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

Satya Prakash Physical Research Laboratory, Ahmedabad, India

Abstract. This study proposes a new mechanism through which electric field perturbations, which map up to the F region base, can be generated by gravity wave winds. These fields are produced efficiently when gravity waves interact with thin E layers or sporadic layers in the low latitude E region. The proposed mechanism operates in two steps. In step 1, eastward zonal winds in the F region produce radially downward electric fields. These fields map down to the E region producing downward transport of plasma. This reduces the thickness of the E layer and help in formation of sporadic layers. In step 2, gravity wave winds interact with the above created layers and produce electric field perturbations. This interaction is through (i) dynamo action and (ii) zonal currents driven by the F region dynamo field flowing in the E region whose conductivities are perturbed. These perturbations are caused when gravity wave winds produce electron density irregularities. This work demonstrates that with a thin plasma layer in the E region, the electric field perturbations are mainly produced through Hall conductivity. In previously proposed studies, the electric field perturbations were mainly attributed to Pedersen conductivity. The electric field perturbations produced through the proposed mechanism are much larger than those through earlier mechanisms and have characteristics that are suitable for the growth of seed irregularities in the F region base. The characteristics of these seed irregularities are such that they can grow into the equatorial Spread F irregularities through the Rayleigh Taylor instability mechanism.

1. Introduction

Equatorial spread F has been extensively studied in the last 5 decades with ground-based as well as space-borne techniques. Clemesha [1964], using an 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 gravity waves can produce wave- like structures at spatial resonance. Klostermeyer [1978] used spatial resonance mechanism to explain large-amplitude wave- like bottomside disturbances of ionization observed by Rottger. Kelley et al. [ 1981 ], Tsunoda and White [ 1981 ], and Hysell et al. [1990] presented observations which display strong evidence of gravity wave initiation of equatorial spread F. Many of these studies assume that gravity waves present in the F region initiate equatorial spread F irregularities.

Dagg [ 1957] pointed out the importance of small-scale electric fields to explain equatorial spread F irregularities. Farley [1960] studied the generation of small-scale electrostatic fields using a qyqtom nf hori7cmtnl winclq in tho /7 region and thoir m•,,,,i,,•, into the F region. He found that the coupling will be weakest at the equator, and the 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, the plasma in the F region will move with gravity wave winds, while Huang et al. [ 1993] assumed V.Jñ = 0 for the determination of the electric field perturbations. Here Jñ is the

Copyright 1999 by the American Geophysical Union.

Paper number 1999JA900028 0148-0227/99/1999JA900028509.00

current in the plane perpendicular to B. Both these assumptions imply that the electric field perturbations due to gravity wave winds are not electrically loaded; i.e., the current parallel to B is zero. The flow of current, due to gravity wave winds, takes place via the geomagnetic field lines because of the linkage between regions of positive and negative potentials produced by gravity wave winds. Prakash and Pandey [ 1979] showed that the 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 generation of these fields is large when the region with the gravity wave winds where k• = 0, overlaps with the region of high Pedersen conductivity [Prakash and Pandey, 1980, 1985]. Similarly, Jacobson and Bernhardt [1985] found that uncompensated charges could be generated by a vortex-like acoustic gravity wave only when its wave number is sufficiently perpendicular to the geomagnetic field line. Thus it can be seen that gravity wave winds in the F region can produce electric field perturbations and thereby initiate equatorial spread F irregularities only when they propagate in the near zonal direction. In all these mechanisms, electric field perturbations are produced through Pedersen conductivity.

From Kelley et al. [1981] and Hysell et al. [1994],VHF radar maps over Jicamarca show that on many strong equatorial spread F days the 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 plumes are initiated by gravity wave winds, this would require some of 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.

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