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