Engineering Papers⌕ Search

Engineering topics

Moffett, R. J.

Publications and source records attributed to Moffett, R. J..

14 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Response of the thermosphere and ionosphere to geomagnetic storms

Four numerical simulations have been performed, at equinox, using a coupled thermosphere-ionosphere model, to illustrate the response of the upper atmosphere to geomagnetic storms. The storms are characterized by an increase in magnetospheric energy input at high latitude for a 12-hour period; each storm commences at a different universal time (UT). The initial response at high latitude is that Joule heating raises the temperature of the upper thermosphere and ion drag drives high-velocity neutral winds. The heat source drives a global wind surge, from both polar regions, which propagates to low latitudes and into the opposite hemisphere. The surge has the character of a large-scale gravity wave with a phase speed of about 600 m/s. Behind the surge a global circulation of magnitude 100 m/s is established at middle latitudes, indicating that the wave and the onset of global circulation are manifestations of the same phenomena. A dominant feature of the response is the penetration of the surge into the opposite hemisphere where it drives poleward winds for a few hours. The global wind surge has a preference for the night sector and for the longitude of the magnetic pole and therefore depends on the UT start time of the storm. A second phase of the meridional circulation develops after the wave interaction but is also restricted, in this case by the buildup of zonal winds via the Coriolis interaction. Conservation of angular momentum may limit the buildup of zonal wind in extreme cases. The divergent wind field drives upwelling and composition change on both height and pressure surfaces. The composition bulge responds to both the background and the storm-induced horizontal winds; it does not simply rotate with Earth. During the storm the disturbance wind modulates the location of the bulge; during the recovery the background winds induce a diurnal variation in its position. Equatorward winds in sunlight produce positive ionospheric changes during the main driving phase of the storm. Negative ionospheric phases are caused by increases of molecular nitrogen in regions of sunlight, the strength of which depends on longitude and the local time of the sector during the storm input. Regions of positive phase in the ionosphere persist in the recovery period due to decreases in mean molecular mass in regions of previous downwelling. Ion density changes, expressed as a ratio of disturbed to quiet values, exhibit a diurnal variation that is driven by the location of the composition bulge; this variation explains the ac component of the local time variation of the observed negative storm phase.

Fuller-Rowell, T. J.↗
NASA NTRS1992

The temporal evolution of the ionospheric signatures of subauroral ion drifts

The effects of an imposed westward plasma drift on O(+) and molecular ion behavior in the nightside ionosphere are investigated using a model of the ionosphere and plasmasphere. A closed subauroral tube of the plasma is considered, and the velocity input persists for 30 min. The rapid increase in the F-region ion temperature resulting from ion-neutral frictional heating causes an immediate surge in the O(+) field-aligned velocity, upwards in the topside ionosphere and downwards below the F2-peak, but after about 10 min into the event the surge in the topside disappears. After the event there is a return flow of O(+) from the plasmasphere. The relative abundance of O(+) decreases during the event due to the increased rate of conversion of O(+) into NO(+) and O2(+); the decrease is more marked for greater values of the imposed westward ion drift. The implications of these results for satellite observations of subauroral ion drifts events and on EISCAT incoherent scatter radar observations of ion heating events is discussed.

Moffett, R. J.↗
NASA NTRS1992

Comparison of the USU ionospheric model with the UCL-Sheffield coupled thermospheric-ionospheric model

Several physical models of the high-latitude ionosphere have been developed that describe the time-dependent evolution of the E- and F-region plasma density. The models require a variety of inputs, including solar EUV fluxes, magnetospheric convection, auroral precipitation, and neutral atmosphere. Of specific relevance to this study is how the neutral atmosphere is incorporated into the ionospheric models. For the USU ionospheric model, the neutral atmosphere is the MSIS 1986 empirical model, while for the UCL-Sheffield coupled thermospheric-ionospheric model the neutral atmosphere is computed simultaneously with the ionosphere. Both models were run for similar solar and magnetospheric conditions (solar maximum, moderate geomagnetic activity, and winter solstice). Solar maximum conditions ensured a strong coupling between the ionosphere and thermosphere, which provided the possibility of a large ionospheric difference between the two physical models. This was further enhanced by choosing winter conditions so that the densities were not dominated by sunlight. The comparison of the two models indicated that both models predict the same morphological features with similar ionospheric densities, generally within about 30 percent.

Sojka, J. J.↗
NASA NTRS1991

Effects of large zonal plasma drifts on the subauroral ionosphere

A model of the earth's ionosphere and plasmasphere is used to investigate the effects of an imposed westward plasma drift of maximum velocity 2 km/s. A closed subauroral tube of plasma is considered and the velocity spike persists for 10 min. Ion-neutral frictional heating causes rapid elevation of the F-region O(+) temperature. The F-layer O(+) concentration is decreased due to increased O(+) loss rate and rapid ion flows both upward and downward from the F-region. The upward flux of O(+) through the topside ionosphere can each 5 x 10 exp 9/sq cm/s; when the velocity spike ceases there is a return flow of O(+) that tends to replenish the F-layer. Most of the features revealed by the model for the F-region and topside ionosphere are in accord with observations of subauroral ion drifts. Downward flows that are predicted to be persistently present around the 300 km altitude level appear to agree with observations only occasionally; suggestions are made to resolve this discrepancy.

Sellek, R.↗
NASA NTRS1984

The effect of realistic conductivities on the high-latitude neutral thermospheric circulation

The dynamics of the high latitude thermosphere are dominated by the ion circulation pattern driven by magnetospheric convection. The reaction of the neutral thermosphere is influenced by both the magnitude of the ion convection velocity and by the conductivity of the thermosphere. Using a three-dimensional, time-dependent, thermospheric, neutral model together with different ionospheric models, the effect of changes in conductivity can be assessed. The ion density is described by two models: the first is the empirical model of Chiu (1975) appropriate for very quiet geomagnetic conditions, and the second is a modified version of the theoretical model of Quegan et al. (1982). The differences in the neutral circulation resulting from the use of these two ionospheric models emphasizes the need for realistic high latitude conductivities when attempting to model average or disturbed geomagnetic conditions, and a requirement that models should couple realistically the ionosphere and the neutral thermosphere. An attempt is made to qualitatively interpret some of the features of the neutral circulation produced at high latitudes by magnetospheric processes.

Fuller-Rowell, T. J.↗
NASA NTRS1973

Effects of interhemisphere transport on plasma temperatures at low latitudes.

The thermal balance of the equatorial plasma between 300 and 800 km is examined. Steady state nighttime calculations are made for O+, H+, and electrons. The following features are included: collisional heat transfer between ions, electrons, and neutrals; ion and electron thermal conduction along the field lines; curvature of the field lines; nonlinear advection due to field-aligned ion and electron motions; and convective compression or expansion due to field-aligned and E x B motions. The ion velocities necessary to calculate the effects of convection are obtained from the work of Moffett and Hanson, who include a meridional wind across the magnetic equator in their calculations. It is shown that field-aligned interhemisphere plasma flows appreciably affect the plasma temperatures.

Bailey, G. J.↗
NASA NTRS1973

Calculated distributions of hydrogen and helium ions in the low-latitude ionosphere.

The simultaneous time-dependent continuity equations for O(+), H(+) and He(+) in the low latitude F-region are solved. Account is taken of E x B drift, a meridional neutral wind, and ion-ion and ion-neutral drag. The calculated profiles of O(+) and H(+) concentrations at 1630 LT are in fair agreement with the observations of Hanson et al. The He(+) field-aligned velocity is almost matched to the O(+) field-aligned velocity and, above the chemical equilibrium region and around the He(+) peak, the He(+) concentration is determined largely by production and transport. There is disagreement between the theoretical vertical He(+) profile and the profile observed by Hanson et al. Satisfactory agreement is obtained with Taylor's satellite results at fixed height for O(+) and H(+). It is found that the He(+) concentration is greater in the winter hemisphere than in the summer hemisphere, even if the neutral helium distribution is symmetrical about the Equator. The He(+) results are consistent with Taylor's results.

Moffett, R. J.↗
NASA NTRS1973

Ogo 6 measurements of supercooled plasma in the equatorial exosphere.

Plasma measurements performed on Ogo 6 reveal electron and ion temperature values that on occasion appear to be well below the expected neutral gas temperature. The phenomenon is observed only at night above 500 km near the magnetic equator. It is suggested that the expansion cooling of the plasma is accomplished by downward motions of the F region plasma induced by winds or diffusion, by outward E x B drift, or more probably by the upward flow of plasma during interhemisphere transport along magnetic field lines. It is also suggested that preferential cooling of electrons because of their greater thermal conductivity should tend to give ion temperatures greater than electron temperatures during the postsunset cooling of the plasmasphere when photoelectrons are absent.

Hanson, W. B.↗