2-3-7 Ionospheric Dynamo Process
JIN Hidekatsu
Ionospheric electron density varies on various temporal and spatial scales. Because electron
density variations affect radio waves reflecting at and penetrating in the ionosphere, their prediction is an important issue for space weather forecast. Some of the ionospheric day-to-day variations are considered to originate in the lower atmospheric activities. Ionospheric dynamo plays
an 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 density variations. We also introduce our electrodynamics model which represents ionospheric
dynamo process. This model is a part of a whole atmosphere-ionosphere coupled model, whichis expected to reproduce ionospheric day-to-day variations.

Keywords
Ionospheric dynamo, Space weather, Ionosphere, Atmosphere, Simulation

1 Introduction: daily changes in
the ionosphere and the ionospheric dynamo
The ionosphere over Japan, particularly inits southern regions, shows significant
changes on a daily basis. For example, Fig. 1
shows daily changes in the total electron content (TEC) over Okinawa as observed over an
80-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), respectively. The top panel shows the Kp index thatindicates the degree of geomagnetic turbulence. The Kp index during these periods was
generally below 3, indicating geomagneticallyquiet. The Kp index occasionally fluctuates
between 3 and about 5, but the figure apparently suggests that this fluctuation does notcorrelate with changes in TEC. That is, eventhough there are no effects from the magnetosphere, significant daily changes occur in theionosphere at such low latitudes. In terms of

Fig.1 Daily changes in the total electron

content over southern part of JapanThe Kp index during an 80-day period from the
beginning of 2006 (top panel) and changes inthe total electron content in the space over Okinawa (middle panel: average from 11:00 to
15:00LT during the daytime; bottom panel:
average from 17:00 to 21:00LT around sunset)

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209its effects on our society, the TEC is a quantity
directly linked to radio propagation delay
between the ground surface and artificial
satellites, and therefore daily changes in TEC
can give serious impacts on the use of satellite
positioning applications.
These daily changes in the ionosphere
over southern Japan are related to the development of “equatorial ionization anomaly (EIA)”
to mid-latitudes. The EIA refers to a structurethat the largest density region is not located at
the equator where the solar radiation flux
becomes largest but is separated to the south
and north portions across the magnetic equator
as shown in Fig. 2. In the formation of theEIA, electric field generated by dynamo
process (described later) plays a key role.
When the electric field acts on ionosphericplasma, the electric field drift in the direction
orthogonal to the magnetic field lines of theearth (poleward/ upward due to eastward electric field) and diffusion toward thepoles/downward along the magnetic field lines
will generate a distribution of electron densities as shown in Fig. 2. Thus, the dynamo
electric field affects the latitudinal spread of

Fig.2 Formation of the equatorial ioniza-tion anomaly (EIA)
A latitude-altitude distribution of the ionosphere
during the daytime (at 12:00LT). The distributionof the ionosphere was obtained from the empirical international reference ionospheric model(IRI). The white curve represents the magnetic
field lines of the earth. The black arrows show
the movements of ionospheric plasma. The black
vertical lines correspond to the latitudes (near
Okinawa) shown in Fig. 1.

EIA. Southern part of Japan is located on thehigh-latitude side of EIA, so that the F region
electron density during the daytime depends
on the degree to which EIA spreads. In otherwords, changing electric field due to dynamo
process is one of major factors behind dailychanges in electron density at low to mid latitudes.
This paper presents the ionospheric
dynamo that is closely related to changes in
the 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 current system and neutral atmospheric dynamics(e.g., tides, other atmospheric waves) as estimated based on observations of geomagnetic
field variation on the ground. For the detailed
historical review, see Reference［1］ and othermaterials. In recent years, the importance ofionospheric dynamo has been further recognized from several aspects. One aspect isrelated to the recent advances in the observation technique, i.e., global remote sensing
from satellites and GPS receiver network,
these have enabled us to observe broader
ionospheric distribution. This resulted in the
new discovery of an ionospheric longitudinal
structure considered to originate in the lower
atmosphere (see Reference［2］ in his feature
edition). The ionospheric dynamo presumably
plays an important role in the linkage between
the lower atmosphere and ionosphere. Anotheraspect is due to advances in the numerical
technique, that is, global-scale simulation
models that integrate different vertical regions
have become possible. The electrodynamics
model that treats the ionospheric dynamo is an
important component that connects an atmospheric model with an ionospheric model.
Advances made in regionally integrated models have allowed researchers to address suchchallenges as the origin of daily changes in theionosphere, and the models are expected to be
used in future numerical prediction of theupper atmospheric variations. This paper isorganized as follows: Chapter 2 briefly
describes the basic theory of the ionospheric

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dynamo, Chapter 3 describes in detail theelectrodynamics model incorporated in realtime ionosphere and thermosphere simulations
conducted by NICT, and Chapter 4 presents
several examples of calculations performed
with the electrodynamics model, shows therelation between daily changes in the ionosphere at mid-latitudes and the ionospheric
dynamo based on the simulations.

2 Generation of electric fields
and currents due to the ionospheric dynamo
2.1 What is the ionospheric dynamo?
A dynamo generally refers to somethingthat generates power. In the case of ‘ionospheric dynamo’, it means a mechanism thatconverts the kinetic energy of backgroundthermospheric fluid moving against the ionospheric plasma into the electrostatic energy,
whereby currents and electric fields are generated through the collision between the plasma
and the neutral atmosphere. More specifically,
the following two stages are often taken
together when one refers to the ionospheric
dynamo.
(1) When the thermospheric fluid moves
against the ionospheric plasma with collisions
between them in the presence of the Earth’s
magnetic field, the drag force of the neutral
atmosphere 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 spatially uniform in the ionosphere, the divergence and convergence of dynamo current
cause electric charge to be accumulated. The
resultant polarization electric field further drives the plasma drift. With collision with theneutral atmosphere, the drift motion is notunique between ions and electrons, thus resulting in a flow of current (‘polarization current’).
2.2 Ohm’s law in the ionosphere
The direction and magnitude of ion andelectron drifts (described in Section 2.1)
change with altitude. Figure 3 shows the altitude variations in the ion and electron drifts
driven by atmospheric drag and electric field(from Reference［3］). Since the density of neutral atmosphere quickly becomes smaller withaltitude. At lower altitudes, both ions and electrons exhibit dominant motion due to collision
with the neutral atmosphere. At higher altitudes, the drift motion ‘frozen-into’ the geomagnetic field becomes dominant. The altitude at which these effects switch over ismuch lower for electrons than for ions. For
that reason, with respect to the drift due toatmospheric drag, it causes ions to be draggedin the direction of motion u ⊥ (,where u denotes the neutral atmosphericvelocity vector and B the magnetic field vector) 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 the
dynamo current flowing in the same direction
as the ion drift. On the other hand, withrespect to the drift driven by electric field, theion drift changes from the direction E⊥ (where
E 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 Reference［3］)The horizontal direction represents altitude. The
rightward means higher altitude (with values
denoting altitude). The ionospheric E and F
regions are shaded separately. The drift direction
is relative to geomagnetic field B and neutral
wind u⊥ (electric field E⊥) (not the rightward
arrow 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) andin the direction E⊥ (carried by ions) in the Eregion, while current flowing in the directionE⊥ remains in the F region. In the upper F
region, both electrons and ions drift in the
direction E⊥×B, and thus no polarization current flows transverse to the geomagnetic field.
The following numerical expressions summarize the currents generated in the ionospheric dynamo:…（1 a）…（1 b）j⊥ and j|| are the current vectors perpendicularand parallel components to the magnetic fieldlines, respectively. Here, σP and σH denote theelectric conductivities of the ionosphere in
directions perpendicular to the geomagnetic
field (where Pedersen conductivityσP is thecomponent in direction E⊥, and Hall conduc), while σ0 is thetivityσH in direction
electric conductivity of the ionosphere in the
direction 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 atmosphere, respectively. For the derivation of(Equation 1), see References［1］ and［3］. On
the first line of (Equation 1 (a)), the first andsecond terms on the right hand side are thedynamo currents due to the neutral atmospheric drag; the third and fourth terms denotepolarization currents due to polarization electric field. In the expression on the second lineof (Equation 1 (a)), E⊥ + u×B can be considered as an electric field seen in the coordinate
system that moves together with the neutral
atmosphere. Therefore, (Equation 1) expresses
Ohm’s law connecting electric field to current.
2.3 Generation of the current systemand electric field distribution in theionosphereThis section describes how the ionospheric
dynamo presented in the preceding section
generates the ionospheric current system, andthe east-west electric field at low altitude thatinduces EIA in the daytime F-region electron
density distribution. Figure 4 (a) shows theglobal flow of the thermospheric atmosphere
integrated over 100−300km at 0:00 in universal 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 component being extracted. Roughly speaking,one can see that the thermospheric fluid
moves out in a radial direction from the region
near the geographical equator in the afternoon
sector, and that it converges to the region in
the early morning sector at the geographicalequator. Figure 4 (b) and (c) are the results ofdynamo currents calculated by using (Equation 1 (a)) on the basis of those flows. Figure 4(b) indicates the Pedersen current (corresponding 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 (Equation
1 (c)), the ion/electron density and temperature distributions were utilized from the International Reference Ionosphere (IRI)［5］. The
density and temperature distributions of theneutral atmosphere were obtained from theempirical model of the upper neutral atmosphere (NRLMSIS) ［6］. Since electric conductivity is large in the E region during the daytime, the dynamo current in Fig. 4 (b) and (c)also flows mostly during the daytime. As isevident from the figure, the ionospheric cur-

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

Fig.4 Generation of dynamo current due

to thermospheric neutral winds(a) Wind velocity field in the thermosphere (September, 0:00 in UT, diurnal component), (b) Horizontal component of Pedersen dynamo current
driven by the neutral winds in (a), and (c) Horizontal component of Hall dynamo current. The
panels (a) to (c) are all integrated over altitudes
from 100 to 300 km.

rent is not closed with the dynamo current
alone. The discontinuity in the dynamo current
causes charge accumulation, resulting in apolarization electric field. As later shown in
Chapter 4, combining the dynamo and polar-

ization currents will generate a global closed
current system in the ionosphere (called Sq
current) that circulates throughout the low to
mid 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 current
diverges from around the sunset sector at theequator, with negative charge being accumulated. Therefore, an eastward polarizationelectric field is setup at low latitude during thedaytime. This electric field induces polarization current in the E region, and at the sametime it propagates to the F region along thegeomagnetic field lines that can be regarded as
equal-potential. This eastward electric field
drives EIA in the F region as described in
Chapter 1.
As stated above, the ionospheric dynamo
with its origin in the background neutral
atmospheric 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 in
the thermosphere: it usually changes on various spatial and temporal scales, particularly
due to gravity waves, atmospheric tides, andplanetary waves. Many of those waves are
originated in the atmospheric region below thethermosphere and propagated upwards. Therefore, these atmospheric waves cause significant changes in the ionospheric current systemand the development of EIA through theionospheric dynamo. Daily changes in the
low-to-mid latitude ionosphere as presented in
Chapter 1 also presumably occur due to theeffects of atmospheric waves. Investigationsof how and which atmospheric waves lead to
day-to-day changes in the ionosphere thus
pose an important challenge to be addressed inorder to achieve numerical ionospheric forecasts. The simulation model for the electrodynamic process, such as presented in the next
chapter, can be coupled with atmospheric andionospheric models to offer an effective meansof addressing this challenge.
In the above statement, we only refer to

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213‘E-region dynamo’ in which electric current
flows mainly through the E region during thedaytime, resulting in a polarization electricfield. On the other hand, the electric conductivity of the E region declines at nighttime, sothat current flowing in the F region becomes
relatively large and generates polarizationelectric field. This is called the F region
dynamo. Around the sunset, sudden changes
occur from the E region to the F region
dynamo. Although not treated in this paper,
such sudden change causes a dynamic motionof F-region plasma around the sunset, which
eventually leads to the generation of plasma
bubbles (see Reference［7］ and other parts ofthis special issue for details). The occurrence
and development of plasma bubble is also
considered to be affected by atmosphericwaves via the ionospheric dynamo.

3 Numerical models expressing
the ionospheric dynamo
Many researchers have developed numerical models expressing the ionospheric dynamo(ionospheric electrodynamics models). In
early models where the ionosphere wasassumed as being thin layer, the earth’s current
system was solved on a horizontal two-dimensional plane (see Reference［8］). The models
capable of treating the earth’s current systemin three-dimensional space gradually became
mainstream (see References［9］to［11］), and anelectrodynamics model incorporating a realistic configuration of the earth’s magnetic field
emerged［12］. Moreover, it is now possible toincorporate an electrodynamics model as anelement of regionally integrated model comprising the neutral atmospheric region and theionospheric region［13］. This chapter introduces the technique employed in our ionospheric electrodynamics model (referred to as
the HJEDYN model). This chapter also
describes the model in detail, so that readers
not 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 (Equation 1), the following equations for the current
continuity (Equation 2) and for the assumptionthat we treat only electrostatic potential
expressed as S (Equation 3).
…（2）
…（3）

The selection of the appropriate coordinate
system generally makes it easier to treat equations numerically. In the case of ionospheric
dynamo, it is appropriate to use a coordinate
system that matches the configuration of the
Earth’s magnetic field. In the low to mid latitude regions, at altitudes where most current
flows (higher than 90 km), the relation
holds true. Thus, when acharge separation generates along the geomagnetic field lines, a very weak electric field willreadily cause a current to flow so as to reducethe charge separation. Consequently, the geomagnetic field lines can be regarded as
equipotential, so that if one coordinate variable is chosen in the direction of the magnetic
field lines, then the number of dimensions will
be reduced by one, thereby making it easier tosolve the equations as shown later on. In the
HJEDYN 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 electrodynamics model

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

the coordinate system. Note that q1 in the figure is φa (magnetic longitude of the apex of
the magnetic field lines), q2 isλa (magnetic latitude of the point where the magnetic fieldlines intersect the ground surface (here wetake northern portion)), q3 is VB /(r0B0) (indicating location along the magnetic fieldlines, where VB denotes the magnetic potential (B = − V B ) and B 0 the intensity of themain magnetic field of the dipole component)). (Equation 1) to (Equation 3) are developed on the above-mentioned coordinate system. Since direction q1 and direction q2 are notmutually orthogonal, two sets of basis vectors
are necessary. The base vectors are defined asfollows:

tric field (Equation 3) can be expressed in thefollowing equation:…（3 ）Summarizing (Equation 1’) to (Equation 3’) interms of electrostatic potential and integrating
along 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 following magnetic field lines;…（4）

And their mutual relation is set as follows:

…（7）

…（5）

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

where q 3a and q 3b denote the start and end
points 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 follows:…（1 ）Since it is assumed that the magnetic fieldlines are equipotential, the direction along themagnetic field lines (Equation 1 (b)) is therefore not used here. The equation for the current continuity (Equation 2) is as in the following equation:…（2 ）The assumption of treating electrostatic elec-…（8）

Therefore, under (Equation 8), (Equation 6)
becomes a two-dimensional partial differential
equation concerning S(q1,q2).
In order to accurately determine the electric fields and currents in the ionosphere under
the realistic configuration of geomagnetic
field, 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 Geomagnetic Reference Field (IGRF) model canbe used (http://www.ngdc.noaa.gov/IAGA/

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vmod/igrf.html). Simplifying the geomagnetic
field configuration to be a dipole magnetic
field will considerably make easier the calculations. In that case, the directions of base vectors di and ei are identical, and base vectors of
the 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 values
along 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 solved
under the appropriate boundary conditions.
Many researchers have developed numericalmodels that treat the ionospheric dynamo,
most of them basically treating the similar
equation (Equation 6). In the following, thenumerical treatment of the HJEDYN model isintroduced below. The HJEDYN model has
been developed so that it would become anelement of whole atmosphere-ionosphere coupled model (together with an ionosphericmodel［14］ and a whole atmospheric general［16］), and incorcirculation model (GCM)［15］
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 converge
in a stable manner), (ii) high calculation
speed, and (iii) high spatial resolution (savememory at the same time).
In the HJEDYN model, the equations aredifferentiated by using a finite volume technique because it can be well fit to the boundary conditions. Thus, (Equation 6) is changed
as follows:

Fig.6 Distribution of grid points and cur-rents in the electrodynamics model
The distribution of grid points and currents (integral quantities J1, J2) in a two-dimensional (q1,
q 2) calculation space in the electrodynamics
model. The black circles represent the center of
each grid where the quantities such as electrostatic potential are placed. The white circles represent inter-grid boundaries where the currents areplaced.

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 twodimensional space consisting of q1 and q2. The
grid numbers are set as i = 1, 2, ..., imax in the
direction of q1, and as j = 1, 2, ..., jmax in the
direction of q2. As shown in Fig. 6, the electrostatic potential S is defined at the grid center(i,j) and the currents J1 and J2 are defined at
the inter-grid boundaries (i±1/2, j) and(i,j±1/2). The current of (Equation 6’) isexpressed with a differential equation as follows:…（9 a）…（6 ）216 Journal of the National Institute of Information and Communications Technology Vol.56 Nos.1-4 2009

…（9 b）

In the differential term in the J1 equationin the direction of q1 and in the term in the J2equation in the direction of q2, the contributionof the difference between the neighboring
grids with a certain faction is added in order toreduce the truncation error due to the difference between the second-order differentiation
terms. In grids other than the boundaries, thefollowing equation that differentiates the firstline of (Equation 6’) is applied:aries is important to ensure robustness. Defining 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 the
difference equation (Equation 9).
The index [i, j] is put together to express
as m = imax ×(j −1) + i. Then, substituting(Equation 9) into (Equation 10) will produce a
dynamo difference equation using an index m.

…（12）

…（10）

The boundary conditions in the directionof q1 (ϕ a: apex longitude) are periodic boundaries, that is:…（11 a）

So as to satisfy the current closure at the bottom of the ionosphere, the boundary condition
at j = 1 in the direction of q2 (λa: magnetic latitude 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 currents
flowing into and out from the magnetic poles
are balanced. The electrostatic potential of thepoles was also selected as the reference point
and S(i, jmax + 1) = 0 was set. The ionospheric currents should be connected to the magnetospheric currents at the high-latitude region,
which remains a challenge to be addressed in
the future. (Equation 6) is sensitive to theboundary setting and the treatment of bound-

Here, a(m, n) and b(m) are coefficients
expressed by using the integral quantity along
the magnetic field lines from (Equation 7). For
the 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 following linear equation;…（13）

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

4 Reproduction of the ionospheric
dynamo with the electrodynamics modelThis chapter presents some examples ofcalculations using the electrodynamics model
described in Chapter 3. Figure 7 compares the

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Fig.8 Current distribution and electrostatic

Fig.7 Comparison between electrostatic

potential reproduced by the electrodynamics model and an empirical model
Comparison between electrostatic potential reproduced by the electrodynamics model (top panel)and an empirical model［19］ (bottom panel).
Because the universal times (UT) compared are
different, the two figures are shifted in longitude
direction to match the local time (LT).

distribution of electrostatic potential calculated using the HJEDYN model with an empirical model［19］. In the calculation, we used as
the input distributions of electric conductivity
and neutral atmospheric wind velocity, theInternational Reference Ionosphere (IRI)［5］and the extended atmospheric general circula［16］. Although the consideredtion model［15］season is the same (September), the HJEDYN
model treats a medium degree of solar activity(F10.7 = 135), while the empirical model
treats low solar activity, which may lead to thedifference in the range of global potential variation. The overall potential distribution at low
to mid latitudes is similar between both themodels, indicating that the ionospheric
dynamo contributes mainly to the formation ofobserved potential distribution.
Figure 8 shows the current distribution
reproduced by the HJEDYN model with blackarrows and displays the electrostatic potential
with contours. The current is a value integrated along the magnetic field lines and projected
onto the point where the magnetic field lines
intersect the altitude of 70 km in the Northern
Hemisphere. (The calculations in Fig. 7 only

potential reproduced by the electrodynamics model
The current distribution (arrow) and electrostatic potential (contours) reproduced by the electrodynamics model. The currents are integrated
along the magnetic field lines and projected onto
the plane where the magnetic field lines intersect
an altitude of 70 km.

used the diurnal and semidiurnal migrating
tidal winds as input from the atmospheric general circulation model, while the calculation in
Fig. 8 used all tidal components.) As shown in
Fig. 8, the low to mid-latitude regions during
the daytime constitute one larger eddy current.
As described in Section 2.3, the dynamo current driven directly by the neutral atmosphericmotion is discontinuous, but the polarization
current fills to meet the current continuity inthe entire region. The simulation also reproduces 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 Sq
current system during daytime, small eddy
current structures are seen around the sunset
and sunrise at low latitudes, adding complexities to the ionospheric current distribution.
These structures are not constant and exhibit
highly temporal variations. Such fine structures and temporal variations stem from thewind velocity distribution from the atmospheric general circulation model and its temporalchanges. Although verification through comparison with observations is necessary, suchvariations are quite interesting in terms of therelation with the ionospheric day-to-day variations.
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

electric field at low altitudes through the
dynamo process and change the latitudinal
growth of EIA. Future prediction of the electron density variation in the ionosphere at
mid-latitudes requires the reproduction ofthese processes with simulation models. As afeasible study, a series of neutral wind distributions during the 30 days of September
obtained from an atmospheric global circulation model was used as input into the HJEDYN 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 distribution was calculated. Figure 9 (a) and (b)Fig.9 Comparison of the ionospheric den-sity variations calculated based on
the electric field obtained from theelectrodynamics model with NmF2
at Okinawa(a), (b): Results from ionospheric simulation
using the input electric field variations from theelectromagnetic model; (a) represents the temporal change in the electron density at an altitude of
300 km and at the longitude of Japan (at 135˚ E
geographic longitude). (b) represents simulated
NmF2 (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

show part of the results. Figure 9 (a) is a fiveday variation of F-region electron density distribution at the low-to-middle latitudes. There
are separate regions of high density across themagnetic equator (EIA), and the growth of
EIA in latitude varies on the day-to-day basis.
The top panel in Fig. 9 (b) is a 20-day
variation of electron density at mid-latitude
reproduced by the same simulation. This figure displays NmF2 at 27˚ N geographic latitude. From the figure, one can see how NmF2
changes daily. For example, from September
27 to 28, the NmF2 maximum during the daytime almost doubled. This is due to the expansion of EIA to the high-latitude as shown in
Fig. 9 (a). Chapter 1 explained the relation
between the EIA growth and the electric field
generated in the dynamo process. The bottom
panel in Fig. 9 (b) displays the east-west electric field at the magnetic equator and at an altitude of 300 km. A comparison with NmF2 inthe top panel of Fig. 9 (b) indicates a goodcorrelation between the intensity of the eastward electric field and NmF2 during the daytime. For example, September 18, 20, 24 and27 experienced smaller eastward electric fields
during the daytime than on the previous or following days, and NmF2 at mid-latitude wasalso smaller. However, there were some days
(such as around September 14) when the correlation was not so good between the equatorial 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 neutral composition of the background thermosphere and meridional wind velocity along themagnetic field lines.
Figure 9 (c) is a five-day (September
24 – 28, 2006) variation of NmF2 observed by
the ionosonde at Okinawa (26.68˚ N geographic latitude). We selected the same monthas that of the simulation shown in Fig. 9 (a)and (b) under the condition of low solar activity and low geomagnetic turbulence. Figure 9(c) shows variations with a period of two to
three days in addition to shorter-scale temporal changes with a period of several hours, andJIN Hidekatsu

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there are even some days when the NmF2
maximum is about double that of the preceding day. Similar daily variations are also
reproduced by the simulation as shown in Fig.
9 (b), and the variations over a period of two
to three days are reproduced both in mid-latitude NmF2 and the equatorial electric field.
Therefore, the origin of daily variations in
NmF2 as observed in Fig. 9 (c) may be considerably from the variations in the low-latitude ionospheric dynamo, as interpreted basedon the simulation results, and the variations in
the ionospheric dynamo originate in the various atmospheric waves propagating from low
altitudes.

5 Conclusion
As presented in this paper, the ionospheric
dynamo plays a significant role in connecting
the background atmospheric motion and theionospheric density variation through the generation of electric fields. Day-to-day changes
in the ionosphere presumably originate inactivities in the lower atmosphere that excite
atmospheric waves. Therefore, this is the subject that regional coupled simulations work. In
recent years, we have coupled the electrodynamics model introduced in this paper with anextended atmospheric model extending fromthe ground surface to the upper thermos［ 1 6 ］ and with an ionospheric
phere［ 1 5 ］
model［14］, thereby developing a model covering the entire atmospheric region of the earth.
This whole atmosphere-ionosphere coupledmodel includes a series of physical processes
described in this paper and will hopefully
reproduce daily changes in the ionosphere ona quantitative basis. The calculations given in
Chapter 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 taken
into account for the future development of thewhole atmosphere-ionosphere model.

Acknowledgments
This study used the calculation results ofthe extended atmospheric general circulation
model developed at Kyushu University andTohoku University. We also used ionosphericmodel SAMI2 developed at the United States
Naval Research Laboratory. The World Data
Center for Geomagnetism at Kyoto University
provided the Kp indices of geomagnetic disturbances. We would like to express our deep
appreciation and thanks to those institutions.

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JIN Hidekatsu, Dr. Sci.
Expert Researcher, Space Environment
Group, Applied Electromagnetic
Research Center
Upper Atmospheric Physics

JIN Hidekatsu

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