Proceedings with Extended Abstracts (single PDF file) - Radio ...

(σ H / σ P )E y (where σ H and σ P are the Hall and Pedersen conductivities inside the Es patch, witha ratio σ H / σ P ≥ 10). E p x in turn can cause strong northward and upward Hall electron driftswhich, in conjunction with ambient meridional density gradients, can destabilize the plasma,even at times they might be strong enough to excite the Farley instability. In addition, they mayalso polarize the patch in the meridional direction, as shown in Figure 1, with a secondarypolarization field E py acting to reduce E p x . A steady state prevails when divergence-freeconditions are established inside the patch through field-aligned current closures. The closuredetails, which depend upon the ionospheric conductivities and the patch dimensions, aredescribed by Shalimov et al. [1998]. Finally, as shown in Figure 1, a westward electric fieldmay also set in inside regions of low electron density as a result of the oppositely chargededges of neighboring E s patches.Figure 1. A new mechanism for generation of mesoscale Spread-F at midlatitudeNext, we postulate that the electric fields inside sporadic-E plasma patches map up along thefield lines to F region altitudes. The effectiveness of mapping depends on the zonal extent ofthe E s patch, l x , and Farley's mapping factor, (σ 0 /σ P ) 1/2 , where σ 0 is the parallel or specificconductivity and σ P is the Pedersen conductivity. So, the mapping distance along B is l z = l x (σ 0/σ P ) 1/2 . If we adopt a rather conservative value near 10 for (σ 0 /σ P ) 1/2 , as suggested by Swartzet al. [2002], then l x needs to be greater than about 15 km for the polarization fields, E p x , tomap to F region altitudes higher than 250 km. These scale sizes are below the mean zonalextents measured for unstable E s layers in various midatitude radar studies, thus we concludethat many of the E s patches are sufficiently large for their polarization fields to map up to Fregion altitudes. Note that the typical zonal E s scales measured by the Valensole radar in thesouth of France range from between 20 and more than 100 km, which seem to compare wellwith the spread-F azimuthal scales measured, for example, by Fukao et al. [1991].The eastward fields mapped to the F region will act upon the magnetized plasma and thusimpact upward and northward ExB drifts, causing F region uplifts and, therefore, spread-F. Ifwe take a magnetic dip angle of about 50 0 and consider an eastward E x p field of 5 mV/m, thiswill cause a northward and upward ExB drift of about 110 m/s to act upon an F region volumeand thereby produce a vertical plasma uplift of 60 km in about 10 minutes. For a largereastward electric field of 12 mV/m, the northward and upward drifts will be 250 m/s, and an Fregion uplift of 60 km will form in only 4 minutes. These estimates are in agreement with themeasurements of Behnke [1979] and Swartz et al. [2002], who quoted uplifts of 30 to 100 km,and also with the large negative (away) Doppler velocities measured by Fukao et al. [1991].88