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The electron density (Ne), ion density (Ni), and electron temperature (Te) statistics recorded by the DEMETER satellite payload ISL (Instrument Sonde de Langmuir) were used to study the disturbance characteristics of the ionosphere before solid earthquakes of magnitude 6 or higher in Japan during the summer of 2005 – 2009 , to provide more information and methods for the coupling mechanism and short-range earthquake prediction. In this paper, the region of ± 10 ° of the epicenter is divided into 1 ° × 1 ° pixels, and the background field of each parameter is constructed using data without earthquakes and relatively calm space weather. We also define a measure of the perturbation intensity of ionospheric parameters relative to the background field during the occurrence of earthquakes. The analysis results of the four Japanese earthquakes from space and time show an excellent synchronization in the time and area of the anomalies in ionospheric parameters of the four cases. All four instances showed Ne abnormalities, and three showed Ni and Te anomalies, in which N e and Ni increased or decreased abnormally, while Te increased abnormally every time, and the anomalies mainly occurred about 9–12 days before the earthquake. This paper eliminates the influence of solar, geomagnetic, and satellite data defects on the experimental results as much as possible in data screening and method selection. The results partially agree with the conclusions reported in the existing literature, and the obtained anomalies are somewhat related to the ionospheric precursors of earthquakes. [ABSTRACT FROM AUTHOR]

The history of the research on the SC (sudden commencement) of magnetic storms before World War II is studied in this paper. Since geomagnetic research activities before World War II are still not yet fully known, this paper aims to reveal some historical facts related to SC investigation at that time. The first conclusion of this paper is the possible first discoverer of the simultaneity of SC at distant locations. We show that a Portuguese scientist had already pointed it out 16 years earlier than believed. The second conclusion is the role and activities of Aikitu Tanakadate as the reporter of the SC investigation committee of STME (Section of Terrestrial Magnetism and Electricity) and IATME (International Association of Terrestrial Magnetism and Electricity) in the IGGU (International Geodetic and Geophysical Union) or IUGG (International Union of Geodesy and Geophysics). Very little was known about his activities as the reporter of this committee. Our investigation at the Tanakadate Aikitu Memorial Science Museum disclosed how he acted and what he thought of SC, based on his frequent letters to and from other scientists. The third conclusion concerns SC research carried out by Japanese scientists during the period of the Second International Polar Year (1932–1933). Not only Tanakadate but also many other Japanese scientists participated in SC research during this international project. This formed a traditional basis of SC investigation in Japan, prompting a number of Japanese scientists to study SC after World War II. [ABSTRACT FROM AUTHOR]

Tephra rings that surround maar craters are typically inferred from field observations to have been emplaced rapidly over a time period of days to years and thus monogenetic, which is, however, rarely assessed quantitatively. This paper describes a polycyclic origin of the Mamiyadake tephra ring (Japan), comparing the paleomagnetic directions obtained from throughout the stratigraphy. The new data show that the paleomagnetic directions change systematically with stratigraphic height through the sections, which is interpreted to record paleosecular variation (PSV) of the geomagnetic field during formation of the tephra ring. The paleomagnetic results, together with using an average rate of PSV in Japan, indicate that the Mamiyadake tephra ring was constructed episodically with five major eruptive episodes, separated by centuries or longer, over a period of at least 1000 years. The findings demonstrate that detailed paleomagnetic characterization can uncover the temporal evolution of tephra rings, providing a useful criterion for identifying time breaks, even where field evidence is lacking, and a minimum estimate of the time interval for their emplacement. The approach used here may be applicable to volcanoes of any type. [ABSTRACT FROM AUTHOR]

Since the appearance and evolution of earthquake ionospheric precursors are expected to show a nonlinear and complex behaviour, the use of nonlinear predictor models seems more appropriate. This paper proposes a new approach based on deep learning as a powerful tool for extracting the nonlinear patterns from a time series of ionospheric precursors. A Long Short-Term Memory (LSTM) network as a type of Recurrent Neural Network (RNN) was used to investigate 52 six-month time series, deduced from the three Swarm satellite (Alpha (A), Bravo (B) and Charlie (C)) measurements, including electron density (Ne), electron temperature (Te), magnetic scalar and vector (X, Y, Z) components, Slant and Vertical Total Electron Content (STEC and VTEC), for day and night periods around the time and location of a seismic event. This new approach was tested on a strong Mw = 7.1 earthquake in Japan on 13 February 2021, at 14:07:50 UTC by comparing the results with two implemented methods, i.e., Median and LSTM methods. Furthermore, clear anomalies are seen by a voting classification method 1, 6, 8, 13, 31 and 32 days before the earthquake. A comparison with atmospheric data investigation is further provided, supporting the lithosphere–atmosphere–ionosphere coupling (LAIC) mechanism as a suitable theory to explain the alteration of upper geolayers in the earthquake preparation phase. In other words, using multi-method and multi-precursor analysis applied to 52 time series and also to the orbit-by-orbit investigation, the observed anomalies on the previous day and up to 32 days before the event in normal solar and quiet geomagnetic conditions could be considered as a striking hint of the forthcoming Japan earthquake. [ABSTRACT FROM AUTHOR]

This paper demonstrates and assesses the capability of the advanced three-dimensional (3-D) ionosphere tomography technique, during severe conditions. The study area is northeast Asia and quasi-Japan-centred. Reconstructions are based on total electron content data from a dense ground-based global navigation satellite system receiver network and parameters from operational ionosondes. We used observations from ionosondes, Swarm satellites and radio occultation (RO) to assess the 3-D picture. Specifically, we focus on St. Patrick's day geomagnetic storm (17–19 March 2015), the most intense in solar cycle 24. During this event, the energy ingested into the ionosphere resulted in Dst and Kp and reaching values ~ − 223 nT and 8, respectively, and the region of interest, the East Asian sector, was characterized by a ~ 60% reduction in electron densities. Results show that the reconstructed densities follow the physical dynamics previously discussed in earlier publications about storm events. Moreover, even when ionosonde data were not available, the technique could still provide a consistent picture of the ionosphere vertical structure. Furthermore, analyses show that there is a profound agreement between the RO profiles/in-situ densities and the reconstructions. Therefore, the technique is a potential candidate for applications that are sensitive to ionospheric corrections. [ABSTRACT FROM AUTHOR]

The paper analyzes the data of manual ionograms processing of hourly measurements of the critical frequency foF2 of the F2 ionospheric layer at the Wakkanai ionospheric vertical sounding station (Japan) in a geomagnetically quiet environment before a series of earthquakes with magnitude M > 6.0, for which the station entered the earthquake preparation zone, in order to detect possible Ionospheric Disturbances Preceding Earthquakes (IDPE), and to determine their quantitative characteristics. Detected IDPE, in the opinion of the authors, can be related to the processes of preparation of the corresponding earthquakes, i.e., to be Ionospheric Precursors of Earthquakes (IPE). [ABSTRACT FROM AUTHOR]

Surface atmospheric electric field (AEF) measurements were conducted using a Kelvin water dropper equalizer by the Japan Meteorological Agency's geomagnetic observatory at Kakioka, Ibaraki Prefecture, Japan, from 1929 to February 28, 2021. To obtain AEF data successive to the water dropper measurement, we independently installed the field mills at Kakioka. In this present study, we estimated the calibration coefficients based on comparisons between the water dropper and the field mills through parallel observations over 3 days. The results revealed that the new observations could be regarded as continuous data. Our new data from the field mills in this study have begun the process of being distributed in GLObal Coordination of Atmospheric Electricity Measurements (GloCAEM) as both archived and real‐time data in 1 min and 1 s samplings. [ABSTRACT FROM AUTHOR]

The state of the ionosphere affects the operational stability of satellite radio navigation systems. The dense network of GEONET GPS sites on the territory of Japan allows to conduct a thorough investigation of the stability of system operation at mid-latitudes in different heliogeophysical conditions. This paper considers deterioration in the accuracy of GPS system positioning due to the effect of a large-scale ionospheric disturbance (an ionospheric bubble) over Japan on February 12, 2000. [ABSTRACT FROM AUTHOR]

Vulcanian activity is one of the most common eruption styles of arc andesitic volcanism on Earth. It ejects and deposits volcanic bombs around the source crater. Although paleomagnetic studies of volcanic bombs are limited, such studies can potentially provide more opportunities for high-resolution paleomagnetic dating of volcanic activity. In this study, paleomagnetic dating was applied to large (> 1 m) volcanic bombs around active craters in the Azuma volcano group, NE Japan. Oriented samples were collected from the interior parts of five large volcanic bombs situated on gentle slopes, a few hundred meters from the source crater. More than six core samples were collected from each bomb and all samples were subjected to a range of rock magnetic experiments, including anisotropy of magnetic susceptibility (AMS) and thermal/alternating field demagnetization (THD/AFD) analyses. The Characteristic Remanent Magnetization (ChRM) directions for specimens from all bombs were well-defined, have small α95 (< 2.5º), and are in close agreement with each other. Comparing our measured overall mean direction (Dm = 355.5º, Im = 49.8º, α95 = 1.6º) with modeled geomagnetic field estimates and a reference secular variation curve for this area (using MATLAB-based archaeomagnetic dating tool), we suggest that the volcanic bombs were produced in the historical Meiji period (1893–1895 CE) eruption. In addition, a combination of the data of ChRM, AMS, thermomagnetic analyses, hysteresis measurement, and XRF analysis indicates that the volcanic bombs were derived from a plug of lava in the conduit under the solidification point (ca. 800 °C), but above the Curie point of the titanomagnetite remanence carrier (around 300 °C). We show that volcanic bombs can be powerful for paleomagnetic dating if certain sampling conditions, such as quantity, situation, size and portion are satisfied. [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via the original article. [ABSTRACT FROM AUTHOR]

We present new absolute archaeointensity data from six archaeological sites situated in the Okayama Prefecture, Japan. The materials studied are well-dated fragments from pottery, ceramic coffins and haniwa artefacts. Their ages range from 160 AD to 675 AD, covering the Late Yayoi and Kofun periods. Rock magnetic experiments suggest the presence of magnetite and/or Ti-magnetite as the main carrier of the remanence, with a possible minor contribution of higher coercivity minerals. After thermal demagnetization experiments, the most magnetically stable samples were selected for archaeointensity analysis performed following the double-heating method proposed by Thellier and modified by Coe. Partial thermoremanent magnetization (pTRM) checks and pTRM tail-checks were performed for monitoring possible chemical alterations during heating. All measurements were corrected for both anisotropy and cooling-rate effects. Successful archaeointensity determinations, following rigorous selection criteria, were obtained for samples from all the investigated archaeological sites. Compared with literature data from Japan, the new high-quality data show significantly lower intensity values. They also reveal possible fast secular variation changes during the Late Yayoi period and very weak geomagnetic intensity field around 630 AD. Such values offer evidence of a possible recurrence of weak intensity field in East Asia, suggesting an ancient recurrence of the West Pacific Anomaly. The new data might change the archaeomagnetic field models interpretations in the area, even though more data are still necessary to better understand the secular variation in Japan and the temporal evolution of the geomagnetic field's behaviour in East Asia. [ABSTRACT FROM AUTHOR]

Data set from three geomagnetic observatories located along the African meridian has been used to carry out an extensive study on the seasonal variability of Sq in horizontal H, declination D, and vertical Z intensity. The stations were managed by the International Centre for Space Weather Science Education ICSWSE, Kyushu University, Japan. Twelve months of data of only quiet days between August 2008 and September 2009 were used in the analysis following Lloyd's seasonal classification. Seasonal variability was found to exist in all magnetic elements with equinoctial maxima and D-season minimum in the H field. The observed Sq in both D and Z fields has maximum J-season amplitude and minimum during September equinox. Results of the correlation coefficient show strong and positive correlation values between the same elements at different seasons. A poor correlation exists between the H field and other elements at all seasons but D and Z fields are well correlated in some seasons. This depicts some level of the interrelationship among geomagnetic elements at different seasons. Also, the variability in ionospheric current and shift in the relative position of Sq focus is assumed to be responsible for the seasonal variations. [ABSTRACT FROM AUTHOR]

A numerical model of the influence of a strong earthquake on disturbances to the geomagnetic field is proposed. The impact on the epicentral region of the Earth's ionosphere of wave atmospheric disturbances caused by a seismic event is considered as the main mechanism of the magnetic effect. The estimates made on the basis of the proposed model using the example of a strong earthquake on March 11, 2011 (Japan, M = 9.1), are consistent with the results of instrumental observations. [ABSTRACT FROM AUTHOR]

This study, for the first time, reports the geomagnetically conjugate structure of a plasma bubble extending to the mid-latitudes and the asymmetrical structure of the decay of the plasma bubble during a geomagnetic storm. We investigated the temporal and spatial variations of plasma bubbles in the Asian sector during a geomagnetic storm on March 1, 2013, using Global Navigation Satellite System-total electron content data with high spatiotemporal resolutions. The first important point of our data analysis results is that the plasma bubble extended from the equator to the mid-latitudes with geomagnetic conjugacy along the magnetic field lines. The total electron content data showed that the plasma bubbles appeared in the equatorial regions near 150° E after sunset during the main phase of the geomagnetic storm. From ionosonde data over both Japan and Australia, they suggest that a large eastward electric field existed in the Asian sector. Finally, the plasma bubbles extended up to the mid-latitudes (~ 43° geomagnetic latitude) in both hemispheres, maintaining geomagnetic conjugacy. The second point is that the mid-latitude plasma bubble disappeared 1–2 h earlier in the northern hemisphere than in the southern hemisphere at close to midnight. In the northern hemisphere, the ionospheric virtual height decreased near midnight, followed by a rapid decrease in the total electron content and a rapid increase in the ionospheric virtual height. These results imply that the mid-latitude plasma bubble disappeared as the background plasma density decreased after midnight due to the recombination resulting from the descent of the F layer. Therefore, we can conclude that mid-latitude plasma bubbles can be asymmetric between the northern and southern hemispheres because of the rapid decay of plasma bubbles in one of the hemispheres. [ABSTRACT FROM AUTHOR]

Miyakejima volcano experienced its latest eruption in 2000 with the summit subsidence, and the next event is expected in the near future. An aeromagnetic survey in Miyakejima was conducted in March 2021 in order to investigate the current state of its magnetization structure to identify the potential for another eruption and, thus, mitigate volcanic disaster. The survey flight was conducted using an uncrewed aerial vehicle (UAV), a multirotor drone, to deploy a scalar magnetometer. After processing geomagnetic field data from this survey, in combination with data from previous surveys conducted by using another UAV, an uncrewed helicopter, the average magnetization intensity was determined to be 12.4 A/m. Further, the surrounding area of the crater was relatively highly magnetized; however, the crater rim had a low magnetization intensity. Temporal variation was detected between 2014 and 2021 and dominated the central part of the observation area. Decreased magnetization intensity was identified beneath the caldera, which may become recently demagnetized due to heat supply traveling through fractures in the impermeable layer from the deep heat reservoir. [ABSTRACT FROM AUTHOR]

Abstract: The problem of identifying precursory signals of earthquakes in the hope of mitigate the seismic hazard is a very important topic, but inaccurate documentations of precursory signatures decrease the credibility of this field of research. The statistical analysis by shows that there is tendency of positive total electron content (TEC) anomalies to occur 1–5days before 52 M >6 earthquakes which struck Japan during 1998–2010. also report in detail three selected case studies claiming the occurrence of TEC anomalies possibly related to large and destructive earthquakes. This paper casts doubts on the possibility that in the three cases the TEC disturbances were caused by seismic events suggesting that these TEC changes could be induced by normal variations of the global geomagnetic activity. As a consequence, also the results of the Superimposed Epoch Analysis performed by could be seriously influenced by global magnetospheric signals. [Copyright &y& Elsevier]

Abstract: , hereafter cited as H&H, report the observation of anomalous increases in the ULF geomagnetic field spectral density ratio which the authors claim to be possible precursors of the 2008 Iwate–Miyagi Nairiku earthquake. Here the results of H&H are reviewed taking into account the global geomagnetic activity level by means of ∑Kp index. This paper cast serious doubts on the seismogenic origin of the magnetic signatures documented by H&H showing that the anomalous signals are normal ULF magnetic variations induced by solar–terrestrial interaction. In summary, H&H’s claims that magnetic field disturbances about a month before the Mw 6.9 Iwate–Miyagi Nairiku earthquake on June 13, 2008 are precursors to the earthquake are unlikely to be correct. [Copyright &y& Elsevier]

Abstract: In this paper, we examine pre-earthquake ionospheric anomalies in time series and perform a statistical test by using total electron content (TEC) derived from global ionosphere maps (GIM) around the Japan area for the first time. The normalized GIM-TEC (GIM-TEC*), which is computed based on 15days backward running mean of GIM-TEC, have been investigated for minimizing possible confounding effects of consecutive earthquakes and identify the abnormal signals. Meanwhile, to reduce the effect of strong geomagnetic activities such as geomagnetic storms, the criterion for removing the GIM-TEC data have been adapted; that is when Dst index exceeds −60nT. Temporal variations of GIM-TEC* for large and destructive earthquakes in Japan have been studied; which are the 2004 mid-Niigata Prefecture Earthquake (M6.8), its aftershock (M6.1), the 2007 offshore mid-Niigata Earthquake (M6.8), and the 2008 Iwate–Miyagi Nairiku Earthquake (M7.2). Although there are some positive and negative TEC anomalies before and after the four earthquakes, there is a tendency that positive TEC anomalies appear 1–5days before all the above earthquakes even during the quiet geomagnetic condition. Superposed epoch analyses have been performed for the statistical analysis of TEC anomalies associated with M ⩾6.0 earthquakes during the 12-year period of May 1998–May 2010. The statistical result indicates the significance of the positive TEC anomalies 1–5days before earthquakes within 1000km from the epicenter around Japan. [Copyright &y& Elsevier]

Abstract: The key point of this paper is that for the first time we successfully observed “co-rupturing” Earth''s magnetic field changes due to the piezomagnetic effect caused by earthquake rupturing. Electromagnetic changes associated with earthquakes have been investigated previously. Most of them are “coseismic” step-like offsets in the geomagnetic field, which are regarded as the piezomagnetic effect caused by the total static stress changes resulting from an earthquake. Proton precession magnetometers have so far been mainly used to detect seismomagnetic effects, for which the measurement interval was usually 1 or 10min and the measurement accuracy was 0.1nT. For seismomagnetic observations we employed flux-gate magnetometers whose specifications are the measurement with accuracy of 0.01nT and with the sampling interval of 0.5 or 1s. By virtue of such highly sensitive magnetometers, we show observations of magnetic signals accompanying fault movement. Our observation site happened to be situated at an epicentral distance of 26km from the 2008 Iwate-Miyagi Nairiku earthquake of M 7.2, in NE Japan. Magnetic field components began to change almost simultaneously with the onset of the earthquake rupture and grew non-linearly until the first P wave arrival. Such magnetic signals are most probably generated by the changing stress field due to rupturing, i.e. the piezomagnetic effect, rather than the seismic dynamo effect or electromagnetic induction within the conducting crust, because they lack oscillatory features like seismic waves. We attempt to interpret the observations with the aid of quasi-static piezomagnetic-model calculation which strongly supported our observation results. [Copyright &y& Elsevier]

Abstract: In this paper a model for computing geomagnetically induced currents (GIC) from local geomagnetic field observations carried out in Hokkaido, Japan is constructed. The model is composed of system parameters mapping the horizontal geoelectric field to GIC and of 1D conductivity model. A rigorous model validation is used to show that the model reproduces the observed GIC with a very good accuracy. Statistical occurrence of GIC is computed using the constructed model and geomagnetic field recordings covering years 1986–2008. The modeled GIC is used to generate a list of 10 largest GIC events in Hokkaido, Japan. It is found that the 10 largest events between 1986 and 2008 were associated with various phases of coronal mass ejection driven major geomagnetic storms. It is also shown that although smaller GIC are fairly common, the largest possible GIC are likely limited to the amplitudes of the order of 10A. The constructed 1D ground conductivity model is interpreted in the context of the local geological setting and it is shown that the subduction zone dynamics likely play an important role in the observed GIC and geomagnetic field characteristics. Crustal conductor associated with the subduction zone is the cause for unusual direct relation between GIC and the local geomagnetic field rather than its time derivative and GIC. [Copyright &y& Elsevier]

An increase in geomagnetically induced currents (GICs) is an inevitable result of geomagnetic field disturbances, and is harmful to the power grid, in particular, at high latitudes. At mid and low latitudes, the amplitude of the GICs is, in general, small, but large‐amplitude GICs are often observed during magnetic storms. It is of importance to understand major characteristics and extreme values of GICs at mid and low latitudes. For the geoelectric field disturbances ΔE observed at Kakioka (27.8° geomagnetic latitude) in Japan in 1996–2004, we performed superposed epoch analyses with respect to three types of geomagnetic disturbances: (a) storm sudden commencements (SSCs)/sudden impulses (SIs), (b) main phase of magnetic storms, and (c) bay disturbances. It is shown that the SSCs/SIs and the main phase of the magnetic storms are equally important for causing large‐amplitude disturbances of ΔE at Kakioka. GICs are thought to be amplified when the SIs and/or the bay disturbances occur during the magnetic storms. The maximum value of ΔE tends to be correlated with the maximum value of ΔH during the three types of events, where ΔH is the horizontal component of the geomagnetic field. Assuming that a quasi‐linear relationship between the maximum ΔE and the maximum ΔH is valid, we estimated GICs at three substations in Japan for an extreme SSCs/SIs, and the extreme magnetic storms. This scheme could be applicable to estimate roughly the GICs against extreme events, and to forecast the maximum GICs in a real‐time manner. Plain Language Summary: Geomagnetic fields are known to change in characteristic manners during the magnetic storms and substorms. However, characteristic changes in the geomagnetically induced electric fields on the ground are not well known. We performed superposed epoch analyses of the electric fields observed at Kakioka Magnetic Observatory in Japan to obtain an overview of the effects of associated electric current systems in near‐Earth space on the electric field and geomagnetically induced currents (GICs). Histograms of the maximum amplitude of the electric field are also provided. The correlation between changes in the horizontal component of the geomagnetic field and the electric field is found to be good for each type of disturbance. Using empirical equations to indicate the relationship between them, we can roughly estimate the maximum electric field at Kakioka and GICs flowing at three electrical substations in Japan for extreme disturbances. Key Points: We performed superposed epoch analyses of low‐latitude geoelectric field for storm sudden commencements (SSCs)/sudden impulses (SIs), magnetic storms and bay disturbancesSSCs/SIs and magnetic storms are almost equally important for large‐amplitude disturbances of geoelectric field at low‐latitudeWith empirical equations, we estimated geomagnetically induced currents flowing at three substations in Japan for extreme SSCs and extreme magnetic storms [ABSTRACT FROM AUTHOR]

This study presents the conjugate ionospheric disturbances triggered by the 2011 Tohoku‐Oki reflected tsunami oceanic waves using the ground‐based Global Navigation Satellite System (GNSS) total electron content (TEC) observations. We found that the equatorward and westward propagating nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) occurred over Japan and Australia simultaneously following the tsunami oceanic waves reflected by the Emperor Seamount Chain in the northern hemisphere. The atmospheric gravity waves driven by reflected tsunami oceanic waves are hypothesized to be the source to trigger the conjugate MSTIDs by transporting the polarization electric fields along the field line to the conjugate hemisphere. Moreover, only the southwestward propagating MSTIDs have this conjugate effect, which could be due to the wavefront orientation. The Perkins instability could also be involved in the interhemispheric coupling process. This study provides the first observational evidence that the reflected tsunami can induce conjugate ionospheric disturbances through electrodynamic forcing. Plain Language Summary: The 2011 M 9.1 Tohoku‐Oki earthquake occurred near the east coast of Honshu, Japan, unleashing a savage tsunami traveling across the Pacific Ocean and beyond. Some tsunami oceanic waves were reflected by seafloor topography in the Pacific Ocean, thus propagating backward toward Japan. These reflected tsunami oceanic waves also excited gravity waves propagating upward into the upper atmosphere, causing plasma perturbations at the atmosphere‐space interaction region above Japan. Gravity wave‐driven variations in the neutral wind can move the plasma against the Earth's magnetic field lines, thus generating currents and electric fields. Since the Earth's magnetic field lines are conductive like electric power lines, the generated electric fields can transport along the magnetic field line from Japan to the southern hemisphere, causing plasma disturbances above Australia. This study provides observational evidence that the localized tsunami can also cause space plasma perturbations in the conjugate area. Key Points: Conjugate ionospheric disturbances associated with the 2011 Tohoku reflected tsunami‐driven gravity waves are observed for the first timeReflected tsunami‐driven gravity waves cause the conjugate ionospheric disturbances through polarization electric fieldsResulting conjugate ionospheric effect depends on the propagation direction of gravity waves [ABSTRACT FROM AUTHOR]

Palaeomagnetic records from geological archives provide significant information about the nature of geomagnetic polarity reversals; however, there are few detailed palaeomagnetic records of pre-Pleistocene reversals. The lower Mammoth Subchron boundary (late Pliocene) is recorded in a 10-m interval of a marine succession deposited at high accumulation rates (9–66 cm kyr–1) in the Boso Peninsula, central Japan. Here, we report a continuous palaeomagnetic record of the lower, normal to reverse boundary interval of the Mammoth Subchron, including the geomagnetic field direction and relative palaeointensity, with an average temporal resolution of ca. 800 yr. A hybrid method of thermal demagnetization at 200 °C and progressive alternating field demagnetization were used to effectively extract the primary palaeomagnetic component, which is carried by magnetite. The lower Mammoth transition is characterized by palaeomagnetic direction of instability and decay of the relative palaeointensity, and occurred from late Marine Isotope Stage MG3 (3351 ka) to MG2 (3336 ka) or MG1 (3331 ka), spanning 15–20 kyr. Virtual geomagnetic poles (VGPs), calculated from primary palaeomagnetic directions, rapidly rebounded twice from southern latitudes to northern latitudes within the transition. In contrast to the complex lower Mammoth reversal behaviour recorded in the Boso Peninsula succession, records from a lava sequence in O'ahu (Hawai'i) reveal a rebound following a 180° directional change, and those from a marl succession in Sicily (Italy) indicate a single rapid directional change. Diverse geomagnetic field evolution among these three sections is reflected resolution difference among the records likely in combination with an influence of non-axial dipole field. [ABSTRACT FROM AUTHOR]

Geological records of the Matuyama–Brunhes (M–B) geomagnetic reversal facilitate the development of an age model for sedimentary and volcanic sequences and help decipher the dynamics of the Earth's magnetic field. However, the structure of the geomagnetic field during the M–B geomagnetic reversal remains controversial due to its complex field behavior. In this study, we conducted paleo- and rock-magnetic analyses of samples from the Chiba composite section (CbCS), a continuous and expanded marine succession in Central Japan, to reconstruct the full sequence of the M–B geomagnetic reversal. We define an average stratigraphic position of the M–B boundary and estimate its age based on three sections in the CbCS and a neighboring drill core, TB-2. The average stratigraphic position of the M–B boundary in the CbCS is established at 1.1 ± 0.3 m above a widespread volcanic ash bed (the Byk-E tephra). Assuming a chronological error associated with orbital tuning of 5 kyr and stratigraphic uncertainty of 0.4 kyr, the M–B boundary in CbCS is at 772.9 ± 5.4 ka (1σ). The virtual geomagnetic pole, which is calculated from the paleomagnetic directions, shows several short fluctuations between 783 and 763 ka, with concomitant decreases in geomagnetic field intensity index. After termination of the field instabilities, the field intensity recovered and became higher than before the M–B boundary, with a stable normal polarity direction. The paleomagnetic records in the CbCS exhibit a field asymmetry between the axial dipole decay and field recovery, providing a full sequence of the M–B reversal, suggesting that the non-axial dipole field dominated several times during periods ca. 20 kyr long across the M–B boundary, due to depletion in the main axial dipole component. Our results provide probably the most detailed sedimentary record of the M–B geomagnetic reversal and offer valuable information to further understand the mechanism and dynamics of geomagnetic reversals. [ABSTRACT FROM AUTHOR]

Spatial gradients in the primary geomagnetic fields directly contribute to both the amplitudes and phases of inter-station transfer functions (IS-TFs). This suggests that, for the analysis of subsurface resistivity structures, IS-TFs should be carefully treated by checking the establishment of the plane-wave assumption. Geomagnetic time-series data include various and complicated characteristics and accordingly, time–frequency domain analysis is suitable for the discussion of spatial gradients of time-varying geomagnetic fields. However, such evaluations are complicated by the huge amount of information contained in the spectrograms from several stations. Therefore, we propose a Multi-Channel Nonnegative Matrix Factorization (MC-NMF) method that can decompose raw spectrograms into several components, allowing the spatial gradient of each geomagnetic temporal variation to be identified. We confirm that such components actually affect the estimation of IS-TFs using data acquired at the Kakioka and Memambetsu magnetic observatories in Japan. We derive the year-to-year changes in IS-TFs from each set of paired stations among Kakioka, Kanoya, and Memambetsu observatories. Although the IS-TFs should exhibit opposite polarities (a negative correlation) when the input and output observatories are swapped; surprisingly, some of them have "identical" polarities. The application of MC-NMF shows that the analyzed geomagnetic data include several components that have various spatial gradients. Although IS-TFs sometimes fail to give the expected implication regarding the spatial gradients of geomagnetic temporal variations, MC-NMF can verify whether the IS-TFs exhibit any spatial gradients. Thus, the use of IS-TFs with MC-NMF can yield better implications regarding subsurface resistivity information. [ABSTRACT FROM AUTHOR]

Submillimeter-Wave Limb-Emission Sounder 2 (SMILES-2) is a satellite mission proposed in Japan to probe the middle and upper atmosphere (20–160 km). The main instrument is composed of 4 K cooled radiometers operating near 0.7 and 2 THz. It could measure the diurnal changes of the horizontal wind above 30 km, temperature above 20 km, ground-state atomic oxygen above 90 km and atmospheric density near the mesopause, as well as abundance of about 15 chemical species. In this study we have conducted simulations to assess the wind, temperature and density retrieval performance in the mesosphere and lower thermosphere (60–110 km) using the radiometer at 760 GHz. It contains lines of water vapor (H2O), molecular oxygen (O2) and nitric oxide (NO) that are the strongest signals measured with SMILES-2 at these altitudes. The Zeeman effect on the O2 line due to the geomagnetic field (B) is considered; otherwise, the retrieval errors would be underestimated by a factor of 2 above 90 km. The optimal configuration for the radiometer's polarization is found to be vertical linear. Considering a retrieval vertical resolution of 2.5 km, the line-of-sight wind is retrieved with a precision of 2–5 m s -1 up to 90 km and 30 m s -1 at 110 km. Temperature and atmospheric density are retrieved with a precision better than 5 K and 7 % up to 90 km (30 K and 20 % at 110 km). Errors induced by uncertainties on the vector B are mitigated by retrieving it. The retrieval of B is described as a side-product of the mission. At high latitudes, precisions of 30–100 nT on the vertical component and 100–300 nT on the horizontal one could be obtained at 85 and 105 km (vertical resolution of 20 km). SMILES-2 could therefore provide the first measurements of B close to the electrojets' altitude, and the precision is enough to measure variations induced by solar storms in the auroral regions. [ABSTRACT FROM AUTHOR]

A first quasi-successive paleosecular variation data since ∼5.5 kya, including magnetic inclination and relative geomagnetic field intensity variation were obtained from a 55 m-long sediment core in the Nakdong River delta, southeastern Korean Peninsula. Both magnetic inclination curves and relative paleointensity data showed similar trends with those from Lake Biwa, Japan. They also showed close correlation with continuous global geomagnetic field models. For instance, the present study confirms the existence of archeomagnetic jerks. In addition, the present study supports the evidence of globally high geomagnetic intensity circa 3 kya. [ABSTRACT FROM AUTHOR]

The geomagnetic fields in the atmosphere can be affected by phenomena in the Earth, so that changes of geomagnetic intensity might be used as an indicator of earthquake occurrences. Variations of geomagnetic data have been analyzed in association with the Tohoku Earthquake on March 11th 2011. The Geomagnetic data have been derived from Memambetsu (MMB), Kakioka (KAK) and Kanoya (KNY) Observatories, which are INTERMAGNET observatories. The analysis was performed by calculating the power spectral density (psd) of the ULF signal of Z and H components and then polarizations are observed by comparing the psd of Z and of H. The results showed the difference psd of Z and of H between the KAK observatory (the nearest position to the epicenter) with MMB and KNY (at some greater distance from the epicenter) is quite significant, which can be observed over a period of 10 days before the earthquake occurrence. The polarizations of Z/H in KAK indicate a highest change of intensity which occurred at 18 days prior to the earthquake. [ABSTRACT FROM AUTHOR]

By applying Detrended Fluctuation Analysis (DFA) to the time series of the geomagnetic data recorded at three measuring stations in Japan, Rong et al. in 2012 recently reported that anomalous magnetic field variations were identified well before the occurrence of the disastrous Tohoku Mw9.0 earthquake that occurred on 11 March 2011 in Japan exhibiting increased "non-uniform" scaling behavior. Here, we provide an explanation for the appearance of this increase of "non-uniform" scaling on the following grounds: These magnetic field variations are the ones that accompany the electric field variations termed Seismic Electric Signals (SES) activity which have been repeatedly reported that precede major earthquakes. DFA as well as multifractal DFA reveal that the latter electric field variations exhibit scaling behavior as shown by analyzing SES activities observed before major earthquakes in Greece. Hence, when these variations are superimposed on a background of pseudosinusoidal trend, their long range correlation properties--quantified by DFA--are affected resulting in an increase of the "non-uniform" scaling behavior. The same is expected to hold for the former magnetic field variations. This explanation is strengthened by recent findings showing that the fluctuations of the order parameter of seismicity exhibited an unprecedented minimum almost two months before the Tohoku earthquake occurrence which is characteristic for an almost simultaneous emission of Seismic Electric Signals activity. [ABSTRACT FROM AUTHOR]

Monitoring of telluric current, which is practically a synonym for geoelectric potential difference, was conducted on Kozu-shima Island about 170 km south of Tokyo from May 14, 1997 to June 25, 2000. During the monitoring period, 19 anomalous telluric current changes (ATCs) were observed. Their possible correlation with nearby earthquakes was statistically examined by assuming various lead times for different ranges of magnitude and focal distance. The best correlation may be obtained for earthquakes with a magnitude greater than 3.0 occurring within 20 km of focal distance. There were 23 such earthquakes, of which 11 were preceded by ATCs within 30 d. Of these 11 earthquakes, preceding ATCs of 5 and 6 were positive and negative polarities of telluric current, respectively. Their epicenters were spatially well clustered in the east and west of the island. These facts were clearly beyond those expected by chance and led to a simple speculative model. [ABSTRACT FROM AUTHOR]

There have been many reports on ULF electromagnetic phenomena associated with the 2000 Izu Islands earthquake swarm. In this study, seismo-magnetic anomalies are presented by examining energy variations of signatures at the periods around 100 s. Geomagnetic data observed at three stations in Izu Peninsula from 1 January 2000 to 30 April 2001 have been analyzed and wavelet transform has been performed. In order to indentify anomalous changes from ionosphere disturbances, Kakioka station has been chosen as a reference station, and the similar data analysis has been performed. The results suggest that the unusual energy enhancement of the Z component, which only appears in Izu Peninsula from late June until early November 2000, might possibly be one of electromagnetic phenomena associated with the 2000 Izu Islands earthquake swarm. [ABSTRACT FROM AUTHOR]

On 19 December 2009, the eastern part of Taiwan at approximate depth of 45 km was struck by a strong earthquake (Mw = 6.4). The epicenter was located about 20 km away from the Hualien (HLN) station in Taiwan. By analyzing data from the HLN station and Amami Oshima (AMA) in Japan as a remote reference, the geomagnetic east-west (D) and vertical (Z) components recorded at the HLN station are showing no correlation with those at the AMA station in December 2009. Anomalous variations of ∼10-15 nT started about one week before the occurrence of the earthquake and lasted for about two weeks. Also, an enhanced ULF signal in the range of Pc 3 (10-40 s) was observed a few days before the onset of the earthquake. Moreover, the polarization ratio (Z/H (north-south component)) of the Pc 3 amplitude at the HLN station decreased a few days preceding the earthquake. The mechanism behind these anomalous variations is not fully understood. However, we expect that crustal stress perturbations and underground conductivity changes associated with the earthquake played an important role for generating such observed geomagnetic variations. [ABSTRACT FROM AUTHOR]

Many years ago Rikitake et al. described the anomalous behaviour of the vertical component Z of the geomagnetic solar quiet (Sq) daily variation field at observatories in central and northern Japan - namely about 2 hr shift of the local noontime peak towards morning hours. They suggested that this anomaly is associated with the anomalous distribution of electrical conductivity in the mantle beneath central Japan. Although a few works have been done to confirm or argue this explanation, no clear answer has been obtained so far. The goal of this work is to understand the nature of this anomaly using our 3-D forward solution. The conductivity model of the Earth includes oceans of laterally variable conductance and conducting mantle either spherically symmetric or 3-D underneath. Data from six Japanese observatories at four seasons for two different years of the solar cycle are analysed. As an inducing ionospheric (Sq) current system, we use those provided by the Comprehensive Model (CM4) of Sabaka et al. Our analysis clearly demonstrates that 3-D induction in the ocean is responsible for the anomalous behaviour of Z daily variations in this region. We also show that the effects from a suite of 3-D mantle models that include mantle wedge and subducting slab are minor compared with the ocean effect. [ABSTRACT FROM AUTHOR]

The toroidal magnetic field of ITER is provided by 18 Nb3 Sn superconducting coils that provide 5.3 Tin the Tokamak magnetic centre thanks to a nominal current of 68 kA. Two major procurement packages will be integrated to accomplish the final assembly of the coils: 1) the winding pack which houses the double pancakes in 7 stacked radial plates (RP) and 2) the TF coil case which houses the winding pack (WP) and is the main structural component of the ITER machine. 19 TF coil cases (18+1 spare) will be entirely procured by Japan, whereas the WP and the TF coil assembly will be shared between Japan (9) and Europe (10). The insertion process of the WP inside the coil case, VPI and final closure welding is presented with an emphasis on the R&D still remaining. An appropriate metrology strategy and correct benchmarking of the WP current center line (CCL) in the coil case external walls is essential to allow the correction of the error fields during operation. The TF coils will be assembled and pre-aligned in pairs together with their corresponding VV sector and craned to the Tokamak hall hanging from 2 independent hooks. The integration of the IF coils in the Tokamak primary datum system will have to bc accomplished within the maximum tolerances defined by the error field analysis. As-built 3D data collected during manufacture will be used to ensure the correct positioning of the CCL against the Tokamak datum system and ease the alignment process. Shimming will be used in the gravity support for vertical alignment and in the upper and lower OIS for toroidal alignment. A subsequent survey of the `as-installed' TF coils will determine the final Tokamak Datum System. [ABSTRACT FROM AUTHOR]

Measurements of cosmic-ray neutron dose rates with a balloon in Sanriku, Japan (geographic location: 39° N, 142° E; corresponding geomagnetic latitude: 30° N) were conducted at an altitude from 0.2 to 25 km on 25–26 August 2004 when solar activity was at an average level. Neutron dose rates given as ambient dose equivalent rates (H*(10)) were measured with high-sensitive neutron dose equivalent counters and electronic silicon personal dosimeters (EPDs). The neutron dose rates increased with increasing altitude, but they were saturated around 15–20 km and decreased with increasing altitude beyond 20 km. The neutron ambient dose equivalent rate was 1.5 µSv/h− 1 at 20 km. Measured values were corrected for the deviation of the energy response of the dose equivalent counter from the fluence-to-ambient dose equivalent conversion coefficient, and the corrected values were very close to the calculated values with EPCARD. On the other hand, neutron measurements by the EPDs gave about 10 times overestimation because of the high sensitivity to cosmic-ray protons. [ABSTRACT FROM PUBLISHER]

Studies of palaeodirections and palaeointensities were carried out on andesite lavas from the older part of the late Quaternary Ontake Volcano, Japan. Recently published K-Ar ages of the Older Ontake lavas range from 740 to 393 Ka. Palaeomagnetic samples were collected at 24 sites which are either of the same locality or nearby the dating site. Determination of palaeodirections was straightforward at most of the sites. Exceptions are a few sites in which the site mean palaeodirections were obtained only by the great circle method due to large secondary components, possibly lightning strikes in origin. The obtained VGP angular dispersion of 14.3° around the mean is smaller than the representative value for the site latitude of 36°N, which could be attributed to an insufficient number of sampling sites. Thellier's experiments were successful in 25 specimens from three lavas out of 54 specimens from nine lavas, giving a moderate success rate of 46 per cent. The obtained flow mean palaeointensities of 21, 40 and 53 μT conform well with the worldwide trend of the global palaeointensity during the Brunhes Chron, which indicates possible maxima of the global palaeointensity around 0.7 and 0.35 Ma with following gradual decrease. [ABSTRACT FROM AUTHOR]

Intensive palaeomagnetic and rock magnetic studies clarify the history of multiple block rotational events within an active plate margin. The Cretaceous Kurihashi Granodiorite in northeast Japan preserves multi-component remanent magnetization: high-unblocking-temperature thermoremanent magnetization (TRM) acquired during the initial cooling of the granodiorite, and middle-/low- thermoviscous remanent magnetization (TVRM) acquired during succeeding hydrothermal activities. The observed directional difference between the three magnetic components requires the first counter-clockwise rotation event within a part (>1 km) of northeast Japan as a result of left-lateral plate motion in the Late Cretaceous. Later clockwise rotation, probably related with right-lateral faulting during the course of the Miocene backarc spreading of the Japan Sea, is indicated by significant easterly deflections in the low- component. On the other hand, scatter in the high- TRM declinations implies that the second deformation phase was achieved as non-rigid rotation of fault-bounded blocks as wide as 100 m. The latest and most confined rotation events are confirmed by complicated deflections of the TRM and TVRM in a fault zone. A shear zone as wide as 30 m with dense fractures near the main fault is sharply divided into three compartments, which have been rotated around three orthogonal poles. Systematic block rotations may accommodate differential slips on fault planes within a plutonic body, genetically containing orthogonal fractures as reactivated intrinsic cooling joints. We interpret our results to show that the Kurihashi granodiorite has been progressively segmented since the Cretaceous under varied stresses. [ABSTRACT FROM AUTHOR]

We have submitted a secular variation (SV) candidate model for the thirteenth generation of International Geomagnetic Reference Field model (IGRF-13) using a data assimilation scheme and a magnetohydrodynamic (MHD) dynamo simulation code. This is the first contribution to the IGRF community from research groups in Japan. A geomagnetic field model derived from magnetic observatory hourly means, and CHAMP and Swarm-A satellite data, has been used as input data to the assimilation scheme. We adopt an ensemble-based assimilation scheme, called four-dimensional ensemble-based variational method (4DEnVar), which linearizes outputs of MHD dynamo simulation with respect to the deviation from a dynamo state vector at an initial condition. The data vector for the assimilation consists of the poloidal scalar potential of the geomagnetic field at the core surface and flow velocity field slightly below the core surface. Dimensionless time of numerical geodynamo is adjusted to the actual time by comparison of secular variation time scales. For SV prediction, we first generate an ensemble of dynamo simulation results from a free dynamo run. We then assimilate the ensemble to the data with a 10-year assimilation window through iterations, and finally forecast future SV by the weighted sum of the future extension parts of the ensemble members. Hindcast of the method for the assimilation window from 2004.50 to 2014.25 confirms that the linear approximation holds for 10-year assimilation window with our iterative ensemble renewal method. We demonstrate that the forecast performance of our data assimilation and forecast scheme is comparable with that of IGRF-12 by comparing data misfits 4.5 years after the release epoch. For estimation of our IGRF-13SV candidate model, we set assimilation window from 2009.50 to 2019.50. We generate our final SV candidate model by linear fitting for the weighted sum of the ensemble MHD dynamo simulation members from 2019.50 to 2025.00. We derive errors of our SV candidate model by one standard deviation of SV histograms based on all the ensemble members. [ABSTRACT FROM AUTHOR]