A Change of Solar He II EUV Global Network Structure of the Transition Region as an Indicator of Geo-Effectiveness of Solar Minima

TL;DR

This study examines how changes in the solar He II EUV global network structure during solar minima relate to geo-effectiveness, revealing that structural variations may serve as indicators beyond irradiance levels.

Contribution

It introduces the analysis of global network structure changes in the solar transition region as a new indicator of geo-effectiveness during solar minima.

Findings

Decreased EUV irradiance during the 2008-2011 minimum compared to previous minima.

Larger spatial power concentration in the transition region's network during the 1996 minimum.

Structural variations may indicate geo-effectiveness beyond irradiance levels.

Abstract

Solar activity during 2007--2009 was very low, causing anomalously low thermospheric density. A comparison of solar extreme ultraviolet (EUV) irradiance in the He II spectral band (26 to 34 nm) from the Solar Extreme ultraviolet Monitor (SEM), one of instruments on the Charge Element and Isotope Analysis System (CELIAS) onboard of the Solar and Heliospheric Observatory (SOHO) for the two latest solar minima showed a decrease of the absolute irradiance of about 15 +- 6 % during the solar minimum between Cycles 23 and 24 compared with the Cycles 22/23 minimum when a yearly running mean filter was used. We found that some local, shorter-term minima including those with the same absolute EUV flux in the SEM spectral band show a larger concentration of spatial power in the global network structure from the 30.4 nm SOHO Extreme ultraviolet Imaging Telescope (EIT) images for the local minimum of 1996 compared with the minima of 2008--2011. We interpret this larger concentration of spatial power in the transition region's global network structure as a larger number of larger area features on the solar disk. Such changes in the global network structure during solar minima may characterize, in part, the geo-effectiveness of the solar He II EUV irradiance in addition to the estimations based on its absolute levels.