Revisiting interminima solar EUV change using adjusted SOHO SEM data

Abstract

AbstractThe change of solar EUV irradiance between the 1996 and 2008 minima is still in active debate due to a possible trend in the data from the Solar EUV Monitor (SEM) on board the Solar and Heliospheric Observatory (SOHO). For the period of years 2002–2012, i.e., the descending phase of solar cycle 23 (SC23) and the ascending phase of SC24, concurrent observations from the Solar EUV Experiment (SEE) on board the Thermospheric Ionospheric Mesospheric Energy and Dynamics spacecraft are available. We compare the data from the two instruments in terms of an expected linear relation between the EUV irradiance and the ionospheric peak electron density (NmF2). For the SEE data the NmF2‐EUV function is solar cycle invariant, but for the SEM data the function is found to be different between SC23 and SC24, which is hard to explain at present. An attempt is therefore made to obtain a unbiased SEM data set by using the SEE data as a calibration source. As a result, a temporal sensor drift coefficient of ∼−2%/yr is fitted, and the trend causing the unnatural solar cycle different NmF2‐EUV function is removed from the data. A new estimation of the interminima change of EUV irradiance based on the adjusted SEM data is ∼10% reduction in 2008 compared to 1996, much smaller than previous results. After adjustment the SOHO SEM could continue to be a reliable data source for solar EUV and aeronomic studies over both SC23 and SC24.