A Study of the Semi-annual Anomaly and Annual Asymmetry

AZPILICUETA F.; BRUNINI C.

Barcelona

Simposio; International Beacon Satellite Symposium 2010; 2010

Universitat Politècnica de Catalunya

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. Whatever a 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 June solstice (far larger than that 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, we have found a mean global TEC annual pattern that shows the combined effect of both anomalies. Several works of the specialized literature have analyzed these anomalies as separate effects. Many of them propose explanations associated to an enhancement on the iononization rate by radiation, due to hydro-dynamics processes of the neutral species. In this work, we study both anomalies as one unique phenomenon and provide significant evidences indicating that the process responsible for this is not in the lower atmosphere, but in the region close to the magnetosphere. Finally, we will present some results that propose as the working hypothesis that the TEC annual pattern could be associated to an interaction between the Solar wind with the Earth’s magnetic field.