Characterization of Equatorial Ionospheric Features on the Verge of the Next Solar Cycle Maximum

The analysis of the ionospheric characteristics (and specifically of the equatorial ionosphere) is of real importance due to its impact on the radio signals´ propagation: the ionosphere affects signals broadcast by Global Navigation Satellite Systems (GNSS) by delaying the propagation of the code carried by the signal. In general ionospheric effects in mid-latitude regions are not severe, causing only gradual variations in ionospheric delays (except in the presence of magnetic storms) whose magnitude can be roughly predicted. However in the equatorial region ionospheric effects are more severe: added to the higher ionospheric delays in this area, the presence of the equatorial anomaly (with higher Total Electron Content values and gradients) and other equatorial features (such as ionospheric plasma depletions and scintillations) complicate the ionospheric modeling and make more difficult the prediction of its main features and its effects on GNSS and other applications that depends on the ionospheric status. This paper focused on the characterization of some of the main equatorial ionospheric features in the current solar cycle and to compare them, when feasible, with those obtained in the previous solar cycle. Ionospheric features to be analyzed are global maximum ionospheric Total Electron Content (TEC) values and their gradients (spatial and temporal), as well as ionospheric depletions occurrence and their main characteristics (depth and width). It is expected that the next solar cycle (number 24) will peak in 2013. If predictions are correct, solar cycle 24 will be one of the lowest cycles since the beginning of the 20th century. In order to analyze the equatorial ionospheric characteristics on the verge of the next solar cycle maximum, a 6-months dual frequency GPS dataset from 5 equatorial IGS stations will be processed. Global ionospheric features will be studied in order to have a more complete view of the situation. The selected dataset allows analyzing some of the main features of this solar cycle´s ionosphere. At equatorial latitudes the ionospheric activity can become a limitation on GNSS applications, and therefore it is important to analyze its main features during the current solar cycle and compare them with those observed in previous solar cycles. On the one hand, maximum ionospheric Total Electron Content (TEC) values and gradients will be globally analyzed in the selected time window. On the other hand, dual frequency GPS data will be processed to study the main characteristics of the equatorial rate of TEC (RoT) values as a function of time. The five equatorial IGS stations selected for this purpose are: Malindi, Kourou, Brasilia, Bangalore and Quezon City. The existence of plasma depletions and their main characteristics will be studied in the analysis as well. It is widely known that residual positioning errors may persist in regions of steep TEC gradients. At low latitudes (near the equatorial anomaly) and during periods of solar maximum an enhancement of large-scale TEC gradients is observed. In this paper, maximum global temporal and spatial TEC gradients will be calculated and analyzed using TEC values from the corresponding IONEX files. Ionospheric irregularities can be detected and characterized by calculating the rate of TEC. In this paper RoT values will be computed in the 5 IGS equatorial stations by differencing the phase measurements over 60 seconds after cycle slips removing. Maximum RoT values will be analyzed for different local time hours, elevations and months in order to characterize and quantify the ionospheric irregularities. Plasma depletions (or bubbles) are strong reductions in the ionospheric F-region plasma density due to the appearance of a Rayleigh-Taylor instability in the post-sunset, producing severe radio signal disruptions when crossing them. Most of the plasma depletions are confined on the Appleton Anomaly region, which also shows the presence of strong scintillation activity (having consequently a severe effect on the L band signals propagation). The geographic latitudes of the selected IGS equatorial stations are expected to be frequently affected by the presence of plasma depletions. The local time and monthly distribution of the depletions, as well as their main features (depth and width) will be analyzed for the selected stations. In order to carry out the aforementioned analyses, a dedicated tool to analyze the ionospheric behavior in terms of statistical parameters (by means of the study of different features) developed by GMV will be used. This tool, named magicIONO, consists of a set of modules, each of them focused on the analyses of a certain ionospheric feature, which can be executed either separately or together for the same or different input datasets. At the same time, each module consists of a set of scripts and C programs, and the tool has been designed to be executed on a UNIX platform. Results from magicIONO depletion analysis have also been compared with those achieved using the Ionospheric bubble seeker, another depletion detection algorithm. A comparison (when feasible, at global and single stations´ levels) with similar ionospheric studies from the previous solar cycle has been done to analyze the relative impact of the current solar cycle effects.