Showing papers by "Arthur D. Richmond published in 1999"

Lunar tides in the Thermosphere-Ionosphere-Electrodynamics General Circulation model

Abstract: Lunar semidiurnal tides are introduced at the lower boundary of the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM). The tides are derived from the model of Vial and Forbes [1994] and interesting properties of these tides are found when they are subjected to Hough decomposition; there is considerable hemispherical antisymmetry in the September tides, and the March and September modal compositions are significantly different. A differencing method is used to isolate the lunar tidal effects in the TIEGCM, and these are compared with lunar tidal analyses of ionospheric data. The model reproduces the broad features of the lunar tide in f0F2 (maximum frequency of the F region) with phase changes around 7° magnetic dip latitude during daytime. The model and data analysis both give variations of the amplitude and phase of the lunar tide with local time. Near the equator the variation of phase with local time changes with latitude as the equatorial anomaly develops during the day. Comparison between the model predictions and analyses of data at observatories at midlatitudes produces mixed results. Here the effects of the lunar components of both electrodynamic drifts and of neutral winds need to be taken into account. Several cases of day to night changes in the phase of the lunar tide in f0F2 are noted. Large nighttime amplitudes of the lunar tide in hmF2 (height of the maximum density), more than 4 km, seem to be due to inphase action of the electrodynamic and neutral wind effects while during daytime they are out of phase. The lunar tide in the ratio of oxygen to nitrogen density [O]/[N2]is estimated and found to be of relatively minor importance. Amplitudes of the lunar tide in f0F2 may be measured at more than 0.4 MHz at some local times, but the model values are less than this. Comparison is also made with ion drift measurements made by the San Marco D satellite. The several uncertainties which underlie this work are discussed in detail.

Magnetic mirroring in an incident proton beam

Abstract: We point out that the influence of magnetic-field nonuniformity on redirecting the pitch angle of a particle is independent of the particle's charge and thus is identical for protons and neutral hydrogen atoms Under certain circumstances one can then speak of “magnetic mirroring” of hydrogen atoms as well as of protons In the case of an energetic proton beam incident on the upper atmosphere, the study of the influence of magnetic field on both protons and H atoms can be relevant to inferring information about proton aurora from measurements of upgoing energetic particles observed from space In a model that here neglects collisional angular redistribution of the particles, the total particle and energy albedos are approximately independent of the energy of the incident particles and of the atmospheric temperature However, the separate proton and H atom albedos have a strong dependence on the incident energy We also reinvestigate how to handle energy conservation properly in the presence of a nonuniform magnetic field, to provide a good validation for proton transport models