Jicamarca and Altair radar data [Woodman and scale corresponds approximately to the echo LaHoz, 1976; Tsunoda et al., 1979; Tsunoda and intensity in decibels relative to the incoherent White, 1981], recent rocket results [Szuszczewicz scatter level at an electron density of 106 cm-3 et al., 1980; Rino et al., 1981; Kelley et al., at the same range.

Based on the experiments we consider, we predict that the s/1 variations of Pseed will be found to be similar to those of PEF•, and largely to explain them. Finally, we find reasons, based on the similarity of the DRSF variations to s/1 patterns of the average scintillation index, for not using, as is commonly done, such scintillation patterns as substitutes for PEF• or Pinst patterns.

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.

Using the Atmosphere Explorer satellite AE‐C, we observe large‐scale (10‐ to >200‐km) irregular biteouts of up to three orders of magnitude in the ion concentration Ni in the nighttime equatorial F region associated with small‐scale inhomogeneities in Ni. Similar phenomena were reported by Hanson and Sanatani but without the more complete plasma diagnostics present on AE. Simultaneous plasma velocity observations show irregular upward and westward motion of the order of 150 m s−1 associated with some of these ‘bubbles,’ while others move more slowly or move with approximately the velocity of the background plasma. The plasma composition signatures indicate that most of the bubbles observed have recently moved upward. Several features of recent VHF radar observations can be understood as resulting from these plasma bubbles, e.g., the ‘plume’ features and very high apparent velocities seen on range‐time‐intensity spread F maps and the very complex and/or wide spectral features observed using such radars. Bubbles that move upward typically also move westward with respect to the background plasma. This may explain the east‐west asymmetries seen in studies of equatorial irregularities made using HF radar, HF transequatorial propagation, and detailed 6300‐Å tropical airglow mapping techniques.

We address the question regarding the two‐dimensional shape of equatorial plasma bubbles in the plane transverse to the geomagnetic field. By comparing the east‐west spatial relationship of ion‐density depletions measured in‐situ by the Atmospheric Explorer E (AE‐E) satellite to backscatter plumes measured by the ALTAIR radar, we show that plasma bubbles are vertically elongated depletions that extend upward from the bottom side of the F layer, in the form of tilted wedges, rather than more isotropically shaped but isolated structures. The shape of plasma bubbles is inferred from (1) ion density depletions that exceeded 99% in the ‘neck’ regions of plumes and (2) the eastward drift velocities of the plumes. The expected electrodynamics of vertically elongated plasma bubbles are consistent with the observations of large eastward drift velocities of plumes that are comparable to F region plasma drift measurements made at Jicamarca and to F region neutral wind measurements made at Kwajalein. The results also reveal that the west wall of large‐scale altitude modulations of the bottomside F layer that produces the primary plumes and bubbles becomes structured, and evolves with the generation of secondary plumes and bubbles.