The Morphology of Equatorial Plasma Bubbles - a review

th century, so too were anomalies in radio communication. The association of the radio communication anomalies with the electron density irregularities in the ionosphere was verified after the probing of the ionosphere with an ionosonde. An ionosonde is an instrument that measures the electron density as a function of height by transmitting a sweep of frequencies and receiving delayed, reflected signals from different ionospheric layers. In the absence of irregularities in the electron density, the observations of an ionosonde provide a thin-layer electron-density profile for the frequency-range coordinates. In the presence of irregularities, however, a spread in frequency or range appears in the electron density profile. The spread is caused by the irregular reflection-altitude of the transmitted signal, which indicates the existence of electron density irregularities in the reflecting layer. The spread is most severe at night in the equatorial F-region, so this phenomenon is called ‘equatorial spread F’ (ESF). Because of its significant effect on communication, navigation, and geodesic systems, ESF has been recognized as one of the most important space-weather features in the ionosphere. Various terms have been used to represent the election density irregularities in the equatorial F-region. The generic name ESF originated from observations of the irregularities by ionosonde. With in situ satellite observations, irregularities appear at the places where the electron density is depleted, relative to the background ionosphere, so terms such as “depletions, ”, “bite-outs,” and “plasma holes” were used (e.g., Hanson & Sanatani 1973, McClure et al. 1977). The terms “plumes” or “wedges” stemmed from the morphology of turbulent regions in radar observations (e.g.,