Solar cycle variations of thermospheric composition at the solstices

Abstract

We examine the solar cycle variability of thermospheric composition (O/N2) at the solstices. Our observational and modeling studies show that the summer‐to‐winter latitudinal gradient of O/N2 is small at solar minimum but large at solar maximum; O/N2 is larger at solar maximum than at solar minimum on a global‐mean basis; there is a seasonal asymmetry in the solar cycle variability of O/N2, with large solar cycle variations in the winter hemisphere and small solar cycle variations in the summer hemisphere. Model analysis reveals that vertical winds decrease the temperature‐driven solar cycle variability in the vertical gradient of O/N2 in the summer hemisphere but increase it in the winter hemisphere; consequently, the vertical gradient of O/N2 does not change much in the summer hemisphere over a solar cycle, but it increases greatly from solar minimum to solar maximum in the winter hemisphere; this seasonal asymmetry in the solar cycle variability in the vertical gradient of O/N2 causes a seasonal asymmetry in the vertical advection of O/N2, with small solar cycle variability in the summer hemisphere and large variability in the winter hemisphere, which in turn drives the observed seasonal asymmetry in the solar cycle variability of O/N2. Since the equatorial ionization anomaly suppresses upwelling in the summer hemisphere and strengthens downwelling in the winter hemisphere through plasma‐neutral collisional heating and ion drag, locations and relative magnitudes of the equatorial ionization anomaly crests and their solar cycle variabilities can significantly impact the summer‐to‐winter gradients of O/N2 and their solar cycle variability.

Longitudinal variations of thermospheric composition at the solstices

“…This study concludes our investigations on thermospheric composition at the solstices pertaining to mechanisms for its summer‐to‐winter latitudinal gradient and its annual asymmetry [ Qian et al , ], the solar cycle variability [ Qian et al , ], and the longitudinal variation. Applying this understanding to the ionosphere, we expect that the ionosphere winter anomaly should occur most often and with the strongest intensity in the longitude sector when the magnetic pole is in the winter hemisphere, under December solstice conditions, i.e., the west longitudes in the northern hemisphere winter, at solar maximum.…”

“…This hemispheric asymmetry in the winter enhancement of O/N 2 is consistent with the prevalence of the winter maximum of midday N m F 2 in the northern hemisphere described in Torr and Torr [1973]. QIAN In addition, we examined solar cycle variability of O/N 2 at the solstices and its driving mechanisms [Qian et al, 2016b]. Our observational and modeling studies show that solar cycle variations of O/N 2 are large in the winter hemisphere but small in the summer hemisphere; the summer-to-winter latitudinal gradient of O/N 2 is small at solar minimum but large at solar maximum.…”

“…In addition, we examined solar cycle variability of O/N 2 at the solstices and its driving mechanisms [ Qian et al , ]. Our observational and modeling studies show that solar cycle variations of O/N 2 are large in the winter hemisphere but small in the summer hemisphere; the summer‐to‐winter latitudinal gradient of O/N 2 is small at solar minimum but large at solar maximum.…”

“…As we did in our companion studies [ Qian et al , , ], we utilize recent global O/N 2 data measured by the Global Ultraviolet Imager on board the Thermosphere Ionosphere Mesosphere Energetics Dynamics satellite (TIMED/GUVI, version 08), combined with modeling studies using the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (NCAR/TIE‐GCM) to describe these longitudinal changes in O/N 2 and to determine their causes. Descriptions of the TIMED/GUVI and NCAR/TIE‐GCM are given in Qian et al [] and Qian et al [].…”

Impact of the lower thermospheric winter‐to‐summer residual circulation on thermospheric composition

“…They ran the model with temporal constant eddy diffusion versus the annual/semiannual varying eddy diffusion of Qian et al [2009]. Qian et al [2016b] investigated solar cycle variations of thermospheric composition at the solstices using the TIE-GCM and compared the simulated O/N 2 with the O/N 2 observed by TIMED/GUVI. The simulated NmF 2 using the annual/semiannual varying eddy diffusion significantly improved the model-data agreement in NmF 2 at low latitudes (± 30 o magnetic latitudes); however, the varying eddy diffusion did not solve the discrepancies between the model and data at mid-to mid-high latitudes.…”