Ionospheric total electron contents (TECs) as indicators of solar EUV changes during the last two solar minima

Abstract

During the extended solar minimum from 2007 to 2009 anomalously low densities were observed in the Earth's thermosphere. Solar activity for the minimum of Solar Cycles 23 and 24 was lower than for Solar Cycles 22 and 23. However, a comparison of global daily averaged Total Electron Content (TEC) for these two solar minima does not show any significant difference. Why these significant changes between the 2008 and 1996 minima were observed in the He II EUV absolute irradiance and not in the global TEC for which EUV irradiance is one of the primary drivers is not well understood and may be interpreted as an effect of EUV instrument degradation. In this study, we analyze TEC data from 1995 to 2013 from the Center for Orbit Determination in Europe (CODE) database and compare global daily averaged TEC signal with TEC sectorial spherical harmonics which reflect a narrower temporal and spatial integration of the TEC in the Earth's ionosphere. Our analyses of both TEC data and errors show significant decreases of sectorial TECs for the 2008 and 2009 minimum compared to the minimum of 1996. We also compare the most recent version of Solar and Heliospheric Observatory/Solar EUV Monitor (SOHO/SEM) He II absolute EUV irradiance with the measurements from the Solar Dynamics Observatory/Extreme Ultraviolet Variability Experiment channels. This comparison shows that the EUV He II irradiance was about 12-±-4% lower for the minimum of 2008 and 2009 compared to the minimum of 1996 when a low-pass filter with a 365 day window was used. © 2014. American Geophysical Union. All Rights Reserved.