Physics
Scientific paper
Feb 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000jgr...105.2461s&link_type=abstract
Journal of Geophysical Research, Volume 105, Issue A2, p. 2461-2476
Physics
5
Atmospheric Composition And Structure: Airglow And Aurora, Atmospheric Composition And Structure: Thermosphere-Composition And Chemistry, Atmospheric Composition And Structure: Thermosphere-Energy Deposition
Scientific paper
An analysis is reported of near zenith viewing brightness observations of O I 844.6 nm, N2+ 1NG 391.4 nm, N2 2PG 337.1 nm, O I 555.7 nm, and O I 630.0 nm recorded by photometers onboard the ARIA II rocket launched from Poker Flat, Alaska, on February 12, 1994. The rocket passed through diffuse aurora mixed with intermittent soft precipitation on the upleg followed by much more structured precipitation on the downleg. The most meaningful analysis of the data comes from the upleg portion of the flight and is reported here. The primary objective of the investigation was to quantify possible departures of the density profiles of N2 and O from their representations within the MSIS model. To do so requires knowledge of the electron precipitation responsible for the observed emissions. This was obtained by fitting electron transport results to altitude profiles of in situ electron spectrometer data and N2+ 391.4 nm photometer data. Good agreement with profile shapes was achieved with precipitation characterized by a Maxwellian distribution having a characteristic energy E0 of 1.75 keV. The strength of the precipitation was determined by the brightness of the N2+ 391.4 nm profile which yielded an energy flux Q of 3.08 ergs cm-2 s-1. Simultaneous good fits to the low-altitude N2+ 391.4 nm and O I 844.6 nm data required a scaling of the MSIS-based O density profile by a factor of 0.45. This factor amounts to a significant reduction in O and is close to that obtained from an analysis of coincident ground-based optical data [Christensen et al., 1997]. Perhaps more significant is a clear sign of an abrupt composition change as the rocket approached apogee based on the observed brightness ratios of 844.6/391.4 and 557.7/391.4. Both exhibit the same gross structure as well as fine structure arising from intermittent soft precipitation. Their ratios are observed to drop by more than a factor of 2 along a portion of the trajectory as the rocket passed above 218 km. Upon achieving such heights, a close relationship exists between these ratios and the ratio of O to N2 column densities (O/N2). Full consideration of the relationships between the observed brightness of O I 844.6 nm, O I 557.7 nm, and N2+ 391.4 nm leads to the conclusion that the rocket entered a region of enhanced N2 density with continued depletion of the O density similar to what was observed prior to entering this region. Relative to MSIS, the N2 densities are at least a factor of 2 higher while the O densities are at least a factor of 2 smaller leading to a reduction in MSIS-based O/N2 of more than a factor of 4.
Christensen Andrew B.
Hecht James H.
McEwen Don J.
Strickland Douglas J.
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