Table of Contents
Solar Terrestrial Data
From time to time you might come across an image like this, like those published at n0nbh.com.
This is solar-terrestrial data. Solar-terrestrial charts are used to evaluate radio propagation conditions.
TL;DR Quick Interpretation Tips
Here is a bit longer guide on how to interpret them.
Date and Time
Example: 04 Apr 2025 1800 GMT
This is the timestamp of the data in UTC (GMT). Data is typically refreshed every few hours.
Solar Indicators (Top Left)
These affect the ionosphere, which in turn impacts radio wave propagation.
| Label | Meaning | Notes |
|---|---|---|
| SFI (Solar Flux Index) | 178 | Higher SFI (100–200+) means better HF propagation |
| SN (Sunspot Number) | 124 | More sunspots = more ionization = improved HF conditions. 100-250 is the approximate range during solar maximum - see below for more info. |
| A / K Index | A=26, K=5 | A (avg daily geomagnetic activity), K (3-hour disturbances); lower values are better (A<20, K<4) |
| X-Ray | C2.1 | Solar flare level: A < B < C < M < X (ascending severity) |
| 304A | 143.3 @ SEM | Ultraviolet measurement, proxy for ionization |
| Ptn Flx (Proton Flux) | 928 | High values degrade propagation, especially near poles |
| Elc Flx (Electron Flux) | 15500 | Can impact satellites and VHF communications |
| Aurora | 6.7 @ 1.9 | Indicates auroral activity strength |
| Aur Lat | 60.7° | Estimated latitude of visible aurora |
| Bz | -6.1 | Southward magnetic field = higher chance of geomagnetic storm |
| SW (Solar Wind) | 479.1 km/s | Speeds > 500 km/s may cause disturbances |
VHF Conditions (Middle)
VHF propagation conditions, especially E-skip and meteor scatter.
| Band | Status |
|---|---|
| Aurora | Band Closed |
| 6m EsEU (Europe) | Band Closed |
| 4m EsEU | Band Closed |
| 2m EsEU | Band Closed |
| 2m EsNA (North America) | Band Closed |
| EME Deg (Moonbounce) | Fair |
HF Conditions (Top Right)
Propagation quality on HF bands.
| Band | Day | Night |
|---|---|---|
| 80m–40m | Poor | Poor |
| 30m–20m | Poor | Poor |
| 17m–15m | Fair | Fair |
| 12m–10m | Fair | Poor |
Fair: Usable but not idealPoor: Weak or unreliable propagation
Geophysical Environment
| Item | Meaning |
|---|---|
| Geomag Field | MIN STORM — mild geomagnetic disturbance ongoing |
| Sig Noise Level | S4–S6 — background band noise (S1 = low, S9+ = high) |
| MUF GR Athens | 20.27 MHz — Max usable frequency in that region |
| Solar Flare Prob | 66% — Chance of a solar flare causing disruptions |
MUF Map (Bottom Color Bar)
Visual indicator of Maximum Usable Frequency (MUF) across regions.
- Blue: <6 MHz — poor
- Yellow/Orange: 8–16 MHz — moderate
- Red: >18 MHz — good
Quick Interpretation Tips
- HF Operators:
- Look for: SFI > 100, SN > 50, A < 15, K < 4
- Avoid when X-ray is M-class or higher, flare probability > 50%
- VHF Es Activity:
- Watch for summer months, E-skip openings, and open band indicators
- Aurora:
- More active with Bz negative and solar wind > 500 km/s
Adapted from data provided by n0nbh.com.
Sunspot Number (SN) Overview
The Sunspot Number (SN) is a primary indicator of solar activity and directly influences HF radio propagation. SN varies over the 11-year solar cycle.
Typical Range of Sunspot Numbers
| Solar Cycle Phase | Approx. SN Range | Description |
|---|---|---|
| Solar Minimum | 0–20 | Very quiet sun, little or no sunspots. Weak HF propagation. |
| Rising Phase | 20–100 | Increasing sunspots, improving conditions. |
| Solar Maximum | 100–250+ | High activity, excellent HF propagation. Watch for radio blackouts due to solar flares. |
| Declining Phase | 20–100 | Gradual drop in activity, conditions slowly degrade. |
- All-time peaks have occasionally exceeded 250, though rarely.
- During deep minima, SN can remain near 0 for extended periods.
What Is a "Good" SN for Radio Operators?
SN > 50→ HF band openings become more consistent.SN > 100→ Excellent upper HF band propagation (15m, 12m, 10m).SN < 30→ Poor or unreliable conditions on high HF bands.
SN values are monitored by observatories like NOAA and SIDC. Regular updates available at NOAA Space Weather Prediction Center.