The Current State of Ionospheric Wind Dynamo Theory Is Reviewed

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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 components 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 theoretical 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 ionospheric dynamo mechanism now appears to have been well established. Because the theory links winds, conductivities, electric currents, and electric fields in the ionosphere, 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 magnetosphere (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 appropriate 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 convection 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 conductivities 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 production and loss. The conductivities are calculated from given electron and ion densities, 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, 1973; SMITH et al., 1974; SHEN et al., 1976). Figure 2 shows profiles of the Pedersen conductivity 61 obtained from measured electron densities over the Arecibo incoherent scatter observatory during a typical night. Unlike daytime conditions, the relative importance of i above 200km to that below 200km can be substantial at night (K. MAEDA and MATSUMOTO, 1962; HARPER and WALKER, 1977). The downward-moving layer in the E region seen in Fig. 2 can be explained by a downward-moving convergence zone of plasma produced by neutral wind shear (HARPER and WALKER, 1977).

3.2 Currents Most of the available observations relating to Ionospheric currents are actually measurements of the magnetic perturbations produced by those currents. Figure 3 shows an example of the global perturbation magnetic potential pattern at ground level associated with quiet-day ionospheric currents, derived from the observed horizontal components of magnetic variation at the stations indicated. Roughly speaking, 50kA of current flows overhead in the ionosphere between each pair of contours, counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. The effects are largely concentrated in the daylit hemisphere because of high daytime ionospheric conductivities. For times of low sunspot number, the amount of current flowing may be as little as one-third of that for this period of high sunspot number. See PRICE and WILKINS (1963), PRICE and STONE (1964), MATSUSHITA (1968), PARKINSON (1971), MALIN (1973), and SUZUKI (1973) for de- tailed analyses of quiet-day solar (Sq) and lunar (L) magnetic variations. Figure 3 does not include a narrow feature along the magnetic equator known as the equatorial electrojet, because of insufficient station coverage. This feature can better be examined with data from a chain of stations across the equator (e. g., FAMBITAKOYE and MAYAUD, 1976a, b, c). The electrojet current, which is usually eastward within a band some 600km wide during the day, produces an enhanced variation of the northward magnetic component at stations underneath. Not infre- quently, the current direction may be westward during part of the day, giving rise to a negative H variation and a situation commonly called the "counter-electrojet" or "reversed electrojet," discussed more fully in other papers of this issue. The 292 AD. RICHMOND

Fig. 3. The global ground magnetic potential pattern obtained from geomagnetic data at 0330 UT on the magnetically quiet day of 5 August 1958. The non-equatorial magneto- meter stations are indicated as black dots and the equatorial stations as open circles. The circled cross represents the subsolar point and the stippled bands the terminators. The equipotential lines are spaced at 0.05Wb. m-1, and the maximum and minimum values are shown. The equipotential lines are also approximate flow lines for the equivalent current system, for which the line spacing is approximately 5 x 104A. From SCHIELDGEet al. (1973).

UNH 65-2 UNH 65-5

Fig. 4. Profiles of current density obtained from data published by MAYNARD and CAHILL (1965). The abcissa scale, in pA. m-2, is marked in units of ac- tual current density J and of normalized current density J', corresponding to a 100nT ground variation of H at Huancayo. Flight UNH 65-5 was at the magnetic equator, while flight UNH 65-2 was about 7 north of the magnetic equator. From DAVIS et al. (1967). Ionospheric Wind Dynamo Theory: A Review 293 electrojet has also been examined from above by satellite observations of the magnetic field strength F (e. g., CAIN and SWEENEY, 1973). For information about the vertical distribution of electric currents, rocket flights (see CAHILL, 1969; POGREBNOY and FATKULLIN, 1969; SHUMAN, 1970; SUBBARAYA et al., 1972; CLOUTIER and SANDEL, 1972; YABUZAKI and OGAWA, 1974; BURROWS and SASTRY, 1976; BURROWS et al., 1977; MUSMANN and SEILER, 1978) and incoherent scatter radar observations (SALAH and EVANS, 1977; HARPER, 1977a) have been useful. Figure 4 shows two examples of eastward current densities inferred from rocket magnetometers. Rocket flight UNH 65-5 went through the equatorial electrojet, while flight UNH 65-2 was outside the direct influence of the electrojet. The strong current layer in the electrojet is generally reproducible in other daytime observations, whereas the vertical structure outside the electrojet is quite variable.

3.3 Electric fields The spatial and temporal coverage of electric field observations (see RIEGER, 1971; BALSLEY, 1973; RICHMOND, 1976; BLANC and AMAYENC, 1976; BALSLEY et al., 1976; CARPENTER and SEELY, 1976; KIRCHHOFF and CARPENTER, 1976; HARPER, 1977a; BLANC et al., 1977; WOODMAN et al., 1977; CARPENTER, 1978) is not so good as that for currents. Figure 5 shows the average quiet-day electrostatic potential synthesized from available middle and low latitude F-region data under the assumption that local time and magnetic longitude are interchangeable. As the quantity of data increases, longitudinal, seasonal, and solar-cycle changes in this pattern should emerge.

Fig. 5. Quiet-day F -region electrostatic potential pattern in apex latitude-local time coordinates, synthesized from available electric field observations. The contour interval is 1kV. Zero potential is defined to be the heavy solid line which intersects the equator around 0200 and 1200 LT. Other solid contours represent positive potentials; dotted contours represent negative potentials. The maximum potential of 3.3kV occurs at the equator around 0700LT, while the minimum of -5.0kV occurs at the equator around 2000LT. Values above 65 latitude have no significance. From RICHMOND (1976). 294 A. D. RICHMOND

3.4 Winds wind observations in the thermosphere (e. g., EVANS, 1972; BEDINGER, 1972, 1977; KOCHANSKI, 1973; BARNES, 1973; AMAYENC, 1974; SALAH and HOLT, 1974; BERNARD, KOCHANSKI, 1973; BARNES, 1973; AMAYENC, 1974; SALAH and HARPER et al., 1976; KATO, 1976; KING-HELE and WALKER, 1977; HARPER, 1977b) usually exhibit strong oscillations which can be characterized by frequencies of one per day or some integral multiple thereof. Figure 6 shows some results from a Fourier analysis of southward winds observed over the St. Santin incoherent scatter observatory. Only the prevailing and the diurnal (24hr) components are shown here. Below 250km, nighttime data were unavailable, so that the results of the Fourier analysis are more questionable than they are above 250km. Figure 7 shows time-height profiles of the daytime E-region wind velocities deduced from incoherent scatter data taken at Arecibo. A fairly strong semidiurnal (12hr) variation is apparent, with a downward phase propagation. The periodic nature of observed winds strongly suggests an interpretation in terms of atmospheric tides, which may simply be defined here as long-period global atmospheric oscillations. The main source of the tides is absorption of solar radia- tion (e. g., CHAPMAN and LINDZEN, 1970; KATO, 1971). Classical tidal theory (e. g., DIKII, 1969; CHAPMANand LINDZEN, 1970) finds it convenient to divide the oscil-

AMPLITUDE HOUR OF MAXIMUM Fig. 6. Mean seasonal height structures of the diurnal component amplitude (ms1) and phase (local hour of southward maximum) of the meridional neutral wind from 1971- 1972 observations at St. Santin (note the break in the height scale above 250km). Left top: steady meridional neutral winds (positive values indicate southward winds). From AMAYENC(1974). Ionospheric Wind Dynamo Theory: A Review 295

INFERRED NEUTRAL WIND ARECIBO JAN 4, 1974

EASTWARD WIND Uy SOUTHWARD WIND UK

Fig. 7. Inferred horizontal components of neutral wind over Arecibo for January 4, 1974. Contour levels are 20m-1. Top altitudes could be in error due to assumed collision frequency model. From HARPER Pt al. (1976). lations into separate tidal modes, each with its own characteristic horizontal and vertical structure. Modes which propagate westward around the earth with the apparent solar position are commonly labeled with an S and two subscripts: e. g., S2,4, where the 2 represents 2 wavelengths around the earth in longitude, and the 4 is a label indicating the particular latitudinal structure of the mode. Four modes which are often discussed in connection with thermospheric winds are: a) S1, 2. This diurnal mode does not propagate in the vertical direction, and thus tends not to show much phase variation with height. The diurnal tide above 120km in Fig. 6 appears to be associated with this mode. b) Sl,l. This propagating diurnal tide has a vertical wavelength around 24km in the thermosphere, but is severely dissipated by viscosity above 110km altitude. The diurnal tide below 120km in Fig. 6 and in other observations (e. g., DINES, 1966; HARPER, 1977b) may be associated with this mode. c) S2,2. This semidiurnal mode can propagate vertically above about 110km, where it has a relatively long vertical wavelength. HARPER(1977b) associates a strong semidiurnal wind observed above 125km at Arecibo with this mode. d) S2, 4. This propagating semidiurnal mode has a vertical wavelength around 40km in the thermosphere, but is severely dissipated by viscosity above about 125km. The semidiurnal wind apparent in Fig. 7 and in other midlatitude observations (e. g., BERNARD, 1974; SALAH et al., 1975; LINDZEN, 1976) seems to be associated at least partly with this mode. Although the concept of individual tidal modes is a convenience, the separability of modes found in classical theory is not strictly valid when the effects of mean zonal winds, viscosity, and the Ampere force of ionospheric currents are considered. One relatively simple way to account approximately for the Ampere force is to ignore uE in comparison with v (as uE is roughly one-third to one-half as large as v observationally), to make some simplifying assumptions concerning a1B2/pand a2B2/p, 296 A. D. RICHMOND

Fig. 8. (Top) Wind pattern of the classical S1, 2 tidal mode, with amplitude and phase as employed by TARPLEY (1970b). (Bottom) Wind pattern of the S1, 2 tidal mode modified by a constant ion drag appropriate to 150 km altitude, with amplitude and phase as employed by RICHMOND et al. (1976) at 150km. A vector length equal to 5 of latitude represents a wind speed of 50ms. From RICHMOND et al. (1976). and to solve for the resultant modified tidal mode (JONES, 1971a, b, 1972; ISHIMINE, 1972, 1977; VOLLAND, 1974b, c; VOLLAND and MAYR, 1972a, b, 1974, 1977). Figure 8 compares the unmodified and modified wind patterns associated with the S1, 2 mode using a value of the ion drag parameter Q1B2lpappropriate to mean conditions at 150km. The greatest relative change is at low latitudes. A less restrictive ap- proximation to solve for tides in the presence of zonal winds and dissipative forces is to assume only that uE is negligible and that 61B2/pis constant in longitude, and to abandon the concept of individual modes, solving the tidal equations numerically on a two-dimensional height-latitude grid of points (HONG and LINDZEN, 1976; FORBES and GARRETT, 1976, 1978). Figure 9 is an example of the diurnal tide calculated in this way. Note that the results inferred for 45 north latitude compare satisfactorily with the winter solstice observations in Fig. 6. The importance of the term UEfor tidal theory has been examined by TAFFE (1969) and VoLLAND (1976a, b), by solving the combined tidal and dynamo equations simultaneously under simplified conditions. They found that the feedback on Ionospheric Wind Dynamo Theory: A Review 297

DECEMBER SOLSTICE

Fig. 9. Vertical profiles of northerly velocity for sunspot minimum (top) and maximum (bottom) conditions at latitudes -60 (short dashes), -30 (long dashes), 0 (solid), +30 (dash-dot-dash), and +60 (dash-dot-dot-dash), for December solstice conditions. From FORBES and GARRETT (1978). the tidal winds from the electric field so generated can be rather important under certain conditions. At night, winds in the F region can be important in dynamo theory because of the relatively greater importance of the nighttime F-region conductivities compared to E-region conductivities. Some fairly direct observations of F-region winds are available, as in Fig. 6, but much of our information about F-region winds comes from theoretical studies (see reviews by IZAKOY, 1971; RISHBETH, 1972; EVANS, 1972, 1975; VOLLAND, 1973; MURATA, 1974a; MATUURA, 1974a; DICKINSON, 1975; VOLLAND and MAYR, 1977). 298 A. D. RICHMOND

4. Results of Dynamo Calculations

A number of recent dynamo calculations (RICHMOND et al., 1976; KIRCHHOFF and CARPENTER, 1976; FORBES and LINDZEN, 1976a, b, 1977; SALAH and EVANS, 1977; HARPER, 1977a) have shown that presently available observational and theoretical knowledge of ionospheric winds is reasonably consistent with observed electric currents and fields. Most of the total Sq current flow appears to be due to a diurnal tidal wind (see also H. MAEDA, 1955; KATO, 1956; HIRONO and KITAMURA, 1956; STENING, 1969a; MATSUSHITA, 1969; TARPLEY, 1970b; VOLLAND, 1971b; MURATA, 1974b; MOHLMANN, 1976b) whose dynamo effect is most important in the 120-200km height range during the day (VOLLAND, 1971a; MOHLMANN, 1973; RICHMOND et al., 1976; FORBES and LINDZEN, 1976a). This tide can be identified to a large extent with the S1, 2 mode, modified by ion drag, generated in situ by solar heating. However, semidiurnal tides also make a substantial contribution to the electric currents and fields (RICHMOND et al., 1976; KIRCHHOFF and CARPENTER, 1976; FORBES and LINDZEN, 1976a, b, 1977; SALAH and EVANS, 1977; HARPER, 1977a; STENING, 1977c), and the diurnal S1, 1 tide probably gives a very important

Fig. 10. (Top) Current function in the northern hemisphere produced by the modified S1, 2 tidal mode and the S2,4 mode, with amplitudes and phases as indicated by wind observations. Contours spacing is 10kA, and the current flows clockwise during the day. (Bottom) Sq current function deduced by MATSUSHITA (1968) from IGY geomagnetic data. Contour spacing is 25kA. The difference between the computed and observed current intensities can be largely explained by the difference in the assumed solar cycle conditions (moderate-low) from IGY conditions. From RICHMOND et al. (1976). Ionospheric Wind Dynamo Theory: A Review 299 contribution to the current density below 120km (FORBES and LINDZEN, 1976a, b; HARPER, 1977a). Figure 10 compares northern hemisphere currents computed from the modified S1, 2 mode and the S2,4 mode, as indicated by observations, with the observed equivalent currents derived from magnetic observations. Figure 11 compares the electric field drifts computed from the same winds with observed drifts at various stations. Although the agreement between observation and computation is far from perfect, it is perhaps as good as can be expected from the present quite incomplete state of knowledge concerning Ionospheric winds and electric fields, and their variability. Figure 12 shows computed vertical profiles of electric current at the magnetic equator and at 5 north magnetic latitude. These profiles agree quali- tatively quite well with the observed profiles of Fig. 4. RISHBETH (1971a, b, c) pointed out that F-region winds can have an appreciable dynamo effect, particularly at night, when E-region conductivities are greatly re- duced, and particularly near the magnetic equator, where relatively long segments of the magnetic field lines may run through the F region. He also showed that the mutual dynamic influences between neutral air and plasma motions can be very important in this region. HEELIS et al. (1974) and MATUURA (1974b) performed further calculations substantiating these ideas. Some further interesting results of dynamo calculations can be summarized

E x B DRIFT VELOCITIES -(1 ,-2)＊+(2,4) Observed

Fig. 11. F-region E x B drift velocities perpendicular to the geomagnetic field at various latitudes as a function of local time, computed from the modified S1,_2 tidal mode and the S2,4 mode as in Fig. 10 (solid lines), and indicated by observations at various stations (dashed and dotted lines). From RICHMOND et al. (1976). 300 A. D. RICHMOND

Fig. 12. Eastward current densities at 0N and 5N, respectively, at 0600 LT (dash-dot-dash), 0900 LT (dashes), 1200 LT (solid), 1500 LT (dots), and 1800 LT (dash-dot-dot-dash), computed from the S1, 2, S1, 1, S2, 2, and S2, 4 tidal modes. Note the difference in scales for ON and 5N. From FORBES and LINDZEN (1976b).

rather briefly: a) Lunar tidal theory and observed magnetic variations appear to be consistent (H. MAEDA and FUJIWARA, 1967; STENING, 1969a; TARPLEY, 1970a). b) Certain hypothetical wind distributions produce an electric field but no current, while other distributions can produce currents but no electric field (KATO, 1957; DEWITT and AKASOFU, 1964; PRICE and COCKS, 1968; MOHLMANN and WAGNER, 1970; MOHLMANN, 1971a, 1974a, 1976a; see also JONES, 1973). c) In general, electric currents flow between the northern and southern hemispheres along geomagnetic field lines, with a total current which can amount to a substantial fraction of the current flowing within the ionosphere (DOUGHERTY, 1963; K. MAEDA and MURATA, 1965; VAN SABBEN, 1966, 1968, 1970; STENING, 1969a, 1977b; MISHIN, 1969; MATVEEV, 1971; MISHIN et al., 1971; MOHLMANN, 1971b; SCHIELDGE et al., 1973; H. MAEDA, 1974; see also RICHMOND, 1974). d) Vertical currents within the ionosphere give rise to toroidal magnetic fields not observable from the ground (NISHIDA and FUKUSHIMA, 1959; UNTIEDT, 1967; PRICE, 1968; FUKUSHIMA, 1968; COCKS and PRICE, 1969; SUGIURA and POROS, 1969; RICHMOND, 1973a). e) Longitudinal and seasonal variations of the conductivity can account for much of the corresponding Ionospheric Wind Dynamo Theory: A Review 301

variations of Sq currents and electric fields (H. MAEDA and MURATA, 1968; STENING, 1971, 1973), but universal-time variations of the winds also appear to be important (SCHIELDGE et al., 1973). f) Non-periodic winds can generate significant currents (VAN SABBEN, 1962; H. MAEDA and MURATA, 1968; POGREBNOY and GORDIYENKO, 1970), but do not appear to be the main contributor to Sq currents (STENING, 1969a; RICHMOND et al., 1976). g) Electric fields generated locally can extend over dis- tance of several thousands of kilometers within the ionosphere (GAGNEPAIN et al., 1976; STENING, 1977a). h) Dynamo theory can successfully explain observed features of low-latitude currents, although certain discrepancies remain (UNTIEDT, 1967; STENING, 1969b; SUGIURA and PORos, 1969; KRYLOV et al., 1973; RICHMOND, 1973b; FAMBITAKOYE et al., 1976; ANANDARAO et al., 1977).

5. Conclusions Ionospheric dynamo theory can be a useful tool for extending our knowledge about conductivities, winds, electric currents, and electric fields in the ionosphere. The basic validity of the theory appears to be well established. In order to improve theoretical models of the dynamo mechanism, it will be important to consider more realistically the mutual coupling among winds, currents, electric fields, and conductivities, and to model more realistically the nighttime conditions. Further ad- vances in dynamo theory will also depend heavily on improved temporal and spatial coverage of observations, especially of winds, electric fields, and nighttime conductivities. These observations are particularly important for helping us to understand the great spatial and temporal variability of ionospheric conditions, a subject which has not been covered in this brief review.

The author performed this work under a Resident Research Associateship from the National Research Council.

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Note added in proof References to some recent publications are added here to bring this review up to date. 1. Recent reviews concerning dynamo theory: BLANC, M., Convection du plasma Bans l'ionosphere et la magnetosphere: un circuit electrique et son schema de resolution, Ann. Geophys.,in press, 1979. BLANC, M., Electrodynamics of the ionosphere from incoherent scatter: a review, J. Geomag. Geo- electr., 31, this issue, 1979. EVANS, J. V., Incoherent scatter contributions to studies of the dynamics of the lower thermosphere, Rev. Geophys.Space Phys., 16, 195-216,1978. 2. Discussions of interactions among winds, currents, electric fields, and electron densities: MAEDA, K., Conductivity and drifts in the ionosphere, J. Atmos. Terr. Phys., 39, 1041-1053, 1977. RISHBETH, H., Dynamics of the equatorial F-region, J. Atmos. Terr. Phys., 39, 1159-1168, 1977. 3. Models of ionospheric electron and ion densities: DANILOV, AD. and V. K. SEMENOV, Relative ion composition model at midlatitudes, J. Atmos. Terr. Phys., 40, 1093-1102, 1978. KOHNLEIN, W., Electron density models of the ionosphere, Rev. Geophys. Space Phys., 16, 341-354, 1978. RAWER, K., D. BILITZA, and S. RAMAKRISHNAN, Goals and status of the international reference ionosphere,Rev. Geophys.Space Phys., 16, 177-181,1978. 4. Analyses of Sq magnetic variations: MALIN, S. R. C. and J. C. GUPTA, The Sq current system during the International Geophysical Year, Geophys. J. Roy. Astron. Soc., 49, 515-529, 1977. SUZUKI, A., Geomagnetic Sq field at successive universal times, J. Atmos. Terr. Phys., 40, 449-463, 1978. SUZUKI, A. and H. MAEDA, Equivalent current systems of the daily geomagnetic variation in De- cember 1964, World Data Center C2 for Geomagnetism Data Book No. 1, Kyoto University, 1978. 310 A. D. RICHMOND

5. Rocket measurements of ionospheric currents: SASTRY, T. S. G., K. BURROWS, S. SAMPATH, J. D. STOLARIK, and M. J. USHER, Day-to-day variability of the equatorial electrojet as observed by rocket-borne magnetometers, Space Res., 17, 409-410, 1977. 6. Electric field observations: BLANC, M. and P. AMAYENC, Seasonal variations of the ionospheric E x B drifts above Saint-Santin on quiet days, J. Geophys. Res., 84, in press, 1979. CoRNEC, J. P., Winds and electric fields in the upper E-region over Malvern, J. Atmos. Terr. Phys., 40, 73-79, 1978. FEJER, B. G., D. T. FARLEY, R. F. WOODMAN, and C. CALDERON, Dependence of equatorial F region vertical drifts on season and solar cycle, J. Geophys. Res., 84, in press, 1979. 7. Reviews concerning thermospheric winds: CHAMPION, K. S. W. and J. M. FORGES, Some recent mesospheric and lower thermospheric data and models, Ann. Geophys., 34, 285-300, 1978. EVANS, J. V., W. L. OLIVER, Jr., and J. E. SALAH, Thermospheric properties as deduced from incoherent scatter measurements, J. Atmos. Tcrr. Phys., 41, 259-278, 1979. FORGES, J. M. and H. B. GARRETT, Theoretical studies of atmospheric tides, Rev. Geophys. Space Phys., 17, in press, 1979. KAZIMIROVSKY, ES., V. D. KOKOUROV, and E. I. ZHOVTY, Dynamics of the quiet thermosphere (a review), J. Atmos. Terr. Phys., 41, in press, 1979. LINDZEN, R. S., Atmospheric tides, in Review of Earth and Planetary Sciences, 7, edited by F. A. Donath, pp. 199-225, Annual Reviews, Inc., Palo Alto, Calif., 1979. MAYR, H. G., I. HARRIS, and N. W. SPENCER, Some properties of upper atmosphere dynamics, Rev. Geophys. Space Phys., 16, 539-565, 1978. 8. Tidal theory including coupling between neutral and ion motions: VOLLAND, H. and L. GRELLMANN, A hydromagnetic dynamo of the atmosphere, J. Geophys. Res., 83, 3699-3708, 1978. 9. Dynamo electric field calculations with height-independent winds of classical tidal mode structure: MAEDA, H., T. ARAKI, A. SUZUKI, and M. TAKEDA, Electric fields and neutral winds in the ionospheric dynamo region as deduced from the daily geomagnetic variations in December 1964, World Data Center C2 for Geomagnetism Data Book No. 2, Kyoto University, 1979. MOHLMANN, D., Ionospheric electrostatic fields, J. Atmos. Terr. Phys., 39, 1325-1332, 1977. 10. Dynamo calculations with realistic tidal winds: FORGES, J. M., and H. B. GARRETT, Solar tidal wind structures and the E-region dynamo, J. Geomag. Geoelectr., 31, this issue, 1979. 11. Dynamo calculations of field-aligned current flow between hemispheres: SCHAFER, K., Dynamo-electric field-aligned currents in the plasmasphere, J. Atmos. Terr. Phys., 40, 755-760, 1978. 12. Examination of equatorial electrojet models: GAGNEPAIN, J., M. CROCHET, and A. D. RICHMOND, Comparison of equatorial electrojet models, J. Atmos Terr. Phys., 39, 1119-1124, 1977. 13. Dynamo effects of disturbed thermospheric winds during magnetic storms: BLANC, M. and A. D. RICHMOND, The ionospheric disturbance dynamo, J. Geophys. Res., 84, 1979 (submitted).