Solar Terrestrial Data

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 ideal
Poor: 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.

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