Experimental results obtained with the.Jicamarca radar and a new digital processing system during spread F conditions are presented. The data consist of two-dimensional maps showing backscatter power and samples of frequency spectra of the backscatter signals as a function of altitude and time. Almost simultaneous spread F backscatter power and incoherent scatter observations of electron density and vertical drifts are presented for one occasion. It is shown that spread F can occur at the bottomside, at the topside and the steep bottom of the F region, and in the valley between the F and E regions when the electric field is either positive, negative, or null. The existence of plumelike structures extending hundreds of kilometers in altitude and physically connecting the spread F on the topside with the bottomside is one of the highlights of the experimental results. They are interpreted as evidence of a Rayleigh-Taylor instability. A mechanism involving 'bubbles' or low-density plasma is proposed to extend the instability to the stable regions on the top. Other unstable processes are proposed for spread F at other altitude ranges. The frequency spectra show a large variety of shapes. Simple or multiple peak spectra from a few hertz to a few hundred hertz wide are found. An interpretation of the spectral shapes is presented in terms of turbulent motions and the angular extent of k vector angles of the fluctuation waves with respect to perpendicularity. A puzzling phenomenon, referred to as explosive spread F, which consists of the simultaneous onset of short time enhancements in the backscatter power and involves selected heights in an altitude range of the order of 100 kin, is presented. probes, and propagation of satellite beacons (phase and amplitude scintillation). Ionosondes and satellite scintillations have been used mainly to obtain statistical data regarding temporal and spatial (latitude and longitude) spread F behavior and its relationship to other geophysical phenomena (e.g., magnetic and solar activity). Freemouw and Rino [1971] have collected the satellite scintillation observations into an empirical mathematical model. Ionosonde observations have been reviewed by Skinner and Kelleher [1971]. Most of the results reported in the literature have made use of these techniques. That these techniques are limited is shown by the fact that despite the long and intensive effort to find a theoretical explanation for the physical nature of the phenomenon, this effort has been unsuccessful.

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Incoherent scatter observations of vertical drifts taken at Jicamarca (2° dip) are presented. Vertical drifts are found to be nearly constant as a function of height. These vertical drifts can also be taken as a direct measurement of the east‐west electric fields at the magnetic equator. Their daily and seasonal behavior is presented. The effect of geomagnetic activity is discussed.

Scatter from field‐aligned irregularities associated with equatorial spread F has been studied using the powerful 50 MHz radar at Jicamarca. We find that these irregularities, which have a wavelength of 3 meters, first, at times can be generated anywhere in the F region, no matter with what vertical velocity the region is moving, second, have growth times of a few seconds or less, third, move at velocities comparable to that of the F layer, and fourth, may attain strengths of perhaps 107–108 times the background thermal level. There are also indications that there may be two types of irregularities, classified roughly as strong and weak, and that there may be a minimum threshold altitude that the bottom of the F layer must be above before the irregularities are first generated. These and other observed characteristics of the irregularities seem to rule out currently popular theories of their origin (the vertical drift instability suggested first by Martyn, mechanisms based on coupling between the E and F regions, gravitational instabilities, generation by hydromagnetic waves, and others) and several other well known types of plasma instability. Gradients of density and drift velocity remain as possible sources of instability, but at the moment we can only speculate as to what may ultimately prove to be the correct explanation.

Simultaneous auroral and equatorial electric field data are used along with magnetic field data to study anomalous electric field patterns during disturbed times. During some substorms, accompanied by ring current activity, the worldwide equatorial zonal electric field component reverses from the normal pattern. This is interpreted as a partial closure of high latitude field aligned currents in the dayside, low latitude ionosphere. These currents flow westward across the dayside. In several cases the zonal equatorial electric field component was nearly identical in form to the zonal auroral component, indicating the close electrical coupling between these regions. Less certain, but equally intriguing, is the evidence presented for a close relationship between the zonal equatorial electric field and the time derivative of the ring current induced magnetic field. Another class of events seems related to rapid changes of magnetospheric convection and a temporary imbalance between the field external to the plasmasphere and the shielding charges in the Alfven layer. Examples of both rapid increases and decreases are presented. The latter seems often to be related to a northward turning of the interplanetary magnetic field.