Reading the Solar Numbers
Every morning I check three numbers before deciding whether to bother listening: the solar flux index, the planetary K-index, and the sunspot count. For over a year I have noted them next to each entry without really understanding what they meant for the kitchen table, and this winter I finally sat down with the notes and tried to see what correlated with what.
The solar flux index is a measure of the sun’s radio output at 2,800 MHz, and higher numbers mean the upper layers of the ionosphere are more strongly ionised, which lets higher frequencies be reflected back to earth. Through the past year, near the peak of the solar cycle, the index has been running between about 150 and 230. On the days above 200 the 16 and 13 metre bands were alive well into the evening, with Asian stations audible at 17:00 UTC that on a day of 150 would have been gone by mid-afternoon. Below 140, which happened on a handful of days, 13 metres was dead and even 19 metres closed early.
The K-index measures geomagnetic disturbance on a scale from 0 to 9, and it turned out to matter more than I expected for the lower bands. On days with K at 4 or above, which usually follows a coronal mass ejection a day or two earlier, the 60 and 75 metre bands at dawn were noticeably weaker and more fluttery, and the far northern paths went first. One memorable storm in May took the whole band down to nothing but a hiss for two nights, and the aurora was visible from the balcony, which was some consolation.
What I learned from the correlation is partly that I had been listening at the wrong times. My best logs on the high bands were on high-flux, low-K days in the late afternoon, and my best logs on the low bands were on low-K days at dawn regardless of flux. The worst sessions, the ones where I sat for an hour and heard only static, were almost all on high-K days when I had looked only at the flux and thought the conditions were good.
The numbers are not a forecast of what I will hear, but they are a decent forecast of where to look, which in a hobby with limited evenings is worth a great deal.