Showing papers on "Earth's magnetic field published in 2020"
TL;DR: The CHAOS-7 model as mentioned in this paper is based on magnetic field observations collected by the low-Earth orbit satellites Swarm, CryoSat-2, CHAMP, SAC-C and Orsted, and on annual differences of monthly means of ground observatory measurements.
Abstract: We present the CHAOS-7 model of the time-dependent near-Earth geomagnetic field between 1999 and 2020 based on magnetic field observations collected by the low-Earth orbit satellites Swarm, CryoSat-2, CHAMP, SAC-C and Orsted, and on annual differences of monthly means of ground observatory measurements. The CHAOS-7 model consists of a time-dependent internal field up to spherical harmonic degree 20, a static internal field which merges to the LCS-1 lithospheric field model above degree 25, a model of the magnetospheric field and its induced counterpart, estimates of Euler angles describing the alignment of satellite vector magnetometers, and magnetometer calibration parameters for CryoSat-2. Only data from dark regions satisfying strict geomagnetic quiet-time criteria (including conditions on IMF
$$B_z$$
and
$$B_y$$
at all latitudes) were used in the field estimation. Model parameters were estimated using an iteratively reweighted regularized least-squares procedure; regularization of the time-dependent internal field was relaxed at high spherical harmonic degree compared with previous versions of the CHAOS model. We use CHAOS-7 to investigate recent changes in the geomagnetic field, studying the evolution of the South Atlantic weak field anomaly and rapid field changes in the Pacific region since 2014. At Earth’s surface a secondary minimum of the South Atlantic Anomaly is now evident to the south west of Africa. Green’s functions relating the core–mantle boundary radial field to the surface intensity show this feature is connected with the movement and evolution of a reversed flux feature under South Africa. The continuing growth in size and weakening of the main anomaly is linked to the westward motion and gathering of reversed flux under South America. In the Pacific region at Earth’s surface between 2015 and 2018 a sign change has occurred in the second time derivative (acceleration) of the radial component of the field. This acceleration change took the form of a localized, east–west oriented, dipole. It was clearly recorded on ground, for example at the magnetic observatory at Honolulu, and was seen in Swarm observations over an extended region in the central and western Pacific. Downward continuing to the core–mantle boundary, we find this event originated in field acceleration changes at low latitudes beneath the central and western Pacific in 2017.
148 citations
TL;DR: A review of relevant geomagnetic hazards studies from around the world and their common and unique features can be found in this article, focusing especially on the effects that the Earth's electrical conductivity has on the GICs flowing in the electric power grids.
Abstract: Geomagnetic disturbances cause perturbations in the Earth’s magnetic field which, by the principle of electromagnetic induction, in turn cause electric currents to flow in the Earth. These geomagnetically induced currents (GICs) also enter man-made technological conductors that are grounded; notably, telegraph systems, submarine cables and pipelines, and, perhaps most significantly, electric power grids, where transformer groundings at power grid substations serve as entry points for GICs. The strength of the GICs that flow through a transformer depends on multiple factors, including the spatiotemporal signature of the geomagnetic disturbance, the geometry and specifications of the power grid, and the electrical conductivity structure of the Earth’s subsurface. Strong GICs are hazardous to power grids and other infrastructure; for example, they can severely damage transformers and thereby cause extensive blackouts. Extreme space weather is therefore hazardous to man-made technologies. The phenomena of extreme geomagnetic disturbances, including storms and substorms, and their effects on human activity are commonly referred to as geomagnetic hazards. Here, we provide a review of relevant GIC studies from around the world and describe their common and unique features, while focusing especially on the effects that the Earth’s electrical conductivity has on the GICs flowing in the electric power grids.
52 citations
52 citations
TL;DR: New paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircon have escaped magnetic resetting since their formation are provided.
Abstract: Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth's Hadean liquid iron core.
52 citations
TL;DR: A review of the current understanding of auroral features that appear poleward of the main auroral oval within the polar cap, especially those that are known as Sun-aligned arcs, transpolar arcs, or theta auroras, can be found in this paper.
Abstract: This paper reviews our current understanding of auroral features that appear poleward of the main auroral oval within the polar cap, especially those that are known as Sun-aligned arcs, transpolar arcs, or theta auroras. They tend to appear predominantly during periods of quiet geomagnetic activity or northwards directed interplanetary magnetic field (IMF). We also introduce polar rain aurora which has been considered as a phenomenon on open field lines. We describe the morphology of such auroras, their development and dynamics in response to solar wind-magnetosphere coupling processes, and the models that have been developed to explain them.
47 citations
TL;DR: In this article, the authors quantify the regional variability of dB/dt using closely placed IMAGE stations in northern Fennoscandia and assess the significance of spatial geomagnetic variations to modeling GICs across a transmission line.
Abstract: Faraday's law of induction is responsible for setting up a geoelectric field due to the variations in the geomagnetic field caused by ionospheric currents. This drives geomagnetically induced currents (GICs) which flow in large ground‐based technological infrastructure such as high‐voltage power lines. The geoelectric field is often a localized phenomenon exhibiting significant variations over spatial scales of only hundreds of kilometers. This is due to the complex spatiotemporal behavior of electrical currents flowing in the ionosphere and/or large gradients in the ground conductivity due to highly structured local geological properties. Over some regions, and during large storms, both of these effects become significant. In this study, we quantify the regional variability of dB/dt using closely placed IMAGE stations in northern Fennoscandia. The dependency between regional variability, solar wind conditions, and geomagnetic indices are also investigated. Finally, we assess the significance of spatial geomagnetic variations to modeling GICs across a transmission line. Key results from this study are as follows: (1) Regional geomagnetic disturbances are important in modeling GIC during strong storms; (2) dB/dt can vary by several times up to a factor of three compared to the spatial average; (3) dB/dt and its regional variation is coupled to the energy deposited into the magnetosphere; and (4) regional variability can be more accurately captured and predicted from a local index as opposed to a global one. These results demonstrate the need for denser magnetometer networks at high latitudes where transmission lines extending hundreds of kilometers are present.
45 citations
TL;DR: In this paper, the authors compared the amplitude of the semi-annual variation in geomagnetic activity, as a fraction of the overall mean, to that of the corresponding variation in power input to the magnetosphere, P α, estimated from interplanetary observations.
Abstract: We study the semi-annual variation in geomagnetic activity, as detected in the geomagnetic indices am , aa H , AL , Dst and the four aσ indices derived for 6-hour MLT sectors (around noon, dawn, dusk and midnight). For each we compare the amplitude of the semi-annual variation, as a fraction of the overall mean, to that of the corresponding variation in power input to the magnetosphere, P α , estimated from interplanetary observations. We demonstrate that the semi-annual variation is amplified in the geomagnetic data compared to that in P α , by a factor that is different for each index. The largest amplification is for the Dst index (factor ~ 10) and the smallest is for the aσ index for the noon MLT sector (aσ -noon, factor ≈ 1.1). By sorting the data by the prevailing polarity of the Y -component (dawn-dusk) of the Interplanetary Magnetic Field (IMF) in the Geocentric Solar Equatorial (GSEQ) reference frame, we demonstrate that the Russell-McPherron (R-M) effect, in which a small southward IMF component in GSEQ is converted into geoeffective field by Earth’s dipole tilt, is a key factor for the semi-annual variations in both P α and geomagnetic indices. However, the variability in the southward component in the IMF in the GSEQ frame causes more variability in power input to the magnetosphere P α than does the R-M effect, by a factor of more than two. We show that for increasingly large geomagnetic disturbances, P α delivered by events of large southward field in GSEQ (known to often be associated with coronal mass ejections) becomes the dominant driver and the R-M effect declines in importance and often acts to reduce geoeffectiveness for the most southward IMF in GSEQ: the semi-annual variation in large storms therefore suggests either preconditioning of the magnetosphere by average conditions or an additional effect at the equinoxes. We confirm that the very large R-M effect in the Dst index is because of a large effect at small and moderate activity levels and not in large storms. We discuss the implications of the observed “equinoctial” time-of-year (F ) – Universal Time (UT ) pattern of geomagnetic response, the waveform and phase of the semi-annual variations, the differences between the responses at the June and December solstices and the ratio of the amplitudes of the March and September equinox peaks. We also confirm that the UT variation in geomagnetic activity is a genuine global response. Later papers will analyse the origins and implications of the effects described.
44 citations
TL;DR: In this paper, a multi-parameter global statistical model of extreme horizontal geomagnetic field fluctuations (dBH /dt ) is presented, which are a useful input to models assessing the risk of geomagnetically induced currents in ground infrastructure.
Abstract: This paper presents a multi-parameter global statistical model of extreme horizontal geomagnetic field fluctuations (dBH /dt ), which are a useful input to models assessing the risk of geomagnetically induced currents in ground infrastructure. Generalised Pareto (GP) distributions were fitted to 1-min measurements of |dBH /dt | from 125 magnetometers (with an average of 28 years of data per site) and return levels (RL) predicted for return periods (RP) between 5 and 500 years. Analytical functions characterise the profiles of maximum-likelihood GP model parameters and the derived RLs as a function of corrected geomagnetic latitude, λ . A sharp peak in both the GP shape parameter and the RLs is observed at |λ| = 53° in both hemispheres, indicating a sharp equatorward limit of the auroral electrojet region. RLs also increase strongly in the dayside region poleward of the polar cusp (|λ| > 75°) for RPs > 100 years. We describe how the GP model may be further refined by modelling the probability of occurrences of |dBH /dt | exceeding the 99.97th percentile as a function of month, magnetic local time, and the direction of the field fluctuation, dB H , and demonstrate that these patterns of occurrence align closely to known patterns of auroral substorm onsets, ULF Pc5 wave activity, and (storm) sudden commencement impacts. Changes in the occurrence probability profiles with the interplanetary magnetic field (IMF) orientation reveal further details of the nature of the ionospheric currents driving extreme |dBH /dt | fluctuations, such as the changing location of the polar cusp and seasonal variations explained by the Russell-McPherron effect.
43 citations
TL;DR: The CM6 model as discussed by the authors is the parent model of the NASA/GSFC candidates for the DGRF2015 and IGRF2020 in response to the IGRFA-13 call.
Abstract: From the launch of the Orsted satellite in 1999, through the CHAMP mission from 2000 to 2010, and now with the Swarm constellation mission starting in 2013, satellite magnetometry has provided excellent monitoring of the near-Earth magnetic field regime. The advanced Comprehensive Inversion scheme has been applied to data before Swarm and to the Swarm data itself, but now for the first time to all the satellite data in this new era, culminating in the CM6 model. The highlights of this model include not only a continuous core magnetic field description over the entire time period 1999 to 2019.5 in good agreement with the CHAOS model series, but the addition of two new oceanic tidal magnetic sources: the larger lunar elliptic semi-diurnal constituent $$N_2$$ and the lunar diurnal constituent $$O_1$$. CM6 is also the parent model of the NASA/GSFC candidates for the DGRF2015 and IGRF2020 in response to the IGRF-13 call. This paper provides a full report on the development of CM6.
42 citations
TL;DR: In this paper, the solar wind at Earth is categorized into four types of plasma emanating from different morphological features on the solar surface and the systematic differences between the four plasma types is assessed.
39 citations
TL;DR: The use of geomagnetic field patterns called MP (Magnetic Pattern) with CNN (Convolutional Neural Networks) to perform indoor localization to solve indoor localization problems and results show promising results.
Abstract: Conventional geomagnetic field-based indoor positioning and localization techniques determine the user's position by comparing the database with the geomagnetic field strength collected by the user. However, the magnetic field strength collected from various devices varies significantly. So, the greater the difference between the geomagnetic field strength stored in the database and user collected geomagnetic field strength is, the lower the degree of location accuracy will be. The diversity of smartphone makes it impossible to develop a single database which can work with all the smartphones in the same fashion. Intending to solve these problems, this paper proposes the use of geomagnetic field patterns called MP (Magnetic Pattern) with CNN (Convolutional Neural Networks) to perform indoor localization. The database is constructed using the MP that occurs at the points of measurement while the location is calculated using CNN which matches the user collected MP with the database. A voting mechanism is contrived to combine the predictions from several CNNs and the user's position is finally estimated. To evaluate the performance of the proposed technique, Samsung Galaxy S8 and LG G6 are used in two buildings with different experimental environments and path geometry. The proposed approach is tested by two male and two female users for analyzing the impact of user heights. Experiment results show promising results; furthermore, the comparison analysis with other magnetic indoor localization approaches demonstrate that the proposed approach outperforms them.
TL;DR: In this article, the effects of a space weather event on several longitudinal sectors (Asia, Africa, America, and the Pacific) have been analyzed using various parameters such as total electron content (TEC), geomagnetic field, and column [O/N2] ratio.
Abstract: We present a study concerning a space weather event on 25–29 August 2018, accounting for its ionospheric and magnetic signatures at low latitudes and midlatitudes. The effects of a storm in several longitudinal sectors (Asia, Africa, America, and the Pacific) have been analyzed using various parameters such as total electron content (TEC), geomagnetic field, and column [O/N2] ratio. Positive ionospheric storms are found in all the longitudinal sectors having its maximum effects in the Asian sector, whereas the negative ionospheric storms have been observed in the summer hemisphere (Northern Hemisphere). A large decrease in [O/N2] ratio in the Northern Hemisphere is a possible cause of the observed negative storm effects. Ionospheric F2 region maximum electron density (NmF2) and TEC have shown a positive correlation during this storm. The study suggests that storm time‐generated wind does not have a uniform planetary extension and mainly affects dayside (America and Pacific) and duskside (Africa) sectors. During the space weather event, we observe an asymmetric variation of the magnetic field as a function of the longitude. On the other hand, the magnetic variations at midlatitudes are found to be symmetric in both hemispheres. A signature of the disturbance dynamo (anti‐Sq circulation) has been observed, mainly at low latitudes. We emphasize that the partial ring current (PRC), estimated by the ASYM‐H magnetic index, must also be taken into account along with the SYM‐H index for a better approximation of ionospheric currents. The study further suggests existence of several electric current cells in the ionosphere, which is consistent with the Blanc‐Richmond model.
TL;DR: The existence of a stable stratified layer underneath the core-mantle boundary (CMB) has been recently revived by corroborating evidences coming from seismic studies, mineral physics and thermal evolution models as discussed by the authors.
Abstract: The existence of a stably stratified layer underneath the core-mantle boundary (CMB) has been recently revived by corroborating evidences coming from seismic studies, mineral physics and thermal evolution models. Such a layer could find its physical origination either in compositional stratification due to the accumulation of light elements at the top or the core or in thermal stratification due to the heat flux becoming locally sub-adiabatic. The exact properties of this stably-stratified layer, namely its size $\mathcal{H}_S$ and the degree of its stratification characterised by the Brunt-Vaisala frequency $N$, are however uncertain and highly debated. A stable layer underneath the CMB can have crucial dynamical impacts on the geodynamo. Because of the inhibition of the convective motions, a stable layer is expected to primarily act as a low-pass filter on the magnetic field, smoothing out the rapidly-varying and small-scale features by skin effect. To investigate this effect more systematically, we compute 70 global geodynamo models varying the size of the stably-stratified layer from 0 to 300~km and its amplitude from $N/\Omega = 0$ to $N/\Omega \simeq 50$, $\Omega$ being the rotation rate. We show that the penetration of the convective flow in the stably-stratified layer is controlled by the typical size of the convective eddies and by the local variations of the ratio $N/\Omega$. Using quantitative measures of the degree of morphological semblance between the magnetic field obtained in numerical models and the geomagnetic field at the CMB, we establish an upper bound for the stable layer thickness $\mathcal{H}_s < (N/\Omega)^{-1} d_c$, $d_c$ being the horizontal size of the convective flow at the base of the stable layer. This defines a strong geomagnetic constraint on the properties of a stably-stratified layer beneath the CMB.
TL;DR: Direct observations of a plasmapause surface wave and its impacts during a geomagnetic storm using multi-satellite and ground-based observations show that the surface-wave-driven sawtooth auroras occurred in more than 90% ofGeomagnetic storms during 2014–2018, indicating that they are a systematic and crucial process in driving space energy dissipation.
Abstract: Energy circulation in geospace lies at the heart of space weather research. In the inner magnetosphere, the steep plasmapause boundary separates the cold dense plasmasphere, which corotates with the planet, from the hot ring current/plasma sheet outside. Theoretical studies suggested that plasmapause surface waves related to the sharp inhomogeneity exist and act as a source of geomagnetic pulsations, but direct evidence of the waves and their role in magnetospheric dynamics have not yet been detected. Here, we show direct observations of a plasmapause surface wave and its impacts during a geomagnetic storm using multi-satellite and ground-based measurements. The wave oscillates the plasmapause in the afternoon-dusk sector, triggers sawtooth auroral displays, and drives outward-propagating ultra-low frequency waves. We also show that the surface-wave-driven sawtooth auroras occurred in more than 90% of geomagnetic storms during 2014–2018, indicating that they are a systematic and crucial process in driving space energy dissipation. Theoretical studies suggested that plasmapause surface waves related to the sharp inhomogeneity exist and act as a source of geomagnetic pulsations. Here, the authors show direct observations of a plasmapause surface wave and its impacts during a geomagnetic storm using multi-satellite and ground-based observations.
TL;DR: In this article, the authors reconstructed the time series of geomagnetic activity based on contemporary observational records and showed that potential threats posed by extreme space weather events exist even during weak solar cycles or near their minima.
Abstract: While the Sun is generally more eruptive during its maximum and declining phases, observational evidence shows certain cases of powerful solar eruptions during the quiet phase of solar activity. Occurring in the weak Solar Cycle 14 just after its minimum, the extreme space weather event in 1903 October-November is one of these cases. Here, we reconstruct the time series of geomagnetic activity based on contemporary observational records. With the mid-latitude magnetograms, the 1903 magnetic storm is thought to be caused by a fast coronal mass ejection (≈1500 km s-1) and is regarded as a superstorm with an estimated minimum of the equivalent disturbance storm time index (Dst') of ≈-531 nT. The reconstructed time series has been compared with the equatorward extension of auroral oval (≈44. 1 in invariant latitude) and the time series of telegraphic disturbances. This case study shows that potential threats posed by extreme space weather events exist even during weak solar cycles or near their minima.
TL;DR: In this article, the SHAWQ-IR model was used to investigate the origin of the unusually high fluctuations of the palaeofield of the geomagnetic field in the Iberian Peninsula prior to Late Iron Age times.
TL;DR: In this paper, the authors present a methodology to estimate the total electron content (TEC) of the low-latitude ionosphere based on GPS measurements and compare with results from other geophysical instruments, such as all-sky imagers and ionosondes.
Abstract: The ionosphere over the Brazilian region has particular characteristics due to the large geomagnetic declination angle over most of the territory. Furthermore, the equatorial ionization anomaly southern crest is located over the Brazilian territory. In this region, plasma irregularities may arise in the post-sunset hours. These ionospheric irregularities develop in the form of magnetic field-aligned plasma depletions, known as equatorial plasma bubbles, which may seriously affect radio signals that propagate through them. These irregularity structures can cause amplitude and phase scintillation of the propagating signals, thereby compromising the availability, performance, and integrity of satellite-based communication and navigation systems. Additionally, the total electron content (TEC) introduces propagation delays that can contribute to range measurement errors for global positioning system (GPS) users. The ionospheric characteristics change significantly according to the time of day, season, as well as the solar and geomagnetic activities, among other factors. Indeed, the ionosphere is one of the most significant sources of errors in the positioning and navigation systems based on the GPS satellites. Due to these features, there is a strong interest by the scientific community in better understanding and characterizing the ionospheric behavior. In this context, the TEC analysis has wide applicability for space plasma studies and is a well-established tool for investigating the ionospheric behavior and its potential impact on space-based navigation systems. One of the goals of these studies is the generation of TEC maps for a geographic region based on GPS observations. In the present work, some electrodynamic processes of the low-latitude ionosphere are reviewed and the TEC estimation based on GPS measurements is revisited in detail. A methodology aimed at creating the TEC maps is presented and validated by comparison with results from other geophysical instruments, such as all-sky imagers and ionosondes. Finally, examples of the ionospheric behavior displayed by TEC maps during equatorial plasma bubble events and a geomagnetic storm are fully described and discussed.
TL;DR: In this paper, the authors describe and discuss the preprocessing and calibration steps applied to the magnetic data measured by the three platform magnetometers on-board the CryoSat-2 satellite by comparing the magnetometer sensor readings with magnetic field values for the time and position of the satellite as given by the CHAOS-6 geomagnetic field model.
Abstract: We describe and discuss the preprocessing and calibration steps applied to the magnetic data measured by the three “platform magnetometers” on-board the CryoSat-2 satellite. The calibration is performed by comparing the magnetometer sensor readings with magnetic field values for the time and position of the satellite as given by the CHAOS-6 geomagnetic field model. We allow for slow temporal variations of the calibration parameters by solving for scale values, offsets, and non-orthogonalities in monthly bins, and account for non-linearities as well as the magnetic disturbances caused by battery, solar panel and magnetorquer currents. Fully calibrated magnetic vector data, together with time and position, are provided as daily files in CDF data format at swarm-diss.eo.esa.int. The data show good agreement with Swarm satellite magnetic measurements during close encounters (rms difference between 1 and 5 nT for inter-satellite distances below 300 km).
TL;DR: In this paper, the authors reported a major improvement in the sensing characteristics of "cross" geometry by using pulsed laser ablation grown Permalloy as a sensing material, achieving a field sensitivity of 650 Ω/T with an estimated detection threshold of 5nT in open-loop condition.
TL;DR: A systematic inspection of the magnetic field and electron density, recorded by Swarm three-satellite constellation over the seismic region hit by the 2016-2017 Amatrice-Norcia (Central Italy) seismic sequence, has allowed to identify some possible precursory anomalies, when disturbed periods of the geomagnetic conditions are properly taken into account and/or avoided as mentioned in this paper.
Abstract: A systematic inspection of the magnetic field and electron density, recorded by Swarm three-satellite constellation over the seismic region hit by the 2016–2017 Amatrice-Norcia (Central Italy) seismic sequence, has allowed us to identify some possible precursory anomalies, when disturbed periods of the geomagnetic conditions are properly taken into account and/or avoided. This paper aims at studying and interpreting the electromagnetic phenomena occurred before and during the 2016–2017 Amatrice-Norcia (Central Italy) seismic sequence, in order to look for any possible evidence of precursory anomalies. Results show magnetic field and electron density anomalies of four tracks that precede the major earthquakes of the seismic sequence. After an inspection of the geomagnetic conditions, a Swarm Charlie track, acquired on 20/08/2016 that precedes by 3.2 days the beginning of the whole seismic sequence, remains unexplainable with the normal geomagnetic disturbance phenomena of the Earth’s magnetic field. Furthermore, we carry out a blind study of possible relationship between abnormal magnetic field signals detected by Swarm satellites during geomagnetic quiet conditions and major seismic events from about 4 months before the start of the seismic sequence until about the first 8 months from the seismic sequence (i.e. a total of one year of analysed data). We find a very interesting increase of such anomalies starting about 40 days before the beginning of the seismic sequence. It coincides and follows surface and atmospheric alterations, resulting in a temporal sequence of anomalies from Earth’s surface up to ionosphere, supporting the possibility of lithosphere–atmosphere–ionosphere coupling models.
TL;DR: In this paper, the response of the ionosphere during the intense geomagnetic storms of October 12-20, 2016 and May 26-31, 2017 which occurred during the declining phase of the solar cycle 24.
TL;DR: In this paper, a time-dependent geomagnetic field model from platform magnetometer data is proposed, by co-estimating their calibration parameters with the internal field parameters, which can provide meaningful information on rapid core field variations during the gap period.
Abstract: For the past 20 years, state of the art geomagnetic core field models have relied heavily on magnetic measurements made from space-based instrumentation. These models have revealed rapid global magnetic field variations on sub-decadal timescales originating in Earth’s core. With the end of the CHAMP mission in 2010 and the launch of Swarm in late 2013, there has been a 3-year gap in high-quality satellite measurements of the geomagnetic field. Geomagnetic field models have therefore relied on ground observatory data to fill in this gap period. However, ground observatories are unable to provide a truly global picture of the core field and its temporal changes. Many satellites in operation carry vector fluxgate “platform” magnetometers for attitude control, which can offer an alternative to relying on ground observatory measurements during the gap period. However, these instruments need to be carefully calibrated in order to provide meaningful information on Earth’s core field. Some previous studies attempted to calibrate such instruments with a priori geomagnetic field models. This approach has several disadvantages: (1) errors in the model will introduce errors in the calibration parameters, and (2) relying on an a priori model may not be feasible in the post-Swarm era. In this paper, we develop a novel approach to build a time-dependent geomagnetic field model from platform magnetometer data, by co-estimating their calibration parameters with the internal field parameters. This method does not require an a priori geomagnetic field model, but does require a dataset of previously calibrated data. We use CHAMP, Swarm, and ground observatory measurements to supply this dataset, and incorporate platform magnetic measurements from DMSP and Cryosat-2 during the gap years. We find that the calibration parameters of DMSP and Cryosat-2 can be reliably estimated, and these missions provide meaningful information on rapid core field variations during the gap period.
TL;DR: The proposed MAD method employing an improved orthogonal basis function (OBF) for the magnetic tensor contraction decomposition improves the SNR of the MAD raw signal and is capable of detecting and locating the magnetic anomaly.
Abstract: In certain scenarios, such as detection of unexploded ordnance (UXO), submarines, and intruders, magnetic anomaly detection (MAD) is an effective method due to the magnetic field advantages of small operating power, strong penetrability, and strong anti-interference ability. However, variability in the background geomagnetic field and low signal-to-noise ratio (SNR) in MAD cannot be avoided. In this article, we propose a MAD method employing an improved orthogonal basis function (OBF) for the magnetic tensor contraction decomposition. The orthogonal basis decomposition approach makes the best use of modeling the MAD output signal, whereas the magnetic tensor contraction aid in avoiding the background field problem. Furthermore, by increasing the baseline length of the magnetic gradiometer, the SNR can also be improved in some manner. A dual magnetic gradiometer framework is introduced to fulfill the demand of the magnetic anomaly localization. From our simulation results, the proposed method improves the SNR of the MAD raw signal and is capable of detecting and locating the magnetic anomaly. Moreover, field test has been carried out to evaluate the performance of the proposed method.
TL;DR: The second in a series of papers that investigate the semi-annual, annual and Universal Time (UT) variations in the magnetosphere has been published in this article, where the authors present a varied collection of empirical results that can be used to constrain theories and modelling of these variations.
Abstract: This is the second in a series of papers that investigate the semi-annual, annual and Universal Time (UT ) variations in the magnetosphere We present a varied collection of empirical results that can be used to constrain theories and modelling of these variations An initial study of two years’ data on transpolar voltage shows that there is a semi-annual variation in magnetospheric flux circulation; however, it is not as large in amplitude as that in geomagnetic activity, consistent with the latter showing a non-linear (quadratic) variation with transpolar voltage We find that during the persistent minimum of the UT variation in geomagnetic activity, between about 2 and 10 UT , there is also a persistent decrease in observed transpolar voltage, which may be, in part, caused by a decrease in reconnection voltage in the nightside cross-tail current sheet We study the response of geomagnetic activity to estimated power input into the magnetosphere using interplanetary data from 1995 onwards, an interval for which the data are relatively free of data gaps We find no consistent variation in the response delay with time-of-year F and, using the optimum lag, we show that the patterns of variation in F -year spectrograms are very similar for geomagnetic activity and power input into the magnetosphere, both for average values and for the occurrence of large events The Russell–McPherron (R–M) mechanism is shown to be the central driver of this behaviour However, the (R–M) effect on power input into the magnetosphere is small and there is a non-linear amplification of the semi-annual variation in the geomagnetic response, such that a very small asymmetry in power input into the magnetosphere P α between the “favourable” and “unfavourable” polarities of the IMF B Y component generates a greatly amplified geomagnetic response The analysis strongly indicates that this amplification is associated with solar wind dynamic pressure and its role in squeezing the near-Earth tail and so modulating the storage and release of energy extracted from the solar wind In this paper, we show that the equinoctial pattern is found in the residuals of fits of P α to the am index and that the amplitude of these equinoctial patterns in the am fit residuals increases linearly with solar wind dynamic pressure Similarly, the UT variation in am is also found in these fit residuals and also increases in amplitude with solar wind dynamic pressure
TL;DR: In this article, the authors present new authigenic 10Be/9Be ratio (Be ratio) results covering the 60-20 ka time interval from equatorial core MD05-2920, interpreted as two independent recordings of the geomagnetic dipole moment (GDM) decrease linked to the Laschamp excursion dated at ca 41 ka.
TL;DR: A substantial and diverse set of 3D numerical dynamo simulations and recent observation-based field models are used to derive a power law relationship between the angular dispersion of virtual geomagnetic poles at the equator and the median axial dipole dominance measured at Earth’s surface.
Abstract: A defining characteristic of the recent geomagnetic field is its dominant axial dipole which provides its navigational utility and dictates the shape of the magnetosphere. Going back through time, much less is known about the degree of axial dipole dominance. Here we use a substantial and diverse set of 3D numerical dynamo simulations and recent observation-based field models to derive a power law relationship between the angular dispersion of virtual geomagnetic poles at the equator and the median axial dipole dominance measured at Earth’s surface. Applying this relation to published estimates of equatorial angular dispersion implies that geomagnetic axial dipole dominance averaged over 107–109 years has remained moderately high and stable through large parts of geological time. This provides an observational constraint to future studies of the geodynamo and palaeomagnetosphere. It also provides some reassurance as to the reliability of palaeogeographical reconstructions provided by palaeomagnetism. This study describes how the geomagnetic axial dipole dominance of Earth’s magnetic field remained stable through large parts of the geological time. Since other characteristics of the geomagnetic field have changed substantially on the same timescales, this new observation provides a challenge for future core modeling studies.
TL;DR: In this paper, the distribution characteristics of ionospheric F-region irregularities in the low latitudes were investigated using 16'Hz electron density observations made by a faceplate which is a component of the electric field instrument (EFI) onboard Swarm satellites of the European Space Agency (ESA).
Abstract: . During the night, in the F-region, equatorial ionospheric irregularities manifest as plasma depletions observed by satellites, and they may cause radio signals to fluctuate. In this study, the distribution characteristics of ionospheric F-region irregularities in the low latitudes were investigated using 16 Hz electron density observations made by a faceplate which is a component of the electric field instrument (EFI) onboard Swarm satellites of the European Space Agency (ESA). The study covers the period from October 2014 to October 2018 when the 16 Hz electron density data were available. For comparison, both the absolute ( dNe ) and relative ( dNe∕Ne ) density perturbations were used to quantify the level of ionospheric irregularities. The two methods generally reproduced the local-time (LT), seasonal and longitudinal distribution of equatorial ionospheric irregularities as shown in earlier studies, demonstrating the ability of Swarm 16 Hz electron density data. A difference between the two methods was observed based on the latitudinal distribution of ionospheric irregularities where ( dNe ) showed a symmetrical distribution about the magnetic equator, while dNe∕Ne showed a magnetic-equator-centred Gaussian distribution. High values of dNe and dNe∕Ne were observed in spatial bins with steep gradients of electron density from a longitudinal and seasonal perspective. The response of ionospheric irregularities to geomagnetic and solar activities was also investigated using Kp index and solar radio flux index (F10.7), respectively. The reliance of dNe∕Ne on solar and magnetic activity showed little distinction in the correlation between equatorial and off-equatorial latitudes, whereas dNe showed significant differences. With regard to seasonal and longitudinal distribution, high dNe and dNe∕Ne values were often found during quiet magnetic periods compared to magnetically disturbed periods. The dNe increased approximately linearly from low to moderate solar activity. Using the high-resolution faceplate data, we were able to identify ionospheric irregularities on the scale of only a few hundred of metres.
TL;DR: In this article, the authors used 25 years of data from the northern European International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network to show that the measured dH∕dt is dominated by the signature from the secondary induced telluric currents at nearly all IMAGE stations.
Abstract: . Geomagnetically induced currents (GICs) are directly described by ground electric fields, but estimating them is time-consuming and requires knowledge of the ionospheric currents and the three-dimensional (3D) distribution of the electrical conductivity of the Earth. The time derivative of the horizontal component of the ground magnetic field ( dH∕dt ) is closely related to the electric field via Faraday's law and provides a convenient proxy for the GIC risk. However, forecasting dH∕dt still remains a challenge. We use 25 years of 10 s data from the northern European International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network to show that part of this problem stems from the fact that, instead of the primary ionospheric currents, the measured dH∕dt is dominated by the signature from the secondary induced telluric currents at nearly all IMAGE stations. The largest effects due to telluric currents occur at coastal sites close to high-conducting ocean water and close to near-surface conductivity anomalies. The secondary magnetic field contribution to the total field is a few tens of percent, in accordance with earlier studies. Our results have been derived using IMAGE data and are thus only valid for the stations involved. However, it is likely that the main principle also applies to other areas. Consequently, it is recommended that the field separation into internal (telluric) and external (ionospheric and magnetospheric) parts is performed whenever feasible (i.e., a dense observation network is available).
TL;DR: In this paper, the geomagnetic field model COV-OBS.x2 is presented, which is constrained by observatory series, satellite data, plus older surveys.
Abstract: We present the geomagnetic field model COV-OBS.x2 that covers the period 1840–2020. It is primarily constrained by observatory series, satellite data, plus older surveys. Over the past two decades, we consider annual differences of 4-monthly means at ground-based stations (since 1996), and virtual observatory series derived from magnetic data of the satellite missions CHAMP (over 2001–2010) and Swarm (since 2013). A priori information is needed to complement the constraints carried by geomagnetic records and solve the ill-posed geomagnetic inverse problem. We use for this purpose temporal cross-covariances associated with auto-regressive stochastic processes of order 2, whose parameters are chosen so as to mimic the temporal power spectral density observed in paleomagnetic and observatory series. We aim this way to obtain as far as possible realistic posterior model uncertainties. These can be used to infer for instance the core dynamics through data assimilation algorithms, or an envelope for short-term magnetic field forecasts. We show that because of the projection onto splines, one needs to inflate the formal model error variances at the most recent epochs, in order to account for unmodeled high frequency core field changes. As a by-product of the core field model, we co-estimate the external magnetospheric dipole evolution on periods longer than 2 years. It is efficiently summarized as the sum of a damped oscillator (of period 10.5 years and decay rate 55 years), plus a short-memory (6 years) damped random walk.
TL;DR: A significant ~8.6 year periodic increasing oscillation in length of day and its good link to geomagnetic jerks related to Earth’s core oscillations, which may be used to predict the future jerk timings are found.
Abstract: Earth’s core oscillations and magnetic field inside the liquid outer core cannot be observed directly from the surface, we can infer these information from the intradecadal variations in Earth’s rotation rate defined by length of day. However, the fine time-varying characteristics as well as relevant mechanisms of the intradecadal variations are still unclear. Here we report that the intradecadal variations present a significant 8.6-year harmonic component with an unexpected increasing phenomenon, besides a 6-year decreasing oscillation. More importantly, we find that there is a very good correspondence between the extremes of the 8.6-year oscillation with geomagnetic jerks. The fast equatorial waves with subdecadal periods propagating at Earth’s core surface may explain the origin of this 8.6-year oscillation. Earth rotation variation reflects the physics, dynamics and the magnetic field changes of Earth’s interior. The authors find a significant ~8.6 year periodic increasing oscillation in length of day and its good link to geomagnetic jerks related to Earth’s core oscillations, which may be used to predict the future jerk timings.