W. Swift

Analysis of auroral morphology : Substorm precursor and onset on January 10, 1997

The solar wind interaction with the geomagnetic field is studied using global auroral images obtained by the Ultraviolet Imager (UVI) on Polar. We study the dynam,cs of the poleward and equatorward boundaries of the auroral oval in response to the solar wind IMF on January 10, 1997 using a neural network algorithm to perform an automated morphological analysis. Poleward and equatorward boundaries identified by the algorithm demonstrate a clear growth motion with the southward turning of the IMF and growth and poleward expansion at substorm onset. The area poleward of the oval (polar cap) is found to increase in size coincident with thesouthward turning of the IMF Bz component at 0220 UT and peaks at substorm onset at 0334 UT. The area of the oval, however, decreases continuously throughout the period of the polar cap area increase with a slight recovery observed during the substorm onset. These observations are consistent with the concept that magnetospheric dynamics are directly driven by the solar wind-geomagnetic field interactions.

Mesospheric nightglow spectral survey taken by the ISO Spectral Spatial Imager on ATLAS 1

This paper reports the first comprehensive spectral survey of the mesospheric airglow between 260 and 832 nm taken by the Imaging Spectrometric Observatory (ISO) on the ATLAS 1 mission. We select data taken in the spectral window between 275 and 300 nm to determine the variation with altitude of the Herzberg I bands originating from the vibrational levels v′ = 3 to 8. These data provide the first spatially resolved spectral measurements of the system. The data are used to demonstrate that to within an uncertainty of ± 10%, the vibrational distribution remains invariant with altitude. The deficit reported previously for the v′ = 5 level is not observed although there is a suggestion of depletion in v′ = 6. The data could be used to place tight constraints on the vibrational dependence of quenching rate coefficients, and on the abundance of atomic oxygen.

Sensitivity of the 6300 A twilight airglow to neutral composition

The field line interhemispheric plasma (FLIP) model is used to study the 6300 A line intensity measured during three morning twilights from the McDonald Observatory in Texas. The Imaging Spectrometric Observatory (ISO) measured the 6300 A intensity during the winter of 1987 and the spring and summer of 1988. The FLIP model reproduces the measured intensity and its variation through the twilight well on each day using neutral densities from the MSIS-86 empirical model. This is in spite of the fact that different component sources dominate the integrated volume emission rate on each of the days analyzed. The sensitivity of the intensity to neutral composition is computed by varying the N2, O2 and O densities in the FLIP model and comparing to the intensity computed with the unmodified MSIS-86 densities. The ion densities change self-consistently. Thus the change in neutral composition also changes the electron density. The F2 peak height is unchanged in the model runs for a given day. The intensity changes near 100° SZA are comparable to within 10% when either [O2], [N2] or [O] is changed, regardless of which component source is dominant. There is strong sensitivity to changes in [N2] when dissociative recombination is dominant, virtually no change in the nighttime (SZA ≥ 108°) intensity with [O2] doubled, and sensitivity of over 50% to doubling or halving [O] at night. When excitation by conjugate photoelectrons is the dominant nighttime component source, the relative intensity change with [O] doubled or halved is very small. This study shows the strong need for simultaneous measurements of electron density and of emissions proportional to photoelectron fluxes if the 6300 A twilight airglow is to be used to retrieve neutral densities.

The first negative bands of N2+ in the dayglow from the ATLAS-1 shuttle mission

During November–December 1983 the first spectral images of the N2+ first negative bands in the dayglow were measured by the Imaging Spectrometric Observatory on the Spacelab 1 shuttle mission. These data contained two unexpected characteristics: the intensities were considerably higher than current photochemistry predicts (by factors of 3–5), and the bands showed larger than expected populations in the higher vibrational levels. Both of these characteristics persisted throughout the mission and were independent of vehicle attitude. The spectra were imaged for the second time in the dayglow by the same instrument from the ATLAS-1 shuttle mission in March 1992, providing an opportunity to re-examine these issues. The vibrational distributions measured from ATLAS-1 are found to be rather similar to those measured from Spacelab 1, but with somewhat lower populations in levels, ν′= 2 and 3. The Spacelab 1 and the ATLAS-1 missions were the first opportunities to image the Δν = +1 progression at 3584 A, allowing the first determinations of the populations in levels higher than 2, which earlier studies were not able to address. The integrated column intensities of the 0–0 band emission at 3914 A as measured from ATLAS-1 are found to be in good agreement with model values, while the Spacelab 1 intensities appear to have been significantly affected by vehicle-induced effects. It would appear that the Spacelab 1 mission was highly anomalous in this regard, and the emission spectra from that mission are contaminated by non-ambient features, many of which are from the same species as are found in the thermosphere. The ion environment, in particular, appears to have been very perturbed on that mission. By contrast, the ATLAS-1 mission spectra appear to be very clean and do not show obvious evidence of vehicle effects. It appears that the absence of the large pressurized double-Spacelab module that occupied much of the payload bay for Spacelab 1, and the revised payload and orbiter processing procedures, have resulted in an environment that is substantially improved for remote sensing purposes. Studies of natural emissions, such as this one of the N2+ bands, are feasible and provide good data for modeling of the region.