Large polarization electric fields associated with midlatitude sporadic E

Abstract

Abstract. Recent 50 MHz E region coherent backscatter observations and in situ rocket measurementsestabhshed the existence of enhanced electric fields in the midlatitude ionosphere that can become at times sufficiently large to excite the Farley-Buneman instabihty. To understand the origin of these fields, we present a simple quantitative model that relates to a local polarization process acting inside spatially confined, nighttime sporadic E layers of dense ionization. By including the effects of field-aligned currents in the current continuity equation we estimate the necessary conditions on the relative horizontal E layer extent and the ratio of integrated Pedersen conductivities above and inside the layer for the generation of both zonal and meridionM polarization fields. We show that the polarization process can account for the elevated electric fields of several millivolts per meter, which are implied often from backscatter Doppler measurements during unstable E region conditions at midlatitude. The polarization process can become much more effective for dense and strongly elongated E, layers under the action of an enhanced ambient electric field. In this case, large polarization fields that may be capable of exciting Farley-Buneman plasma waves can be sustained. The stringent requirements for strongly elongated sporadic E layers with sharp boundaries, low ionospheric Pedersen conductivities above the layer in relation to those inside, and relatively large ambient electric fields would explain why type 1 echoes are so rare in midlatitude E region backscatter.

Interaction of plasma cloud with external electric field in lower ionosphere

“…The components of the total field are E int X j + E X j , where E X j = −∂ /∂X j is the induced electric field. Note that our definition of the "induced" field differs from the definition of the "polarization" field by Shalimov et al (1998), which in our notation would be E X j − E (0) X j . The predominantly Hall response to the internal field E int X j on the cloud boundary is unimportant for the total current balance because the corresponding currents on the opposite symmetric sides of the ellipsoid cancel each other (these currents may slightly distort the cloud shape and affect cloud's overall drift, but these effects are beyond the scope of the present paper).…”

“…Expressed in terms of D, the dipolar current flow system is insensitive to the details of the geometry and internal structure of the distant cloud. These assumptions also apply to the slab geometry considered by Shalimov et al (1998). The general asymptotic solution results in a structure of currents that can never be split into two independent systems, like the zonal and meridional current flows.…”

“…(3) of Shalimov et al (1998) is split into their Eqs. (4) and (5) without justification and we argue that this may lead to highly inaccurate estimate for the fields.…”

“…Tsunoda (1998) and Maruyama et al (2000) applied a similar approach to explain the generation of quasi-periodic echoes (Yamamoto et al, 1991;Ogawa et al, 1995;Pan and Tsunoda, 1998). Shalimov et al (1998) and Shalimov and Haldoupis (2005) applied ideas similar to those from the seminal papers by Farley (1959Farley ( , 1960) by including ionospheric current closure and using a more realistic 3-D slab model. However, their theory relied on approximate scaling arguments rather than on a rigorous solution of the underlying differential equations.…”

“…That is why their model describes no spatial distributions of fields and currents. Furthermore, Shalimov et al (1998) and Shalimov and Haldoupis (2005) used artificial equation splitting which is invalid and they ignored vertical slab polarization, as we discuss in Sect. 4.…”