Photochemistry and dynamics in thermospheric intertropical arcs measured by the WIND Imaging Interferometer on board UARS: A comparison with TIE-GCM simulations

Abstract

The intertropical arcs are bands of enhanced nightglow O(1D) 630nm emission observed at 15° north or south of the magnetic dip equator. Ground-based measurements and measurements from early satellites established their main properties, including a seasonal effect that locates the more prominent of the arc structures on the winter side of the geomagnetic equator. Theoretical considerations show that asymmetry of the arc system is caused by transequatorial neutral winds pushing ionization along the magnetic field lines upward and downward into the recipient hemisphere, where recombination and therefore nightglow emission is then enhanced. Seasonal variations of arc intensity and location are a direct consequence of the seasonal variation of the projection of the meridional winds along the magnetic field lines on both large and small scales.The WIND Imaging Interferometer on UARS offers the first opportunity to measure the wind and the nightglow enhancement simultaneously across the magnetic equator, and its measurements confirm these earlier understandings, in particular the longitudinal fine structures shown by the Orbital Geophysical Observatory 4 and 6. Furthermore, such observations are also consistent with simulation of emission rate and wind generated by the thermosphere–ionosphere electrodynamics general circulation model. This verification lends support to equatorial spread-F theories that rely upon transequatorial winds for the suppression of the Rayleigh–Taylor instability.