Meridional electric fields in layer-type and clump-type plasma structures in midlatitude sporadic e region: Observations and plausible mechanisms

The use of multibased line interferometry technique operated by a VHF coherent scatter radar with relatively broad radar beam pattern makes it possible to reveal zonal drift velocity of 3 m field-aligned irregularities (FAIs) inside moving layer-type or clump-type plasma structures at kilometer scale. We present compelling evidences in this article showing that, no matter what types of the plasma structures, the apparent zonal drift velocity of the 3 m FAIs is always linearly dependent on the zonal trace velocity (or bulk velocity) of the kilometer-scale plasma structure. This feature seems to imply that the bulk motion of the large-scale plasma structure may significantly affect the observed 3 m FAI Doppler velocity, which should be removed to obtain true FAI drift velocity. In addition, there is an evident tendency for the 3 m FAIs in the layer-type plasma structure to drift in the same zonal direction as the moving plasma structure. However, the correlation between the drift direction of the 3 m FAIs and the moving direction of the large-scale plasma structure is indistinct for the clump-type plasma structure. The meridional electric fields estimated from the true zonal FAI drift velocities for the layer-type plasma structures moving in the west (east) direction point in the northward/upward (southward/downward) direction. Statistical results show that the mean value of the meridional electric field inside the layer-type plasma structure is approximately 2.7-2.8 mV/m, about one order of magnitude larger than that for the clump-type plasma structure. Mechanisms that are, respectively, responsible for the generations of the layer-type and clump-type plasma structures are discussed in this article. It is believed that neutral wind shear is very likely the main cause of the layer-type plasma structure formation, while the clump-type plasma structure may be associated with propagating gravity wave. Key Points Developing a method to estimte meridional electric field inside plasma structure establishing relation between trace velocity and drift velocity wind shear theory favors the layer-type plasma structure formation