Page 004PropagationŠiauliai
Reading the solar numbers without pretending to be a physicist
Three numbers get quoted constantly and explained rarely: the sunspot number, the solar flux and the K index. This is what each of them actually changes about an evening on the air, written the way I wish somebody had written it for me in 2020.
- Date
- 24 November 2025
- UTC
- 1112
- Band
- 160 m
- Mode
- CW
- Typed up
- 11 March 2026
- Reading
- 7 min
Three numbers and what they are for
The sunspot number counts the spots visible on the face of the sun and serves as a rough measure of how active it is. The solar flux is a radio measurement taken at a fixed frequency, and it tracks the same activity rather more steadily from one day to the next. Both, loosely, tell you how well the upper ionosphere will reflect signals, and therefore how high a frequency will still come back to earth instead of carrying on into space. High numbers open the higher bands. That is nearly the whole of it.
The K index is different in kind, and confusing it with the other two costs beginners a great many evenings. It measures disturbance in the earth's magnetic field over three-hour periods, on a scale running from nought to nine. Where flux describes what the bands could manage, K describes what is being broken. A day can show excellent flux and a K index of five, and the bands will be worse than on a quiet day with half the flux, particularly this far north.
Noted in the margin
- Sunspot number: how active the sun is, counted by eye
- Solar flux: the same activity, measured steadily by radio
- K index: how disturbed the earth's magnetic field is right now
- Flux says what is possible; K says what is being spoiled
The K index is the one that ruins evenings
Šiauliai sits at nearly fifty-six degrees north, close enough to the auroral zone that magnetic disturbance reaches us before it reaches the people writing the guides in southern Europe. A K index of four is an inconvenience down there and an evening indoors up here. Signals arriving over paths that cross high latitudes go first: the north-west, anything towards northern Scandinavia, and the long way round over the pole. What remains is the southern half of the compass, quieter than usual.
Recovery runs slower than onset, which is the part I misread for two seasons. A disturbance peaking on Tuesday afternoon does not leave the bands by Tuesday night. My own pages show the bands here taking between one and three days to come back, with the low bands taking longest of all. Reading a single K figure and expecting the evening to match it is no better than reading one hour of a forecast and planning a week around it.
Grey line across flat country
The grey line is the moving band of twilight dividing day from night across the earth, and along it the ionosphere sits briefly in a useful state: the absorbing lower layer has faded while the reflecting layer above it has not. Signals travelling along that line can go a very long way on very little power. In country as flat as this, with nothing to block the horizon in any direction, the effect is easy to observe and unreasonably easy to become fond of.
In practice it means checking the low bands within twenty minutes either side of sunrise and sunset, rather than assuming the evening has plenty of time in it. The window is short, and at midwinter at this latitude it is shorter still, perhaps half an hour at each end of the day. The contacts I remember best from five years of this all happened in those minutes, with the sky orange over the fields and the tea going cold behind me.
Reading the numbers against your own log
Published figures describe the whole planet, and what you want to know is what happens above your own roof. The way to bridge that gap is dull and effective: write the day's flux and K index at the head of each page, then reread a season at a time. After two winters you will hold your own table of what a given set of numbers means from your own garden with your own antennas, and it will not match anybody's published chart exactly.
Mine says, for instance, that a K index of two or three makes very little difference to the eastern paths on forty metres in the evening and a great deal of difference to anything north of here. It says the higher bands in autumn open earlier on high-flux days but close at very nearly the same hour regardless. Those are small, local facts. They are also the only ones I trust, because I watched them happen from this chair.