The solar cycle and the long-term modulation of the SR

In addition to sudden events, the Sun acts on the SR slowly as well, in the rhythm of the roughly eleven-year solar cycle. The cycle modulates the overall state of the ionosphere — the level of ionization and the properties of the D-region — and thereby introduces a long-term component of variability into the SR parameters Satori 2016.

This modulation is slow and overlaps with many other long-term influences (the seasonal course of lightning activity, climatic trends), so isolating it from the data requires long measurement series and careful statistics. Unlike individual flares, which cause step changes within minutes to hours, the solar cycle manifests itself as a gradual shift of the background across years.

When interpreting multi-year SR records it is therefore necessary to treat the phase of the solar cycle as one of the factors influencing the state of the upper boundary of the cavity.