Interhemispheric Field-aligned Currents Research Articles

Variations of Inter‐Hemispheric Field‐Aligned Currents: Observations From Ground Geomagnetic Measurements

AbstractThe east–west component of the geomagnetic field (Y‐component) at the Trivandrum (TRD) station, an equatorial site, is used to investigate the characteristics of Inter‐Hemispheric Field‐Aligned Currents (IHFACs) in the Indian sector during solar cycles 21 and 22. The observations reveal an annual variation of IHFAC in the dawn/noon sector with positive/negative peaks around northern summer. Furthermore, it is noted that IHFAC amplitudes are modulated by solar activity, superimposed with seasonal effects, with consistently higher amplitudes during the northern summer months. The dusk‐time IHFAC also shows a pattern modulated by seasons and solar cycles, with negative peaks around northern summer only during the low solar active years. The seasonal pattern of IHFAC shows a similar trend from April to September, where dawn/noon IHFAC is south/north directed, and for the remaining months, both are northward directed. The dusk‐side IHFAC is mostly south‐directed irrespective of the season. Moreover, it is demonstrated that the seasonal pattern of dawn and noon‐time IHFAC amplitudes show a strong to moderate correlation with amplitudes of Sq asymmetries between two hemispheres, but dusk‐time IHFAC does not follow this trend. The IHFAC directions at TRD follow the Fukushima model for May to September during the dawn‐noon sector for 70%–95% of the days and less than 50% of the days for remaining months. The noon‐dusk sector shows Fukushima model‐type polarity for ∼60% of days in June–July and less than 50% for remaining months. The global analysis of IHFAC, using monthly averages of hourly values from eight observatories spanning Asia, Africa, and America, reveals spatial differences in amplitude and directions. The directions of noon‐time IHFAC in the Indian‐East Asian and East‐African sectors follow similar trends, and the amplitude variations of noon‐time IHFAC can be explained by DE‐3 tidal effects. The IHFAC from the South American and West African sectors show intense southward and northward currents compared to other sites, indicating the influence of the South Atlantic Anomaly (SAA).

Modeling the Climatology of Low‐ and Mid‐Latitude F‐Region Ionospheric Currents Using the Swarm Constellation

AbstractWe present a new empirical model of quiet‐time F‐region ionospheric currents and associated magnetic fields. This model is designed to accurately represent these currents and fields at low and mid latitudes. For each individual Swarm satellite, the preprocessed data is represented as a non‐potential toroidal magnetic field using the Mie representation in a thin‐shell and spherical harmonic expansions. This approach allows to fully separate spatial and climatological variations as well as to assess the robustness of the model with respect to both measurement errors and data sampling with local time. The obtained model describes the toroidal magnetic fields and the associated radial poloidal electric currents at two distinct altitudes in the ionosphere F region. Clear signatures of low‐ and mid‐latitude interhemispheric field‐aligned currents (IHFACs) are identified. The model reproduces well‐known characteristics of the climatology of IHFACs and provides new insights, for example, on the average daily variations of IHFACs during winter in the Northern Hemisphere. It also well recovers the variations of IHFACs with longitude. The potential driving mechanisms of these variations, such as longitudinal variations of the main field and modulation by upward propagating atmospheric tides, are discussed. The new model can be used to analyze the relationship between atmospheric tides and IHFACs. It can also be used to investigate the connection between the magnetic fields and electric currents from the ionospheric E and F regions in order to improve the separation of these fields as well as our understanding of the overall ionospheric electric current system.

Global Characteristics of Improved Interhemispheric Field‐Aligned Currents and of F‐Region Meridional Currents Observed by the Swarm Dual‐Spacecraft

AbstractThe Swarm satellite constellation provides an excellent opportunity to explore ionospheric current systems. In this study, we have reanalyzed the ionospheric currents derived by the Swarm dual‐satellite approach, to investigate the characteristic of inter‐hemispheric field‐aligned currents (IHFAC). One major improvement is that the influence of the ambient magnetic field on the IHFAC intensity has been taken into account, and the derived IHFAC densities are normalized to their ionospheric E‐region footprints. In addition, we have extended the analysis to the middle latitudes within ±60° MLat, which show IHFAC features different from those at low latitudes. For the first time the tidal features at mid‐latitudes are studied. They are dominated by longitudinal wave‐1 and wave‐2 patterns. A superposition of these tidal components reflects a confinement of the IHFAC tidal modulation to daytime in the western hemisphere. The tidal signatures at middle latitudes are better organized in universal time than in local time. The strongest IHFACs are appearing around 18–19 UT, near noon in the American sector. This is related to the overlap with the South Atlantic Anomaly. For the first time we define and analyze the F‐region meridional currents (FMC), interpreting the remaining part of the ionospheric radial current (IRC), which is not covered by IHFAC. The mean FMCs are found to be highly symmetric with respect to the magnetic equator and only amounts to one‐third or half of the IHFAC density, however, they show neither typical longitudinal, diurnal variations, or tidal signatures.

Effect of the Non‐Dipole Field on the Seasonal Variation of the Geomagnetic Sq(Y)

AbstractThe annual mean (Sq0), annual (Sq1), and semiannual (Sq2) components of the monthly averaged daily amplitude of the geomagnetic Y component during quiet period over several solar cycles at 25 mid‐low latitudes observatories were separated by means of Fourier analysis. No obvious distinction is found for the morphology of the spatial distribution of these Sq(Y) components during solar quiet and active periods, except that they are more intense in high solar activity. We found that the longitudinal variations in the Southern Hemisphere were primarily attributed to the differences in the annual component (Sq1), which is stronger at the Eurasia and Australia anomaly zones than at the South Atlantic Ocean anomaly (SAA) zones. The positive correlation between Sq0 and geomagnetic vertical component Z suggests that the convection electric fields in the dynamo region play a key role in controlling the annual mean (Sq0). On the other hand, the Sq1 exhibits a positive correlation with geomagnetic horizontal component H, implying the interhemispheric field‐aligned currents (IHFACs) may contribute to difference of the annual variation amplitude at the different observatories. Sq2 is most prominent in the SAA region while rather weak at other positive anomaly zones (in total field), which indicates there is a negative correlation between the total field anomaly and the semiannual variations.

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

The Universal Time Variations of the Intensity of Afternoon Aurora in Equinoctial Seasons

AbstractThe afternoon auroral emissions are investigated in the equinoxes for geomagnetically quiet conditions (Kp = 1) using auroral images from ultraviolet imager (UVI) aboard the Polar satellite. They are compared with solar illumination effects (the solar zenith angle [SZA] and the consequent ionospheric conductivity) and the dipole tilt angle, as well as the observational region 1 upward field‐aligned currents (FACs) from AMPERE data. The averaged afternoon auroral emissions have pronounced universal time (UT) variations with valley (2.8 photons/cm2/s) at around 01:00–03:00 UT and peak (4.7 photons/cm2/s) at around 17:00–19:00 UT. They generally vary with the solar illumination, the dipole tilt angle and the observed region 1 upward FACs as a function of UT. The afternoon auroral intensity is anticorrelated with the SZA and positively proportional to the solar EUV‐produced Pedersen conductivity, region 1 upward FACs and dipole tilt angle. Additionally, they depend weakly on solar flux under geomagnetically quiet conditions. These results suggest that in the afternoon auroral region, the peak auroral emissions are closely associated with the peak conductivity and the maximum upward FACs. Other mechanisms, such as the dipole tilt angle, may also contribute. Further comparison between the northern afternoon aurora and the FACs in the two conjugate hemispheres suggests little contributions on the auroral UT variations from the interhemispheric FACs in the equinoxes.

Diagnosing low-/mid-latitude ionospheric currents using platform magnetometers: CryoSat-2 and GRACE-FO

Electric currents flowing in the terrestrial ionosphere have conventionally been diagnosed by low-earth-orbit (LEO) satellites equipped with science-grade magnetometers and long booms on magnetically clean satellites. In recent years, there are a variety of endeavors to incorporate platform magnetometers, which are initially designed for navigation purposes, to study ionospheric currents. Because of the suboptimal resolution and significant noise of the platform magnetometers, however, most of the studies were confined to high-latitude auroral regions, where magnetic field deflections from ionospheric currents easily exceed 100 nT. This study aims to demonstrate the possibility of diagnosing weak low-/mid-latitude ionospheric currents based on platform magnetometers. We use navigation magnetometer data from two satellites, CryoSat-2 and the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), both of which have been intensively calibrated based on housekeeping data and a high-precision geomagnetic field model. Analyses based on 8 years of CryoSat-2 data as well as ~ 1.5 years of GRACE-FO data reproduce well-known climatology of inter-hemispheric field-aligned currents (IHFACs), as reported by previous satellite missions dedicated to precise magnetic observations. Also, our results show that C-shaped structures appearing in noontime IHFAC distributions conform to the shape of the South Atlantic Anomaly. The F-region dynamo currents are only partially identified in the platform magnetometer data, possibly because the currents are weaker than IHFACs in general and depend significantly on altitude and solar activity. Still, this study evidences noontime F-region dynamo currents at the highest altitude (717 km) ever reported. We expect that further data accumulation from continuously operating missions may reveal the dynamo currents more clearly during the next solar maximum.

Latitude Dependence of Interhemispheric Field‐Aligned Currents (IHFACs) as Observed by the Swarm Constellation

Key Points Two clearly different systems of IHFACs exist at low and middle latitudes, respectively, with a boundary near ±35° in magnetic latitudes The latitudinal and longitudinal structures of IHFACs depend strongly on wave forcing from the lower atmosphere, below 30 km Midlatitude IHFACs have similar climatology in equinoxes/June solstice, while those of December solstice exhibit different behavior

Interhemispheric field-aligned currents at the edges of equatorial plasma depletions

A comprehensive description of electromagnetic processes related to equatorial plasma depletions (EPDs) is essential for understanding their evolution and day-to-day variability. Recently, field-aligned currents (FACs) flowing at both western and eastern edges of EPDs were observed to be interhemispheric rather than anti-parallel about the dip equator, as suggested by previous theoretical studies. In this paper, we investigate the spatial and temporal variability of the FACs orientation using simultaneous measurements of electron density and magnetic field gathered by ESA’s Swarm constellation mission. By using empirical models, we assess the role of the Pedersen conductance in the preference of the FACs to close either in the northern or southern magnetic hemisphere. Here we show that the closure of the FACs agrees with an electrostatic regime determined by a hemispherical asymmetry of the Pedersen conductance. That is, the EPD-related FACs close at lower altitudes in the hemisphere with the highest conductivity. The evidence of this conclusion stands on the general agreement between the longitudinal and seasonal variability of both the conductivity and the FACs orientation.

Excursions of Interhemispheric Field‐Aligned Currents in Africa

AbstractThe interhemispheric field‐aligned currents (IHFACs) are the most evident currents connecting the solar quiet (Sq) current systems in the Northern and Southern Hemispheres. Understanding these coupling currents has practical importance for space weather forecast of the magnetosphere‐ionosphere interactions and their potential impacts on low‐orbiting spacecraft. In this paper, we have examined digital magnetic records from September 2008 to August 2009 at nine Magnetic Data Acquisition System (MAGDAS) stations in order to uniquely determine the variability of IHFACs in terms of their excursions in the African sector. By excursions, we mean the direction in which the IHFACs are moving as a function of local time or latitude. Different from theoretical prediction, we found prominent positive IHFACs around dusk in February and March equinox. A novel feature of this study is the semidiurnal variation control of the coupling between the Northern and Southern Hemispheres. In addition, we found terdiurnal variation in September with their associated strong diurnal variations from October through December. Quite unexpectedly, in January, a significant latitudinal depletion in the intensities of IHFACs was observed from dawn to dusk, compared to any other months under investigation, accompanied with a marked reduction of the prominent diurnal variations at the magnetic equator (Addis Ababa, AAB). The fact that the excursions of IHFACs in January experience an unusual retardation with respect to their intensities points to a relation with the effects of sudden stratospheric warming.

Interaction of Saturn’s dual rotation periods

We develop models of the interaction of Rossby wave disturbances in the northern and southern ionospheres of Saturn. We show that interhemispheric field-aligned currents allow the exchange of vorticity, modifying the background Rossby wave propagation speed. This leads to interaction of the northern and southern Rossby wave periods. In a very simple symmetric model without a plasma disk the periods merge when the overall conductivity is sufficiently high. A more complex model taking account of the inertia of the plasma disk and the asymmetry of the two hemispheres predicts a rich variety of possible wave modes. We find that merging of the northern and southern periods can only occur when (i) the conductivities of both hemispheres are sufficiently low (a criterion that is fulfilled for realistic parameters) and (ii) the background Rossby wave periods in the two hemispheres are identical. We reconcile the second criterion with the observations of a merged period that also drifts by noting that ranges of Rossby wave propagation speeds are possible in each hemisphere. We suggest that a merged disturbance in the plasma disk may act as an ‘anchor’ and drive Rossby waves in each hemisphere within the range of possible propagation speeds. This suggestion predicts behaviour that qualitatively matches the observed merging and splitting of the northern and southern rotation periods that occurred in 2013 and 2014. Low conductivity modes also show long damping timescales that are consistent with the persistence of the periodic signals.

Effect of interhemispheric currents on equivalent ionospheric currents in two hemispheres: Simulation results

AbstractIn this research, we used numerical simulation to study the effect of interhemispheric field‐aligned currents (IHCs), going between two conjugate ionospheres in two hemispheres, on the equivalent ionospheric currents (EICs). We computed the maps of these EICs in two hemispheres during summer‐winter conditions, when the effect of the IHCs is especially significant. The main results may be summarized as follows. (1) In winter hemisphere, the IHCs may significantly exceed and be a substitute for the local R1 currents, and they may strongly affect the magnitude, location, and direction of the EICs in the nightside winter auroral ionosphere. (2) While in summer polar cap the EICs tend to flow sunward, and in winter polar cap the EICs turn toward dawn due to the effect of the IHCs. (3) The well‐known reversal in the direction of the EICs in the vicinity of the midnight meridian, in winter hemisphere, is observed not at the polar caps boundary (as usually expected) but equatorward of this boundary in the region of the IHCs location. (4) The IHCs in winter hemisphere may be, in fact, not only a substitute for the R1 currents but also the major source of the Westward Auroral Electrojet, observed in both hemispheres during substorm activity.

Effect of Interhemispheric Field-Aligned Currents on Region-1 Currents

An asymmetry in ionospheric conductivity between two hemispheres results in the formation of additional, interhemispheric field-aligned currents flowing between conjugate ionospheres within two auroral zones. These interhemispheric currents are especially significant during summer-winter conditions when there is a significant asymmetry in ionospheric conductivity in two hemispheres. In such conditions, these currents may be comparable in magnitude with the Region 1 field-aligned currents. In this case, the R1 current is the sum of two FACs: one is going from to the solar wind, and another is flowing between conjugate ionospheres. These interhemispheric currents can also cause the formation of auroras extended along the nightside polar cap boundary, which may be related to the so-called double auroral oval. In this study, we present the results of analytical and numerical solutions for the interhemispheric currents and their effect on the Region 1 currents.

Climatology of the inter-hemispheric field-aligned currents system over the Nigeria ionosphere

Records of the declination (D) magnetic field data for the year 2009 from the Magnetic Data Acquisition System (MAGDAS) facilities at University of Ilorin were employed for this work. From the minutes value of the D-component, the deduced hourly values of the D-component (Sq(QD)) were used to estimate its diurnal (Sq(D)) values with the most five quietest days identified. The monthly mean (MSq(D)) of the most five quietest days and their seasonal (SSq(D)) variabilities were investigated. The inter-hemispheric field aligned currents (IHFACs) exhibit downward and upward inter-hemispheric field-aligned sheet current that appears as a pair at all local times of the Sq(D), MSq(D), and SSq(D) variations. From these variabilities, the IHFACs were observed to flow from the winter to summer hemisphere during noon and dusk sector and flowing in opposite direction during the dawn sector. The Sq(D) variability patterns that were observed in May, June, August September are gentle compared to the disturbed variabilities in January, February, March and November. The highest positive (∼1.7arc-min) and negative (∼−2.7arcmin) MSq(D) maxima values were observed in August during the dawn and noon sectors respectively. These values indicated that the IHFACs flow in August is strongly southbound (positive) and northbound (negative) in the dawn and noon sectors respectively. Dusk-side IHFACs as can be observed by MAGDAS are weakly northbound in all the seasons. The direction of IHFACs does not flip at the equinoxes but in June and November and does not become largest at solstices but in August. The IHFACs was observed to exhibit longitudinal variability, which indicated that larger amplitude of winter-to-summer IHFACs is observed to be greater in June solstice (northbound/negative IHFACs) than in the December solstice (southbound/positive IHFACs) during the noon sector.

Climatology of the inter-hemispheric field-aligned current system in the equatorial ionosphere as observed by CHAMP

Abstract. From geomagnetic field observations of CHAMP during 2001–2009 we extracted characteristic signatures of inter-hemispheric field-aligned currents (IHFACs) in the equatorial ionosphere. The results are in general agreement with previous observations. Nighttime IHFACs are negligibly small. Solstitial IHFACs flow from the summer to winter (from winter to summer) hemisphere at dawn (around noon). Duskside IHFACs flow southbound irrespective of season. We have also found some new IHFAC properties, which may have been predicted by theories, but are not yet given observational support. IHFACs clearly exhibit a longitude dependence, which is modulated by the South Atlantic Anomaly, the offset between geographic and magnetic equators, and tidal waves. IHFACs show little dependence on the solar cycle. We provide a comprehensive assessment of the IHFAC modulation by non-migrating tides.

Inter-hemispheric Trans-equatorial Field-aligned Currents Deduced from MAGDAS at Equatorial Zone

Inter-hemispheric Trans-equatorial Field-aligned Currents Deduced from MAGDAS at Equatorial Zone

Mid-latitude solar eclipses and their influence on ionospheric current systems

Abstract. Using CHAMP magnetic field data we study the behaviour of the geomagnetic field during two mid latitude eclipses on 21 June 2001 and 22 September 2006. The possible influence of the eclipses on different ionospheric current systems, as seen in the magnetic field measured by CHAMP, is discussed. It is expected that the blocking of solar radiation during an eclipse causes a reduction of the ionospheric conductivity and therefore has an effect on the different current systems. We address in particular the effects of the eclipses on the inter-hemispheric field-aligned currents and on the Sq current system. The two events studied occur under different seasonal conditions, e.g. June solstice and September equinox, therefore quite different aspects can be investigated. We find that the eclipses might affect the direction and intensity of the inter-hemispheric currents and possibly influence the direction of zonal winds, therefore changing the direction of the prevailing F-region dynamo currents. The eclipse in the Southern Hemisphere during September equinox caused inter-hemispheric currents similar to those observed in northern summer. Reverse inter-hemispheric currents were recorded after the end of the eclipse. A large variety of atypical currents was observed during the June event. Most of them might be related to a reversed F-region dynamo in the morning sector and an enhanced conductivity difference between the hemispheres. The eclipse in the south seems to enhance the June solstice conditions considerably.

Seasonal and local time dependences of the interhemispheric field‐aligned currents deduced from the Ørsted satellite and the ground geomagnetic observations

The precise magnetic field measurements by the Ørsted satellite revealed a distinct spatial variation of the eastward component residual (ΔBϕ) at mid and low latitudes on the dayside. This magnetic field variation was attributed to the interhemispheric field‐aligned currents (IHFACs) at mid and low latitudes based on its comparison with ground geomagnetic field variations. These IHFACs were found to flow from the summer hemisphere to the winter hemisphere in the dawn sector, and from the winter hemisphere to the summer hemisphere in the noon and the dusk sectors. These IHFACs flip their direction not at equinoxes but in May and November. The current intensity of the IHFACs is largest not at solstices but in February and August. The ground‐based geomagnetic field observations at 17 low latitude stations also support the seasonal dependence and local time distribution of the IHFACs.

Field‐aligned currents between conjugate hemispheres

Interhemispheric field‐aligned currents flowing between dayside conjugate ionospheres are calculated for different seasons. The currents are a result of redistribution of original three‐dimensional currents because of different ionospheric conductivity in dark and sunlit conjugate hemispheres. Interhemispheric currents and related patterns for the electric potential distribution are computed for a reasonable model of the ionospheric conductivity. The interhemispheric field‐aligned currents are attached to the terminator position, and they close a part of ionospheric currents, flowing in the summer high‐latitude ionosphere, through the conjugate ionosphere of the opposite hemisphere. Although the interhemispheric currents are more significant for summer/winter conditions, they can be observed for equinoctial conditions as well because of a diurnal variation in the terminator position with respect to the geomagnetic poles. Interhemispheric currents in the winter ionosphere are generated mainly by sources in the summer hemisphere. An interesting consequence of the existence of interhemispheric currents is that auroral events can be observed not in the hemisphere with incident field‐aligned currents from a magnetospheric source but in the opposite hemisphere.