Solar cycle variations of thermospheric O/N₂ longitudinal pattern from TIMED/GUVI

Thermospheric composition (O/N₂ ratio) is well known to have a great impact on the variation of daytime ionospheric electron density. This study aims to investigate the local time, seasonal, and solar cycle variations of the O/N₂ longitudinal pattern in both hemispheres during daytime in solstices. The O/N₂ data used are from TIMED/Global Ultraviolet Imager observations made over a solar cycle for geomagnetically quiet conditions. The main findings are as follows: (1) The O/N₂ longitudinal patterns are generally similar during 10:00–14:00 LT and between solar minimum and maximum, although the O/N₂ values change with local time and solar cycle. (2) The winter O/N₂ subauroral enhancement is unexpectedly smaller in the longitudes where the magnetic pole is (near-pole longitudes), rather than in the longitudes far from the magnetic pole, especially during solar maximum, and consequently, the longitudinal pattern of O/N₂ depends on latitude in local winter. (3) The winter O/N₂ subauroral enhancement generally moves to more poleward latitudes during solar maximum, as compared to solar minimum. (4) At higher midlatitudes (~45°–60°N and ~40°–50°S in geographic latitudes) in solar minimum, the winter-to-summer ratio of O/N₂ in each hemisphere has an obvious minimum in near-pole longitudes. This minimum becomes more evident during solar maximum. The National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model simulations indicate that in the winter hemisphere, the unexpected O/N₂ longitudinal pattern in higher midlatitudes is mainly associated with high-latitude Joule heating under the impact from ion convection and auroral precipitation.