Electromagnetic Drivers in the Upper Atmosphere: Observations and Modeling

Abstract

This chapter presents and discusses some of the most recent measurements obtained by the Electron Drift Instrument (EDI) on “Cluster,” the accelerometer on board the low-Earth-orbiting satellite CHAMP, and global maps of total electron content (TEC) gathered by the fleet of GPS satellites. It aims at a better understanding of the globally interconnected complex plasma physical and electrodynamic processes of the Earth’s upper atmosphere by means of first-principle numerical modeling using the Upper Atmosphere Model (UAM). The study results show ionospheric electric fields, generated by magnetospheric and seismogenic sources, and reveal their influence on the thermospheric dynamics and the TEC pattern. UAM simulations of the thermospheric neutral wind at high latitudes are compared with CHAMP observations for varying interplanetary magnetic field (IMF) conditions on 28 October 2003, the day before the famous Halloween superstorm of the previous solar cycle. The simulations show the immediate response of the upper atmosphere and its high sensitivity to IMF changes in strength and orientation. Investigation of the ‘lithosphere–atmosphere–ionosphere’ coupling problem allowed statistically describing GPS-measured TEC variations treated as precursors to earthquakes as (1) anomalous strong (30–90% and more) TEC-positive or -negative deviations relative to the quiet conditions before the forthcoming seismic event, not less than M5 by magnitude, linked to the near-epicenter area. (2) The typical zone of the anomaly maximum manifestation extends more than 1,500 km in latitude and 3,500–4,000 km in longitude. (3) Anomaly living time is from several hours or days to couple of weeks before the earthquake release moment. (4) Analogous effects at the magnetically conjugated area are often reported. (5) In the case of strong low-latitudinal earthquakes, there are effects related to the modification of the ionospheric F2-region equatorial anomaly. From the analysis of the TEC deviations before a few strong recent seismic events (12 January 2010, Haiti; 1 and 2 January 2011, Argentina and Chile; 11 March 2011, Japan), these pre-earthquake TEC signatures are extended with terminator and ‘ban’-time effects. We strongly believe that the main reason for the appearance of these TEC anomalies is the vertical drift of F2-region ionospheric plasma under the influence of a zonal electric field of seismic origin. Increase of the atmospheric radioactivity level during earthquake preparation leads to enlargement of the ionization and electric conductivity of the near-ground atmosphere. Another (and possibly more effective) ionization mechanism proposed is the so-called positive holes. Changes of resistance of the underlying atmosphere lead to the generation of an external electric current flowing between the Earth and the ionosphere and to the corresponding disturbances of the ionospheric electric field and TEC. These disturbances were modeled by UAM and compared with the GPS TEC observations. Comparison shows satisfactory agreement between the model and observations. Methodical recommendations for detection of ionospheric earthquake precursors are given.