disk’s magnetic field configuration is well understood.

SUMER Measurement of the Fe x 3p 43d 4D5/2,7/2 Energy Difference

Abstract Recent studies have shown that magnetic fields in the solar corona are strong enough to significantly mix the two 3p 43d 4D5/2,7/2 levels in Cl-like Fe x. This mixing gives rise to a magnetically induced transition (MIT) component in the bright Fe x 257.3 Å line, commonly observed by current instrumentation, that can be used for coronal magnetic field diagnostics. This line, commonly observed by the still operational EIS spectrometer on board the Hinode satellite since 2007, opens a new window into the coronal magnetic field. However, the strength of this MIT transition depends on the square of the energy difference ΔE of the two levels, so that an accurate determination of ΔE is of critical importance to accurately measure coronal magnetic field strengths. In the present work we present a new measurement of ΔE obtained determining the separation of the two component of the Fe x doublet close to 1603.3 Å from deep-exposure spectra of a quiescent streamer at the solar limb taken with the SUMER instrument on board SoHO. Our measurement of ΔE = 2.29 ± 0.50 cm−1 agrees with, and improves upon, an earlier measurements by Judge et al. by decreasing its uncertainty from 80% to approximately 20%, improving the attainable accuracy of magnetic field strength measurements obtainable with the Fe x 257.26 Å line.

Electromagnetic Fields As An Element of Spacecraft Construction

The article suggests the best option for designing a spacecraft. The process of activity of proposed structure of a spacecraft in outer space must be based on the acceleration due to the electromagnetic fields of the Solar System and the Milky Way Galaxy. Purpose of the study. Suggest a design solution to create a model of a spacecraft that is accelerated in motion using space electromagnetic fields. The relevance of the chosen topic is based on the need to address issues related to the limited resources of the planet, significant levels of overpopulation, problems of demography, ecology, sociology and political life. Novelty. It is determined that in order to obtain calculations of the acceleration of 1 g of spatial arbitrary direction, it is necessary to make measurements along the chain and to deduce the amount of charge on the useful surface of the object. Methodology. Magnetic field strength measurement method, electromagnetic measurement method. Results and conclusions. The article presents a scheme of creating a modified magnetic type engine, the result its process of activity will be an acceleration of 0.05 g. It will be achieved by free movement in outer space with the appropriate current.

Crustal and time-varying magnetic fields at the InSight landing site on Mars

Magnetic fields provide a window into a planet’s interior structure and evolution, including its atmospheric and space environments. Satellites at Mars have measured crustal magnetic fields indicating an ancient dynamo. These crustal fields interact with the solar wind to generate transient fields and electric currents in Mars’s upper atmosphere. Surface magnetic field data play a key role in understanding these effects and the dynamo. Here we report measurements of magnetic field strength and direction at the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) landing site on Mars. We find that the field is ten times stronger than predicted by satellite-based models. We infer magnetized rocks beneath the surface, within ~150 km of the landing site, consistent with a past dynamo with Earth-like strength. Geological mapping and InSight seismic data suggest that much or all of the magnetization sources are carried in basement rocks, which are at least 3.9 billion years old and are overlain by between 200 m and ~10 km of lava flows and modified ancient terrain. Daily variations in the magnetic field indicate contributions from ionospheric currents at 120 km to 180 km altitude. Higher-frequency variations are also observed; their origin is unknown, but they probably propagate from even higher altitudes to the surface. We propose that the time-varying fields can be used to investigate the electrical conductivity structure of the martian interior. The magnetic field measured by the InSight lander on Mars varies daily and is ten times stronger than expected. The field is inferred to originate from components of basement rocks magnetized by an ancient dynamo of Earth-like strength.