Trends in the Equatorial Electrojet due to secular variations in the Earth?s magnetic field

BRUNO S. ZOSSI; ANA G. ELIAS; MARTA ZOSSI DE ARTIGAS; BLAS F. DE HARO BARBAS

Praga

Congreso; 26th IUGG General Assembly (International Union of Geodesy and Geophysics); 2015

IUGG - IAGA - IAMAS

The ionosphere over the equatorial region is controlled by solar ionizing radiation and atmosphere-ionosphere interaction in the environment of the geomagnetic field. The portion of this field generated in the Earth?s core, which accounts for more than 90% of the field observed at Earth?s surface, presents secular variations in strength and orientation. Due to this field is horizontal at the dip equator, there is an enhancement of the Cowling conductivity (Sc), which results in an enhanced eastward current, called the equatorial electrojet (EEJ), flowing along the dayside dip equator. This current is one of the very few phenomena in the upper atmosphere that is highly sensitive to the strength and distribution of the Earth´s core field. It depends, among other variables, on the ionospheric conductivity and the dip equator geographical location. The conductivity in turn depends on the Earth?s main magnetic field, which is mainly dipolar, and the electron concentration in the E region, measured by foE. Since the field dipole moment (M) shows secular variations, induced long-term changes in EEJ might be expected. To detect if M variations and the dip equator secular displacement are able to induce detectable trends in EEJ, the variation of the horizontal intensity (H) at the equatorial geomagnetic station Trivandrum (geographic coordinates: 8.5N, 77.0E) is assessed for the period 1957?1999. During this period, the dip equator latitude at 77.0°E moved from ~9°N to ~8°N, and M decreased by ~3%. Taking also into account the approximated scaling on M of Sc and EEJ strength (~M-5/3 and ~M-2/3 respectively), the expected and experimental trends are analyzed. Another possible mechanism that would be able to induce EEJ trends is the increasing concentrations of greenhouse gases that produce a cooling effect in the upper atmosphere with a consequent lowering of the height level of the region where the EEJ flows, and an increasing trend in foE. The implications of long term trends originating from the Earth?s main magnetic field variations, such as the present study, contributes for a better understanding and evaluation of the overall trends observed in the ionosphere.