Global and regional trends in ionospheric total electron content

A statistically significant positive trend of 0.6 +/- 0.3 total electron content unit (TECU; 1 TECU = 10(16) el m(-2)) per decade is detected in the 15 year record of daily averaged global total electron content obtained from multiple GPS observations between 1995 and 2010. The trend is extracted using a multiple regression analysis that simultaneously accounts for the comparatively larger global ionospheric responses to solar irradiance variability, solar-modulated annual and semiannual ionospheric oscillations, and geomagnetic activity. Geographical maps of regional trends in total electron content reveal both positive and negative local secular change during the past 15 years, with an overall larger increase in the Northern Hemisphere than in the Southern Hemisphere. Largest regional changes of as much as +/- 3 TECU per decade occur in the vicinity of 60 W to 60 E longitude and 15 degrees S to 30 degrees N latitude. TEC trend magnitude depends sensitively on the specification of solar EUV irradiance variations in the multiple regression model. The +0.6 TECU per decade trend pertains to equal solar irradiance levels during the 1996 and 2008 solar activity minima. When the specified solar irradiance is 15% lower in 2008 than in 1996, the derived global ionospheric trend increases to 3 TECU per decade. We contend that such a large global trend is implausible and that the associated anomalously low level of EUV irradiance in cycle 2008 minimum, reported in earlier publications, is unlikely to be real.