Showing papers by "Arthur D. Richmond published in 1978"

Gravity wave generation, propagation, and dissipation in the thermosphere

Abstract: This is the first in a series of papers examining large-scale gravity waves in the thermosphere and their ability to transfer energy from high to low latitudes during magnetic disturbances. The gravity wave source is assumed to be either the Lorentz force of auroral electrojet currents or else a heat input due to energetic particle precipitation or to Joule heating. It is pointed out that the characteristic vertical width of the gravity wave source should usually lie between 2 and 4 pressure scale heights, placing constraints on the vertical wavelengths and horizontal velocities of the generated waves. A simplified analytic model of small-amplitude wave generation by a current source shows how wave energy production depends on the temporal and spatial dimensions of the source, on the electric field strength, and on the electron density enhancement. The steep thermospheric temperature gradient in the vicinity of the source altitude strongly influences the properties of upward and downward propagating waves compared with waves generated in an isothermal atmosphere. Waves produced by the Lorentz force of Hall currents, by the Lorentz force of Pedersen currents, and by Joule heating are influenced quite differently by this temperature gradient. Because upgoing waves above the source are combinations of waves originally launched upward and waves originally launched downward but reflected around 110 km altitude, the mean effective source altitude is about 110 km for the far field response in the thermosphere. Large-scale traveling ionospheric disturbances observable at middle latitudes are most likely produced primarily by Pedersen, rather than Hall, currents. The temperature structure of the thermosphere generally causes gravity wave packets to refract upward; waves traveling with a horizontal component of velocity faster than 250 m/s and with an initial downward component of group velocity will always be reflected upward in the lower thermosphere. The effects of viscosity, heat conduction, and Joule dissipation tend to filter out shorter-period and slower moving waves from observation points at some distance from the source, so that only long-period fast moving waves can reach low latitudes from an auroral source. For example, a wave with a 94-min period moving horizontally at 605 m/s is largely dissipated by the time it has traveled 4000 km from a typical auroral source. A numerical simulation using a fairly realistic thermospheric model illustrates many of the points described from analytic considerations.

Ionospheric effects of the gravity wave launched by the September 18, 1974, sudden commencement

Abstract: A sudden commencement of a geomagnetic storm occurred at 1434 UT on September 18, 1974. A traveling ionospheric disturbance was detected about an hour later by the incoherent scatter radar at Millstone Hill, Massachusetts (42.6°N, 71.5°W), and still later by the incoherent scatter radar at Arecibo, Puerto Rico (18.3°N, 66.7°W). Measurements of the vertical distribution of electron temperature, ion temperature, and electron density made during the passage of the disturbance at both stations showed significant perturbations in comparison to the geomagnetic quiet time values obtained on the previous day. We calculated characteristics of the thermospheric gravity wave from a time-dependent dynamic model of the thermosphere. The wave source is assumed to be enhanced electric currents and particle heating over the polar cap region. The gravity wave generated by the impulsive heating propagates equatorward, and its characteristic structure changes as it progresses to lower latitudes. The calculated neutral temperature and wind perturbations are added to the quiet time variations in a diurnal model of the mid-latitude ionospheric F region to calculate the time-dependent properties of the ionosphere during the passage of the wave over both incoherent scatter radar stations. By requiring agreement between the calculated and observed ionospheric structure we estimated the magnitude of the gravity wave energy source. For the event on September 18, 1974, we obtained reasonably good agreement between the calculated and observed ionospheric structures for a total energy input of about 2 × 1015 J, occurring over a period of 3 hours.

The nature of gravity wave ducting in the thermosphere

Abstract: It has been widely held that the steep temperature gradient in the lower thermosphere can support one or more surface modes of long-period gravity wave propagation, thereby acting as a ducting mechanism. This paper shows that such a ducting mechanism in fact does not operate in the earth's thermosphere. It is shown that the thermospheric gravity modes computed by Francis [1973] can instead be explained by internal waves undergoing total reflections in the lower thermosphere and weak partial reflections in the upper thermosphere due to viscosity and heat conduction effects. This ducting mechanism is, however, so weak that the usefulness of the concept of ducted thermospheric gravity modes appears to be limited. An alternative concept of freely propagating waves appears to have more practical applications.

Low‐latitude E region ionization by energetic ring current particles

Abstract: Energetic neutral particles resulting from the charge exchange of ring current ions with geocoronal hydrogen are known to strike the atmosphere at low latitudes. We have evaluated the ionization rates caused by these particles in the low-latitude upper E region ionosphere. We find that these particles are an important quiet time source of E region ionization at night and that they can account for the observed increases in nighttime ionization that correlate with geomagnetic activity. This ionization has previously been attributed to 1- to 10-keV electron precipitation. However, sufficiently intense precipitating electron fluxes have not been measured at low latitudes, and we know of no source of such electrons.