Long-term relationships between sunspots, solar faculae and the ionosphere

Abstract

Evidence is presented which shows that the relationship between the ionospheric critical frequencies and the sunspot numbers has changed significantly on a secular time-scale. From the date of the earliest ionospheric observations in 1933, the critical frequency of the F2-layer corresponding to a fixed sunspot number has varied in such a way that the lowest values occurred around 1955 when the frequencies were about 1 MHz less than they had been 15 years earlier. Corresponding effects occur in the E-layer, and the phenomenon is observed in both hemispheres. Various types of solar data covering several decades are used to show that the varying relationship between the critical frequencies and the sunspot numbers is not introduced by any failure of the sunspot numbers to represent solar activity correctly. Results are presented which lead to the conclusion that the ionospheric effects arise because a significant proportion of the solar ionizing flux is associated with photospheric faculae and experiences a time-variation linked to the total facular area. The importance to the ionosphere of this faculae-associated radiation is not negligible; its contribution is normally about 40%, but can at times exceed 50%, of the contribution associated with sunspots. It is shown that in the long-term average the ionizing flux associated with a given facular area is 15–20% of that associated with the same area of sunspots. These results have implications for solar physics, and also for ionospheric forecasting systems that do not include the effects of the faculae-associated radiation.