Evidence for electrodynamic linkages between spread-F, ion rain, the intermediate layer, and sporadic E: results from observations and simulations

High resolution ISR observations of the evening and nighttime E- and F-regi ons at Arecibo during both "normal" low-activity periods and during an acti ve spread-F event reveal apparent electrodynamic links between the low-lyin g layers, ion-rain, and the spread-F instability process. In particular, du ring a 22 November 1994 event, streaks of ionization-ion rain [Mathews et a l., Geophys. Res. Lett. 24 (1997) 1411]-appear to descend from the highly s tructured F-region base onto the 100-110 km Tidal Ion Layer (TIL) system. T he evening intermediate tidal ion layer, if present, is almost totally disr upted while the spread-F event is characterized by deep, almost vertical fo lds in electron concentration. The amplitudes of these structures are 100% with fold-structures extending from the nominal F-region base to well above the nominal peak of the F-region. Apparent periods range from less than 10 min for small-scale structures to about 40 min for the largest-scales. We suggest that the E-region structures are a manifestation of polarization el ectric fields that "map" into the E-region and are well in excess of 1 mV/m in contrast to low-activity period small-scale fields of order 1 mV/m. The se E-fields are hypothesized to be generated in the "field" of linear (low- activity) and/or non-linear (high-activity spread-F periods) Perkins-like i nstabilities that are individually of relatively small horizontal-scales bu t occur over large horizontal distances in the bottom-side of the nighttime F-region. We present numerical simulations of 3-dimensional ion trajectori es in tidal wind and electric field structures similar to those implied by these observations. These simulations extend the results of Machuga and Mat hews (J. Atmos. Solar-Terres. Phys., 2001, 63(14), 1519-1528, this issue.) (Paper 1) and demonstrate the formation of parallel sheets of ionization th at form in the tilted plane oriented transverse to the bulk flow of the F-r egion that sweeps similar to1 mV/m amplitude E-fields quasiperiodically thr ough a fixed location in the E-region. A fixed pointing radar sees these io nization sheets as ion-rain-like features. (C) 2001 Elsevier Science Ltd. A ll rights reserved.