Abstract.Using more than 350 ionospheric images reconstructed tomographically, studies on the motion of the anomaly crest and its geophysical implications are carried out. On an average day, the crest forms at 09:00 LT and,,in the next two hours, moves poleward with a speed of about 1 ø per hour as it intensifies. This poleward motion is slowed as the crest reaches its highest latitude where it stays for several hours until early afternoon. Thereafter, the crest starts to weaken as it recedes with a speed of about 0.5 ø per hour equatorward. During 12:00-14:00 LT, the crest latitude is found to correlate with the fountain strength and the total number of electrons in a cross sectional plane at the observational longitude of the whole equatorial ionosphere.
[1] Observations of energetic electrons (10 -300 keV) by NOAA/POES and DMSP satellites at heights <1000 km during the period from 1999 to 2010 allowed finding abnormal intense fluxes of~10 6 -10 7 cm À2 s À1 sr À1 for quasi-trapped electrons appearing within the forbidden zone of low latitudes over the African, Indo-China, and Pacific regions. Extreme fluxes appeared often in the early morning and persisted for several hours during the maximum and recovery phase of geomagnetic storms. We analyzed nine storm time events when extreme electron fluxes first appeared in the Eastern Hemisphere, then drifted further eastward toward the South-Atlantic Anomaly. Using the electron spectra, we estimated the possible ionization effect produced by quasi-trapped electrons in the topside ionosphere. The estimated ionization was found to be large enough to satisfy observed storm time increases in the ionospheric total electron content (TEC) determined for the same spatial and temporal ranges from global ionospheric maps. Additionally, extreme fluxes of quasi-trapped electrons were accompanied by the significant elevation of the low-latitude F-layer obtained from COSMIC/FORMOSAT-3 radio occultation measurements. We suggest that the storm time ExB drift of energetic electrons from the inner radiation belt is an important driver of positive ionospheric storms within low-latitude and equatorial regions.
Abstract. The equatorial anomaly in ionization density has been imaged using the computerized ionospheric tomography technique applied to data from a low-latitude ionospheric tomography network. Examples of images representative of typical conditions during equinox and low solar flux are presented and shown to exhibit some characteristic features which have not been observed directly previously. The EA core, comprising the highest density region of the EA, is shown to exhibit a characteristic structure and asymmetry. These characteristics are quantified using measures of the ionospheric slab thickness and altitude, and they are discussed in light of the fountain mechanism which is responsible for formation of the EA.
We study a magnetosphere-ionosphere coupling at low latitudes during a moderate (corotating interaction regions/high-speed solar wind streams-driven) geomagnetic storm on 22 July 2009. Recently, it has been shown that during major (coronal mass ejection-driven) storms, quasi-trapped >30 keV electrons largely enhance below the radiation belt in the forbidden zone and produce an additional ionization in the topside ionosphere. In this work, we examine a case of the recurrent storm when the magnetosphere-ionosphere coupling through the quasi-trapped electrons also may take place. Data from NOAA/Polar-orbiting Operational Environmental Satellite and Japanese Greenhouse gases Observing Satellite were used to identify the forbidden electron enhancement (FEE). We find a positive vertical gradient of the electron fluxes that indicates to the radiation belt as a source of FEE. Using global ionospheric maps, radiotomography reconstructions from beacon data and COSMIC/FORMOSAT-3 radio occultation measurements, we have observed an unusually large area in the nighttime ionosphere with increased total electron content (TEC) and prominent elevation of the F layer at low latitudes that coincides with FEEs spatially and temporarily. Ionizing particles are considered as an addition source of ionization along with generally accepted mechanisms for storm time TEC increase (a positive ionospheric storm). We discuss relative contributions of the FEE and disturbance dynamo electric field in the TEC increases during the storm recovery phase.
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