The Electrical State of the Upper Atmosphere*

By Tatsuzo OBAYAsHI

Ionosphere Research Laboratory Kyoto University (Read May 10, 1963; Received Sept. 1, 1963)

Abstract

The electrical nature of the upper atmosphere is reviewed with an emphasis on those problems which mighti be considered as belonging fiothe regime of space elcc- tricity. The physical structure and electrodynarnic behaviour of the ionosphere are explained in terms of an interacting ternary gas of electrons, ions and neutral particles under the influence of the geomagnetic field. The concept of an atmospheric dynamo is important, producing strong currents and electric polarization fields in the lower ionosphere. In the exosphere, the behaviour of gas is essentially hydromagnetic. Possible mechanisms for generating electric fields by magnetospheric convective motions are discussed.

1. Introduction

This paper will be concerned with the electrical state of the earth's upper atmo- sphere, which may be of prime interest for the study of "space electricity". fihe electrical nature of the atmosphere arises from space charges carried by free electrons and ions which are maintained by a complicated photochemical balance of ionization and loss processes and dynamical movements of the gas itself. In the vicinity of our atmosphere within the regions of the troposphere and stratosphere, electrons formed by ionizing radiation will attach themselves to molecules so rapidly that their effect is almost negli- gible, and only ions play an important role in atmospheric electricity. In the atmosphere above about 80km, the concentration of electrons becomes appreciable, because of the increasing flux of ionizing agencies and also because of the long life of electrons due to the prevailing condition of extremely Iow air density. This region, where the behaviour of free electrons is dominant and they are strongly influenced by the geomagnetic field, is called the ionosphere. It extends up to a height of a few thousand kilometers, where it merges gradually into the exosphere, the uppermost regions of the earth's atmosphere. In contrast to the ionosphere, which is composed mostly of heavier atoms and molecules in a weakly ionized state, the exospheric gas consists mainly of protons and electrons, i.e., afully ionized plasma imbedded in the geomatnetic field. In the study of atmospheric electricity, much of the work in the past has been con- cerned with problems of the lower atmosphere such as fair weather, rain, and thunderstorm

* Read at the Third InternationalConference on Atmospheric and Space ElectricityMay 6-10, 1963, Montreux, Switzerland

(133) 134 T. OBAYASHI electricity.However it has been emphasized in recent years that the concept of atmo- spheric electricityshould be extended to include problems in the ionosphere and beyond. It is the purpose of this paper to review the present knowledge and existing problems of the upper atmosphere which might be considered as belonging to the regime of "Space Electricity".The physical structure of the ionosphere and the exosphere will be described firstand the electrodynamic behaviour of these regions is explained in terms of the inter- acting ternary gas of electrons,ions and neutral particlesunder the influence of the geo- magnetic feld. The concept of an atmospheric dynamo is important in the lower ionosphere, producing strong electric currents and an electric polarization field. The generated electrostaticfield is communicated to the upper ionosphere, thereby causing drift motions of the ionized plasma. The dynamical behaviour in the exosphere is essentiallyhydro- magnetic, any gas motions there being closely coupled with those of the geomagnetic field lines.fihe effectof interactingexospheric gas and interplanetary plasma generating electric fieldsin the earth'souter atmosphere will also be discussed briefly.

2. Structure of the Upper Atmosphere

2.1 Constitution of the Ionosphere

The particle density, temperature and constituents of the atmosphere aye the most fundamental physical quantities.The height distributionsof these quantitiesup to 1000km have been fairly well established by recent rocket and satellitemeasurements, and are illustratedin Figure 1. The atmosphere below the ionosphere consistsof N2 and O2 with a constant mean molecular weight of 28.97. The atmosphere is in hydrostatic equilibrium under the gravitationalforce, and the number density of particles,n, above the reference level,where z=z0 and n=n0, is given by

Fig. 1. Altitude distributionsof the temperature T, mean molecular Weight M, and particledensity (n0 for neutral particlesand ne for electrons) in the upper atmosphere. The Electrical State of the Upper Atmosphere 135

(1) whereT isthe temperature, H the scale height given by k Boltzman'sconstant, m the mean molecular mass and g the gravitational acceleration.

The solar radiation contains sufficient energy at ultraviolet wavelengths to cause photo- dissociation (Schuman-Runge 1300-1750 Å for O2) and photo-ionization (Lyman continuum and X rays) of the gas in the high atmosphere. This gives rise to a partially ionized region known as the ionosphere, identified by several distinct layers D, E, F1 and F2. The electron density profile is rather complicated owing to various intricate electron loss processes, and varies considerably with the time of day, season of the year, as well as geographical posi- tion. The principal constituents in the F region, at heights of 200-500km, are O atoms and

N2 molecules. At still greater heights, He and H atoms become more dominant than any other constituents, because the effect of diffusion overcomes any mixing of the air above the 200km level.

The electron density above the F2 peak may be approximated by the relation

(2)

where nm=106cm-3 the peak electrondensity and Z=h-hm/H is the normalized height measured from hm=300km in units of the scale height H=100km for typicalday-time conditions(Wright, 1961). The exosphericeleetron density above 1000-2000km levelsfalls offvery slowly with increasingheight, and is given empiricallyby the equation

(3)

where n0=104cm-3, and is the geocentric distance measured in units of the earth's

radius a=6370km. An important property of the exosphere is that the gas consists of fully

ionized hydrogen atoms and electrons, having a high kinetic temperature of the order of

104°K. The region is strongly influenced by the geomagnetic field, and thus is often referred

to as the magnetosphere. The exosphere terminates in a fringe region at distances of 10-20

earth radii, where the effects of interplanetary gas or solar winds will overcome those of

the geomagnetic field and atmospheric gas.

2.2 Collisions and Gyro-frequency

The presence of electrons and ions in the ionosphere makes this region electrically

conductive. This electrical nature is largely controlled by the concentration of neutral

particles as well as that of charged particles, because collisions of charged particles restrict

their movement under the action of any impressed electric field. A further complication

is brought about by the existence of the geomagnetic field as it restricts the motion of

charged par. titles across the magnetic field and therefore males the conductivity aniso-

tropic. 135 T. OBAYASHI

For the major part of the ionosphere, the gas is a neutral ternary mixture consisting of electrons, ions and neutral particles. Three types of collision frequency, between elec-

Irons and neutral particles, ven, ions and neutral particles, νin, and electrons and ions, νei, are important,

(4)

where T is the electron temperature and M the mean molecular weight. An important parameter related to the magnetic field in an ionized gas is the gyro-frequency. For the j-th constituent of charged particles, the angular gyro-frequ-ency ωj is given by

(5)

where ej is the charge in emu (positiveor negative) and B the magnetic induction. Typical height curves of these parameters are shovvriin Figure 2. Collisions of an electron or ion with neutral particlesare very large in the lower ionosphere, but decrease

Fig. 2. Collision frequencies νin, νen and νei, gyrofrequencies ωe and

ωi, and electrical nature of the ionosphere.

rapidly with altitude. The Coulomb collision νei is dominant in the upper part of the iono-

sphere above the 200km level. The gyro-frequency ω/2π has a typical value of approxi-

mately 1mc/s for an electron, decreasing with geocentric distance r as (a/r)3. An important

deduction from Figure 2 is that the ionosphere may be sub-divided into two electrical

regions; a hydrornagnetic region and a dynamo region. The two regions are characterized

by the relative importance of fhe cyclotron motion of charged particles in the geomagnetic

field and the effect of collisions with neutral particles. In the higher atmosphere, the The Electrical State of the Upper Atmosphere 137 collisionfrequency of a charged particlewith neutral particlesis exceedingly small corn- pared with its gyro-frequency. This means that electrons or ions may execute many cyclotron motions before they hit any neutral particle,indicating that the magnetic field has a much stronger influence on charged particlesthan impact forces due to collisions with neutral particles. Such a region, in which the behaviour of charged particlesand also the motion of the bulk of the plasma itselfare profoundly influenced by the magnetic field,is designated the hydromagnetic region. On the other hand, in the lower atmosphere, collisionaleffects so predominate that na effectof the magnetic fieldis apparent. The transitionregion between these two extreme conditions is situated at a height of about 80km for electrons and of about 120km for ions. The dynamo region is such a transition domain, in which both magnetic and collisioneffects are important and intense currents may flow owing to the dynamo-action of neutral air movements.

2.3 /Ulotionof Chaaged Particlesin the Ionosphere

The motion of gas in the ionosphere consisting of electrons,ions and neutral particles is described by tree momentum equations (in e.m.u.)given by

(6)

(7)

(8)

where V is the mean velocity of the gas, F the non-electrical forces such as gravitational forces and pressure terms, and the K's the coefficients of friction given by

(9)

According to Nines (1963), the dominant forces that determine the mean motion of

the gas in the ionosphere are electrodynamic and collisional. Denoting the time-scale of

motion by τ, the gyration-time of particles around the magnetic field by τBj=ωj-1, and

the frictional collision-time by τjn=Kjn-1, the equation of motion of the j-th constitutent

of charged particles may be reduced to the simpler form

(10)

This equation ignores the F term, and the electron-ion frictional collision term* which

are unimportant or negligible for the present purpose. Furthermore, in most motions of

interest, the inertial term on the left hand side of equation (10) is negligible, because the

typical geophysical time-scale τ is much longer than τBj or τjn which are certainly less

than a second.

* For electrons νei≪ ωe , and for ions the collision with electrons is also ineffective. 138 T. OBAYASHI

Under such circumstances, in the hydromagnetic region defined in the precious section, the Lorentz force term dominates over the collisional term because τjn≫ τBj; provided that

Vn≦Vj, and the equation of motion approximates to

E+Vj×B=0 (11) and

(12) where VD is called the drift velocity. The electric field is only that which results from the accumulation of space charge to offset the Lorentz force, and this will limit the type of velocity field in which V must everywhere lie in the Local equipotential surface S, which is given by

grad S=-E=VD×B (13)

The electrons and ions convert along the equipotential surface with the same velocity VD as does the magnetic tube of flux. Therefore, in this region, the hydrornagnetic concept of a "frozen field" is quite applicable to the gross dynamics of ionisation.

In the dynamo region whereτin≦ τBi, the ions are subject to complicated motions cou- pled with those of the neutral particles, though the electrons are still frozen with the magnetic field. The mean velocity of the ions is now impeded by the neutral particles, being given by

(14)

which is much less than the velocity of the electrons Ve=VD=eE/miτBi. This differential motion between electrons and ions gives rise to a strong current. The electriccurrent, which is thus driven mainly by electrons,can flow in this region down to a height of about 80km, where the motion of the electrons themselves is impeded by the frictionalforce of the neutral particles. Below a height of 80km, the collisionterms are dominant both for electrons and ions, and charged particlesare then simply converted by the neutral gas in its motion. For the motion of the neutral gas, the simplest case is when the collisioninteraction is the only force exerted on the neutral particles.From equation (8),

(15)

where τni=nn/niτin (the electron-neutral collisions being ineffective). The collision force is normally smaller trRan the gravitational force, so xhat its vertical component will have but little effect, although its horizontal component remains as a driving force. This equation reveals that Vn will grow in magnitude and became comparable with Vi, if the time scale

τ≧ τni. τni is some tens of minutes or less in the hydromagnetic region and a few hours in the dynamo region. The Electrical State of the Upper Atmosphere 139

2.4 Electrical Conductivity

In the region, where the ionized gas is pervaded by the magnetic field,the electrical conductivity of the gas is anisotropic.Three specificconductivities are defined, which are given by the expressions

(16)

(17)

(18)

60 is that which exists parallel to, or in the absence of, a magnetic field, sa that it is often referred to as the longitudinal conductivity. σ1 is called the Pedersen conductivity and is perpendicular to the direction of the magnetic field, while σ2 is the Hall conductivity and is perpendicular to both the magnetic field and the applied electric field. Then Ohm's law, relating the current i with the electric field E through the conductivity tensor, may be written as

(19)

in which the direction of the magnetic field is taken along the z-axis. The computed conductivity distributions are shown in Figure 3. The conductivity in

Fig. 3. Electrical conductivity profile of the ionosphere; σ0, σ1, σ2 and σ3 are the longitudinal, Pedersen, Hall and Cowling

conductivities, respectively. 140 T. UBAYASHI

the lower atmosphere is isotropic and its value is of the order of 10-15mho cm-1. It rises rapidly above the bottom of the ionosphere, obtaining typical peak values for σ1 and σ2 of

10-5mho cm-1 at 100-150km levels, and σ0=1mho cm-1 at greater heights. At the very lowest levels σ0≧ σ1≫ σ2, in the dynamo region σ0>σ2≫ σ1, and in the region above σ0≫ σ1≫ σ2.

The electric current flowing in an anisotropic conducting medium is most easily ex- pressed in terms of compone:nts in the direction of the magnetic fieid alld perpendicular to it. Thus

(20)

(21)

However, complications arise in the case of the actual ionosphere, because the ionosphere is inhomobeneous and the vertical scale is much smaller than the horizontal one so no vertical current flows. If the coordinates x, y and z are directed towards the south, east and vertically upwards, respectively, the horizontal current for a thin layer is given by

(22) where

(23)

(24)

(25)

and I is the geomagnetic dip angle. The approximation holds for I>3°. Near the geomag- netic equator, I<3°, σxx=σ0, σyy=σ1+σ22/σ1≡ σ3 andσxy=0. The conductivity σ3 is known as the Cowling conductivity,and is very Large at the 100km level.

3. Electrodynamics in the Upper Atmosphere

3.1 The concept of the atmospheric dynamo and motor

At some levels of the Lower ionosphere the neutral has is moved by tidal forces,and possibly also by forces of thermal origin. Charged particlesshare this motion, and since they behave like a conductor moving through the geomagnetic field,a current is induced as in a "dynamo". As early as 1882 Stewart proposed that this dynamo current is respon- Biblefor the geomagnetic daily variations observed at ground level. The atmospheric dynamo theory of geomagnetic variations has been developed by Chapman (1940) in his extensive treatiseon "Geomagnetism". The geomagnetic variations are attributed to currents in the ionosphere which are set in motion by an inductive elec- tromotive force,generated in turn by the movement of ionized gas in the presence of the geomagnetic field.The electricfield which drives the current consistsof two distinctparts The Electrical State of the Upper Atmosphere 141

one is due to the motion of the atmosphere with velocity v0, producing an induction field

Ei=v0×B, and the other is the electrostatic field Es resulting from the accumulation of

polarization charges. The electrostatic field may reach other parts of the ionosphere, caus-

ing currents to flow there. The geomagnetic field, acting on these currents, then causes the electron-ion plasma. in these more distantregions to move bodily. It is con- venient to give the name "atmospheric motor" to this part of the ionosphere. As is shown schematically in Figure 4, it appears that in general the dynamo is situated in the E region, at an altitudeof 80-120km, and the motor in the F region and above. However, it should be re- Fig. 4. Schematic concEpt of atmospheric dynamo and motor in the ionosphere. marked that there is also an important

motor effect in the dynamo region itself, since currents also flow there, which give rise to

movements of the E layer.

Since the ionosphere may be approximated by a thin highly conductive layer, the current flows only in horizontal directions. The horizontal current components, ix (south- wards) and iy (eastwards) may be derived from the geomagnetic variations ΔX and ΔY, making an appropriate correction for the induction effect within the earth. Thus

(26) which may be derived from a current function R expressed in spherical coordinates (r=a,

θ, φ), as

(27)

On the other hand, the induced electricfield Ei(Ex, Ey) caused by a horizontal wind vo (vx,vy) is given by Ex=vyBz, Ey=-vxBz (28) when charges flow, an electrostatic potential, S, builds up so that electric current closure results. Hence from Ohm's law

i=[σ] [Ei+Es] (29) where

(30)

Using equations (22),(27), (28) and (30),

(31) and 142 T. OBAYASHI

(32)

If S is eliminated, there results the dynamo differential equation,

(33)

where σ3=σ1+σ22/σ1 andε=σyy/σxx which is nearly unity except at the equatorial belt where

ε=σ3/ σ0. The observed magnitude of a geomagnetic variation is of the order of 10-50 gammas,

and this in turn leads to a current of 20-100amp/km and an electrostatic field of 1-5 volts/ km. The estimated value of v0 is 10m/s for a reasonable value of the conductivity, and the

drift velocity in the F region resuiting from such an electrostatic field is of the order of

100m/s. Such movements are in fairly good agreement with those observed in the iono-

sphere.

3.2 Global Electrostatic Field genemted by Dynamo-Action

The world-wide distribution of Es due to currents flowing in the ionosphere may be

deduced from the dynamo theory. Using data from world-wide geomagnetic variations,

Maeda (1955) derived a global pattern of the electrostatic field both for the geomagnetic

quiet variation Sq, as well as for the disturbed day variation SD. His result for the Sq-field

is reproduced in Figure 5, which also shows the computed vertical electrostatic field dis-

tributian across the noon meridian. The vertical field arises in such a way that the electric

field is communicated upwards along the geomagnetic field lines since they are equipoten一

Horizontal Electrostotic Field Vertical Field

Fig. 5. Global distributionof the electrostaticfield in the ionosphere deduced from the geamagnetic Sq variation (afterMaeda, 1955). The Electrical State of the Upper Atmosphere 143 tials,and, therefore, it can be computed from the meridional horizontal component (Es)x, i.e.

(34)

Near the geomagnetic equator, this situation is offset by the strong Hall effect and the resulting vertical field is given

(35)

This particularly intense vertical field exists along a narrow equatorial belt of width ±3°, being confined to a limited altitude range within the dynamo region.

Some prominent features of this electrostatic field are the large diurnal variation of the horizontal field vector in middle and high latitudes and also a pronounced vertical field in low latitudes. This electrostatic field must be communicated along the geomagnetic field lines, and hence may be mapped at greater heights. In the F region this electrostatic

field then produces a convective drift motion of the ionized gas and, with-some time-lag,

of the neutral gas as well. It has been suggested by several workers that these drift motions

of the atmosphere have an important influence on the formation of the F2 layer. In parti-

cular, during geomagnetic disturlaances enhanced drift motions may distort considerably

the electron density profile in the F region, which may be an important factor in causiDg

ionospheric storms (Gbayashi 1963).

The effectiveness of transporting such Es fields generated in the dynamo region to

higher altitudes has been discussed in greater detail by Farley (1959) and Spreiter and Briggs

(1961). Their results show that Es fields, even on a very small horizontal scale (tens of

kilometers), may be transmitted to F region heights without much attenuation, and in fact

are very effective in controlling large-scale movements in the upper atmosphere.

3.3 Electrodynamics of the Magnetosphere

The exosphere is filled with an ion-electron plasma imbedded in the strong geomag-

netic field, and is often called the magnetosphere. In contrast to conditions in the iono-

sphere where the dominant forces arise from collisions or are electrodynamic, the nature

of the magnetosphere is essentially eollisionless and hydromagnetic. Any gas motion is

strongly coupled with the magnetic field. An important consequence of this is the identifi-

cation of a tube of magnetic flux with the ionization it contains. The gas convects together

with the tube of magnetic flux, and is known as the "frozen field" concept of hydromag-

netics (Gold, 1959).

One of the important Large scale motions in the magnetosphere is the co-rotation of

the atmospheric gas with the earth. As has been pointed out by Gold, the viscaus drag

imposed on the neutral gas would enforce the co-rotation of atmospheric gas at least up to

the region of the ionosphere. But the ionized gas must rotate similarly at all higher levels

which are linked to it magnetically, in accordance with the frozen field concept in the

magnetosphere. Such gas motions in the geomagnetic field lead to an induction electro- 144 T. OBAYASHI

motive force Ei=vr×B where vr is the rotational velocity vector. A steady state can be achieved only if and when Es+Ei=0, for only then would currents cease to flow. In other words, the polarization field must be set up in such a way as to offset the Lorentz force so that the motion can proceed. The horizontal and vertical components of this palarizatian field are v0B0(a/ r)2sin2θ and v0B0(a/r)2sin2θ, respectively, and v0=450m/s, B0=0.31 gauss. The totalintegrated fieldfrom the equator to the pole is about 88kv, being directed equator- wards (or verticallydownwards).

It has beon proposed by Johnson (1960) and

others that the geomagnetic field is confirled

within a cavity due to the interaction with a

constant solar plasma stream and that the boun-

dary is tear-drop shaped, being elongated towards

the night side. In such a model, the polar field

lines are swept back towards the geomagnetic

tail in the form. of a magnetic arch, as depicted

in Figure 6. The ionized gas which is linked

magnetically to low and medium latitudes rotates

with the earth, but the gas which is linked to the

Fig. 6. Model of the magnetosphere, polar regions (θ<25°) counter-rotates in the geo- a) Meridional section of geomagnetic magnetic tail. fieldlines, b) Polar view (north) showing rota- Axford and Hfines (1961) showed that in the tional and convective motions. geomagnetic tail a large-scale convective motion

nay arise from the viscous interaction between the solar stream and the plasma in the magnetosphere. Inward convection from the tail can proceed into the magnetosphere, by setting up a polarizationfield directed westwards. Although the velocity of this motion may be a small fraction of that of the solar stream, it may cause a polarizationfield as rnuch as a few kilovoltsfor a typicalscale of 1000km. Twisting of magnetic field Lines may also arise when some of the fieldlines in the polar regions are extended into interplanetary plasma where co-rotation is inhibited.It has been shown that the associated electricfield is directed radially outwaids from the geomagnetic poles. The maximum possible potentialdifference developed baythis effectis of the order of tokv (Harrison, 1962), since this electricfield is directed nearly parallel to the magnetic fieldlines, an interesting consequence is that electrons in outer space are dragged down into the polar atmosphere;electrons tend to run away when the applied electricfield exceeds a certain criticalvalue.. This criticallimit is determined by equating the electricfield acceleration to the moderating force on electrons by collisions,

(36)

The runaway condition of electrons may easily be attained by a slightly twisted magnetic

field, provided that ne=104cm-3 and T=104°K. Such runaway electrons in the Kev range

may precipitate into polar ionosphere, thereby causing auroral displays. The Electrical State of the Upper Atmosphere 145

Fejer (1963) has recently discussed another type of electricfield which may arise from the excess of charges due to differentialmotions between trapped high energy particles and the ambient magnetospheric plasma. He postulates a rotational connective motion of the low energy plasma and an invariant drift motion of trapped energetic protons. fihe low evergy plasma convects in the magnetosphere with the rotation of the earth in such away that the plasma moves inwards on the morning side and outwards on the evening side,since the magnetosphere on the day side is compressed and on the night side expands due to the action of solar streams. On the other hand, high energy particlesare little affectedby such distortionof the geamagnetic fieldand theiradiabatic motion makes them follow linesof approximately constant magnetic fieldin the equatorial plane, being slightly farther from the earth on the day side where the compression of the fieldis intense.There- fore, if high energy trapped protons were in much larger numbers than electronsin the same energy range at a certain altitude,(which in fact has been borne out by satellite observations) the convective motion affecting only the low energy particlesproduces an excess accumulation of negative charge on the outside of the proton belt.For outward convection (an expanding motion on the evening side, as shown in Figure 7, the accumu- lacing space charges produce an electricfield directed polewards, centered somewhere in

Fig. 7. Electrostaticfield and neutralizing current caused by the outward convection of the magnetosphere in the presence of a belt of energetic trapped protons (afterFejer 1963). high latitudes.Similarly an electricfield directed equatorwards is establishedby inward convection on the morning side. According to Fejer, the electricfield set up by this mech- nism near the auroral zones will be sufficientto produce the auroral electrojet,which shows the correct sense of current flow predicted by this mechanism. 146 T. OBAYASHI

4. Concluding Remarks

To summarize the discussion,the gross features of the electricfield existing in the earth's atmosphere are described. A somewhat idealizedheight distributionof the electro- staticfield is illustratedin Figure 8. In the lower atmosphere there exists a downwards electric field(positive for the sign convention used in atmospheric electricity),since the

Fig. 8. Electrostatic field in the upper atmosphere. earth is negatively charged during fair-weather conditions.This electricfield is maintained by convective ion currents which may be provided by source currents from the areas sufferingstormy weather. The vertical electricfield intensity is about 100 v/nl near the ground, decreasing very considerably with increasing height. The measurements of can- ductivity show a corresponding increase with height. Since-theidea of a quasi-static,steady state charge distributionduring fair-weather conditions entailsthe existence of an equal vertical current density at all levels,the increase of conductivity at higher altitudeswould result in a decrease in the fieldand in positive space charges. The fieldintensity decreases exponentially With increasing height, and the total potentialdifference between the earth and the ionosphere is of the order of 106 volts, which has been estimated from air-earth current measurements. At ionospheric levels,the electricfield is generated by a dynamo action. The magnitude of the fieldranges from 1 to 10volts/km, depending on the velocity of tidalair movements and the conductivity in the region. The electricfield is horizontal as well as vertical,and large diurnal changes of amplitude and direction are the general rule in this dynamo region, It is not certain yet, however, whether this ionospheric electricfield is intimately connected with that of the lower atmosphere. In the magnetosphere, above the dynamo-region, the electricfield may be transported upwards from the dynamo region along conductive magnetic fieldlines. The electricfield The Electrical State of the Upper Atmosphere 147

may also be generated by rotational and convective motions in the magnetosphere itself.

The field originating from the dynamo region in the ionosphere will become insignificant with increasing distance, because of the diverging nature of the geomagnetic field lines, while the polarization charge effect by convective movements dominates in the outer part of the magnetosphere. Most of the field in the magnetosphere is generated in such a way that the Lorentz force is compensated by the polarization charge in order to preserve the convective motion; i.e., Es=-v×B. The representative rnagnitude may be of the order

of 1volt/km.

In conclusion we wish to express our thanks to Mr. S. C. Coroniti and the scientists in

atmospheric electricity who attended the Montreux conference for their valuable comments

and discussions on the problem of space electricity. Part of this paper has been finished

at the Institute of Earth Sciences, University of British Columbia, Vancouver, Canada,, and

lam indebted to Prof. J. A. Jacobs for his support of this work.

References

Axford, W. I., and Hines, C. O. A unifying theory of high-latitude geophysical phenomena and geomag.

netic storms. Canad. J. Phys., 39, 1433-1464, 1961.

Chapman, S., and Bartels, J. Geomagnetism, Clarendon Press, Oxford, Vol. 2, 1940.

Farley, D. T. A theory of electrostatic fields in a horizontally stratified ionosphere. J. Geaphys. Res.,

64, 1225-1233, 1959, and 65, 869-877, 1960.

Fejer, J. A. Theory of auroral electrojet. J. Geophys. Res., 68, 2147-2157, 1963.

Gold, T. Motions in the magnetosphere of the earth, J. Geophys. Res., 64, 1219-1224, 1959.

Harrison, E. R. The earth's distant magnetic field. Geophys. J. Roy. Astro. Soc., 6, 479-491, 1962.

Hines, C. O. The upper atmosphere in motion. Quart. J. Roy. Meteor. Soc., 89, 1-41, 1963.

Johnson, F. S. fihe gross character of the geomagnetic field in the solar wind., J. Geophys. Res. 65,

3049-3051, 1060.

Maeda, H. Horizontal wind systems in the ionosphere E region deduced from the dynamo theory of

the geomagnetic Sq variation. J. Geomag. Geoelect. 7, 121-132, 1955.

Obayashi, T. Morphology of storms in the ionosphere. Presented at the Symposium on Results of IGY/

IGC, Los Angeles, August, 1963.

Spreiter, J. R. and Briggls, B. R. Theory of electrostatic fields in the ionosphere. J. Geophys. Res., 66,

1731-1744, and 2345-2354, 1961.

Wright, J. W. A model of the F region above hmax F2. J. Geophys. Res., 65, 185-191, 1961.