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.

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