Showing papers by "Arthur D. Richmond published in 1996"

Assimilative mapping of ionospheric electrodynamics in the thermosphere‐ionosphere general circulation model comparisons with global ionospheric and thermospheric observations during the GEM/SUNDIAL period of March 28–29, 1992

Abstract: Satellite and ground-based observations from March 28 to 29, 1992, were combined in the assimilative mapping of ionospheric electrodynamics (AMIE) procedure to derive realistic global distributions of the auroral precipitation and ionospheric convection which were used as inputs to the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere general circulation model (TIGCM). Comparisons of neutral model winds were made with Fabry-Perot measurements and meridional winds derived from ionosondes. The peak equatorward winds occurred 1–2 hours later in the model. Gravity waves launched from high-latitude Joule heating sources reached the equator in about 2 hours and agreed with observed variations in the height of the maximum electron density (hmF2) and in the meridional winds. Joule heating events produced minima in the O/N2 ratio that moved equatorward and usually westward in longitudinal strips which lasted about a day. Changes in the O/N2 ratio and in the peak electron density (NmF2) were strongly correlated so the observed daytime NmF2 values for stations near 50° magnetic latitude were generally reproduced by AMIE-TIGCM on the second day of the simulation. The AMIE-TIGCM underestimated the electron density after midnight by up to a factor of 2 in midlatitudes, while the modeled F2 layer was about 35 km lower than the observations at midnight. Shifting the model winds 2 hours earlier at night could double the NmF2 at 0400 LT and increase hmF2 by 20 km. NmF2 could also be increased at night by realistically increasing the TIGCM nighttime downward fluxes of O+ at the upper boundary.

Ionospheric drift similarities at magnetic conjugate and nonconjugate locations

Abstract: Suggestions in the literature indicate that the observed similarity in ionospheric drifts between Arecibo-summer and Shigaraki-winter and again between Arecibo-winter and Shigaraki-summer are due to conjugate effects owing to the circumstance that the Shigaraki magnetic conjugate point has the same latitude as does Arecibo (in the opposite hemisphere). Here we develop this theory further from one of association to one of cause and effect. We base our explanation on the control of drifts at conjugate locations resting with the location of higher electrical conductance and suggest that this usually results in practice in summer hemisphere control of the winter hemisphere at night, with the daytime case being more complicated. Consequences of our theory are that the Arecibo-summer Shigaraki-winter similarity is, indeed, due to conjugate effects but that the Arecibo-winter Shigaraki-summer similarity has little to do with conjugacy. We combine empirical models of thermosphere and ionosphere structure and dynamics to test our conceptual ideas. We make predictions for such reverse-season similarities for other pairs of incoherent scatter radar locations.

Relationship of the ionospheric convection reversal to the hard auroral precipitation boundary

Abstract: Plasma drift and particle measurements from the DMSP spacecraft for the three Geospace Environmental Modeling (GEM) campaign periods (January 27–29, March 28–29, and July 20–21, 1992) have been used to study the relationship between the plasma convection reversal and the poleward boundary of the diffuse auroral zone characterized by hard electron precipitation with energies greater than 0.45 keV. This boundary, named the hard auroral precipitation boundary (or the HAP boundary) in this paper, is often regarded as the ionospheric footprint of the boundary between the central plasma sheet (CPS) and the boundary plasma sheet (BPS). By examining simultaneous ion drift and particle measurements from about 500 satellite passes we find that the large-scale plasma flow in the morning and evening sectors changes its direction within the auroral oval at the HAP boundary. However, exceptions are found in the early morning sector between 0300 and 0600 MLT, where the convection reversal is sometimes (in 30% of the DMSP crossings) displaced poleward relative to the HAP boundary. It is shown that the shape of the region bordered by the HAP boundary can be roughly represented by a circle, whose size is influenced by the IMF Bz component. There is roughly a linear correlation between the diameter of this circle and the cross-polar-cap potential drop, with the best correlation coefficient of 0.65 for winter season. Our study suggests that the HAP boundary corresponds to the magnetospheric boundary between the quasi-dipolar region and the region with more stretched field lines, and the source of the region 1 field-aligned current is located near the HAP boundary. A By-dependent shift of the HAP boundary with respect to the noon-midnight meridian is also found. In the northern hemisphere, the shift is dawnward for positive By and duskward for negative By in the southern hemisphere, the shift is opposite to that in the northern hemisphere.

Space weather research prompts study of ionosphere and upper atmospheric electrodynamics

Abstract: Because our society is becoming increasingly dependent on technological systems that can be affected by ionospheric phenomena during geomagnetic storms, the ionosphere, its electrodynamics, and its coupling with the neutral atmosphere and the magnetosphere are being studied as part of a coordinated program of “space weather” research. This research seeks to characterize the variability of ionospheric density and electric currents during magnetic storms, and to determine to what extent valid predictions of those phenomena and their effects can be made.