E.A. Essex

The Weddell sea anomaly observed with the Topex satellite data

This paper introduces the complete image of the Weddell Sea Anomaly, observed with the over-the-ocean ionospheric total electron content (TEC) values obtained from the TOPEX satellite data with an almost unlimited coverage over the oceans, the first time according to the literature; and investigates its development. With a series of TOPEX TEC maps, this paper demonstrates the diurnal variations of both the night-time and the day-time Weddell Sea Anomaly, which appeared as a night-time TEC enhancement and as a day-time TEC depletion, during the near sunspot maximum period of 1998 and 1999 investigated. Several TOPEX passes, plotted in geomagnetic latitudes, are also presented to demonstrate the longitudinal variations of the Weddell Sea Anomaly, and also to show other ionospheric features appearing such as the southern-hemisphere mid-latitude day-time and night-time trough, the northern-hemisphere mid-latitude night-time trough and the equatorial anomaly. This paper demonstrates how large the anomaly is in reality situated west of the Faraday ionosonde station over the Bellinghausen Sea and not over the Weddell Sea that is east of Faraday. Thus the correct name should be Bellinghausen Sea Anomaly. Based upon the review paper of Dudeney and Piggott (1978), the development of the Weddell Sea Anomaly is explained with the combined effects of solar ultraviolet radiation and thermospheric neutral winds.

Using observations from the GPS and TOPEX satellites to investigate night-time TEC enhancements at mid-latitudes in the southern hemisphere during a low sunspot number period

Abstract The state of the ionization of the upper atmosphere at low and mid latitudes in the Australian region has been studied by investigating the total electron content (TEC) obtained by a dual-frequency group path and phase path GPS technique. For the low sunspot number time period of March 1995–February 1996, one week of data centred on the Priority Regular World Day for each month have been used to investigate night-time mid-latitude peaks occurring around midnight in the Australian region. TEC from TOPEX provided additional information related to the formation of the night-time peaks. Although night-time TEC enhancements have been observed previously, there is no general agreement on their origin. From the results of the present study, the development of midnight TEC enhancements coincided with the low latitude processes occurring at around the time of vertical E×B drift velocity reversal. The TOPEX results confirmed that the upward E×B drift velocity reversal and the downward plasma flow from greater heights producing the night-time peaks at mid latitudes are triggered from a common source: the westward electric field.

The southern-hemisphere mid-latitude day-time and night-time trough at low-sunspot numbers

The diurnal, seasonal, spatial and magnetic activity variations of the southern-hemisphere mid-latitude trough has been studied by using GPS and TOPEX satellite techniques during the low-sunspot number period of February 1995–February 1996. The ionospheric total electron content (TEC) values were obtained from the raw satellite data to observe the trough both locally in the Australian longitude region and globally over the world oceans. Various trough features were observed and investigated under different magnetic conditions, and were compared to the results of other researchers employing different techniques at Macquarie Island (−54.5°N; 154.95°E, geographic and the magnetic shell parameter (L) is 5.38). The ionization build-up on the equatorward trough wall, which has not been investigated since Foster (1993), is discussed in detail with the ΔTEC parameter utilized to characterize it. Some of the GPS findings were confirmed with the TOPEX results. Comparisons with the model generated TEC plots indicated that the model did not reproduce the trough. Several day-time and night-time global TOPEX TEC maps, showing also the geomagnetic and dip equators, were constructed for the season of the 1995 autumnal equinox to observe the trough globally, the first time in the literature, with other large-scale ionospheric formations. The magnetic alignment of these large-scale ionospheric formations is obvious. On the night-time maps the empirical position of the trough were plotted for various local time values at Kp=0 and 6, and indicated a very good agreement between the experimental and theoretical results.

A dynamic diffusive equilibrium model of the ion densities along plasmaspheric magnetic flux tubes

Abstract A new approach, based in part on diffusive equilibrium, has been developed to model the dynamic H + content of plasmaspheric magnetic flux tubes. Called the Global Plasmasphere Ionosphere Density (GPID) model, its aim is to reproduce the main dynamic features of the more complete Field Line Interhemispheric Plasma (FLIP) model, with a simpler formulation that allows for more rapid computation of the ions temporal variation in a magnetic flux tube. By simultaneously modelling several thousand flux tubes, a global representation of the plasmasphere can be obtained in a realistic time frame. A global model allows the plasmaspheric contribution to the Total Electron Content (TEC), measured from Global Position System (GPS) satellite radio signals, to be determined and removed, hence permitting the underlying ionosphere to be observed. This paper presents the approach used to model a single magnetic flux tube. GPID assumes an ionosphere/plasmasphere composition of O + and H + , and includes chemical processes and simple diffusive transport. In almost all situations, GPID reproduces the predictions of the FLIP model. Comparisons between GPID and the refilling fluxes obtained from whistler observations generally show good agreement, both in diurnal variations and during prolonged refilling of empty flux tubes. Comparisons to seasonal variations during solar maximum at L =2.5 show good agreement with observations, but at solar minimum there is variable agreement.