Comparative analysis of extreme ultraviolet solar radiation proxies during minimum activity levels

Abstract

Four extreme ultraviolet (EUV) solar radiation proxies (Magnesium II core-to-wing ratio (MgII), Lyman α flux (Fα), 10.7-cm solar radio flux (F10.7), and sunspot number (Rz)) were analyzed during the last four consecutive solar activity minima to investigate how they differ during minimum periods and how well they represent solar EUV radiation. Their variability within each minimum and between minima was compared by considering monthly means. A comparison was also made of their role in filtering the effect of solar activity from the critical frequency of the ionospheric F2 layer, foF2, which at mid to low latitudes depends mainly on EUV solar radiation. The last two solar cycles showed unusually low EUV radiation levels according to the four proxies. Regarding the connection between the EUV “true” variation and that of solar proxies, according to the foF2 filtering analysis, MgII and Fα behaved in a more stable and suitable way, whereas Rz and F10.7 could be overestimating EUV levels during the last two minima, implying they would both underestimate the inter-minima difference of EUV when compared with the first two minima.