near-Earth Magnetic Field Research Articles

Some Features of the Present-day Transition Period in Solar Activity

Various geophysical indices and their prognostic value were analyzed in (Obridko et al., 2013; Kirov et al., 2013, 2015, 2017; Georgieva et al., 2015, 2018). Two indices have been selected for the analysis (total annual Dst values, and duration of a storm). It is important to note that, unlike the other indices of solar and geophysical activity, the summary Dst index does not show violation of the Gnevyshev-Ohl rule. The analysis of annual mean Dst values reveals a clearly pronounced decrease in activity, at least since the 1980-ies. This allows us to suggest that we are at the descending branch of a secular cycle or on the threshold of a Grand Minimum. The decrease in solar activity is corroborated by the analysis of Forbush effects, solar wind speed, and intensity of the near-Earth magnetic field.

Convolutional Neural Networks for Predicting the Strength of the Near-Earth Magnetic Field Caused by Interplanetary Coronal Mass Ejections

In this paper, we explore the potential of neural networks for making space weather predictions based on near-Sun observations. Our second goal is to determine the extent to which coronal polarimetric observations of erupting structures near the Sun encode sufficient information to predict the impact these structures will have on Earth. We focus on predicting the maximal southward component of the magnetic field (-Bz) inside an interplanetary coronal mass ejection (ICME) as it impacts the Earth. We use Gibson&Low (G&L) self-similarly expanding flux rope model (Gibson&Low 1998), which allows to consider CMEs with varying location, orientation, size, and morphology. We vary 5 parameters of the model to alter these CME properties, and generate a large database of synthetic CMEs (over 36k synthetic events). For each model CME, we synthesize near-Sun observations, as seen from an observer in quadrature (assuming the CME is directed Earthwards), of either three components of the vector magnetic field (“Experiment 1”), or of synthetic Stokes images, (“Experiment 2”). We then allow the flux rope to expand and record max(-Bz) as the ICME passes 1AU. We further conduct two separate machine learning experiments and develop two different regression-based deep convolutional neural networks (CNNs) to predict max(-Bz) based on these two kinds of the near-Sun input data. Experiment 1 is a proof of concept, to see if a 3-channel CNN (hereafter CNN1), similar to those used in RGB image recognition, can reproduce the results of the self-similar (i.e. scale-invariant) expansion of the G&L model. Experiment 2 is less trivial, as Stokes vector is not linearly related to B, and the line-of-sight integration in the optically thin corona presents additional difficulties for interpreting the signal. This second CNN (hereafter CNN2), although resembling CNN1 in Experiment 1, will have a different number of layers and set of hyperparameters due to a much more complicated mapping between the input and output data. We find that, given vector B, CNN1 can predict max(-Bz) with 97% accuracy, and for the Stokes vector as input, CNN2 can predict max(-Bz) with 95%, both measured in the relative root square error.

CM6: a comprehensive geomagnetic field model derived from both CHAMP and Swarm satellite observations

From the launch of the Ørsted 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.

Extracting planetary waves from geomagnetic time series using Empirical Mode Decomposition

Empirical Mode Decomposition is presented as an alternative to traditional analysis methods to decompose geomagnetic time series into spectral components. Important comments on the algorithm and its variations will be given. Using this technique, planetary wave modes of 5-, 10-, and 16-day mean periods can be extracted from magnetic field components of three different stations in Germany. In a second step, the amplitude modulation functions of these wave modes can be shown to contain significant contribution from solar cycle variation through correlation with smoothed sunspot numbers. Additionally, the data indicate connections with geomagnetic jerk occurrences, supported by a second set of data providing reconstructed near-Earth magnetic field for 150 years. Usually attributed to internal dynamo processes within the Earth's outer core, the question of who is impacting whom will be briefly discussed here.

Predicting space climate change

Galactic cosmic rays and solar energetic particles can be hazardous to humans in space, damage spacecraft and satellites, pose threats to aircraft electronics, and expose aircrew and passengers to radiation. A new study shows that these threats are likely to increase in coming years as the Sun approaches the end of the period of high solar activity known as “grand solar maximum,” which has persisted through the past several decades. High solar activity can help protect the Earth by repelling incoming galactic cosmic rays. Understanding the past record can help scientists predict future conditions.Barnard et al. analyzed a 9300‐year record of galactic cosmic ray and solar activity based on cosmogenic isotopes in ice cores as well as on neutron monitor data. They used this to predict future variations in galactic cosmic ray flux, near‐Earth interplanetary magnetic field, sunspot number, and probability of large solar energetic particle events. The researchers found that the risk of space weather radiation events will likely increase noticeably over the next century compared with recent decades and that lower solar activity will lead to increased galactic cosmic ray levels. (Geophysical Research Letters, doi:10.1029/2011GL048489, 2011)

Geospheric effects of the solar flare of December 13, 2006

Disturbances in the magnetic field and magnetospheric and ionospheric plasma registered on December 14–16, 2006, during a strong magnetic storm caused by a solar flare of 4B/X3.4 class are studied. It is shown that in the north of Yakutia, interactions between the Earth’s magnetosphere and the region of high dynamic pressure of the solar wind led to the formation of sporadic layers in the ionospheric E and F regions, depletion of the critical frequency of the F2 layer, and total absorption. At the end of the magnetic storm’s main phase, anomalously high values of foF2 exceeding the quiet level by a factor of 1.5–1.7 were detected. It was found that the disturbances detected by ground-based observatories had developed on the background of changes in the temperature, density, and the pitch-angle distribution of particles at the geostationary orbit manifesting radial shifts of magnetospheric structures (magnetopause, cusp/cleft, and plasma sheet) relative to the observation points. A conclusion is drawn that in this case, changes in the near-Earth plasma and magnetic field manifest the dynamics of the physical conditions at the magnetospheric boundary and diurnal rotation of the Earth.

Solar cycle dependence of quiet-time magnetospheric currents and a model of their near-Earth magnetic fields

Satellite measurements from high-resolution magnetic field mapping missions over almost a complete solar cycle have been used to investigate the variability of major magnetospheric currents during quiet times. As judged from near-Earth magnetic field observations, the magnetotail current system exhibits no long-term trend. Activity-dependent enhancements decay within hours. A suitable quantity to parameterize the temporal variation is the merging electric field, delayed by 60 minutes with respect to its value at the bow shock. Conversely, the ring current strength shows pronounced solar cycle dependences. We show for the first time that a solar cycle dependent bias has to be added to DST to represent the actual ring current strength. It ranges from −15 nT at solar maximum to zero at solar minimum. The scaled solar flux index, F10.7, delayed by 20 months, is found to be a suitable proxy for predicting the bias. This empirical result reflects no physical relation between F10.7 and ring current bias. A spherical harmonic model valid at Earth’s surface is presented that reflects the quiet-time magnetospheric fields.

An inspection of the long-term behaviour of the range of the daily geomagnetic field variation from comprehensive modelling

This paper attempts to reveal whether long-term trends in the ionosphere are reflected in the amplitude range of the geomagnetic daily variation recorded at ground level. The smooth and regular variation observed in the magnetograms on magnetically quiet days is induced by the ionospheric currents flowing in the dynamo region. So it is likely that trends in the conductivity or in the dynamics of this region could produce changes in the current densities, and consequently in the range of the geomagnetic variation. The crucial aspect is how to separate the changes produced by the geomagnetic activity itself, or by secular changes of the Earth's magnetic field, from the part of the variation produced by factors affecting trends in the ionosphere, which could have an anthropogenic origin. To investigate this, we synthesized for several geomagnetic observatories the daily ranges of the geomagnetic field components with a comprehensive model of the quiet-time, near-Earth magnetic field, and finally we removed the synthetic values from the observed ranges at those observatories. This comprehensive model accounts for contributions from Earth's core, lithosphere, ionosphere, magnetosphere and coupling currents, and, additionally, accounts for influences of main field and solar activity variations on the ionosphere. Therefore, any trend remaining in the residuals, assuming that all the contributions mentioned above are properly described and thus removed by the comprehensive model, should reflect the influence of other sources. Results, based on series of magnetic data from observatories worldwide distributed, are presented. Trends in the X and Z components are misleading, since the current system changes in form as well as in intensity, producing changes of the focus latitude in the course of a solar cycle and from one cycle to another. Some differences exist between the long-term trends in the Y component between the real and modelled ranges, suggesting that other non-direct solar causes to the amplitude changes of the solar quiet geomagnetic variation should not be ruled out. Nevertheless, the results also reflect some short-comings in the way that the comprehensive modelling accounts for the influence of the solar activity on the range of the daily geomagnetic variation.

3-D Global Induction in the Oceans and Solid Earth: Recent Progress in Modeling Magnetic and Electric Fields from Sources of Magnetospheric, Ionospheric and Oceanic Origin

Electromagnetic induction in the Earth’s interior is an important contributor to the near-Earth magnetic and electric fields. The oceans play a special role in this induction due to their relatively high conductivity which leads to large lateral variability in surface conductance. Electric currents that generate secondary fields are induced in the oceans by two different processes: (a) by time varying external magnetic fields, and (b) by the motion of the conducting ocean water through the Earth’s main magnetic field. Significant progress in accurate and detailed predictions of the electric and magnetic fields induced by these sources has been achieved during the last few years, via realistic three-dimensional (3-D) conductivity models of the oceans, crust and mantle along with realistic source models. In this review a summary is given of the results of recent 3-D modeling studies in which estimates are obtained for the magnetic and electric signals at both the ground and satellite altitudes induced by a variety of natural current sources. 3-D induction effects due to magnetospheric currents (magnetic storms), ionospheric currents (Sq, polar and equatorial electrojets), ocean tides, global ocean circulation and tsunami are considered. These modeling studies demonstrate that the 3-D induction (ocean) effect and motionally-induced signals from the oceans contribute significantly (in the range from a few to tens nanotesla) to the near-Earth magnetic field. A 3-D numerical solution based on an integral equation approach is shown to predict these induction effects with the accuracy and spatial detail required to explain observations both on the ground and at satellite altitudes.

Normal waves of the anisotropic Earth—ionosphere waveguide in the VLF-ULF range

[1] The problem on determination of the electromagnetic field of a near-Earth horizontal electric dipole in the frequency range 10−1 − 10 Hz at distances 10−1 − 10 km in the Earth–ionosphere waveguide has been solved. The ionosphere is assumed to be horizontally stratified and anisotropic, and the Earth is assumed to be homogeneous to some depth and infinitely conducting below. The representation of the electromagnetic field as expansion in waveguide modes is given. The zero mode is regarded as anisotropic. Local modes are given in the isotropic approximation of the anisotropic ionosphere due to inclusion of an effective exponential conductivity profile. An analytical description of the field reflection for vertical and horizontal polarizations has been obtained for this model. At short distances from the source, the solution coincides with the known solution for the case of the absence of the ionosphere. The range of distances beginning from which the influence of the ionosphere on the near-Earth magnetic field manifests itself has been found.

Effect of Tailward Flows From the Ionosphere on the Near-Earth Magnetic Field

探测一号(TC-1)卫星的观测结果表明,尾向流能够拉伸近地磁尾的磁力线,从而导致磁场位形改变.尾向流具有垂直于磁场的速度分量,这种垂直磁场的速度分量会导致磁力线向尾向拉伸,磁场的结构由偶极型变为非偶极型.而随尾向流的终止,地向流的出现,磁场的结构由非偶极型变为偶极型,磁力线恢复原状.另外在磁场的结构由非偶极型变为偶极型的过程中,伴随磁能的释放热离子温度的迅速升高,温度由各向同性逐渐趋向各向异性.其次,观测结果显示来自电离层的尾向流对磁场By分量有重要的影响,能够引起磁场By分量的显著增强.上述分析结果表明来自电离层的尾向流对近地磁尾动力学过程有着重要的影响.

Modeling the Sun's open magnetic flux

Context. The heliospheric magnetic field can be extrapolated from the photospheric field distribution using models based upon various approximations. Such models are required for the reconstruction of the open solar magnetic flux prior to the time of direct measurements on the basis of surface flux transport simulations. Aims. We evaluate the consistency of extrapolation models with direct measurements of the heliospheric magnetic field. Furthermore, we study whether extrapolations on the basis of a surface flux transport model for the photospheric magnetic field reproduce the temporal evolution of the measured near-Earth magnetic field. Methods. We use the potential field source surface (PFSS) model and the current sheet source surface (CSSS) model to extrapolate the heliospheric field on the basis of the Wilcox Solar Observatory (WSO) synoptic maps of the solar surface field from 1976-2005. The results are compared with the near-Earth measurements of the radial heliospheric field and its independence of latitude found with Ulysses. Furthermore, we determine extrapolations on the basis of photospheric flux distributions computed with a surface flux transport code, using as input sunspot group areas from the SOON database. Results. The CSSS model based upon WSO data, with a source surface located at >10 R ⊙ and cusp surface at 1.7 R ⊙ . yields the best agreement with the measurements. The flux transport simulations reproduce the observed surface flux together with the open flux if the tilt angle of emerging bipolar magnetic regions is smaller than commonly assumed, but consistent with sunspot observations. Conclusions. The CSSS model with a source surface in the vicinity of the Alfvenic point of the solar wind leads to a good extrapolation of the heliospheric field from solar surface data. A surface flux transport model based upon sunspot data with consistently calibrated tilt angles reproduces the observed evolution of the solar total open flux.

CHAOS-a model of the Earth's magnetic field derived from CHAMP, Ørsted, and SAC-C magnetic satellite data

SUMMARY We have derived a model of the near-Earth magnetic field (up to spherical harmonic degree n = 50 for the static field, and up to n = 18 for the first time derivative) using more than 6.5 yr of high-precision geomagnetic measurements from the three satellites Orsted, CHAMP and SAC-C taken between 1999 March and 2005 December. Our modelling approach goes in several aspects beyond that used for recent models: (i) we use different data selection criteria and allow for higher geomagnetic activity (index Kp ≤ 2o), thus we include more data than previous models; (ii) we describe the temporal variation of the core field by splines (for n ≤ 14); (iii) we take magnetometer vector data in the instrument frame and co-estimate the Euler angles that describe the transformation from the magnetometer frame to the star imager frame, avoiding the inconsistency of using vector data that have been aligned using a different (pre-existing) field model; (iv) we account for the bending of the CHAMP optical bench connecting magnetometer and star imager by estimating Euler angles in 10 day segments and (v) we co-estimate degree-1 external fields separately for every 12 hr interval. The model provides a reliable representation of the static (core and crustal) field up to spherical harmonic degree n = 40, and of the first time derivative up to n = 15.

Modeling and analysis of solar wind generated contributions to the near-Earth magnetic field

Abstract Solar wind generated magnetic disturbances are currently one of the major obstacles for improving the accuracy in the determination of the magnetic field due to sources internal to the Earth. In the present study a global MHD model of solar wind magnetosphere interaction is used to obtain a physically consistent, divergence-free model of ionospheric, field-aligned and magnetospheric currents in a realistic magnetospheric geometry. The magnetic field near the Earth due to these currents is analyzed by estimating and comparing the contributions from the various parts of the system, with the aim of identifying the most important aspects of the solar wind disturbances in an internal field modeling context. The contribution from the distant magnetospheric currents is found to consist of two, mainly opposing, contributions from respectively the dayside magnetopause currents and the cross-tail current. At high latitudes the field-aligned component is of partidular interest in connection with internal field-modelling. In the altitude regime of 400–800 km (typical for low Earth orbit satellites) the ionospheric currents are found to contribute significantly to the disturbancance, and account for more than 90% of the field-aligned disturbance. The magnetic disturbance field from field-aligned currents (FACs) is basically transverse to the main field, and they therefore contribute with less than 2% to the disturbance in total field intensity. Inhomogeneity in ionospheric conductance is identified as the main cause of main-field disturbance in the field-aligned direction. These disturbances are associated with the ionospheric Pedersen currents, and may introduce systematic errors in internal field models.

Extending comprehensive models of the Earth's magnetic field with Ørsted and CHAMP data

SUMMARY A new model of the quiet-time, near-Earth magnetic field has been derived using a comprehensive approach, which includes not only POGO and Magsat satellite data, but also data from the Orsted and CHAMP satellites. The resulting model shows great improvement over its predecessors in terms of completeness of sources, time span and noise reduction in parameters. With its well separated fields and extended time domain of 1960 to mid-2002, the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997. Because all sources are coestimated in a comprehensive approach, intriguing north–south features typically filtered out with other methods are being discovered in the lithospheric representation of the model, such as the S Atlantic spreading ridge and Andean subduction zone lineations. In addition, this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection. The F-region currents, through which the satellites pass, are now treated as lying within meridional planes, as opposed to being purely radial. Results are consistent with those found previously for Magsat, but an analysis at Orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite. Besides the model, a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset. This has proven very useful in diagnosing the cause of peculiar artefacts in the Magsat vector data, which seem to suggest the presence of a small misalignment bias in the vector magnetometer.

A comprehensive model of the quiet-time, near-Earth magnetic field: phase 3

SUMMARY The near-Earth magnetic field is caused by sources in the Earth’s core, ionosphere, magnetosphere, lithosphere and from coupling currents between the ionosphere and the magnetosphere, and between hemispheres. Traditionally, the main field (low degree internal field) and magnetospheric field have been modelled simultaneously, with fields from other sources being modelled separately. Such a scheme, however, can introduce spurious features, especially when the spatial and temporal scales of the fields overlap. A new model, designated CM3 (Comprehensive Model: phase 3), is the third in a series of efforts to coestimate fields from all of these sources. This model has been derived from quiet-time Magsat and POGO satellite and observatory hourly means measurements for the period 1960‐1985. It represents a significant advance in the treatment of the aforementioned field sources over previous attempts, and includes an accounting for main field influences on the magnetosphere, main field and solar activity influences on the ionosphere, seasonal influences on the coupling currents, a priori characterization of the influence of the ionosphere and the magnetosphere on Earth-induced fields, and an explicit parametrization and estimation of the lithospheric field. The result is a model that describes well the 591 432 data with 16 594 parameters, implying a data-to-parameter ratio of 36, which is larger than several popular field models.

Comparison of efficiency of artificial neural networks for forecasting the geomagnetic activity index Dst

Using artificial neural networks (ANN), we study problems of forecasting the evolution of the geomagnetic D st index on the basis of parameters of the solar-wind plasma and magnetic field obtained from the OMNI satellite system. Using this sample problem, we compare the neural network modifications with error backpropagation, such as classic network and feedback network. We analyze the efficiency of a feedback in the hidden layer of the Elman neural network which has a dynamic memory due to the time delay. The role of a delay in the input layer of a neural network is studied. It is shown that the forecasting efficiency increases significantly if a feedback in the hidden layer and the time delay in the input layer are used. The hidden-layer feedback and the input-layer time delay reproduce the time delay in the physical problem considered. Using thresholds in activation functions does not influence the ANN efficiency. The solar-wind parameters that have the strongest influence on the geomagnetic-storm formation are found. The influence of the number of hidden-layer neurons on the quality of forecasting the global disturbances in the near-Earth magnetic field is studied. It is pointed out that the networks with error backpropagation allow, after the relevant tuning and learning, fairly efficient forecasting of phenomena similar to geomagnetic storms several hours before a large-scale magnetospheric event.

Hot plasma pressure variations on the geostationary orbit on the base of Gorizont satellite data

Plasma and energetic particles pressure distribution is studied using data from the plasma and energetic particle experiment (0.1 – 133 keV) onboard the Gorizont-35 geostationary satellite for the period from 11 to 25 March 1992. The analysed period consists of relatively quiet time, small geomagnetic storms, SC and the time of the northern orientation of the IMF. The calculations show that the basic contribution to the total particle pressure was made by ions at the energy from 0.1 to 12.4 keV. The derived average value of the calculated pressure (∼1 nPa) points to the important role of the geostationary orbit plasma population in the formation of the magnetosphere pressure balance and of the near-Earth magnetic field distortion.

Geostationary orbit plasma pressure variations according to gorizont satellite data

Plasma and energetic particles pressure distribution is studied using data from the plasma and energetic particle experiment (0.1 –133 keV) onboard the Gorizont-35 geostationary satellite for the period from 11 to 25 March 1992. The analysed period consists of relatively quiet time, small geomagnetic storms, SC and the time of the northern orientation of the IMF. The calculations show that the basic contribution to the total particle pressure was made by ions at the energy from 0.1 to 12.4 keV. The derived average value of the calculated pressure (≈1 nPa) points to the important role of the geostationary orbit plasma population in the formation of the magnetopause pressure balance and of the near-Earth magnetic field distortion.

The missing dimension in Magsat and POGO anomaly studies

The development of a comprehensive model of the near‐Earth magnetic field in terms of sources in the core, ionosphere, and magnetosphere has allowed the isolation of heretofore unknown north‐south trending anomalies of lithospheric origin. Previously, along‐track filtering of satellite magnetic field data had been necessary in order to remove unmodeled magnetospheric and ionospheric magnetic fields. These fields change between adjacent passes by tens of nanoTeslas and have wavelengths which overlap the crustal spectrum. An example from Australia illustrates the additional information that can be extracted from the Magsat data set when this ‘missing dimension‧ is included. A north‐south trending low covers most of eastern Australia, in agreement with the aeromagnetic map of Australia. This feature is seen in both the dawn and dusk subsets collected from Magsat. The center of this low is roughly coincident with the Eromanga Basin but extends for almost 2000 km in a N‐S direction. The low corresponds to a region of almost featureless magnetic field on the aeromagnetic map. It is also consistent with an elevated geotherm previously proposed from studies of xenoliths in southeastern Australia. Filtering the new map in a north‐south direction reproduces the previous best estimate of the Australian satellite anomaly field. We expect that this comprehensive model will provide a new window into the magnetic field of the earth's lithosphere. The model continues to be enhanced.