HF Propagation FAQ

Above about 100 the upper bands (15, 12 and 10 metres) open regularly. Between 80 and 100, 20 and 17 metres are good in daylight. Below 80 the upper bands are mostly closed, but the lower bands can still give excellent DX at night. See the solar flux index guide.

The K-index (and the planetary Kp) measures geomagnetic disturbance on a scale of 0 to 9. Kp 0 to 2 is quiet and good for propagation, Kp 4 is active, and Kp 5 or more is a geomagnetic storm that degrades high-latitude paths and lowers the usable frequencies. See K-index and A-index explained.

K is measured at a single observatory for three hours. Kp is the planetary average of many observatories. ap is a linear version of Kp, and the A-index (Ap) is the daily mean of the eight ap values. The Kp to A-index converter translates between them.

The high bands depend on strong ionisation of the F layer, which is driven by sunlight. After sunset the F layer loses ionisation and its maximum usable frequency drops, so 15, 12 and 10 metres close and the lower bands take over. See HF bands by day and night.

The gray line is the twilight zone at the day/night terminator. Absorption in the D layer is low there while the F layer still reflects, so low-band signals travel unusually far along it. Use the live gray line map and the sunrise and sunset calculator.

A strong solar flare (class M or X) increases ionisation in the D layer on the sunlit side of the Earth and absorbs HF signals for minutes to hours. NOAA rates them R1 to R5. Night-side paths are not affected by the flare itself. See solar flares and radio blackouts.

They are estimated from the solar flux index, the planetary Kp index and the GOES X-ray flux using a simplified model of the usable frequency, with penalties for geomagnetic disturbance and flares. They are indicative and do not replace real-time spots or a professional ionospheric forecast. See the methodology.

Solar flux, sunspot number, Kp, X-ray flux, solar wind, magnetic field, forecasts and alerts come from the NOAA Space Weather Prediction Center. Solar images come from NASA SDO, SOHO and NOAA. The data is refreshed about every 5 minutes.

The page data is refreshed about every 5 minutes. The solar flux index itself is measured once a day, with updated values published a few times a day, while Kp, X-ray flux and solar wind change continuously.

Yes. Use the free banners or the JSON, XML and RSS data feeds. Please keep the link back to this site.

No. Radio aurora depends on the geomagnetic activity (Kp), and sporadic-E depends on local atmospheric conditions, with a strong seasonal pattern in May to August. Neither follows the solar flux. See aurora and sporadic-E on VHF.