What is Solar Proton Flux?
Solar proton flux refers to the flow of high-energy protons emitted by the Sun, especially during solar flares and coronal mass ejections (CMEs). These energetic particles travel at high speeds and can reach Earth within minutes to hours after a solar event.
Solar proton flux is measured in terms of the number of protons per square centimeter per second per steradian (particles/cm²/s/sr) and is tracked continuously by satellites like NOAA’s GOES series.
How Solar Proton Events Happen
During major solar flares (especially X-class flares) and CMEs, enormous amounts of charged particles — including protons — are ejected into space.
If these protons are directed toward Earth, they can:
- Penetrate deep into the polar regions of the atmosphere
- Increase ionization at lower altitudes
- Trigger geomagnetic storms alongside other space weather effects
These are called Solar Proton Events (SPEs) or Proton Storms.
How Solar Proton Flux Affects Ham Radio
Solar proton storms mainly affect HF communications (3–30 MHz) and primarily over polar regions. The key impacts include:
1. Polar Cap Absorption (PCA) Events
- High-energy protons greatly increase ionization in the D-layer of the ionosphere at high latitudes (near the poles).
- This causes extreme absorption of HF radio waves.
- HF communication becomes very difficult or impossible across polar paths — for example, flights and operations involving polar routes are severely affected.
2. Signal Fadeout and Loss
- Even mid-latitude paths can experience noticeable fading and increased background noise during strong proton events.
- Lower HF frequencies (80m, 40m) suffer the most during these absorption events.
3. Navigation and Timing Errors
- GPS and GNSS systems can be slightly degraded, causing timing errors.
- This can indirectly affect modes that rely on precise timing like WSPR, FT8, or APRS position reporting.
Quick Summary of Effects
| Solar Proton Activity | Ham Radio Impact |
|---|---|
| Minor Increase | Slight signal fading at high latitudes |
| Moderate Storm | HF blackouts over polar regions |
| Major Storm | Widespread HF blackout over polar and high mid-latitudes |
Monitoring Solar Proton Flux
Hams interested in long-distance (DX) work, polar routes, or contesting should watch for proton events:
- NOAA SWPC Alerts — for real-time solar proton flux warnings
- Solar Flux Monitors — which report proton density and energy levels
- Propagation Websites — that display warnings like “PCA Event in Progress”
Typical threshold for concern:
Proton flux above 10 pfu (particles >10 MeV) at 1 pfu (proton flux unit) starts to degrade polar HF paths.
Proton flux above 100 pfu usually causes a polar blackout.
Summary
Solar proton flux is an important but often overlooked aspect of space weather for ham operators. While solar flares disrupt signals quickly via X-ray bursts, proton storms create longer-lasting absorption across polar regions that can last hours to days.
Smart operators monitor proton events, especially if planning communications over the poles or working international HF contests.