Space weather
| Space weather | |
|---|---|
| What it is | Changing conditions on the sun and in near-Earth space that affect radio and technology |
| Main causes | Solar flares, coronal mass ejections, high-energy protons, fast solar wind from coronal holes |
| Forecast by | NOAA Space Weather Prediction Center (SWPC), Boulder, Colorado |
| NOAA scales | R1–R5 radio blackouts, S1–S5 radiation storms, G1–G5 geomagnetic storms |
| Key numbers | |
| K index | 0–9, geomagnetic disturbance over three hours |
| A index | 0–400, daily geomagnetic average |
| Solar flux index | Solar radio output at 10.7 cm; higher favors the upper HF bands |
| For hams | Lower K and A mean steadier HF; storms bring aurora on VHF |
Space weather is the term for changing conditions on the sun and in the space around Earth, and their effects on technology here. For radio amateurs it explains why a band that was wide open yesterday can be silent today. The sun constantly sends out light, radio noise and a stream of charged particles called the solar wind, and bursts of activity on top of that steady output, more frequent in the active years of the solar cycle, can strengthen, weaken or wipe out HF propagation for minutes to days.
Solar flares and radio blackouts
A solar flare is a sudden burst of radiation from an active region on the sun. Flares are graded by their X-ray output as class A, B, C, M or X, each step ten times stronger than the one before. The X-rays travel at the speed of light, reaching Earth about eight minutes after leaving the sun and at the same moment the flare itself is seen, so there is no warning.
On the sunlit side of Earth those X-rays strengthen the absorbing D layer of the ionosphere, causing a sudden ionospheric disturbance or radio blackout. HF signals fade out, the lowest frequencies first and most deeply, sometimes within a minute or two. A small flare may cause only a dip in signal strength; a large one can silence the whole of HF on the daylit side for an hour or more. The night side is unaffected.
Some flares and eruptions also accelerate protons to very high energies. These solar radiation storms reach Earth within tens of minutes to hours and pour into the atmosphere over the polar regions, where they cause polar cap absorption that can close paths over the poles for days.
CMEs and geomagnetic storms
A coronal mass ejection (CME) is a vast cloud of magnetized gas thrown off by the sun. One aimed at Earth usually arrives one to three days later, the fastest in less than a day. When it strikes Earth's magnetic field, particularly if its own magnetic field points southward, it can trigger a geomagnetic storm. Coronal holes, regions where the sun's magnetic field opens into space, release fast streams of solar wind that cause milder storms, and because the sun turns about once every 27 days as seen from Earth, these tend to recur on that schedule.
Satellites about a million miles sunward of Earth measure the solar wind before it arrives, giving roughly 15 to 60 minutes of warning that a disturbance is about to hit.
K and A indexes
Geomagnetic activity is reported with two numbers. The K index measures how much Earth's magnetic field was disturbed over a three-hour period, on a roughly logarithmic scale from 0 to 9. The A index is a daily figure on a linear scale from 0 to 400, worked out from the eight three-hour readings. The planetary versions, Kp and Ap, combine magnetometers around the world. For both, lower is better for HF: a K of 0 to 2 is quiet, and a K of 5 or more is a geomagnetic storm.
NOAA scales
The Space Weather Prediction Center rates events on three five-step scales, from 1 (minor) to 5 (extreme).
| Scale | Event | Based on | Main radio effect |
|---|---|---|---|
| R1–R5 | Radio blackout | X-ray flare strength (R1 is an M1 flare, R3 an X1, R5 an X20) | HF fadeout on the sunlit side |
| S1–S5 | Solar radiation storm | Flux of high-energy protons (10 MeV and above) | HF blackout over the polar regions |
| G1–G5 | Geomagnetic storm | Kp index (G1 is Kp 5, G5 is Kp 9) | Lower MUF, poor high-latitude paths, aurora |
The storm of May 2024 reached G5, the first storm at that level since 2003, and brought auroras into the southern United States.
Effects on amateur radio
A geomagnetic storm upsets the F2 layer, the main long-distance layer of the ionosphere. Its first hours can bring a brief improvement, but the usual result is a depleted layer and a lower maximum usable frequency (MUF) for a day or two, so the higher bands close early or not at all. Paths that cross high latitudes suffer most, which is why contacts between North America and Europe or Asia are often the first to go. Absorption in the auroral zone also hurts the low bands for stations in the far north.
Storms have an upside. They push the aurora toward lower latitudes, and VHF operators can bounce signals off it on 6 and 2 meters, a technique described under Auroral propagation. Quiet space weather, with a low K index and a high solar flux, is what produces the best days on the higher HF bands.
The SWPC publishes current values, alerts and forecasts, including a three-day forecast and a 27-day outlook. Many logging programs and ham websites display the key numbers, and propagation prediction tools use them.