[1] GPS radio occultation measurements from CHAMP, GRACE-A and FORMOSAT-3/COSMIC are used to derive global information on small-scale ionospheric irregularities such as sporadic E layers between January 2002 and December 2007. The investigations are based on the analysis of amplitude variations of the GPS radio occultation signals. The global distribution of ionospheric irregularities shows strong seasonal variations with highest occurrence rates during summer in the middle latitudes. The long-term data set of CHAMP allows for first climatological studies, while the data coverage increases significantly with the combination of CHAMP, GRACE and FORMOSAT-3/ COSMIC measurements. This allows for global maps of sporadic E occurrence rates of very high spatial resolution where the influence of the Earth's magnetic field becomes visible in global sporadic E maps for the first time.

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Abstract. GPS (Global Positioning System) Radio occultation (RO) measurements from CHAMP, GRACE and FORMOSAT-3/COSMIC satellites at Northern Hemisphere midlatitides (50 • -55 • N) are analysed to obtain the diurnal variation of sporadic E layer occurrence frequency in 2006 and 2007. Interconnections with zonal wind shears measured by meteor radar at Collm (51.3 • N, 13 • E), Germany, are investigated. According to theory, maximum E s occurrence is expected when the zonal wind shear, which is mainly produced by the semidiurnal tide in midlatitudes, is negative. This is confirmed by the present measurements and analysis.

AB STRACTThe Tai wan/US FORMOSAT-3/COS MIC (FORMOsa SAT el lite mis sion -3/Con stel la tion Ob serv ing Sys tem for Me te o rol ogy, Ion o sphere and Cli mate) sat el lite con stel la tion was suc cess fully launched on 14 April 2006. It is ex pected to le ver age the use of the GPS (Global Po si tion ing Sys tem) ra dio occultation data for at mo spheric and ion o spheric re search to im prove global weather fore casts and aid cli mate change re lated stud ies. FORMOSAT-3/COS MIC, to gether with the Eu ro pean MetOp, Ger man CHAMP and US/Ger man GRACE-A sat el lites, form a 9 sat el lite con stel la tion for pre cise at mo spheric sound ing on a global scale. This sat el lite con stel la tion is expected to provide about 3500 occultation measurements daily.Re cent re sults and the sta tus of the CHAMP and GRACE-A or bit and occultation data anal y sis are re viewed and com ple mented with a re view of ini tial re sults from FORMOSAT-3/COS MIC at GFZ. The sig nif i cantly in creased po ten tial of the CHAMP, GRACE-A and FORMOSAT-3/COS MIC con stel la tion for at mo spheric stud ies, com pared to sin gle sat el lite mis sions, is dem on strated for se lected ap pli ca tions such as global mon i tor ing of wa ter va por dis tri bu tions, tropo pause parameters and ionospheric irregularities.

Metallic ion layers are commonly observed in the ionosphere. In particular, those which occur at E-region heights show high plasma concentrations, and they can cause anomalous high frequency (HF) radio propagation, known as Es. The characterization of the phenomenon and the formation mechanism for Es layers have been a subject of study for many decades (see, e.g., reviews by Haldoupis, 2011;Mathews, 1998;Whitehead, 1989). Observations of Es have been made using various instruments and techniques, including ionosondes (e.g., Taguchi, 1961), sounding rockets (e.g., Smith, 1966), incoherent scatter radars (e.g., Miller & Smith, 1978), lidars (e.g., Gardner et al., 1993, and radio occultation measurements (e.g., Hocke et al., 2001). It is now well established that Es layers occur most frequently at mid latitudes (20°-50° latitude) in the summer hemisphere at altitudes of 90-120 km.The formation of an Es layer requires the convergence of ion flux. The wind shear theory (Axford, 1963;Whitehead, 1961) suggests that the required ion flux convergence can be realized by the vertical shear of horizontal neutral winds. The following simple expression can be obtained for the vertical component of the ion velocity, under the assumption of balance between the Lorentz force due to ion motions across the ambient geomagnetic field and the frictional force due to ion-neutral collisions (e.g., Haldoupis, 2011):