Plasma bubbles and irregularities in the equatorial ionosphere

Abstract

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.

Nonlinear equatorial spread F: Spatially large bubbles resulting from large horizontal scale initial perturbations

“…However, observations by McClure et al [1977] also indicate ionospheric ion density "biteouts" of much larger horizontal extent (10 -> 200 km) and greater intensity (ion density depletions up to three orders of magnitude) than indicated by our small scale simulations.…”

Some new features of electron density irregularities over SHAR during strong spread F

“…The di erence in ascent and descent values is likely to be much enhanced during a spread F event. Thus, the present observations, which were conducted during a well-developed spread F epoch, showing maximum change in electron density by a factor of 20 around 210 km altitude, indicate the presence of enhanced horizontal electron density gradients throughout the 150±300 km altitude region during fully developed spread F. Similar horizontal gradients were observed by McClure et al (1977), in AE satellite data. Recently, Gdalevich et al (1998) reported the presence of horizontal gradients in plasma density, using a cylindrical Langmuir probe in the rocket experiment`Vertical-10', and attributed these to large-scale disturbances propagating as internal gravity waves (IGWs), in the upper atmosphere.…”

A Review of Recent Results on Spread F Theory

“…The NRL group has also added neutral wind effects to these simulations and found that an eastward neutral wind results in a westward motion of the bubbles in addition to its rise . This is also in agreement with observations of bubble motion (see McClure et al, 1977 (14) and (15) were utilized 'with n = n + ?~, etc . This yields the following coupled nonlinear…”

“…It should be emphasized that this paper will deal only with equatorial Spread F theory and , in addition , dwell only on those theories using plasma mechanisms as a basis. For experimental results, the worXs of Farley et al (1970), Dyson et al • (1974) , Kelley et al (19 76) , Woodman and La Hoz (1976) , Morse et al (1977), and McClure et al (1977) should serve as good references for the interested reader.…”