1 Introduction: daily changes inthe ionosphere and the ionos-pheric dynamo

The ionosphere over Japan, particularly inits southern regions, shows significantchanges on a daily basis. For example, Fig. 1shows daily changes in the total electron con-tent (TEC) over Okinawa as observed over an80-day period from the beginning of 2006.The middle and bottom panels in Fig. 1 showchanges during the daytime (i.e., average from11:00 to 15:00 LT), and changes around sunset(i.e., average from 17:00 to 21:00LT), respec-tively. The top panel shows the Kp index thatindicates the degree of geomagnetic turbu-lence. The Kp index during these periods wasgenerally below 3, indicating geomagneticallyquiet. The Kp index occasionally fluctuatesbetween 3 and about 5, but the figure appar-ently suggests that this fluctuation does notcorrelate with changes in TEC. That is, eventhough there are no effects from the magnetos-phere, significant daily changes occur in theionosphere at such low latitudes. In terms of

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2-3-7 Ionospheric Dynamo Process

JIN Hidekatsu

Ionospheric electron density varies on various temporal and spatial scales. Because electrondensity variations affect radio waves reflecting at and penetrating in the ionosphere, their predic-tion is an important issue for space weather forecast. Some of the ionospheric day-to-day varia-tions are considered to originate in the lower atmospheric activities. Ionospheric dynamo playsan important role in converting from atmospheric waves into the variations in electric field andcurrent.This paper describes the basic theory of ionospheric dynamo, and the generations ofglobal ionospheric current system and electric field distribution which leads to ionospheric densi-ty variations. We also introduce our electrodynamics model which represents ionosphericdynamo process. This model is a part of a whole atmosphere-ionosphere coupled model, whichis expected to reproduce ionospheric day-to-day variations.

KeywordsIonospheric dynamo, Space weather, Ionosphere, Atmosphere, Simulation

Fig.1 Daily changes in the total electroncontent over southern part of JapanThe Kp index during an 80-day period from thebeginning of 2006 (top panel) and changes inthe total electron content in the space over Oki-nawa (middle panel: average from 11:00 to15:00LT during the daytime; bottom panel:average from 17:00 to 21:00LT around sunset)

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its effects on our society, the TEC is a quantitydirectly linked to radio propagation delaybetween the ground surface and artificialsatellites, and therefore daily changes in TECcan give serious impacts on the use of satellitepositioning applications.

These daily changes in the ionosphereover southern Japan are related to the develop-ment of “equatorial ionization anomaly (EIA)”to mid-latitudes. The EIA refers to a structurethat the largest density region is not located atthe equator where the solar radiation fluxbecomes largest but is separated to the southand north portions across the magnetic equatoras shown in Fig. 2. In the formation of theEIA, electric field generated by dynamoprocess (described later) plays a key role.When the electric field acts on ionosphericplasma, the electric field drift in the directionorthogonal to the magnetic field lines of theearth (poleward/ upward due to eastward elec-tric field) and diffusion toward thepoles/downward along the magnetic field lineswill generate a distribution of electron densi-ties as shown in Fig. 2. Thus, the dynamoelectric field affects the latitudinal spread of

EIA. Southern part of Japan is located on thehigh-latitude side of EIA, so that the F regionelectron density during the daytime dependson the degree to which EIA spreads. In otherwords, changing electric field due to dynamoprocess is one of major factors behind dailychanges in electron density at low to mid lati-tudes.

This paper presents the ionosphericdynamo that is closely related to changes inthe ionosphere at low to mid latitudes. Studyof the ionospheric dynamo has a long history.It initiated mainly during the 1950s on amechanism that connects the ionospheric cur-rent system and neutral atmospheric dynamics(e.g., tides, other atmospheric waves) as esti-mated based on observations of geomagneticfield variation on the ground. For the detailedhistorical review, see Reference［1］and othermaterials. In recent years, the importance ofionospheric dynamo has been further recog-nized from several aspects. One aspect isrelated to the recent advances in the observa-tion technique, i.e., global remote sensingfrom satellites and GPS receiver network,these have enabled us to observe broaderionospheric distribution. This resulted in thenew discovery of an ionospheric longitudinalstructure considered to originate in the loweratmosphere (see Reference［2］ in his featureedition). The ionospheric dynamo presumablyplays an important role in the linkage betweenthe lower atmosphere and ionosphere. Anotheraspect is due to advances in the numericaltechnique, that is, global-scale simulationmodels that integrate different vertical regionshave become possible. The electrodynamicsmodel that treats the ionospheric dynamo is animportant component that connects an atmos-pheric model with an ionospheric model.Advances made in regionally integrated mod-els have allowed researchers to address suchchallenges as the origin of daily changes in theionosphere, and the models are expected to beused in future numerical prediction of theupper atmospheric variations. This paper isorganized as follows: Chapter 2 brieflydescribes the basic theory of the ionospheric

Fig.2 Formation of the equatorial ioniza-tion anomaly (EIA)A latitude-altitude distribution of the ionosphereduring the daytime (at 12:00LT). The distributionof the ionosphere was obtained from the empiri-cal international reference ionospheric model(IRI). The white curve represents the magneticfield lines of the earth. The black arrows showthe movements of ionospheric plasma. The blackvertical lines correspond to the latitudes (nearOkinawa) shown in Fig. 1.

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dynamo, Chapter 3 describes in detail theelectrodynamics model incorporated in real-time ionosphere and thermosphere simulationsconducted by NICT, and Chapter 4 presentsseveral examples of calculations performedwith the electrodynamics model, shows therelation between daily changes in the ionos-phere at mid-latitudes and the ionosphericdynamo based on the simulations.

2 Generation of electric fieldsand currents due to the ionos-pheric dynamo

2.1 What is the ionospheric dynamo?A dynamo generally refers to something

that generates power. In the case of ‘ionos-pheric dynamo’, it means a mechanism thatconverts the kinetic energy of backgroundthermospheric fluid moving against the ionos-pheric plasma into the electrostatic energy,whereby currents and electric fields are gener-ated through the collision between the plasmaand the neutral atmosphere. More specifically,the following two stages are often takentogether when one refers to the ionosphericdynamo.(1) When the thermospheric fluid movesagainst the ionospheric plasma with collisionsbetween them in the presence of the Earth’smagnetic field, the drag force of the neutralatmosphere drives the drift motion of plasma.The resultant difference in the drift motion ofions and electrons causes a current to flow(‘dynamo current’).(2) Since the electric conductivity is not spa-tially uniform in the ionosphere, the diver-gence and convergence of dynamo currentcause electric charge to be accumulated. Theresultant polarization electric field further dri-ves the plasma drift. With collision with theneutral atmosphere, the drift motion is notunique between ions and electrons, thus result-ing in a flow of current (‘polarization cur-rent’).

2.2 Ohm’s law in the ionosphere The direction and magnitude of ion and

electron drifts (described in Section 2.1)change with altitude. Figure 3 shows the alti-tude variations in the ion and electron driftsdriven by atmospheric drag and electric field(from Reference［3］). Since the density of neu-tral atmosphere quickly becomes smaller withaltitude. At lower altitudes, both ions and elec-trons exhibit dominant motion due to collisionwith the neutral atmosphere. At higher alti-tudes, the drift motion ‘frozen-into’ the geo-magnetic field becomes dominant. The alti-tude at which these effects switch over ismuch lower for electrons than for ions. Forthat reason, with respect to the drift due toatmospheric drag, it causes ions to be dragged

in the direction of motion u⊥ ( ,

where u denotes the neutral atmosphericvelocity vector and B the magnetic field vec-tor) in the E region. At higher altitudes, theion drift begins to move in the direction ofu×B, while electrons remain trapped in thegeomagnetic field, which results in thedynamo current flowing in the same directionas the ion drift. On the other hand, withrespect to the drift driven by electric field, theion drift changes from the direction E⊥ (whereE denotes the electric field vector) to thedirection E⊥×B toward the F region, while

Fig.3 Altitude dependence of ion andelectron drifts due to atmosphericdrag and electric field (from Refer-ence［3］)The horizontal direction represents altitude. Therightward means higher altitude (with valuesdenoting altitude). The ionospheric E and Fregions are shaded separately. The drift directionis relative to geomagnetic field B and neutralwind u⊥ (electric field E⊥) (not the rightwardarrow indicating the upward drift).

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the electron drift is in the direction E⊥×B.Therefore, the polarization currents flow in the

direction (carried by electrons) and

in the direction E⊥ (carried by ions) in the Eregion, while current flowing in the directionE⊥ remains in the F region. In the upper Fregion, both electrons and ions drift in thedirection E⊥×B, and thus no polarization cur-rent flows transverse to the geomagnetic field.

The following numerical expressions sum-marize the currents generated in the ionos-pheric dynamo:

…（1 a）

…（1 b）

j⊥ and j|| are the current vectors perpendicularand parallel components to the magnetic fieldlines, respectively. Here,σP andσH denote theelectric conductivities of the ionosphere indirections perpendicular to the geomagneticfield (where Pedersen conductivityσP is thecomponent in direction E⊥, and Hall conduc-tivityσH in direction ), whileσ0 is theelectric conductivity of the ionosphere in thedirection along the magnetic field lines,expressed as follows;

…（1 c）

where denotes the electron density, e theelementary charge, and Ωi (Ωe) and vin (ven)the gyro frequency of the ions (electrons) andcollision frequency with the neutral atmos-phere, respectively. For the derivation of(Equation 1), see References［1］and［3］. Onthe first line of (Equation 1 (a)), the first andsecond terms on the right hand side are thedynamo currents due to the neutral atmospher-ic drag; the third and fourth terms denote

polarization currents due to polarization elec-tric field. In the expression on the second lineof (Equation 1 (a)), E⊥ + u×B can be consid-ered as an electric field seen in the coordinatesystem that moves together with the neutralatmosphere. Therefore, (Equation 1) expressesOhm’s law connecting electric field to current.

2.3 Generation of the current systemand electric field distribution in theionosphere

This section describes how the ionosphericdynamo presented in the preceding sectiongenerates the ionospheric current system, andthe east-west electric field at low altitude thatinduces EIA in the daytime F-region electrondensity distribution. Figure 4 (a) shows theglobal flow of the thermospheric atmosphereintegrated over 100－300km at 0:00 in univer-sal time (UT) in autumn equinox (September).This wind velocity field is obtained from anempirical model (HWM93［4］) and, for thesake of simplicity, with only the diurnal com-ponent being extracted. Roughly speaking,one can see that the thermospheric fluidmoves out in a radial direction from the regionnear the geographical equator in the afternoonsector, and that it converges to the region inthe early morning sector at the geographicalequator. Figure 4 (b) and (c) are the results ofdynamo currents calculated by using (Equa-tion 1 (a)) on the basis of those flows. Figure 4(b) indicates the Pedersen current (correspond-ing to the first term on the RHS of the firstline of Equation 1 (a)); Fig. 4 (c) indicates theHall current (similarly corresponding to thesecond term). In the calculations of (Equation1 (c)), the ion/electron density and tempera-ture distributions were utilized from the Inter-national Reference Ionosphere (IRI)［5］. Thedensity and temperature distributions of theneutral atmosphere were obtained from theempirical model of the upper neutral atmos-phere (NRLMSIS)［6］. Since electric conduc-tivity is large in the E region during the day-time, the dynamo current in Fig. 4 (b) and (c)also flows mostly during the daytime. As isevident from the figure, the ionospheric cur-

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rent is not closed with the dynamo currentalone. The discontinuity in the dynamo currentcauses charge accumulation, resulting in apolarization electric field. As later shown inChapter 4, combining the dynamo and polar-

ization currents will generate a global closedcurrent system in the ionosphere (called Sqcurrent) that circulates throughout the low tomid latitudes in the daytime sector.

Figure 4 (b) shows an intense convergenceof dynamo current to the sunrise sector at themagnetic equator. Here, positive charge isaccumulated. In contrast, dynamo currentdiverges from around the sunset sector at theequator, with negative charge being accumu-lated. Therefore, an eastward polarizationelectric field is setup at low latitude during thedaytime. This electric field induces polariza-tion current in the E region, and at the sametime it propagates to the F region along thegeomagnetic field lines that can be regarded asequal-potential. This eastward electric fielddrives EIA in the F region as described inChapter 1.

As stated above, the ionospheric dynamowith its origin in the background neutralatmospheric motion plays essential roles forthe generation of EIA in the F region and theglobal ionospheric current system. On theother hand, atmospheric motion is not static inthe thermosphere: it usually changes on vari-ous spatial and temporal scales, particularlydue to gravity waves, atmospheric tides, andplanetary waves. Many of those waves areoriginated in the atmospheric region below thethermosphere and propagated upwards. There-fore, these atmospheric waves cause signifi-cant changes in the ionospheric current systemand the development of EIA through theionospheric dynamo. Daily changes in thelow-to-mid latitude ionosphere as presented inChapter 1 also presumably occur due to theeffects of atmospheric waves. Investigationsof how and which atmospheric waves lead today-to-day changes in the ionosphere thuspose an important challenge to be addressed inorder to achieve numerical ionospheric fore-casts. The simulation model for the electrody-namic process, such as presented in the nextchapter, can be coupled with atmospheric andionospheric models to offer an effective meansof addressing this challenge.

In the above statement, we only refer to

Fig.4 Generation of dynamo current dueto thermospheric neutral winds(a) Wind velocity field in the thermosphere (Sep-tember, 0:00 in UT, diurnal component), (b) Hori-zontal component of Pedersen dynamo currentdriven by the neutral winds in (a), and (c) Hori-zontal component of Hall dynamo current. Thepanels (a) to (c) are all integrated over altitudesfrom 100 to 300 km.

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‘E-region dynamo’ in which electric currentflows mainly through the E region during thedaytime, resulting in a polarization electricfield. On the other hand, the electric conduc-tivity of the E region declines at nighttime, sothat current flowing in the F region becomesrelatively large and generates polarizationelectric field. This is called the F regiondynamo. Around the sunset, sudden changesoccur from the E region to the F regiondynamo. Although not treated in this paper,such sudden change causes a dynamic motionof F-region plasma around the sunset, whicheventually leads to the generation of plasmabubbles (see Reference［7］and other parts ofthis special issue for details). The occurrenceand development of plasma bubble is alsoconsidered to be affected by atmosphericwaves via the ionospheric dynamo.

3 Numerical models expressingthe ionospheric dynamo

Many researchers have developed numeri-cal models expressing the ionospheric dynamo(ionospheric electrodynamics models). Inearly models where the ionosphere wasassumed as being thin layer, the earth’s currentsystem was solved on a horizontal two-dimen-sional plane (see Reference［8］). The modelscapable of treating the earth’s current systemin three-dimensional space gradually becamemainstream (see References［9］to［11］), and anelectrodynamics model incorporating a realis-tic configuration of the earth’s magnetic fieldemerged［12］. Moreover, it is now possible toincorporate an electrodynamics model as anelement of regionally integrated model com-prising the neutral atmospheric region and theionospheric region［13］. This chapter intro-duces the technique employed in our ionos-pheric electrodynamics model (referred to asthe HJEDYN model). This chapter alsodescribes the model in detail, so that readersnot interested in the simulation may skip it.

3.1 Equations and coordinate system Equations treated in the ionospheric elec-

trodynamics model include Ohm’s law (Equa-tion 1), the following equations for the currentcontinuity (Equation 2) and for the assumptionthat we treat only electrostatic potentialexpressed as S (Equation 3).

…（2）

…（3）

The selection of the appropriate coordinatesystem generally makes it easier to treat equa-tions numerically. In the case of ionosphericdynamo, it is appropriate to use a coordinatesystem that matches the configuration of theEarth’s magnetic field. In the low to mid lati-tude regions, at altitudes where most currentflows (higher than 90 km), the relation

holds true. Thus, when acharge separation generates along the geomag-netic field lines, a very weak electric field willreadily cause a current to flow so as to reducethe charge separation. Consequently, the geo-magnetic field lines can be regarded asequipotential, so that if one coordinate vari-able is chosen in the direction of the magneticfield lines, then the number of dimensions willbe reduced by one, thereby making it easier tosolve the equations as shown later on. In theHJEDYN model, the apex coordinates areused similar to Reference［12］. Figure 5 shows

Fig.5 The earth’s magnetic field lines andthe coordinate system of the elec-trodynamics model

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the coordinate system. Note that q1 in the fig-ure is φa (magnetic longitude of the apex ofthe magnetic field lines), q2 isλa (magnetic lat-itude of the point where the magnetic fieldlines intersect the ground surface (here wetake northern portion)), q3 is VB/(r0B0) (indi-cating location along the magnetic fieldlines, where VB denotes the magnetic poten-tial (B =－ VB) and B0 the intensity of themain magnetic field of the dipole compo-nent)). (Equation 1) to (Equation 3) are devel-oped on the above-mentioned coordinate sys-tem. Since direction q1 and direction q2 are notmutually orthogonal, two sets of basis vectorsare necessary. The base vectors are defined asfollows:

…（4）

And their mutual relation is set as follows:

…（5）

These equations can be used to develop Ohm’slaw (Equation 1) as in the following equation:

…（1’）

Since it is assumed that the magnetic fieldlines are equipotential, the direction along themagnetic field lines (Equation 1 (b)) is there-fore not used here. The equation for the cur-rent continuity (Equation 2) is as in the fol-lowing equation:

…（2’）

The assumption of treating electrostatic elec-

tric field (Equation 3) can be expressed in thefollowing equation:

…（3’）

Summarizing (Equation 1’) to (Equation 3’) interms of electrostatic potential and integratingalong the magnetic field lines will produce thefollowing equation:

…（6）

Each factor on the left hand side of(Equation 6) is an integral value along the fol-lowing magnetic field lines;

…（7）

where q3a and q3

b denote the start and endpoints of integration along the magnetic fieldlines, respectively. If the range of integrationis set sufficiently large so that the start andend are taken at the bottom of the ionospherein the Northern and Southern Hemispheres,the current vanishes to 0 and expressed as fol-lows:

…（8）

Therefore, under (Equation 8), (Equation 6)becomes a two-dimensional partial differentialequation concerning S(q1,q2).

In order to accurately determine the elec-tric fields and currents in the ionosphere underthe realistic configuration of geomagneticfield, one must properly treat the set of basevectors di and ei and (Equation 7) as statedabove. With respect to the actual configurationof geomagnetic field, the International Geo-magnetic Reference Field (IGRF) model canbe used (http://www.ngdc.noaa.gov/IAGA/

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vmod/igrf.html). Simplifying the geomagneticfield configuration to be a dipole magneticfield will considerably make easier the calcu-lations. In that case, the directions of base vec-tors di and ei are identical, and base vectors ofthe same kind are mutually orthogonal (di dj

= 0, i ≠ j) and further can be expressed byusing polar coordinate variables (r, θ, ϕ) thatmake it easier to calculate integral valuesalong the magnetic field lines (Equation 7)(such as Reference［9］).

3.2 Numerical techniques When neutral atmospheric wind velocity

and electric conductivity are given globally,the integral values along the magnetic fieldlines from (Equation 7) can be calculated.Then, (Equation 6) can be numerically solvedunder the appropriate boundary conditions.Many researchers have developed numericalmodels that treat the ionospheric dynamo,most of them basically treating the similarequation (Equation 6). In the following, thenumerical treatment of the HJEDYN model isintroduced below. The HJEDYN model hasbeen developed so that it would become anelement of whole atmosphere-ionosphere cou-pled model (together with an ionosphericmodel［14］and a whole atmospheric generalcirculation model (GCM)［15］［16］), and incor-porated into a real-time simulation［17］ forspace weather forecasts. For that reason, themodel has been developed in consideration of(i) robustness (so calculations always convergein a stable manner), (ii) high calculationspeed, and (iii) high spatial resolution (savememory at the same time).

In the HJEDYN model, the equations aredifferentiated by using a finite volume tech-nique because it can be well fit to the bound-ary conditions. Thus, (Equation 6) is changedas follows:

…（6’）

J1 and J2 are the q1 and q2 components ofcurrent integrated along the magnetic fieldlines (when multiplied by an area element),respectively. Figure 6 shows the distributionsof grid point and physical quantities in a two-dimensional space consisting of q1 and q2. Thegrid numbers are set as i = 1, 2, ..., imax in thedirection of q1, and as j = 1, 2, ..., jmax in thedirection of q2. As shown in Fig. 6, the electro-static potential S is defined at the grid center(i,j) and the currents J1 and J2 are defined atthe inter-grid boundaries (i±1/2, j) and(i,j±1/2). The current of (Equation 6’) isexpressed with a differential equation as fol-lows:

…（9 a）

Fig.6 Distribution of grid points and cur-rents in the electrodynamics model

The distribution of grid points and currents (inte-gral quantities J1, J2) in a two-dimensional (q1,q2) calculation space in the electrodynamicsmodel. The black circles represent the center ofeach grid where the quantities such as electro-static potential are placed. The white circles repre-sent inter-grid boundaries where the currents areplaced.

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…（9 b）

In the differential term in the J1 equationin the direction of q1 and in the term in the J2

equation in the direction of q2, the contributionof the difference between the neighboringgrids with a certain faction is added in order toreduce the truncation error due to the differ-ence between the second-order differentiationterms. In grids other than the boundaries, thefollowing equation that differentiates the firstline of (Equation 6’) is applied:

…（10）

The boundary conditions in the directionof q1 (ϕ a: apex longitude) are periodic bound-aries, that is:

…（11 a）

So as to satisfy the current closure at the bot-tom of the ionosphere, the boundary conditionat j = 1 in the direction of q2 (λa: magnetic lat-itude at the intersection of the magnetic fieldlines with the ground surface) is;

…（11 b）

together with S(i,0) = S(i,1). At j = jmax,

…（11 c）

was set in order to ensure that the currentsflowing into and out from the magnetic polesare balanced. The electrostatic potential of thepoles was also selected as the reference pointand S(i, jmax + 1) = 0 was set. The ionospher-ic currents should be connected to the magne-tospheric currents at the high-latitude region,which remains a challenge to be addressed inthe future. (Equation 6) is sensitive to theboundary setting and the treatment of bound-

aries is important to ensure robustness. Defin-ing the current at the inter-grid boundaries asin (Equation 9) will facilitate the treatment ofboundary conditions as in (Equation 11 (a)) to(Equation 11 (c)). Concerning robustness, it isimportant to reduce the truncation error in thedifference equation (Equation 9).

The index [i, j] is put together to expressas m = imax×(j－1) + i. Then, substituting(Equation 9) into (Equation 10) will produce adynamo difference equation using an index m.

…（12）

Here, a(m , n) and b(m) are coefficientsexpressed by using the integral quantity alongthe magnetic field lines from (Equation 7). Forthe boundary conditions in (Equation 11 (a))to (Equation 11 (c)), substituting (Equation 9)into them will make it a linear equation withrespect to electrostatic potential S. Therefore,the equation can be transformed into the fol-lowing linear equation;

…（13）

Indexes m and n each take a range from 1to mmax (mmax=imax×jmax). (Equation 13)can be solved numerically by using generalappropriate methods. After distribution ofelectrostatic potential is solved, the distribu-tions of electric fields and currents can bederived by (Equation 1’), (Equation 3’) and(Equation 6).

4 Reproduction of the ionosphericdynamo with the electrody-namics model

This chapter presents some examples ofcalculations using the electrodynamics modeldescribed in Chapter 3. Figure 7 compares the

distribution of electrostatic potential calculat-ed using the HJEDYN model with an empiri-cal model［19］. In the calculation, we used asthe input distributions of electric conductivityand neutral atmospheric wind velocity, theInternational Reference Ionosphere (IRI)［5］and the extended atmospheric general circula-tion model［15］［16］. Although the consideredseason is the same (September), the HJEDYNmodel treats a medium degree of solar activity(F10.7 = 135), while the empirical modeltreats low solar activity, which may lead to thedifference in the range of global potential vari-ation. The overall potential distribution at lowto mid latitudes is similar between both themodels, indicating that the ionosphericdynamo contributes mainly to the formation ofobserved potential distribution.

Figure 8 shows the current distributionreproduced by the HJEDYN model with blackarrows and displays the electrostatic potentialwith contours. The current is a value integrat-ed along the magnetic field lines and projectedonto the point where the magnetic field linesintersect the altitude of 70 km in the NorthernHemisphere. (The calculations in Fig. 7 only

used the diurnal and semidiurnal migratingtidal winds as input from the atmospheric gen-eral circulation model, while the calculation inFig. 8 used all tidal components.) As shown inFig. 8, the low to mid-latitude regions duringthe daytime constitute one larger eddy current.As described in Section 2.3, the dynamo cur-rent driven directly by the neutral atmosphericmotion is discontinuous, but the polarizationcurrent fills to meet the current continuity inthe entire region. The simulation also repro-duces an intensified eastward current at themagnetic equator during the daytime, i.e., theequatorial electrojet current (EEJ) (see［20］ inthis special issue). In addition to a large Sqcurrent system during daytime, small eddycurrent structures are seen around the sunsetand sunrise at low latitudes, adding complexi-ties to the ionospheric current distribution.These structures are not constant and exhibithighly temporal variations. Such fine struc-tures and temporal variations stem from thewind velocity distribution from the atmospher-ic general circulation model and its temporalchanges. Although verification through com-parison with observations is necessary, suchvariations are quite interesting in terms of therelation with the ionospheric day-to-day varia-tions.

Chapter 1 described how fluid waves inthe thermosphere modulated the east-west

218 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

Fig.7 Comparison between electrostaticpotential reproduced by the elec-trodynamics model and an empiri-cal modelComparison between electrostatic potential repro-duced by the electrodynamics model (top panel)and an empirical model［19］(bottom panel).Because the universal times (UT) compared aredifferent, the two figures are shifted in longitudedirection to match the local time (LT).

Fig.8 Current distribution and electrostaticpotential reproduced by the elec-trodynamics model The current distribution (arrow) and electrosta-tic potential (contours) reproduced by the electro-dynamics model. The currents are integratedalong the magnetic field lines and projected ontothe plane where the magnetic field lines intersectan altitude of 70 km.

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show part of the results. Figure 9 (a) is a five-day variation of F-region electron density dis-tribution at the low-to-middle latitudes. Thereare separate regions of high density across themagnetic equator (EIA), and the growth ofEIA in latitude varies on the day-to-day basis.

The top panel in Fig. 9 (b) is a 20-dayvariation of electron density at mid-latitudereproduced by the same simulation. This fig-ure displays NmF2 at 27˚ N geographic lati-tude. From the figure, one can see how NmF2changes daily. For example, from September27 to 28, the NmF2 maximum during the day-time almost doubled. This is due to the expan-sion of EIA to the high-latitude as shown inFig. 9 (a). Chapter 1 explained the relationbetween the EIA growth and the electric fieldgenerated in the dynamo process. The bottompanel in Fig. 9 (b) displays the east-west elec-tric field at the magnetic equator and at an alti-tude of 300 km. A comparison with NmF2 inthe top panel of Fig. 9 (b) indicates a goodcorrelation between the intensity of the east-ward electric field and NmF2 during the day-time. For example, September 18, 20, 24 and27 experienced smaller eastward electric fieldsduring the daytime than on the previous or fol-lowing days, and NmF2 at mid-latitude wasalso smaller. However, there were some days(such as around September 14) when the cor-relation was not so good between the equatori-al electric field and mid-latitude NmF2. Thisis because the ionospheric density is notaffected only by the growth of EIA due toelectric fields but also by changes in the neu-tral composition of the background thermos-phere and meridional wind velocity along themagnetic field lines.

Figure 9 (c) is a five-day (September24–28, 2006) variation of NmF2 observed bythe ionosonde at Okinawa (26.68˚ N geo-graphic latitude). We selected the same monthas that of the simulation shown in Fig. 9 (a)and (b) under the condition of low solar activi-ty and low geomagnetic turbulence. Figure 9(c) shows variations with a period of two tothree days in addition to shorter-scale tempo-ral changes with a period of several hours, and

electric field at low altitudes through thedynamo process and change the latitudinalgrowth of EIA. Future prediction of the elec-tron density variation in the ionosphere atmid-latitudes requires the reproduction ofthese processes with simulation models. As afeasible study, a series of neutral wind distrib-utions during the 30 days of Septemberobtained from an atmospheric global circula-tion model was used as input into the HJE-DYN model, and the 30-day variation in theelectric field distribution was calculated.Moreover, the distributions of electric fieldand neutral wind were utilized as input intoanother ionospheric model (SAMI2)［21］, anddaily change in the ionospheric density distri-bution was calculated. Figure 9 (a) and (b)

Fig.9 Comparison of the ionospheric den-sity variations calculated based onthe electric field obtained from theelectrodynamics model with NmF2at Okinawa (a), (b): Results from ionospheric simulationusing the input electric field variations from theelectromagnetic model; (a) represents the tempo-ral change in the electron density at an altitude of300 km and at the longitude of Japan (at 135˚ Egeographic longitude). (b) represents simulatedNmF2 (F layer maximum electron density) at 27˚N and the east-west electric field at 300 km heightat the magnetic equator. (c) NmF2 observed by the ionosonde at Okinawa(26.88˚ N) on September 24–28, 2006

220 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

there are even some days when the NmF2maximum is about double that of the preced-ing day. Similar daily variations are alsoreproduced by the simulation as shown in Fig.9 (b), and the variations over a period of twoto three days are reproduced both in mid-lati-tude NmF2 and the equatorial electric field.Therefore, the origin of daily variations inNmF2 as observed in Fig. 9 (c) may be con-siderably from the variations in the low-lati-tude ionospheric dynamo, as interpreted basedon the simulation results, and the variations inthe ionospheric dynamo originate in the vari-ous atmospheric waves propagating from lowaltitudes.

5 Conclusion

As presented in this paper, the ionosphericdynamo plays a significant role in connectingthe background atmospheric motion and theionospheric density variation through the gen-eration of electric fields. Day-to-day changesin the ionosphere presumably originate inactivities in the lower atmosphere that exciteatmospheric waves. Therefore, this is the sub-ject that regional coupled simulations work. Inrecent years, we have coupled the electrody-namics model introduced in this paper with anextended atmospheric model extending from

the ground surface to the upper thermos-phere［15］［16］ and with an ionosphericmodel［14］, thereby developing a model cover-ing the entire atmospheric region of the earth.This whole atmosphere-ionosphere coupledmodel includes a series of physical processesdescribed in this paper and will hopefullyreproduce daily changes in the ionosphere ona quantitative basis. The calculations given inChapter 4 show such example. However,reproducing and predicting ionosphericchanges that match the actual ionosphericbehaviors necessitate the incorporation ofobservational data into the model. Therefore,such technique for that purpose must be takeninto account for the future development of thewhole atmosphere-ionosphere model.

Acknowledgments

This study used the calculation results ofthe extended atmospheric general circulationmodel developed at Kyushu University andTohoku University. We also used ionosphericmodel SAMI2 developed at the United StatesNaval Research Laboratory. The World DataCenter for Geomagnetism at Kyoto Universityprovided the Kp indices of geomagnetic dis-turbances. We would like to express our deepappreciation and thanks to those institutions.

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JIN Hidekatsu, Dr. Sci.

Expert Researcher, Space EnvironmentGroup, Applied ElectromagneticResearch Center

Upper Atmospheric Physics