Horizontal Magnetic Components Research Articles

Electrokinetic effect combined with surface-charge assumption: a possible generation mechanism of coseismic EM signals

According to field observations, electromagnetic (EM) signals accompanying seismic waves can be recorded. The orders of magnitude of observed coseismic electric and magnetic signals associated with earthquakes are usually around 1 to 101 mV km−1 and 10−2 to 1 nT, respectively. In this paper, we carry out numerical simulation of coseismic EM signals associated with seismic waves due to electrokinetic effect and compare with field observations. The seismic source is represented by a finite fault measuring 15 × 9 km2 with a max slip displacement 1.5 m, corresponding to a Mw 5.9 earthquake. While using the EM surface boundary condition of continuous horizontal EM components, the magnetic signals only accompany the late-arriving S waves at receiver near the ground surface. This is obviously different from field observations. Thus, we adopt another EM surface boundary condition which assumes the ground surface carries surface charge. For the used half-space model, a surface-charge density magnitude |Qsc| in excess of 10−4 C m−2 is sufficient to make horizontal magnetic components clearly show up at the whole time duration of seismic waves. When |Qsc| increases, the contribution of surface-charge density to coseismic EM signals becomes more and more dominant comparing with that of the seismically induced streaming-current. We estimate the Qsc expected at the Earth's surface might be a value between −5 × 10−4 and −0.1 C m−2 by the comparison between numerical results and field observations. The vertical magnetic signals only accompany the late-arriving seismic waves, because they are theoretically only induced by SH wave. The field observation results of vertical magnetic signals may be resulted from the scattering effect or the seismic dynamo effect. We conclude that electrokinetic effect combined with surface-charge assumption is one possible generation mechanism of the observed coseismic EM signals.

Annual variations of the amplitude characteristic of the noise magnetic field in the ELF frequency range based on the results of high-latitude observations

The annual variation of a number of amplitude characteristics of the noise electromagnetic field in the frequency range near the first Schumann resonance (and in particular the mean values of the modulus of horizontal magnetic components and parameters of the distribution function of noise pulses over amplitudes) was obtained and analyzed based on the results of three-year measurements performed at the high-latitude magnetic observatory Lovozero. The summer increase in mean and median values of the magnetic component of noises associated with electrical storms and the lack of annual variations of the exponent that empirically describes the distribution of amplitudes of noise signals were found. The relationship between the seasonal (winter/summer in the northern hemisphere) increase in the mean value of the modulus of the horizontal magnetic field component and seasonal increase in the global number of lightning discharges is shown by the correlation of the results of satellite observations and measurements of the magnetic component.

Erratum to: Schumann resonance observation in China and anomalous disturbance possibly associated with Tohoku M9.0 earthquake

Schumann resonance (SR) is an electromagnetic resonance phenomenon in the Earth–ionosphere cavity exited by global lightning activities when the wavelength matches the circumference of the Earth, and the lowest four peak frequencies of SR are about 8, 14, 20, and 26 Hz. This article presents the new observational data of SR in China. The observations of two horizontal magnetic components (BNS and BEW) in the frequency band range of 3–29 Hz at Yongsheng observatory (26.7°N, 100.8°E) in southwestern China were mainly analyzed. It is found that the SR amplitudes at peak frequencies in BNS and BEW components all showed diurnal and seasonal variations, and that the SR amplitude in BNS component is always higher than that in BEW component. Diurnal variation of SR amplitude around equinoxes and solstices in BNS component is related to active intervals of three global thunderstorm centers, while SR amplitude in BEW component is the most significant at around 16 LT, corresponding to Asian center. SR amplitudes both in BNS and BEW components increase in the rainy season from May to September. In addition, the SR anomalies in association with the 2011 Japan earthquake are exhibited. The anomalous effect was characterized by an increase in amplitude at the lowest four SR modes beginning at 4 days before this earthquake. Upon analyzing the wave interference between the direct wave and disturbed wave scattered by localized modification of lower ionosphere over the epicenter, Asian and African thunderstorm centers are found to contribute to anomalous effect observed at Yongsheng station. Modeling results of SR regular and disturbed spectra at different local times led to the similar conclusion.

Analysis of One-Point Methods for Lightning-Discharge Passive Location

We consider different algorithms for one-point determination of the coordinates of a high-current lightning discharge within the framework of its model as an arbitrarily oriented electric dipole. The considered algorithms are based on simultaneous processing of the vertical electric and horizontal magnetic components of the electromagnetic field of the discharge. Model estimations of the errors of some algorithms, which are caused by the influence of external atmospheric and industrial interference, are presented. The results allow one to formulate the requirements for instrumental equipment of lightning direction and range finders and estimate the boundaries of their operative application under conditions of reaching the necessary accuracy. We discuss the data obtained during field tests of functional prototypes of such devices in the actual lightning-storm environment, which confirm their applicability for one-point passive location of thunderstorm cells.

Tsunami source of the 2011 Tohoku earthquake detected by an ocean-bottom magnetometer

Tsunami-induced magnetic signals resulting from the 11 March 2011 Tohoku earthquake (M 9.0) were identified from an examination of the record of an ocean-bottom electromagnetometer emplaced 50 km east of the Japan Trench. The impulsive variation of the vertical magnetic field component directly indicates the tsunami wave height and arrival time at the station. The impulsive variation of the horizontal magnetic components indicates east–southeast as the tsunami-propagation direction. These data indicate that the tsunami source was located along the Japan Trench at approximately 39.0°N latitude, which is about 100 km northeast from the main rupture area. Joint analysis of the observed magnetic variations and offshore sea-level data supports this source location and indicates that the tsunami origin time was about 1 min after the earthquake origin time. In addition, the magnetic field variation and the sea level change indicate that the source area was elongated in the trench direction and had a narrow width.

Two‐dimensional simulations of the tsunami dynamo effect using the finite element method

Conductive seawater moving in the geomagnetic main field generates electromotive force in the ocean. This effect is well known as the “oceanic dynamo effect.” Recently, it has been reported that tsunamis are also associated with the oceanic dynamo effect, and tsunami‐induced electromagnetic field variations were actually observed on the seafloor. For instance, our research group succeeded in observing tsunami‐induced magnetic variations on the seafloor in the northwest Pacific at the time of the 2011 Tohoku earthquake. In this study, we developed a time domain tsunami dynamo simulation code using the finite element method to explain the tsunami‐induced electromagnetic variations observed on the seafloor. Our simulations successfully reproduced the observed seafloor magnetic variations as large as 3 nT. It was also revealed that an initial rise in the horizontal magnetic component prior to the tsunami arrival as large as 1 nT was induced by the tsunami.

Spatial structure of the electromagnetic field inside the ionospheric Alfvén resonator excited by atmospheric lightning activity

We have theoretically estimated ULF spectra on the ground and at ionospheric altitudes in the frequency range of the ionospheric Alfvén resonator (IAR). The IAR has been considered to be excited either by a separate lightning stroke or stochastic global thunderstorm activity. The spectra of both horizontal magnetic and electric components are shown to reveal the spectral resonant structure in the upper ionosphere. The IAR excitation for different ionospheric conditions has been compared. The IAR eigenfrequencies latitudinal inhomogeneity results in the smoothing and shift of the spectral resonance structure. The feasibility of the IAR signature detection by low‐orbiting satellites with magnetic or electric sensors is discussed.

RAPID TRANSITION OF UNCOMBED PENUMBRAE TO FACULAE DURING LARGE FLARES

In the past two decades, the complex nature of sunspots has been disclosed with high-resolution observations. One of the most important findings is the "uncombed" penumbral structure, where a more horizontal magnetic component carrying most of Evershed Flows is embedded in a more vertical magnetic background (Solanki & Montavon 1993). The penumbral bright grains are locations of hot upflows and dark fibrils are locations of horizontal flows that are guided by nearly horizontal magnetic field. On the other hand, it was found that flares may change the topology of sunspots in $\delta$ configuration: the structure at the flaring polarity inversion line becomes darkened while sections of peripheral penumbrae may disappear quickly and permanently associated with flares (Liu et al. 2005). The high spatial and temporal resolution observations obtained with Hinode/ SOT provide an excellent opportunity to study the evolution of penumbral fine structure associated with major flares. Taking advantage of two near-limb events, we found that in sections of peripheral penumbrae swept by flare ribbons, the dark fibrils completely disappear, while the bright grains evolve into faculae that are signatures of vertical magnetic flux tubes. The corresponding magnetic fluxes measured in the decaying penumbrae show stepwise changes temporally correlated with the flares. These observations suggest that the horizontal magnetic field component of the penumbra could be straightened upward (i.e., turning from horizontal to vertical) due to magnetic field restructuring associated with flares, which results in the transition of penumbrae to faculae.

NONLINEAR FORCE-FREE MODELING OF MAGNETIC FIELDS IN A SOLAR FILAMENT

We present a striking filament pattern in the nonlinear force-free (NLFF) chromospheric magnetic field of the active region NOAA 10956. The NLFF chromospheric field is extrapolated from the Hinode high-resolution photospheric vector magnetogram using the weighted optimization method. The modeled structure is characterized by a highly sheared field with strong horizontal magnetic components and has a virtually identical shape and location as the filament seen in Hα. The modeled field strength agrees with the recent He I 10830 A observations by Kuckein et al.. The unequivocal resemblance between the NLFF extrapolation and the Hα observation not only demonstrates the ability of the NLFF field to reproduce chromospheric features, but also provides a valuable diagnostic tool for the filament magnetic fields.

ON THE DOPPLER SHIFT AND ASYMMETRY OF STOKES PROFILES OF PHOTOSPHERIC Fe I AND CHROMOSPHERIC Mg I LINES

We analyzed the full Stokes spectra using simultaneous measurements of the photospheric (FeI 630.15 and 630.25 nm) and chromospheric (MgI b2 517.27 nm) lines. The data were obtained with the HAO/NSO Advanced Stokes Polarimeter, about a near disc center sunspot region, NOAA AR 9661. We compare the characteristics of Stokes profiles in terms of Doppler shifts and asymmetries among the three spectral lines, which helps us to better understand the chromospheric lines and the magnetic and flow fields in different magnetic regions. The main results are: (1) For penumbral area observed by the photospheric FeI lines, Doppler velocities derived from Stokes I (Vi) are very close to those derived from linear polarization profiles (Vlp) but significantly different from those derived from Stokes V profiles (Vzc), which provides direct and strong evidence that the penumbral Evershed flows are magnetized and mainly carried by the horizontal magnetic component. (2) The rudimentary inverse Evershed effect observed by the MgI b2 line provides a qualitative evidence on its formation height that is around or just above the temperature minimum region. (3) Vzc and Vlp in penumbrae and Vzc in pores generally approach their Vi observed by the chromospheric MgI line, which is not the case for the photospheric FeI lines. (4) Outer penumbrae and pores show similar behavior of the Stokes V asymmetries that tend to change from positive values in the photosphere (FeI lines) to negative values in the low chromosphere (MgI line). (5) The Stokes V profiles in plage regions are highly asymmetric in the photosphere and more symmetric in the low chromosphere. (6) Strong red shifts and large asymmetries are found around the magnetic polarity inversion line within the common penumbra of the Delta spot. This study thus emphasizes the importance of spectro-polarimetry using chromospheric lines.

Reply to comment by R. Dhanya and S. Gurubaran on “Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system”

Reply to comment by R. Dhanya and S. Gurubaran on “Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system”

Multistation observations of ELF/VLF whistler mode chorus

An array of seven ELF/VLF receivers in Alaska is utilized for direction finding and determination of ionospheric exit point of whistler mode chorus waves from the Earth's magnetosphere. Each receiver records both orthogonal horizontal magnetic components of the chorus waves. All sites use GPS‐synchronized sampling, allowing for the localization of ionospheric exit points utilizing both arrival azimuth and time of arrival lag between sites. Results show two distinct groups of cases, emissions with singular ionospheric exit points and those with multiple exit points. Singular exit point cases exhibit migration and mode conversion as a function of propagation distance from the source point. The multiple exit point case shows chorus waves impinging on the ionosphere over a spread in magnetic latitude, suggesting nonducted propagation. Ray tracing for this case is unable to reproduce the observations unless a cold plasma density different than that predicted by geomagnetic conditions is used. It is proposed that chorus elements cross the transionospheric boundary after experiencing scattering from meter‐scale irregularities. Additionally, subionospheric VLF remote sensing is used to detect precipitation onto the ionosphere of energetic radiation belt electrons that have been pitch angle scattered by individual chorus emission packets. VLF perturbation signatures are consistent with precipitation fluxes being dominated by electrons with energies less than 1 MeV.

17 GHz Neutral Line Associated Sources: Birth, Motion, and Projection Effect

New results and the confirmation of some older suggestions concerning the nature of solar microwave Neutral Line associated Sources (NLSs) were obtained by using data acquired with the Nobeyama Radioheliograph at 17 GHz along with coronal magnetic field magnetograms extrapolated from full-disk longitudinal magnetograms. The used method of extrapolation is applicable for active regions with rather arbitrary position on the solar disk, and allows us to avoid projection effects in observed longitudinal magnetograms. An application of this method was demonstrated for the period of 2003 October–November. We found NLSs to be widespread at 17 GHz. The NLSs are located just above, or in close vicinity to, the inversion line of the radial component of the coronal magnetic field in the place of a close contact of opposite magnetic polarities, where a horizontal magnetic component has local maxima. We briefly address the birth, motions of NLSs, their transformations into sunspot-associated sources, and vice versa. We also add a set of observational facts in evidence for the presence of 17 GHz NLSs in active regions, which produce powerful flares.

Analysis of Propagation Conditions of ELF Radio Waves on the “Zeus”–Transbaikalia Path

We consider the results of measurements of the absolute values of horizontal electric and magnetic components of the electromagnetic field of the “Zeus” ELF radio facility at frequencies 33, 44, 82, and 188 Hz, carried out in Transbaikalia. Electromagnetic-field variations during 17 semidiurnal sessions (about 204 hours of synchronous records) are analyzed. The length of the “Zeus”–Transbaikalia path is approximately 4000 km.

Local time and longitude dependence of the equatorial electrojet magnetic effects

An empirical model of the equatorial electrojet (EEJ), including local time and longitude dependence, has been constructed based on the surface magnetic data recorded at 26 stations located in six different longitude sectors that were set up or augmented during the international equatorial electrojet year (IEEY). The model reproduces the characteristic signatures of the EEJ-associated horizontal and vertical magnetic components at ground level. The model-predicted variations at the orbit of the POGS satellite are generally in good agreement with the onboard magnetic signatures, although strong discrepancies are also often seen. The nature of the differences suggests that the global scale magnetospheric or field-aligned current systems may sometimes dominate the satellite data. The nature of the longitudinal inequalities in the EEJ strength indicates that the equatorial electrojet is strongest in South America (80°–100°W) and weakest in the Indian sector (75°E) with a secondary minimum and a maximum centered, respectively, in the Atlantic Ocean (30°W) and in western Africa (10°E). The EEJ strength is shown to be inversely correlated with the main field intensity along the dip-equator.

Seasonal and diurnal variation of geomagnetic activity: Revised Dst versus external drivers

Daily and seasonal variability of long time series of magnetometer data from Dst stations is examined. Each station separately shows a local minimum of horizontal magnetic component near 18 local time (LT) and weakest activity near 06 LT. The stations were found to have different baselines such that the average levels of activity differed by about 10 nT. This effect was corrected for by introducing a new “base method” for the elimination of the secular variation. This changed the seasonal variability of the Dst index by about 3 nT. The hemispheric differences between the annual variation (larger activity during local winter and autumn solstice) were demonstrated and eliminated from the Dst index by addition of two Southern Hemisphere stations to a new index termed Dst6. Three external drivers of geomagnetic activity were considered: the heliographic latitude, the equinoctial effect, and the Russell–McPherron effect. Using the newly created Dst6 index, it is demonstrated that these three effects account for only about 50% of the daily and seasonal variability of the index. It is not clear what drives the other 50% of the daily and seasonal variability, but it is suggested that the station distribution may play a role.

Improved treatments for evaluating horizontal magnetic components through the 3-D FDM in E-polarization induction problems

To improve the accuracy of the numerical evaluation through the 3-D finite difference method, the surface boundary conditions are added to modify the old program. The author has tested the new program by making calculations for the model constructed by Wanamaker, et al (1984). The comparison between the numerical results obtained from this paper and those by Wannamaker, et al (1984) indicates that a pronounced improvement is realized in the evaluation of the horizontal magnetic components. Moreover, better calculations for the vertical magnetic components are also obtainable by using the new program.

Variations of the lunar geomagnetic tide in the Indian region

The seasonal variations of the lunar geomagnetic tide are analysed at a group of stations around the longitude of India. The position of the focus of the lunar ionospheric current system is found to move with season and to disappear in northern winter. A year-to-year variability of the phases is also demonstrated. The phase relationships indicate the presence of antisymmetric atmospheric tidal modes of various strengths in different seasons. The different lunar Hough mode amplitudes, derived earlier, may also explain the observed September maximum in the lunar horizontal magnetic component at the equator.

Three types of 2-D lateral magnetoresistive sensors with p+-implant confinement

The paper presents three types of 2-D magnetic sensors fabricated by an industrial 0.8 /spl mu/m CMOS process. They are designed to measure the horizontal magnetic component B/sub x/ and the vertical magnetic component B/sub y/, simultaneously. The devices with p/sup +/-implant confinement have been investigated experimentally. According to experimental results, the magnitudes of the cross-coupling noises are proportional to the device size. Both the enlarged device size and the narrowed device conductive thickness with p/sup +/-implant confinement have increased sensitivity. The measured absolute voltage sensitivity S/sub A/, the absolute current sensitivity S/sub l/, and the supply-current-related sensitivity S/sub Rl/ are 0.741 V/T, 493.7 /spl mu/A/T and 439.4 V/AT, respectively.

The magnetic daily variation in Australia: dependence of the total-field signal on latitude

The quiet daily magnetic variation, denoted Sq, occurs as a background signal during magnetic surveying. The morphology of Sq is dependent on a number of factors, particularly latitude. Type curves that describe the latitudinal dependence of the horizontal and vertical components of Sq on a global scale appear in many texts. This paper first describes a recent compilation of global Sq curves for the total magnetic field, which is the component of most relevance to magnetic mapping.An analysis is then made of total-field variations from a north-south line of stationary recording magnetometers which operated across central Australia as part of the AWAGS experiment of 1989-1990. These data are analysed for information on the magnetic daily variation across the Australian continent, particularly its latitudinal variation. Observations cover a full year and their analysis is divided into four seasons to show variation with season as well as with latitude. The observed data show a minimum in the total-field signal in the geomagnetic latitude band 20°–30°, in support of the global Sq total-field curves. There is also clear evidence in the AWAGS data for the path of the Sq focus across Australia, identified by an amplitude minimum in the daily variation of the horizontal magnetic north component. The band in which the total-field Sq variations are generally reduced is termed the total-field “doldrums”; this band is on the equator-side of the path of the Sq focus.