External Current Systems Research Articles

P‐68: Oxide Semiconductor Thin‐Film Transistor‐Based Micro‐LED Pixel Circuit with External Current Setting System

We propose an oxide semiconductor thin‐film transistor‐based micro light‐emitting diode pixel circuit with external current setting system. The proposed circuit achieved stable operation using pulse width modulation with maximum error rates 1.2 % and 3.2 % under threshold voltage variations and mobility variations, respectively.

Equivalent Current Systems of Quiet Ionosphere during the 24th Solar Cycle Derived from the Geomagnetic Records in China

External and internal equivalent current systems of solar quiet (Sq) were obtained using the spherical harmonic analysis (SHA) method with a “mirror” technique based on geomagnetic records from 46 stations in China during 2008–2019. It is the first attempt to investigate Sq currents using so many stations in the China region for a long period. On the basis of criterion Kp ≤ 2+, geomagnetic vector data were selected to represent monthly Sq variations. After calculating the equivalent currents for each month and each Lloyd season, Sq variation was analyzed in relation to the solar cycle and season. The intensities of both external and internal Sq currents were found consistent with solar activity for the same month or season, while the positions of the current foci were evidently unaffected by solar activity. The intensities of Sq currents also exhibited primary semiannual (annual) variation in the periods of high (low) solar activity. The latitude of the internal current vortex showed evident seasonal variation in Lloyd seasons with high (low) values in the D (J) season, while the external current vortex exhibited no obvious seasonal variation. The strongest correlation between external and internal foci was found in D season, and the internal current foci usually appeared 20–40 min earlier than the external ones. Owing to the complex mechanisms behind Sq variation, some findings will need further analysis in the future.

Solar and lunar daily geomagnetic variations and their equivalent current systems observed by Swarm

This paper describes solar and lunar daily variations of the geomagnetic field over low- and mid-latitude regions, using vector magnetometer data from Swarm satellites at altitudes of sim500 km during the solar minimum years of 2017–2020. The average solar variation of the geomagnetic field is within the range of ±14 nT, while the lunar variation is within ±2 nT. The latter is comparable to the ocean tidal field. A spherical harmonic analysis is performed on the solar and lunar variations to evaluate their internal and external equivalent current systems. The results show that both the solar and lunar variations are mainly of internal origin, which can be attributed to combined effects of ionospheric dynamo currents and induced underground currents. Global patterns of the internal solar and lunar current systems are consistent with the corresponding external current systems previously reported based on ground observations. The Swarm external currents are mainly in the meridional direction, and are likely associated with interhemispheric field-aligned currents. Both the internal and external current systems depend on the season and longitude.Graphical

Seasonal Variation Characteristics of Geomagnetic Sq External and Internal Equivalent Current Systems in East‐Asia and Oceania Regions

AbstractGeomagnetic data from 20 observatories in East‐Asia and Oceania regions were used to investigate the seasonal variation characteristics of the external and internal Sq current systems by spherical harmonic analysis (SHA). The seasonal current models of D (northern winter) season, E (equinox) season, and J (northern summer) season have been constructed respectively, and the current parameters of these models were analyzed. The main results are obtained as follows: (1) The local time T of the northern vortex moves from local noon to prenoon from D to E to J season for both external and internal currents; meanwhile, in the southern hemisphere, the T moves from local noon to afternoon. (2) The vortex intensity J increases from local winter to equinox then to local summer. The interseasonal change rate of internal current is larger than that of external current. (3) The latitude Φ of the external current vortex exhibits seasonal variation in the southern hemisphere, while in the northern hemisphere the latitude Φ of the internal current vortex shows obvious seasonal changes. (4) The correlations between the internal and external currents are further analyzed. In the E season the correlation of T is the highest, and in the local summer and winter, the correlation of J is the strongest. The different seasonal behavior of the external and internal current systems is unexpected, and its mechanism remains to be identified in future study.

Solar quiet (Sq) and conductivity-depth structure of Asia sub-region using solar quiet field variation

The study used some selected magnetic field records to establish the equivalent external and internal ionospheric current system for the moderate solar quiet year 2012. The month-by-month behaviour of the equivalent current for both external and internal current appears stronger in local summer months (May-August) and weaker in local winter months (November-February). The current vortex appears earlier in local summer than in local winter months and disappeared in December. In each of the month, the latitudinal position of the internal Sq current vortex appeared at a considerable higher latitude than the external. A regression line fitted to the estimated conductivity values gives a profile that starts at 0.02 S/m at a depth of about 30 km and rose gradually to about 0.03 S/m seen at ∼120 km depth. Consequently the conductivity profile increases significantly to about 0.07 S/m at depth of about 400 km followed by a sharp steep increase through the transition zone and reached its peak value ∼0.3 S/m at depth of about 770 km. An evidence of discontinuity was observed at depth of about 370-440 km and 603-671 km. The sharp steep increase in conductivity profile at depth of about 400 km may be due to phase change of the mantle minerals from olivine to spinel structure.

A Dynamic Model of Mercury's Magnetospheric Magnetic Field.

Mercury's solar wind and interplanetary magnetic field environment is highly dynamic, and variations in these external conditions directly control the current systems and magnetic fields inside the planetary magnetosphere. We update our previous static model of Mercury's magnetic field by incorporating variations in the magnetospheric current systems, parameterized as functions of Mercury's heliocentric distance and magnetic activity. The new, dynamic model reproduces the location of the magnetopause current system as a function of systematic pressure variations encountered during Mercury's eccentric orbit, as well as the increase in the cross‐tail current intensity with increasing magnetic activity. Despite the enhancements in the external field parameterization, the residuals between the observed and modeled magnetic field inside the magnetosphere indicate that the dynamic model achieves only a modest overall improvement over the previous static model. The spatial distribution of the residuals in the magnetic field components shows substantial improvement of the model accuracy near the dayside magnetopause. Elsewhere, the large‐scale distribution of the residuals is similar to those of the static model. This result implies either that magnetic activity varies much faster than can be determined from the spacecraft's passage through the magnetosphere or that the residual fields are due to additional external current systems not represented in the model or both. Birkeland currents flowing along magnetic field lines between the magnetosphere and planetary high‐latitude regions have been identified as one such contribution.

A global electric circuit model within a community climate model

AbstractTo determine the complex dependencies of currents and electric fields within the Global Electric Circuit (GEC) on the underlying physics of the atmosphere, a new modeling framework of the GEC has been developed for use within global circulation models. Specifically, the Community Earth System Modeling framework has been utilized. A formulation of atmospheric conductivity based on ion production and loss mechanisms (including galactic cosmic rays, radon, clouds, and aerosols), conduction current sources, and ionospheric potential changes due to the influence of external current systems are included. This paper presents a full description of the calculation of the electric fields and currents within the model, which now includes several advancements to GEC modeling as it incorporates many processes calculated individually in previous articles into a consistent modeling framework. This framework uniquely incorporates effects from the troposphere up to the ionosphere within a single GEC model. The incorporation of a magnetospheric potential, which is generated by a separate magnetospheric current system, acts to modulate or enhance the surface level electric fields at high‐latitude locations. This produces a distinct phasing signature with the GEC potential that is shown to depend on the observation location around the globe. Lastly, the model output for Vostok and Concordia, two high‐latitude locations, is shown to agree with the observational data obtained at these sites over the same time period.

MESSENGER observations of Mercury's magnetic field structure

We present a baseline, time‐averaged model for Mercury's magnetosphere, derived from MESSENGER Magnetometer data from 24 March to 12 December 2011, comprising the spacecraft's first three Mercury years in orbit around the innermost planet. The model, constructed under the approximation that the magnetospheric shape can be represented as a paraboloid of revolution, includes two external (magnetopause and magnetotail) current systems and an internal (dipole) field and allows for reconnection. We take advantage of the geometry of the orbital Magnetometer data to estimate all but one of the model parameters, and their ranges, directly from the observations. These parameters are then used as a priori constraints in the paraboloid magnetospheric model, and the sole remaining parameter, the dipole moment, is estimated as 190 nTRM3from a grid search. We verify that the best fit dipole moment is insensitive to changes in the other parameters within their determined ranges. The model provides an excellent first‐order fit to the MESSENGER observations, with a root‐mean‐square misfit of less than 20 nT globally. The results show that the magnetopause field strength ranges from 10% to 50% of the dipole field strength at observation locations on the dayside and at nightside latitudes north of 60°N. Globally, the residual signatures observed to date are dominated by the results of magnetospheric processes, confirming the dynamic nature of Mercury's magnetosphere.

An empirical model of the quiet daily geomagnetic field variation

[1] An empirical model of the quiet daily geomagnetic field variation has been constructed based on geomagnetic data obtained from 21 stations along the 210 Magnetic Meridian of the Circum-pan Pacific Magnetometer Network (CPMN) from 1996 to 2007. Using the least squares fitting method for geomagnetically quiet days (Kp ≤ 2+), the quiet daily geomagnetic field variation at each station was described as a function of solar activity SA, day of year DOY, lunar age LA, and local time LT. After interpolation in latitude, the model can describe solar-activity dependence and seasonal dependence of solar quiet daily variations (S) and lunar quiet daily variations (L). We performed a spherical harmonic analysis (SHA) on these S and L variations to examine average characteristics of the equivalent external current systems. We found three particularly noteworthy results. First, the total current intensity of the S current system is largely controlled by solar activity while its focus position is not significantly affected by solar activity. Second, we found that seasonal variations of the S current intensity exhibit north-south asymmetry; the current intensity of the northern vortex shows a prominent annual variation while the southern vortex shows a clear semi-annual variation as well as annual variation. Thirdly, we found that the total intensity of the L current system changes depending on solar activity and season; seasonal variations of the L current intensity show an enhancement during the December solstice, independent of the level of solar activity.

Dawn‐dusk oscillation of Saturn's conjugate auroral ovals

We present observations of the oscillation of the dawn‐dusk locations of Saturn's northern and southern UV auroral ovals obtained using the Hubble Space Telescope during the 2009 equinoctial campaign. We determine the dawn‐dusk locations of the centers of the southern and northern auroral ovals from the mean of the dawnward and duskward extents of the emission, and order the computed locations by the phases of the respective SKR oscillations for each hemisphere. We show that statistically significant ∼1–2° oscillations of the dawn‐dusk location of the auroral ovals are evident, with the duskward displacement being in lagging quadrature with the SKR power. These results, the first to indicate that the location of the northern auroral oval oscillates, show that the cause of the oscillation is an external magnetospheric current system, and the sense of the oscillations is consistent with the expected displacement caused by magnetic perturbations observed throughout Saturn's magnetosphere.

The interplanetary magnetic field environment at Mercury's orbit

Mercury is exposed to the most dynamic heliospheric space environment of any planet in the solar system. The magnetosphere is particularly sensitive to variations in the interplanetary magnetic field (IMF), which control the intensity and geometry of the magnetospheric current systems that are the dominant source of uncertainty in determinations of the internal planetary magnetic field structure. The Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has made extensive magnetic field observations in the inner heliosphere over the heliocentric distances of Mercury's orbit, between 0.31 and 0.47 AU. In this paper, Magnetometer data from MESSENGER, obtained at rates of 2 and 20 vector samples per second, are used together with previous observations in the inner heliosphere by Helios and at Earth by the Advanced Composition Explorer, to study the characteristics of IMF variability at Mercury's orbit. Although the average IMF geometry and magnitude depend on heliocentric distance as predicted by Parker, the variability is large, comparable to the total field magnitude. Using models for the external current systems we evaluate the impact of the variability on the field near the planet and find that the large IMF fluctuations should produce variations of the magnetospheric field of up to 30% of the dipole field at 200 km altitude, corresponding to the planned periapsis of MESSENGER's orbit at Mercury. The IMF fluctuations in the frequency range 10 − 4 < f < 10 − 1 Hz are consistent with turbulence, whereas evidence for dissipation was observed for f > 1 Hz . The transition between the turbulent and dissipative regimes is indicated by a break in the power spectrum, and the frequency of this break point is proportional to the IMF magnitude.

Moderate geomagnetic storm (21–22 January 2005) triggered by an outstanding coronal mass ejection viewed via energetic neutral atoms

An outstanding flare on 20 January 2005 was accompanied by a coronal mass ejection which arrived at the magnetopause at ∼1712 UT, 21 January, and produced a strong compression‐pressure pulse. Enhanced magnetospheric activity was stimulated. The associated development between &lt;1800 UT and &gt;02.19 UT on 21–22 January, 2005 of a ring current disturbance in energetic neutral atom (ENA) data, recorded aboard both the Double Star and the IMAGE (Imager for Magnetopause‐to‐Aurora Global Exploration) spacecraft, is described. A magnetic storm from ∼1712 UT, 21 January, reached minimum Dst = ∼−101 nT at ∼0600 UT, 22 January, and its recovery phase endured until 27 January. ENA data indicate that the ring current experienced a deep injection of H+ and O+ ions at ∼1830 UT when IMF Bz was oriented southward. At this time, the ring current was strongly asymmetric, although later it became more symmetric. Bz turned northward at 1946 UT. From ∼0224 to ∼0612 UT on 22 January, Bz fluctuated such that it intermittently pointed southward (±10 nT). The moderate but extended response of the geomagnetosphere to the strong pressure pulse is explained by a slow evolution in the orientation of Bz under conditions of enhanced plasma sheet density. Modeling of dynamical parameters that represent various current systems that contributed to Dst revealed their individual characteristics. The changing geomagnetic field was also modeled. Comparisons with ENA data show that early asymmetric enhancements recorded in hydrogen and oxygen were accompanied by intensified external current systems that produced a magnetic field related compression of the magnetosphere. The gradual reduction in ring current asymmetry was complemented by the largely symmetrical configuration displayed by the corresponding, still intensified, modeled magnetic field.

Polarization and phase of planetary‐period magnetic field oscillations on high‐latitude field lines in Saturn's magnetosphere

We examine magnetic field data obtained by the Cassini spacecraft on a sequence of high‐latitude orbits in Saturn's magnetosphere spanning October 2006 to May 2007 to determine whether planetary‐period oscillations are present on polar open field lines, such as have been found previously in near‐equatorial magnetic field data. Such oscillations are found generally to be present with amplitudes ∼0.5–1 nT, somewhat smaller than the few nT amplitudes typical of the quasi‐dipolar equatorial region. The polarization characteristics in the northern and southern polar regions are determined and found to differ significantly from those in the equatorial region. The phases of the oscillations in the northern and southern hemispheres are also determined relative to the equatorial oscillations, and hence relative to each other, requiring extension of the equatorial oscillation phase model to the end of 2007, spanning the interval of high‐latitude orbits. The results show that the overall pattern of field oscillations is not consistent with a rotating external current system that mimics a rotating transverse dipole in the outer regions. Rather, we suggest that the overall field perturbations are associated with a rotating partial ring current and its field‐aligned closure currents, the latter favoring the southern ionosphere during the southern summer conditions examined. A physical picture is presented that links together observed planetary‐period modulations in the middle and outer magnetospheric field, plasma, and radio emissions that may be subject to further test and makes predictions as to how these phenomena will evolve during future Saturn equinox and northern summer conditions.

Oscillation of Saturn's southern auroral oval

Near‐planetary‐period oscillations in the Cassini plasma and magnetic field data have been observed throughout Saturn's magnetosphere despite the fact that Saturn's internal magnetic field is apparently highly axisymmetric. In addition, the period of the Saturn kilometric radiation has been shown to vary over time. In this paper we present results from the recent Hubble Space Telescope observations of Saturn's southern ultraviolet auroral emission. We show that the center of the auroral oval oscillates with period 10.76 h ± 0.15 h for both January 2007 and February 2008, i.e., close to the periods determined for oscillations in other magnetospheric phenomena. The motion of the oval center is described for 2007 by an ellipse with semimajor axis ∼1.4° ± 0.3° oriented toward ∼09–21 h LT, eccentricity ∼0.93, and center offset from the spin axis by ∼1.8° toward ∼04 h LT. For 2008 the oscillation is consistent with an ellipse with semimajor axis ∼2.2° ± 0.3° oriented toward ∼09–21 h LT, eccentricity ∼0.99, and a center offset from the spin axis by ∼2.2° toward ∼03 h LT. The motion of the auroral oval is thus highly elliptical in both cases, and the major oscillation axis is oriented toward prenoon/premidnight. This result places an independent constraint on the magnitude of the planet's dipole tilt and may also indicate the presence of an external current system that imposes an asymmetry in the ionospheric field modulated close to the planetary period.

Equatorial electrojet as a diagnostic tool of geomagnetic field models

Abstract The equatorial electrojet (EEJ) is a unique feature of the Earth’s external current systems because it must flow along the dip equator. This provides us with a tool to determine the nature of the variations imposed by competing main field models on the equatorial region. First we show that for certain regions a comparison between scalar geomagnetic measurements that use different models to remove the main field may not be reasonable. Next we found the intrinsic error in the determination of the possible location of the dip equator was ±9.8 km (0.088°) at 108 km altitude for the models shown here. Using scalar measurements from over 14,000 CHAMP satellite passes, the latitude of the maximum of the EEJ field at the satellite altitude was determined by subtracting four different models of the main field. We find that the location can be statistically determined to within ±0.5° of the dip equator (calculated at 108 km altitude) irrespective of longitude, time of the measurement, degree of magnetic activity, and subtracted model. However, variations of the latitude of the maximum EEJ field with longitude are sometimes caused by the actual model and are not always a physical phenomenon. By choosing one model, and assuming it is the best representation of the main field, we have also shown that the accuracy of determination of the position of the EEJ signal is reduced in the morning and evening hours and that a morning and evening shift in the location of the EEJ found using ground measurements is also seen here. There exists a clear annual variation in the position of the EEJ regardless of longitude: it is south of the dip equator in December which is in agreement with the findings of all previous studies.

Local time stacking of geomagnetic solar daily variations

Regular geomagnetic solar daily variations, the Sq signal, is used in electromagnetic induction studies to investigate the electrical conductivity structure of Earth's upper mantle. Traditionally, Sq induction studies employ observatory magnetic data from a few quiet days in which the overprint on the regular Sq ionospheric current vortices from various irregular disturbances is minimal. Active days are typically discarded, a procedure which results in a gappy time series that is difficult to analyze. Furthermore, the quiet day selection is largely arbitrary since daily geomagnetic activity levels range continuously from very quiet to very active. This fact precludes the establishment of rigorous quiet/active thresholds. The average local time variation on active days resembles typical quiet day variations except for a multiplicative scaling factor. A multiyear stack of yearlong geomagnetic time series aligned with respect to local time incorporates “regular active” days while greatly attenuating the effects of irregular geomagnetic disturbances. External and internal current systems that rise and fall in step with the Sun's transit across the sky above an observatory contribute coherently to the local time stack. All other current systems are incoherent and tend to stack out. An exact 1‐year‐long stack preserves the seasonal variation. The resulting stacked time series contains no gaps, is dominated by regular solar daily variations, and can be used for improved Sq induction studies and comprehensive geomagnetic field modeling. Hourly geomagnetic values from Niemegk, Boulder, Mbour, and Meanook observatories between 1970 and 2002, along with the ap index, are used in this study.

Multisheet modelling of the electrical conductivity structure in the Fennoscandian Shield

Electromagnetic multisheet modelling is a powerful tool for large model areas, if they can be approximated by a multilayered heterogeneous conductivity structure of small vertical dimension in comparison with the penetration depth of electromagnetic fields. In this paper, thin sheet technique is applied to the whole Fennoscandian (Baltic) Shield, whose upper mantle conductivity structure is the objective of the long period electromagnetic array experiment BEAR (Baltic Electromagnetic Array Research). Three thin sheets, each of about 120,000 model cells with the base length of 10 km, describe a-priori crustal inhomogeneities in terms of conductances. The three sheets represent i) upper crust from surface to the depth of 10 km including continental and ocean bottom sediments and seawater, ii) middle crust ranging from 10 km to 30 km and iii) lower crust from 30 km to 60 km. Thus, modelling is taking into account distortions caused by crustal conductivity anomalies. Additionally, distortions due to inhomogeneous external current systems are investigated by introducing an equivalent current system of a polar electrojet model at ionosphere height. Modelling results are illustrated by induced current distribution at different depth levels and by various electromagnetic transfer functions. The latter demonstrate the resolution of crustal conductivity anomalies and their screening effect even at long periods. The predicted behavior of transfer functions in the very complex conductivity structure is compared with the experimental BEAR data, showing qualitatively a first order agreement for most of the sites. Modeled phases for periods of a few thousands of seconds are considerably biased in comparison with experimental data if the background 1-D model has monotonously decreasing resistivity. However, the bias from phases is removed if a conducting asthenosphere having a resistivity of 20 Ωm is emplaced between the depths of 200 km and 300 km. Thus, multisheet modelling indicates a well conducting upper mantle under the Fennoscandian Shield. All modelling has been performed using a multisheet code by Avdeev, Kuvshinov and Pankratov.

Initial results from the POLAR magnetic fields investigation

The POLAR magnetic field investigation was designed to provide a high temporal resolution and high sensitivity measurement of the magnetic field along the POLAR orbit from perigee at 2 R E to apogee at 9 R E. The objectives of the investigation include probing the field aligned currents that couple the solar wind interaction and the tail dynamics to the auroral ionosphere. The launch of the spacecraft on February 24, 1966 was nominal and the turn on of the instruments very successful. Comparisons with Tsyganenko's 1995 model of the external current systems show that at quiet times it reproduces the field well. POLAR is clearly observing the field aligned current systems and the ring current. On one occasion when the IMF was strongly northward and the solar wind dynamic pressure stronger than usual the high altitude, high latitude magnetosphere became filled with magnetosheath type plasma. This event was probably an encounter with the polar cusp near the cusp-magnetosheath interface.

Cosmic ray cutoff rigidities in the Tsyganenko magnetospheric field model at middle latitudes

The cutoff rigidities of cosmic rays are determined in the magnetospheric magnetic field simulated by the model of external current system proposed by Tsyganenko (1987) (Ts87) [ Planet Space Sci. 35, 1347]. The diurnal variations of cosmic ray vertical cutoffs are obtained for the midlatitude stations Moscow, Irkutsk, Tbilisi and Alma-Ata, at the moment of summer solstice. The cutoffs received are compared with those obtained using the model by Tsyganenko and Usmanov (1982) (TsU82) [ Planet Space Sci. 30, 985] for K p = 0, 2 and 4. The results show that the differences between daily averaged geomagnetic cutoff rigidities in the magnetospheric fields of the Ts87 and TsU82 models are not more than 0.03 GV.

Quiet ionospheric currents of the southern hemisphere derived from geomagnetic records

This work describes the month‐by‐month behavior of the equivalent ionospheric current systems derived from spherical harmonic analyses of the quiet time geomagnetic field daily variations in 1965 at selected observatories representing the three southern global half‐sector regions separately encompassing South America, Africa, and Australia. These external Sq current patterns were vortices having mid‐latitude foci with midday summertime amplitudes reaching 16.2×104 A above the midnight level. The wintertime amplitudes were about 10×104 A smaller. At low latitudes there was a large intrusion of the opposite hemisphere external Sq current system into the wintertime hemisphere at prenoon hours, displacing the primary current vortex to later postnoon hours. The behavior of the southern hemisphere external currents were found to be seasonally similar to those of the northern hemisphere for the same year. The quiet year behavior was compared to the results for the 1958 active year determined earlier by Matsushita. The winter‐to‐summertime increase in focus current was found to be similar in amplitude for the 2 years. The active year summertime and equinoctial current focus amplitudes were about 2.3 times the amplitudes of corresponding months in the quiet year.