JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104,NO. A5, PAGES 10,051-10,069, MAY 1, 1999 Production of electric field perturbations by gravity wave winds in the E region suitable for initiating equatorial spread F SatyaPrakash Physical Research Laboratory, Ahmedabad, India Abstract. This study proposes a new mechanism through which electric field perturbations, whichmapup to the F region base, canbe generated by gravitywave winds. These fields are produced efficientlywhengravitywaves interact with thin E layers or sporadic layers in the low latitude E region.The proposed mechanism operates in two steps. In step1, eastward zonal winds in the F region produce radiallydownward electric fields.These fieldsmapdownto the E region producing downward transport of plasma. This reduces the thickness of theE layer andhelp in formation of sporadic layers. In step 2, gravitywavewinds interact with the above created layers and produce electric field perturbations. This interaction is through (i) dynamo action and(ii) zonalcurrents drivenby the F regiondynamo field flowing in the E regionwhose conductivities areperturbed. These perturbations arecaused when gravity wavewinds produce electron density irregularities. This work demonstrates that with a thin plasma layerin theE region, the electric field perturbations aremainly produced through Hall conductivity. In previously proposed studies, theelectric field perturbations were mainlyattributed to Pedersen conductivity. The electric field perturbations produced through theproposed mechanism aremuch larger than those through earliermechanisms and havecharacteristics that are suitable for the growthof seed irregularities in theF region base. The characteristics of these seed irregularities aresuch thattheycangrowinto theequatorial Spread F irregularities through theRayleigh Taylorinstability mechanism. 1. Introduction Equatorial spread F hasbeenextensively studied in the last 5 decades with ground-based as well as space-borne techniques. Clemesha [1964], usingan 18 MHz backscatter radar observed that equatorial spread F irregularities occur in patches. Rottger [1973, 1978] presented observations of large-scale wave-like F region irregularities from a transequatorial propagation experiment and suggested that gravitywaves canproduce wave- like structures at spatialresonance. Klostermeyer [1978] used spatial resonance mechanism to explain large-amplitude wave- like bottomside disturbances of ionization observedby Rottger. Kelley et al. [ 1981 ], Tsunoda and White [ 1981 ], and Hysell et al. [1990] presented observations whichdisplay strong evidence of gravitywave initiation of equatorial spread F. Many of these studies assume thatgravitywaves present in the F region initiate equatorial spread F irregularities. Dagg[ 1957] pointed outtheimportance of small-scale electric fields to explain equatorial spread F irregularities. Farley [1960] studied the generation of small-scale electrostatic fieldsusinga qyqtom nf hori7cmtnl winclq in tho/7 region and thoir m•,,,,i,,•, into the F region.He foundthat the coupling will be weakest at the equator, andthe strength of the source field will be considerably less than the (•SWxB) field, where •SW is the wind velocity amplitude and B is the geomagnetic field vector. Klostermeyer [1978] assumed that at night when the conductivity of the E region is small, theplasma in theF region will move with gravity wavewinds, whileHuanget al. [ 1993]assumed V.Jñ= 0 for the determination of the electric field perturbations. Here Jñ is the Copyright 1999 by theAmerican Geophysical Union. Paper number 1999JA900028 0148-0227/99/1999JA900028509.00 currentin the plane perpendicular to B. Both theseassumptions imply that the electric field perturbations due to gravity wave windsare not electrically loaded;i.e., the current parallel to B is zero. The flow of current, due to gravity wave winds,takesplace via the geomagnetic field lines because of the linkage between regions of positiveand negative potentials produced by gravity wavewinds. Prakash and Pandey [ 1979] showed thatthe currents parallel to B (J•) can be zero only when the wave front of a gravity wave in the meridian plane is parallel to B; i.e. k• = 0. The efficiency for generationof these fields is large when the region with the gravity wave winds wherek• = 0, overlaps with the regionof high Pedersen conductivity [Prakash and Pandey, 1980, 1985]. Similarly, Jacobsonand Bernhardt [1985] found that uncompensated charges could be generated by a vortex-like acoustic gravity wave only when its wave numberis sufficiently perpendicular to the geomagnetic field line. Thus it can be seen that gravity wave winds in the F region can produce electricfield perturbations and thereby initiate equatorial spread F irregularities only when they propagate in the near zonal direction.In all thesemechanisms, electric field perturbations are produced through Pedersen conductivity. From Kelley et al. [1981] and Hysell et al. [1994],VHF radar maps over Jicamarca showthat on many strong equatorial spread F daysthe plumes are uniformly spaced with a separation of 50 to 150 km. Vertical scale sizes of striations associated with the plumes observed by Hysell et al. [ 1994] are even smaller.If the equatorial spread F plumesare initiatedby gravity wave winds, this would require someof the gravity waves to have horizontal wavelengths as small as 50 km. In the F region the existence of wavelengths of such magnitude have not been established conclusively.In the E region, the gravity waves have a wide range of scale sizes covering the above range of wavelengths. 10,051