Space Weather and the Solar Cycle

Some days the HF bands are wide open and other days they sound almost empty. Most of that change comes from the Sun, which builds the ionosphere that bends your signal home and sometimes shakes it hard.

You do not need a physics degree to follow space weather. A handful of numbers tell you most of what matters, and NOAA’s Space Weather Prediction Center (SWPC) publishes them free every day.

Sunspots and the Solar Cycle

Sunspots are dark patches on the Sun’s visible surface. They form where strong magnetic fields push up from inside, making the gas there cooler and darker than its surroundings. Most flares and eruptions start in sunspot groups.

The number of sunspots rises and falls over about 11 years, from solar minimum to solar maximum and back. Cycles are numbered, and each differs in strength and length, so treat 11 years as an average, not a timetable.

Solar Cycle 25 began at the solar minimum of December 2019. In October 2024, NASA and NOAA announced that the Sun had reached the solar maximum period of this cycle. For where things stand today, check the solar cycle pages on the NOAA Space Weather Prediction Center site rather than predictions you read on forums.

How the Sun Shapes the Ionosphere

Ultraviolet and X-ray light from the Sun strips electrons from atoms high in the atmosphere, creating the D, E and F regions of the ionosphere. The F region does most of the work of bending long-distance HF signals back to Earth, and the more strongly it is ionized, the higher the frequency it can return.

This is why the higher HF bands, such as 15, 12 and 10 meters, come alive for worldwide contacts in the years around solar maximum and can stay quiet for long stretches near minimum. The lower bands, such as 80 and 40 meters, depend much less on a busy Sun, so they stay useful through the whole cycle.

The night side of Earth gets far less ionization, so the highest usable frequency drops after dark. Our page on gray line propagation covers the special openings along the sunrise and sunset line.

Solar Flux, K and A

The solar flux index, often written SFI or F10.7, measures radio noise from the Sun at 10.7 centimeters (2800 MHz). Canada has measured it since 1947, today at Penticton, British Columbia. It follows the sunspot number closely and tracks the extreme ultraviolet output that feeds the ionosphere.

SWPC notes that over a cycle it can range from below 50 to above 300 solar flux units. A higher solar flux usually means higher usable frequencies, but the trend over several days tells you more than one reading.

The K index measures how disturbed Earth’s magnetic field is over a three-hour period. It runs from 0, which is quiet, to 9, which is severely disturbed. The planetary version, called Kp, describes the disturbance across the whole globe, and a Kp of 5 or more counts as a geomagnetic storm.

The A index is a daily measure of the same disturbance, and the planetary value is called Ap. For HF, you want K and A low: a quiet magnetic field usually means steadier signals, especially on paths near the poles.

Flares and Radio Blackouts

A solar flare is a sudden burst of energy on the Sun that lasts minutes to hours. Flares are graded by their X-ray output with the letters A, B, C, M and X, and each letter is ten times stronger than the one before it.

Flare energy travels at the speed of light, so the sunlit side of Earth feels it the moment the flare is seen. The extra X-rays charge up the lowest layer, the D region, where HF signals lose energy or are absorbed completely. A busy band can go quiet at once, while the night side carries on as usual.

NOAA calls this a radio blackout and rates it on the R scale, which starts at an M1 flare. An R3, for example, means a wide area HF blackout on the sunlit side for about an hour.

CMEs and Geomagnetic Storms

A coronal mass ejection, or CME, is a huge cloud of plasma and magnetic field thrown off the Sun. It travels far slower than light. SWPC says the fastest Earth-directed CMEs can arrive in as little as 15 to 18 hours, while slower ones take several days. That delay gives you warning.

A CME whose magnetic field points south for a long stretch can set off a geomagnetic storm when it reaches Earth. Fast solar wind from coronal holes can cause milder storms, and since a coronal hole can last several 27-day solar rotations, those storms may repeat.

Big eruptions can also hurl energetic protons toward Earth in tens of minutes or less. They pour into the atmosphere near the poles and absorb HF there, sometimes for days. This is a solar radiation storm.

NOAA’s R, S and G Scales

NOAA sums all this up in three scales. The HF effects below follow NOAA’s own descriptions.

• R for Radio Blackouts: Caused by X-rays from flares. Effects are on the sunlit side of Earth, from minor degradation at R1 to a complete HF blackout lasting hours at R5.
• S for Solar Radiation Storms: Caused by energetic protons. Effects are mostly on paths through the polar regions, from minor impacts at S1 to a possible complete HF blackout through the polar regions at S5.
• G for Geomagnetic Storms: Tied to the Kp index, with G1 starting at Kp 5. HF can fade at higher latitudes at G2, become intermittent at G3 and sporadic at G4, and may be impossible in many areas for one to two days at G5.

Putting It to Use

A quick check before you sit down at the radio explains a lot of what you hear.

• Look at the current R, S and G levels, the solar flux and the Kp on the SWPC site once a day.
• If HF goes quiet all at once in daylight, check for a flare before you start checking your coax.
• During a geomagnetic storm, favor paths that stay away from the poles and try weak-signal modes such as FT8 or CW.
• Keep notes on what you hear next to the day’s numbers. After a few months, you will know what your own station can do.

When a term is new to you, look it up in our ham dictionary. For turning these numbers into a plan for tonight, read our companion article on reading propagation forecasts.