Solar Indices
Solar indices are measurements that help us predict HF propagation conditions. Understanding these numbers is essential for planning your radio contacts.
Solar Flux Index (SFI)
The Solar Flux Index measures the Sun's radio emission at 2800 MHz (10.7 cm). It's the main indicator of solar activity.
| SFI | Activity | Effect on Bands |
|---|---|---|
| <70 | Very Low | Only low bands (40m-160m) usable |
| 70-90 | Low | Mid bands (20m-30m) occasionally open |
| 90-120 | Moderate | Good propagation up to 15m |
| 120-150 | High | Excellent propagation, 10m-12m open |
| >150 | Very High | All bands active, exceptional DX conditions |
K-Index
The K-index measures geomagnetic activity on a 0-9 scale. Lower values indicate stable conditions and good propagation.
| K-index | Conditions | Effect |
|---|---|---|
| 0-1 | Quiet | Optimal propagation |
| 2-3 | Calm | Good propagation |
| 4 | Active | Possible polar degradation |
| 5-6 | Minor Storm | Degraded propagation |
| 7-9 | Severe Storm | HF blackout possible |
A-Index
The A-index is the daily average of geomagnetic activity, derived from K values. It provides a broader view of conditions.
- A < 7: Quiet conditions, excellent for DX
- A 7-15: Normal conditions
- A 15-30: Elevated activity, variable propagation
- A > 30: Geomagnetic storm, avoid higher bands
Newell Coupling Index
The Newell Coupling Index (or Newell Coupling Function) is an advanced parameter that quantifies the effectiveness of the interaction between the solar wind and Earth's magnetosphere.
The formula combines:
- Solar wind speed (v)
- Transverse interplanetary magnetic field (Bt = √(By² + Bz²))
- Clock angle (θc) of the magnetic field
Formula: dΦ/dt = v^(4/3) × Bt^(2/3) × sin^(8/3)(θc/2)
| Newell Coupling | Activity | Effect on Propagation |
|---|---|---|
| <2000 | Low | Stable conditions, good propagation |
| 2000-5000 | Moderate | Possible degradation at high latitudes |
| 5000-8000 | High | Likely geomagnetic disturbance |
| >8000 | Very High | Imminent geomagnetic storm |
This index is particularly useful for predicting geomagnetic storms a few hours in advance, allowing radio amateurs to plan their activities.
How to Use the Indices
To optimize your contacts:
- High SFI + Low K: ideal conditions for all bands
- Low SFI + Low K: focus on low bands (40m, 80m, 160m)
- High K: wait for it to stabilize before operating
- High Newell Coupling: storm incoming, monitor the evolution
- Monitor trends: rising SFI promises improvements in coming days