Francisco Azpilicueta

A New Ionosphere Monitoring Technology Based on GPS

Although global positioning system (GPS) was originally planned as a satellite-based radio-navigation system for military purposes, civilian users have significantly increased their access to the system for both, commercial and scientific applications. Almost 400 permanent GPS tracking stations have been stablished around the globe with the main purpose of supporting scientific research. In addition, several GPS receivers on board of low Earth orbit satellites fitted with special antennas that focus on Earths horizon, are tracking the radio signals broadcasted by the high-orbiting GPS satellites, as they rise and set on Earth horizon. The data of these ground and space-born GPS receivers, readily accessible through Internet in a ‘virtual observatory’ managed by the International GPS Service, are extensively used for many researches and might possibly ignite a revolution in Earth remote sensing.By measuring the changes in the time it takes for the GPS signals to arrive at the receiver as they travel through Earths atmosphere, scientists can derive a surprising amount of information about the Earths ionosphere, a turbulent shroud of charged particles that, when stimulated by solar flares, can disrupt communications around the world. This contribution presents a methodology to obtain high temporal resolution images of the ionospheric electron content that lead to two-dimensional vertical total electron content maps and three-dimensional electron density distribution. Some exemplifying results are shown at the end of the paper.

Toward a SIRGAS Service for Mapping the Ionosphere’s Electron Density Distribution

SIRGAS is responsible of the terrestrial reference frame of Latin America and the Caribbean. To fulfil this commitment it manages a continuously operational GNSS network with more than 200 receivers. Although that network was not planed for ionospheric studies, SIRGAS attempted to exploit it by establishing, in early 2008, a regular service for computing regional maps of the vertical Total Electron Content. This paper describes an effort for developing a new SIRGAS product, concretely, a 4-dimensional (space and time) representation of the free electron distribution in the ionosphere. The working methodology is based on the ingestion of dual-frequency GNSS observations into a global electron density model in order to update its parameters. Preliminary results are presented and their quality is assessed by comparing the electron density computed with the methodology here described and the one estimated from totally independent observations. A preliminary analysis reveals that the performance of the electron density model improves by a factor greater than 2 after data ingestion.

Improving SIRGAS Ionospheric Model

The IAG Sub-Commission 1.3b, SIRGAS (Sistema de Referencia Geocentrico para las Americas), operates a service for computing regional ionospheric maps based on GNSS observations from its Continuously Operating Network (SIRGAS-CON). The ionospheric model used by SIRGAS (named La Plata Ionopsheric Model, LPIM), has continuously evolved from a “thin layer” simplification for computing the vTEC distribution to a formulation that approximates the electron density (ED) distributions of the E, F1, F2 and top-side ionospheric layers.

Semi-annual Anomaly and Annual Asymmetry on TOPEX TEC During a Full Solar Cycle

During the first decades of ionospheric research, the physical description of the ionospheric free electron vertical density was mainly given by the Chapman theory in which the main driving parameters were the solar irradiance level and the solar zenith distance from the observation point. Any new observed phenomenon that could not be explained by the Chapman theory was considered an ‘anomaly’. After more than 50 years of continuous aeronomic research, many of these phenomena then called ‘anomalies’ were physically explained but some of them are still open to discussions, like the so-called Semi-annual Anomaly that produces global mean TEC values larger for equinoxes than for solstices; and the Annual Asymmetry that causes larger mean global TEC during the December than the June solstice (far larger than the 7% that would be expected from the change on the Sun–Earth relative distance). Using the high-precision TEC 13-year data series provided by the TOPEX/Poseidon mission, the main finding of this work is the characterization of the annual variation of the ionospheric daily mean TEC that reflects the combined effects of the both mentioned anomalies. The analysis of this annual pattern allows a precise quantification of the level of the effects of both anomalies, and suggests that the semi-annual anomaly does not have a half-year period, instead might be considered as another annual anomaly with two maxima separated by 220 days.