Decameter mid-latitude sporadic-E irregularities in relation with gravity waves

Abstract. HF radar observations of mid-latitude spo- radic-E irregularities carried out with the Valensole radar in South France are compared with simultaneous ionosonde measurements underneath the irregularity zones. In a previous study of Valensole radar data, it has been shown that HF backscatter from the night-time mid-latitude E region is usually associated with large- scale wave-like modulations. To obtain more informa- tion on the geophysical conditions prevailing during backscatter events, a new experiment was performed which also included a vertical ionosonde beneath the scattering region. The data to be presented here are from two periods when radar scattering appeared simulta- neously with large variations in the virtual height and the Doppler velocity of F-layer re¯ected echoes mea- sured with the vertical ionosonde, indicating very clearly the passage of atmospheric gravity waves (AGWs). The e.ect of the atmospheric waves on the sporadic-E layer is not always as marked as it is in the F region. In the ®rst event, the passage of the AGWs is accompanied by an upward followed by a downward movement of the Es-layer. The apparent descending movement of the Es-layer from 135 to 110 km in less than 10 min corresponded to a positive (downward) Doppler velocity of 35 m/s measured by the vertical ionosonde, and was accompanied by a range variation in the radar scattering region with a negative rate of about 90±110 m/s. In the second event, the Es-layer is not as strongly disturbed as in the previous one, but, nevertheless, the range varia- tions of the scattering region can still be associated with height ¯uctuations of the Es-layer.