[1] The ionosphere's total electron content (TEC) is a parameter widely used in studies of the near-Earth plasma environment. The scientific use of TEC appeared early in the artificial satellite era, and among its many contributions were fundamental insights into how the ionosphere responds to geomagnetic storms. While many excellent reviews of solar-terrestrial disturbances exist in the literature, none have concentrated on the TEC parameter per se. With new TEC data sets increasingly available from the Global Positioning System (GPS), a comprehensive summary of pre-GPS storm studies is needed to set the base for progress in the GPS era. This review summarizes past case studies, describes statistical occurrence pattern, and identifies responsible mechanisms validated via modeling. It presents a new set of results of TEC disturbance patterns during 180 geomagnetic storms to describe seasonal and solar cycle effects. It concludes with a set of open questions that require additional study.Citation: Mendillo, M. (2006), Storms in the ionosphere: Patterns and processes for total electron content, Rev. Geophys., 44, RG4001,

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[1] Imaging measurements of a bright wave event in the nighttime mesosphere were made on 14 November 1999 at two sites separated by over 500 km in the southwestern United States. The event was characterized by a sharp onset of a series of extensive wavefronts that propagated across the entire sky. The waves were easily visible to the naked eye, and the entire event was observed for at least 5 1 2 hours. The event was observed using three wide-angle imaging systems located at the Boston University field station at McDonald Observatory (MDO), Fort Davis, Texas, and the Starfire Optical Range (SOR), Albuquerque, New Mexico. The spaced imaging measurements provided a unique opportunity to estimate the physical extent and time history of the disturbance. Simultaneous radar neutral wind measurements in the 82 to 98 km altitude region were also made at the SOR which indicated that a strong vertical wind shear of 19.5 ms À1 km À1 occurred between 80 and 95 km just prior to the appearance of the disturbance. Simultaneous lidar temperature and density measurements made at Fort Collins, Colorado, $1100 km north of MDO, show the presence of a large ($50 K) temperature inversion layer at the time of the wave event. The observations indicated that the event was most probably due to an undular mesospheric bore, a relatively uncommon disturbance which has only recently been reported [Taylor et al., 1995a]. Evidence is also shown to suggest that a large east-west tropospheric frontal system lying over the northern United States was the origin of the disturbance.

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All‐sky imagers located at Tucumán, Argentina (26.9°S, 65°W, 14.2°S dip latitude), and Arequipa, Perú (16.5°S, 71.5°W, 2.7°S dip latitude), are used to track 630 nm airglow depletion motions in the first use of multisite airglow imagers for studies of low‐latitude plasma dynamics. A new image analysis technique yields a consistent determination of nighttime zonal plasma drifts from all‐sky images of the depletion motions. The observed eastward plasma drifts are smaller at Arequipa than at Tucumán in the postsunset period. During the postmidnight hours, the opposite pattern occurs. These observations are interpreted using the simple plasma drift model and coupled ionosphere‐electric field model of Eccles [1998a, 1998b]. The observed zonal plasma drifts result from low‐latitude electrodynamics with a mix of influences from E and F region conductivities and neutral wind shears in altitude and latitude. Analysis of the observations suggests that postsunset zonal drifts near the magnetic equator (Arequipa) are strongly influenced by the E region dynamo, while the F region dynamo is the main cause of zonal drifts observed closer to the Equatorial Ionization Anomaly (Tucumán). The observed altitude‐latitude behavior of the plasma drifts gives the first two‐dimensional evidence for the so‐called F region plasma vortex's influence at equatorial and low latitudes obtained using optical imaging techniques. With this framework a synthesis is offered for seemingly inconsistent zonal drift observations in the published literature.

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A new low‐light‐level, all‐sky, 6300 Å airglow imaging system has been developed for ground‐based studies of the optical signatures of equatorial plasma depletions. The system was designed for narrowband interference filters (6 Å full width at half power) with interchangeable all‐sky (180°) and narrow‐field (60°) lenses and records images photographically by using a standard 35 mm camera. A field test of the apparatus was conducted on Ascension Island (8.0°S, 14.4°W) from January 24 to February 10, 1981. The photographs were analyzed for broad morphological characteristics by using simple overlay techniques augmented by digital image processing methods to extract more quantitative detail. The initial results to come from these observations show that airglow depletions occur most often during the 2030–2330 LT period, often covering one‐fourth of the visible sky; they extend from the magnetic equator to beyond the crest of the equatorial anomaly, with east‐west cross sections of ∼100 km to several hundred kilometers; they are usually not aligned with magnetic meridians, showing a progressive skewness to the west at further distances from the equator; several cases of apparently twisting, overlapping, and bifurcating depletions were observed; the depletions drifted to the east with speeds that decreased from ∼190 m/s at 2100 LT to ∼80 m/s at 0100 LT; the airglow intensities within depletions were typically 40–50% of the background levels, and the western walls of the depletions usually had a steeper gradient than the corresponding eastern walls.