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

Abstract Thermospheric composition (O/N 2 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/N 2 longitudinal pattern in both hemispheres during daytime in solstices. The O/N 2 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/N 2 longitudinal patterns are generally similar during 10:00–14:00 LT and between solar minimum and maximum, although the O/N 2 values change with local time and solar cycle. (2) The winter O/N 2 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/N 2 depends on latitude in local winter. (3) The winter O/N 2 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/N 2 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/N 2 longitudinal pattern in higher midlatitudes is mainly associated with high‐latitude Joule heating under the impact from ion convection and auroral precipitation.