J. Geomag. Geoelectr., 31, 287-310, 1979 Ionospheric Wind Dynamo Theory: A Review A. D. RICHMOND SpaceEnvironment Laboratory, National Oceanic and Atmospheric Administration,Boulder, Colorado 80302, U. S. A. (Accepted June 10, 1978) The current state of ionospheric wind dynamo theory is reviewed. Observation- al and theoreticaladvances in recent yearshave permitted more accurate models of the dynamo mechanismto be presentedthan previously,which have lent further credenceto the validity of dynamo theory as the main explanation for quiet-day ionosphericelectric fields and currents at middle and low latitudes. The diurnal component of the wind in the upper E region and lower F region appears to be primarily responsiblefor averagequiet-day currents, although other wind compo- nents give significantcontributions. Observationally,there is a need for better spatial and temporal coverage of wind and electric field data. Theoretically, there is a need for further considerationof the mutual dynamiccoupling among winds, conductivities,electric fields, and electric currents, and for better modeling of nighttimeconditions. 1. Introduction This paper is intended to review the present state of knowledge concerning the theory of the ionospheric wind dynamo. The main emphasis is on current theoreti- cal conceptions, with historical aspects, observational evidence, and treatments of temporal and spatial variability covered more briefly. For further information on dynamo theory and ionospheric currents, previous reviews (K. MAEDA and KATO, 1966; MATSUSHITA, 1967, 1968, 1971a, 1973, 1975, 1977; H. MAEDA, 1968; PRICE, 1969a, b; WAGNER, 1971; AKASOFU and CHAPMAN, 1972; MATSUSHITA and MOZER, 1973; VOLLAND, 1974a; FATKULLIN, 1975; KANE, 1976) will be found useful. 2. Formulation of Dynamo Theory The basic features of ionospheric wind dynamo theory can be formulated as follows. Atmospheric winds at ionospheric heights, by moving the electrically con- ducting fluid through the earth's magnetic field, generate electromotive forces which result in electric current flow, buildup of polarization charges, and electrostatic fields. At middle and low geomagnetic latitudes (below 60) the dynamo mechanism is believed to be the primary source of ionospheric currents and electric fields, as well as ground-level magnetic variations, on geomagnetically quiet days. Although other mechanisms for causing quiet-day ionospheric electric currents and fields have been advanced (e. g., MATSUSHITA, 1971a, b, 1972; GLUSHAKOV and SAMOKHIN, 287 288 A. D. RICHMOND 1974, 1975; LYATSKIY and MAL'TSEV, 1975), the dominant importance of the iono- spheric dynamo mechanism now appears to have been well established. Because the theory links winds, conductivities, electric currents, and electric fields in the iono- sphere, none of whose distributions in space and time is completely known, it can be a useful tool to synthesize and extend the available information on these quantities. The central equation relating the winds, electric fields, and electric currents is a form of Ohm's Law: J=oEii+a1(El+vxB)+albX(E1+vXB) (1) where J is the current density; E11 and El are the components of the electric field parallel and perpendicular to the geomagnetic field B; v is the wind velocity; b is a unit vector in the direction of B; a is the conductivity parallel to B; a1 is the Pedersen conductivity; and a2 is the Hall conductivity. For time scales longer than a minute or so, the electric field can be assumed to be electrostatic, E=-7φ (2) where is the electrostatic potential. Throughout the ionosphere and plasmasphere, the parallel conductivity 6o is usually so large that the parallel electric field E11is almost completely shorted out. Thus it is reasonable to assume in dynamo theory that magnetic field lines are equipotentials along their entire length between conju- gate points, and that electric fields can be mapped between the ionosphere and mag- netosphere (FARLEY, 1960; SPREITER and BRIGGS, 1961; DOUGHERTY, 1963; H. MAEDA, 1964, 1966a, b, 1971, 1974; DEWITT and AKASOFU, 1964; HINES, 1964; K. MAEDA, 1964; REID, 1965; VAN SABBEN, 1966, 1969, 1970; STENING, 1968, 1973; MATSUSHITA and TARPLEY, 1970; MATSUSHITA, 197lb; GUREVICH et al., 1972, 1974; KRYLOV and SHCHERBAKOV, 1972; KRYLOV et al., 1973; SCHIELDGE et al., 1973; RICHMOND, 1973a; KRYLOV, 1973; JONES, 1974; GAGNEPAIN etal., 1976; ANANDARAO et al., 1977; MOHLMANN, 1977). Experimental evidence supporting this assumption has been found by PETERSON et al. (1977). Because of this fact, it is often convenient to use dipole coordinates in performing dynamo calculations (STENING, 1968; HEELIS et al., 1974; MATUURA, 1974b; MOHLMANN, 1974b). If the conductivity and wind distributions are specified, it is a straightforward matter to calculate the electric field and current. In addition to (1) and (2), the following equation is required, expressing the fact that the current is divergenceless: ∇J=0. (3) It is also necessary to specify a high-latitude boundary condition on E or J, taking into account the effects of current flow between the magnetosphere and ionosphere due to solar wind-magnetosphere interactions which do not obey (1). The appro- priate specification of this boundary condition is beyond the scope of this paper, and in fact has generally been circumvented in dynamo calculations by ignoring the current flow between the ionosphere and magnetosphere at high latitudes. Observationally, our present knowledge of ionospheric winds, conductivities, electric fields, and currents is far from complete (WAGNER, 1971), so that the speci- Ionospheric Wind Dynamo Theory: A Review 284 fication of certain of these in order to compute the others from dynamo theory is subject to considerable uncertainty. Therefore, it is useful to invoke additional theoretical constraints, such as the facts that the winds must obey hydrodynamic equations and that the electron and ion densities must obey continuity equations including production and loss. As described by MATUURA (1968), KOHL (1969), MOHLMANN (1974c), ANDERSON and ROBLE (1974), VOLLAND (1976b), and KATO (1976), these additional equations are mutually coupled with each other and with the dynamo Eqs. (1)-(3). When the characteristic electron lifetime is longer than several minutes, as in the F region and the nighttime E region, the ionospheric plasma upon which the conductivity depends is subject to significant redistribution due to winds and electric fields (e. g., COLE, 1969; EVANS, 1972, 1975; RISHBETH, 1972; FUJITAKA and TOHMATSU, 1973; MURATA, 1974a; MATUURA, 1974a; KOHL, 1976). The plasma tends to assume the same component of velocity parallel to the magnetic field as the neutral wind, and perpendicular to the magnetic field as the convection velocity uE, defined as ue=k×D.ng (4) When the electron density is comparable to or greater than 1011rn-3, as in the F region and the daytime E region, the neutral wind can be significantly influenced by the Ampere acceleration due to electric currents, given by J×b=-σ1b4(Vl-uE)-σ2b4b×(v-uE) (5) (PIDDINGTON, 1954), where p is the mass density of the air. The first term on the right-hand side of (5) is usually the more important term, and acts as a drag on the wind, tending to bring the wind motion into conformance with the electric field con- vection velocity uE (e.g., BAKER and MARTYN, 1953; DOUGHERTY, 1961; AKASOFU and DEWITT, 1965; K. MAEDA and KATO, 1966; RISHBETH, 1971a, b, c, 1972; HEELIS et al., 1974; VOLLAND, 1976a). A completely self-consistent treatment of dynamo theory should in principle take these mutual coupling effects into account. 3. Present Knowledge of Parameters Used in Dynamo Theory Before discussing the results of dynamo calculations, in the next section, it will be useful to summarize the state of our knowledge about conductivities, currents, electric fields, and winds in the ionosphere. 3.1 Conductivities The daytime distributions of the conductivities ao, a1, and a2are reasonably well known. The transverse conductivities a1 and a2 are important primarily below 200 km, where the electron and ion densities behave in a fairly regular manner (e.g., DAVIES, 1965; RAWER and SUCHY, 1967; CHING and CHIU, 1973, 1974; NARCISI, 1974; OLIVER, 1975; KOREN'KOV et al., 1975; MIRTOV and STARKOVA, 1976; TORR 290 A. D. RICHMOND Fig. 1. Electron density N0, and parallel, Pedersen, and Hall conduc- tivities oo, o1, and 62, for an overhead sun of moderate activity, with a magnetic field strength Bo of 2.93 x 10-5T. From RICHMOND (1973a). PEDERSEN CONDUCTIVITY APRIL 17-18, 1974 TIME (AST) Fig. 2. Pedersen conductivity over Arecibo for the night of 17-18 April 1974. Note change in altitude scale at 165km. From HARPER and WALKER (1977). Ionospheric Wind Dynamo Theory: A Review 291 and HARPER,1977) and are governed mainly by a simple balance between produc- tion and loss. The conductivities are calculated from given electron and ion densi- ties, neutral densities (e. g., CIRA 1972; MOE, 1973; U. S. Standard Atmosphere, 1976; HEDIN et al., 1977a, b), and experimental or theoretical effective electron- neutral and ion-neutral collision frequencies (e. g., MASON, 1970; ITIKAWA, 1971; HILL and BOWHILL, 1977), using standard formulas (e. g., BAUER, 1973). Figure 1 shows typical noontime low-latitude profiles of the conductivities. Nighttime conductivities are far less well understood, because the E-region electron densities are somewhat difficult to measure and are found to be highly variable (e. g., WAKAI, 1971; KNIGHT, 1972; ROWE, 1973, 1974a, b; RoWE and MATHEWS,