I frequently hear talk about SSNs in Amateur Radio, and somehow if the number is high, that's a good thing. I just don't really understand what they are, and what they mean, and I'd like to better understand them. What is the big deal about SSNs, what does it mean when they are high and low, etc?
It refers to 'smoothed' sunspot number, and is needed by many software utilities to calculate optimal propagation. This refers to the current number of dark spots on the sun associated with high magnetic activity. When the number is high, the whole HF range up to 30 MHz can be used usually around the clock and F2 propagation up to 50 MHz is observed frequently depending upon daily solar flux 10.7cm radiation values. During solar minima, or minimum sunspot counts down to zero, propagation of frequencies above 15 MHz is generally unavailable.
In other words, high levels of sunspot activity lead to better signal propagation on HF bands, although they also increase the levels of solar noise and ionospheric disturbances (caused by impact of the increased level of solar radiation on the ionosphere).