Irradiance Variations During This Solar Cycle Minimum

TL;DR

This study analyzes solar irradiance measurements during the deep 2007-2009 minimum, revealing lower irradiance levels compared to 1996, and discusses implications for understanding long-term solar variability.

Contribution

It provides the first comprehensive comparison of satellite-based solar irradiance data across two minima, highlighting secular changes and associated solar magnetic activity.

Findings

Lower total solar irradiance in 2008 compared to 1996

Reduced EUV irradiance indicating decreased solar activity

Coronal hole prevalence correlates with irradiance decline

Abstract

The current cycle minimum appears to be deeper and broader than recent cycle minima, and this minimum appears similar to the minima in the early 1900s. With the best-ever solar irradiance measurements from several different satellite instruments, this minimum offers a unique opportunity to advance our understanding of secular (long-term) changes in the solar irradiance. Comparisons of the 2007-2009 irradiance results to the irradiance levels during the previous minimum in 1996 suggest that the solar irradiance is lower in this current minimum. For example, the total solar irradiance (TSI) indicates lower irradiance in 2008 than in 1996 by about 200 ppm, and the SOHO Solar EUV Monitor (SEM) 26 to 34 nm irradiance is about 15% lower in 2008 than in 1996. However, these irradiance variations have 30-50% uncertainties due to these results depending strongly on instrument degradation trends over 12 years. Supporting evidence for lower irradiance include that the solar magnetic fields are lower and that there are more low-latitude coronal holes during this current minimum.