K.D. Cole

Energy deposition in the thermosphere caused by the solar wind

Abstract The subject of joule dissipation of ionospheric currents and movement of the thermosphere by electric fields is briefly reviewed and the depositions of energy into the thermosphere by charged particles and electric fields are compared. Dissipation of electric fields is a major source of energy for the thermosphere. The injection of energy into the atmosphere during geomagnetic storms is negligible compared to what is needed to modify the weather except in the thermosphere and perhaps a few scale heights below it. A simple explanation of the correlation of magnetic disturbances and weather in the lower atmosphere, propounded earlier by the author, is included here.

FORMATION OF FIELD-ALIGNED IRREGULARITIES IN THE MAGNETOSPHERE.

Abstract The general physics of field-aligned irregularities in the magnetosphere is studied. Then a specific mechanism is proposed for formation of irregularities. It involves a spatially varying electrostatic field orthogonal to B and operates in conjunction with irregularities of conductivity in the E region, and field-aligned currents caused by mismatch of conjugate ionospheric potentials. The mechanism functions most easily at night and in equatorial regions. An application to the formation of spread F is discussed.

A quartic dispersion equation for internal gravity waves in the thermosphere

Abstract A new quartic dispersion equation in the square of the complex vertical wave number is derived by employing the ‘shallow atmosphere’ approximation and an ion drag approximation. These approximations allow the coefficients of the quartic equation to be given in terms of the corresponding cubic equation, which neglects the Coriolis force and the zonal ion drag component, but modified to take into account these neglected effects. Coupling between the extraordinary viscosity wave mode and the other three wave modes is highlighted and numerical solutions are compared for this quartic equation, an exact eighth order equation and the cubic equation. For the first time the validity of using the ‘shallow atmosphere’ approximation to describe internal gravity wave motions is demonstrated.

A numerical model for gravity wave dissipation in the thermosphere

Abstract Two simplified models of internal gravity wave dissipation due to viscosity, thermal conduction and ion-drag, in a multilayered, isothermal thermosphere are developed. Each of these models uses the WKB approximation, ray theory and the time-averaged equation of energy conservation, but whereas one of the models (A) employs all of the gravity wave equations appropriate to a dissipative atmosphere, the other (B) does not. Results derived from these models for one particular wave are compared to each other and also to some previously published results of Klostermeyer, which employed a full-wave, model. A breakdown of the WKB approximation in the lower, non-isothermal thermosphere leads to models A and B underestimating the total dissipation there. In the middle thermosphere model A estimates the dissipation reasonably well, while model B grossly overestimates the dissipation. In the upper thermosphere model A underestimates the total upward energy flux, probably as a result of the neglect of coupling into the dissipative waves at these levels, while no energy remains in model B. Results from model A show that when dissipation due to viscosity and thermal conduction are included correctly and simultaneously, the dissipation due to viscosity can exceed that due to thermal conduction by a factor of three. It is argued that ray theory may either overestimate or underestimate the energy flux reaching the upper boundary of a thermospheric model depending on both its height and the particular thermospheric model used.

ATMOSPHERIC EXCITATION AND IONIZATION BY IONS IN STRONG AURORAL AND MAN- MADE ELECTRIC FIELDS.

Ions gyrating in crossed electric (E) and magnetic (B) fields may acquire an energy of 2mE2B2. In electric fields found near auroras, ions acquire sufficient energy to excite and sometimes also to ionize molecules. On the assumption that the subsequent emission contributes significantly to the glow outside of discrete auroral forms, cross-sections for excitation of atmospheric species by low energy ions (1–30 eV) are found. When these cross-sections become known, the electric field may be estimated from ground-based measurements of this glow. At the greater field strengths (≳ 250 mV/m) ionization of the air and also excitation of helium to the 23S state may occur. The ionization may be sufficient to sustain the ionosphere near the aurora. An experiment to test the theory is proposed in which Ba+ (a heavy ion) is injected into the auroral ionosphere. The need for cross-sections for excitation and ionization by ions is evident to understand the physics of strong electric fields in the ionosphere and vice versa, field and photometric measurements in the auroral atmosphere may provide useful guide-lines in designing laboratory experiments. The application of strong (by natural standards) electric fields to the upper atmosphere from rockets or orbiting vehicles and study of the resultant glows should be of value in understanding not only aspects of the aurora but also of the electrodynamics of the thermosphere.

A numerical model of the ionospheric dynamo—I. Formulation and numerical technique

A new method of numerically solving a suitably formulated ionospheric wind dynamo equation for electrostatic potential and field is developed. Unlike in many other dynamo models, the upper boundary does not exist and the formulation asymptotically approaches the equatorial boundary condition. Therefore, it naturally incorporates the symmetric, asymmetric E- and F-region dynamo actions in any given ionosphere and any given global or local wind field. It also enables the equation to be posed as an initial value problem and solved numerically using an efficient, accurate, stable and fast integration method of ordinary differential equations. The numerical technique can be extended to compute three dimensional dynamo-generated electric currents in the ionosphere.

The geoelectric field at Davis station, Antarctica

Abstract An electric field mill is used to measure the vertical component of the geoelectric field at Davis station, Antarctica (68.6°S, 78.0°S, geographic coordinates; 74.6°S magnetic latitude). Local influences on the measurements are determined. Approximately a year of data is subjectively examined to determine periods when the ‘fair-weather’ electric field is expected to be dominant. Using a ‘cumulation of consecutive differences’ method, small intervals of data are combined to determine winter, spring and autumn diurnal ‘fair-weather’ electric field curves. A paucity of intervals not locally influenced precludes determination of a summer diurnal curve. The seasonal-diurnal curves each show a peak between 19 UT and 22 UT that is similar in temporal location and relative magnitude to the global, fair-weather, seasonal diurnal curves (see Reiter, 1992, p. 130). A local influence persists between 03 UT and 10 UT and precludes determination of a magnetospheric influence on the geoelectric field for these data.

A numerical model of the ionospheric dynamo—III electric current at equatorial and low latitudes

Abstract Using the general dynamo model and its special cases derived in a previous paper, the distributions of three dimensional electric current density in a magnetic meridional plane in the equatorial and low latitude ionosphere are computed. The winds generating the ionospheric dynamo are tide-like and locally periodic, similar to those in an internal gravity wave. Very large (several μA m−2) field-aligned current density is obtained in the equatorial region at places of sharp vertical gradients of the wind velocity. The currents generated by locally periodic winds of latitudinal wavelength less than several hundred kilometers do not significantly affect the normal equatorial electrojet.

Energy transfer by gravity wave dissipation

Abstract This paper reviews the subject of the dissipation of internal gravity waves in the thermosphere and shows how this is related to propagation. Differences of dissipation and heating rates in quiet and disturbed atmospheres are discussed, and the ranges of waves for different source heights in these atmospheres are calculated. Despite heavy damping of the waves, they may explain T.I.D.s and related airglow observations in middle and low latitudes.

Energetics of and a source of energy for equatorial spread-F events☆

Abstract It is shown that in spread- F events the consumption of energy may be comparable to that put into the atmosphere by solar EUV at the same altitude. Waves generated by the supersonic motion of the terminator with respect to atmospheric winds are suggested to be a significant source of energy for the events.