Showing papers by "Arthur D. Richmond published in 1979"

Large‐amplitude gravity wave energy production and dissipation in the thermosphere

Abstract: This paper extends some of the analysis of gravity wave generation and dissipation in the thermosphere given in a previous paper [Richmond, 1978] by defining wave energy for gravity waves of arbitrary amplitude and by analyzing several numerical simulations. Wave energy as defined in this paper is closely related to the sum of kinetic energy and available potential energy of the atmosphere. In the absence of production or dissipation mechanisms, wave energy is approximately, but not strictly, conserved. Its dissipation is related to, but in general is not the same as, entropy generation. Numerical simulations substantiate the findings of Richmond (1978) that both the Lorentz force and the Joule heating of auroral currents can generate substantial gravity wave energy but that the waves produced by Joule heating are more capable of traveling to middle and low latitudes. The simulations also reveal that molecular dissipation of wave energy can be relatively less important, in comparison to daytime Joule dissipation, than the simple treatment of Richmond [1978] indicated.

Ionospheric Wind Dynamo Theory: A Review

Abstract: The current state of ionospheric wind dynamo theory is reviewed. Observational and theoretical advances in recent years have permitted more accurate models of the dynamo mechanism to be presented than previously, which have lent further credence to the validity of dynamo theory as the main explanation for quiet-day ionospheric electric 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 responsible for average quiet-day currents, although other wind components give significant contributions. Observationally, there is a need for better spatial and temporal coverage of wind and electric field data. Theoretically, there is a need for further consideration of the mutual dynamic coupling among winds, conductivities, electric fields, and electric currents, and for better modeling of nighttime conditions.

The Quiet-Time Equatorial Electrojet and Counter-Electrojet

Abstract: Using ground magnetometer data from several stations near the dip equator, the magnetically quiet-time characteristics of the daytime normal electrojet and the morning and afternoon counter-electrojets are established and illustrated. In particular, the solar cycle, seasonal, longitudinal and day-to-day variations of counter-electrojets as well as their dependence on lunar phase are studied. In order to investigate the influence of solar tidal winds in the production of counter-electrojets, the equatorial electric fields generated by such winds through the dynamo mechanism are calculated, using a numerical dynamo simulation model developed earlier at UCLA. From these calculations it is found that westward electric fields responsible for the afternoon counter-electrojets can be produced in the local summer solstice (when these events are most common) by a combination of the solar semidiurnal (2, 2) tide and the diurnal (1, -2) tide. An example from January 1964 is discussed to illustrate that afternoon counter-electrojet events occurring at all longitudes on the same day are likely to be associated with abnormalities in the global Sq or SR current system. It is suggested that the lunar semi-diurnal tide is a more important causative agent for the morning rather than the afternoon counter-electrojets.

Thermospheric heating in a magnetic storm: Dynamic transport of energy from high to low latitudes

Abstract: During a magnetic storm, energy can be transferred from high to low latitudes in the thermosphere by two dynamical processes: gravity waves and meridional circulation. This paper evaluates the relative importance of these two mechanisms with the aid of a numerical simulation. The appropriate auroral region conditions to be used as input parameters for such a simulation are a matter of considerable uncertainty, but even when these are chosen so as to be relatively favorable to gravity wave energy transfer, meridional circulation is found to be more important in heating the low-latitude thermosphere.

Magnetic Substorm Characteristics Described by Magnetic Potential Maps for 26 – 28 March 1976

Abstract: Magnetograms of ground-based geomagnetic variation are a useful, yet cumbersome data source for substorm studies. In this study geomagnetic variations from 52 northern hemisphere observatories are used to create magnetic potential contour plots representing so called “equivalent ionospheric current” systems. Contour plots of 10 isolated and storm-time substorms for 26 – 28 March 1976 demonstrate the dynamic behavior of the westward and eastward electrojets in the auroral zone. Substorms may be ordered by the preferred longitude of the dominant westward electrojet during their main phase and fall into three categories, morning, evening and midnight, which may occur simultaneously or independently. During recovery of these ten substorms the westward electrojet is always in the morning sector.