Meridional winds derived from COSMIC radio occultation measurements

Meridional winds at the magnetic meridian in the upper thermosphere are derived from the peak height and density of the ionospheric F-2 layer as retrieved by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites for 4 months from November 2006 to February 2007. These winds (referred to as COSMIC winds) are first validated by comparison at multiple locations with winds obtained from ground-based incoherent scatter radar (ISR) and Fabry-Perot interferometer (FPI) measurements. Then longitudinal variations of these winds are investigated and compared with simulations by the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (NCAR TIE-GCM). The results show generally good agreement between COSMIC winds, ground-based measurements, and the simulations. Significant longitudinal variations are presented in the COSMIC winds. At 40 degrees N (local winter), around midnight the COSMIC winds exhibit stronger and longer-duration equatorward velocities within 110 degrees W-20 degrees W (large negative magnetic declination sector) than those in other longitudes; during late morning hours the poleward winds show similar longitudinal variations. At 40 degrees S (local summer), during the daytime the poleward winds are stronger within 120 degrees E-110 degrees W (large positive magnetic declination sector) than in 60 degrees W-90 degrees E (large negative magnetic declination sector), while during the nighttime the maximum equatorward winds shifts about 2 h later in 60 degrees W-90 degrees E than in 120 degrees E-110 degrees W. Analysis of the TIE-GCM simulations suggests that the longitudinal variation of meridional winds is mainly induced by magnetic declination due to the contribution of geographic zonal wind.