Annual and semiannual variations of vertical total electron content during high solar activity based on GPS observations

Abstract

Annual, semiannual and seasonal variations of theVertical Total Electron Content (VTEC) have been investigatedduring high solar activity in 2000. In this work we useGlobal IGS VTEC maps and Principal Component Analysisto study spatial and temporal ionospheric variability. The behaviorof VTEC variations at two-hour periods, at noon andat night is analyzed. Particular characteristics associated witheach period and the geomagnetic regions are highlighted.The variations at night are smaller than those obtained atnoon. At noon it is possible to see patterns of the seasonalvariation at high latitude, and patterns of the semiannualanomaly at low latitudes with a slow decrease towards midlatitudes. At night there is no evidence of seasonal or annualanomaly for any region, but it was possible to see the semiannualanomaly at low latitudes with a sudden decrease towardsmid latitudes. In general, the semiannual behavior showsMarch–April equinox at least 40% higher than Septemberone. Similarities and differences are analyzed also with regardto the same analysis done for a period of low solar activity.