Photoelectron impact excitation of OI 8446 Å emission observed from Arecibo Observatory

Photoelectron (PE) impact on thermospheric oxygen atoms is a major source of OI 8446 Å emission excitation at midlatitudes. However, historical discrepancies between observed twilight emission brightnesses and PE model predictions have not only prompted speculation regarding secondary sources of excitation but also precluded the use of observed brightness as a much-needed diagnostic of thermospheric O density. In an effort to improve understanding of the physical mechanisms responsible for its excitation, this paper presents new photometric measurements of twilight OI 8446 Å emission brightness, together with calculations using the Field Line Interhemispheric Plasma (FLIP) PE model, acquired from Arecibo Observatory under geomagnetically quiet conditions during three winter campaigns from 1999-2002. Winter is a particularly favorable season for twilight 8446 Å observation in the northern hemisphere since significant PE production, and thus 8446 Å excitation, persists for several hours in the fully illuminated, geomagnetic conjilgate hemisphere in the absence of any local thermospheric illumination. The winter 8446 A brightness data are fully consistent with a dominant PE impact excitation source. However, the new data also confirm the Lancaster et al. (2000) report of excess early morning brightness at Arecibo with respect to PE models which use a tilted-dipole approximation to the geomagnetic field. By using the International Geomagnetic Reference Field (IGRF) geomagnetic field model to refine Arecibo's conjugate point location specified in the FLIP PE model, we demonstrate significantly improved agreement between the modeled and observed brightness decay profiles during both morning and evening twilight intervals.