ANGEO Annales Geophysicae ANGEO 1432-0576 Copernicus GmbH Göttingen, Germany 10.5194/angeo-23-753-2005 Simultaneous observations of the main trough using GPS imaging and the EISCAT radar Meggs R. W. 1 Mitchell C. N. 1 Howells V. S. C. 2 Department of Electronic and Electrical Engineering, University of Bath, BA2 7AY Bath, UK Rutherford-Appleton Laboratory, Chilton, OX11 0QX, UK 30 03 2005 23 3 753 757 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.ann-geophys.net/23/753/2005/angeo-23-753-2005.html The full text article is available as a PDF file from http://www.ann-geophys.net/23/753/2005/angeo-23-753-2005.pdf

We use a digisonde at Jicamarca and a chain of GPS receivers on the west side of South America to investigate the effects of the pre-reversal enhancement (PRE) in <i><b>E</b></i>x<i><b>B</b></i> drift, the asymmetry (<i>I<sub>a</sub></i>) of equatorial ionization anomaly (EIA), and the magnetic activity (<i>K<sub>p</sub></i>) on the generation of equatorial spread F (ESF). Results show that the ESF appears frequently in summer (November, December, January, and February) and equinoctial (March, April, September, and October) months, but rarely in winter (May, June, July, and August) months. The seasonal variation in the ESF is associated with those in the PRE <i><b>E</b></i>x<i><b>B</b></i> drift and <i>I<sub>a</sub></i>. The larger <i><b>E</b></i>x<i><b>B</b></i> drift (&gt;20m/s) and smaller |<i>I<sub>a</sub></i>| (&lt;0.3) in summer and equinoctial months provide a preferable condition to development the ESF. Conversely, the smaller <i><b>E</b></i>x<i><b>B</b></i> drift and larger |<i>I<sub>a</sub></i>| are responsible for the lower ESF occurrence in winter months. Regarding the effects of magnetic activity, the ESF occurrence decreases with increasing <i>K<sub>p</sub></i> in the equinoctial and winter months, but not in the summer months. Furthermore, the larger and smaller <i><b>E</b></i>x<i><b>B</b></i> drifts are presented under the quiet (<i>K<sub>p</sub></i>&lt;3) and disturbed (<i>K<sub>p</sub></i>&ge;3) conditions, respectively. These results indicate that the suppression in ESF and the decrease in <i><b>E</b></i>x<i><b>B</b></i> drifts are mainly caused by the decrease in the eastward electric field.