SSN Range Solar Activity Level HF Propagation
0 – 29 Very Low Poor
30 – 100 Low to Moderate Fair to Good
101 – 200 High Very Good
201+ Very High Excellent

Sunspot Number

SSN and HF Comms (0-30 MHz)

It’s important, and you need to know. 

The Sunspot Number (SSN) is a critical solar weather index that directly determines the quality of your HF communications. When sunspots appear on the solar surface, they indicate increased solar activity, which enhances the ionosphere’s ability to refract HF radio waves back to Earth.

Here’s why SSN matters for your HF operations:

Ionospheric Impact: Higher sunspot activity increases ionization in the F-layer of the ionosphere, particularly the F2 layer. This enhanced ionization raises the Maximum Usable Frequency (MUF), allowing higher HF frequencies to propagate via skywave. During high SSN periods, bands like 15m, 12m, and 10m become viable for long-distance communication.

Band Behavior by SSN Level:

  • SSN 0-29 (Very Low/Poor): Expect poor propagation conditions. The higher HF bands (20m-10m) will be largely unusable for DX. Stick to 40m and 80m for reliable communication, though even these may be limited to regional contacts.
  • SSN 30-100 (Low to Moderate/Fair to Good): 20m becomes your primary DX band. 17m and 15m may open sporadically during daylight hours. 40m remains solid for regional and some DX work.
  • SSN 101-200 (High/Very Good): Excellent conditions across most HF bands. 15m, 12m, and 10m open regularly for DX. 20m provides consistent worldwide propagation. This is prime time for contest operations and DXpedition support.
  • SSN 201+ (Very High/Excellent): Outstanding propagation conditions. All HF bands from 10m-80m can support long-distance communication. 10m may stay open well into the evening hours, and even 6m can exhibit occasional skip propagation.

Practical Operating Tips: Monitor SSN trends rather than daily values. Rising SSN indicates improving conditions over weeks and months. When SSN exceeds 100, check the higher bands first – you’ll often find them more productive than the traditional 20m and 40m frequencies.

The Wolf Sunspot Number and Solar Observation

The Sunspot Number, originally developed by Rudolf Wolf in 1849, represents the oldest continuous solar activity index. Wolf’s formula establishes the relationship:

SSN = k(10g + s)

Where:

  • g = number of sunspot groups
  • s = total number of individual sunspots
  • k = observatory reduction factor (accounts for observing conditions and equipment)

Data Sources and Methodology: The World Data Center for the Sunspot Index (WDC-SILSO) in Brussels, Belgium, maintains the international sunspot number database. Daily observations from a network of approximately 80 observatories worldwide contribute to the official SSN calculation. The Royal Observatory of Belgium coordinates this effort, ensuring data continuity and standardization.

Historical Context: Wolf established his observatory in Zurich, creating the “Zurich sunspot number” that became the international standard. The 10:1 weighting factor in his formula reflects the observational reality that sunspot groups are more reliably counted than individual spots, especially under marginal viewing conditions.

Scientific Significance: SSN correlates strongly with other solar activity indicators including solar flux index, geomagnetic indices, and solar irradiance variations. The 11-year solar cycle, characterized by SSN variations from near-zero to peaks exceeding 200, drives terrestrial ionospheric behavior and consequently HF radio propagation conditions.

Modern Applications: Contemporary space weather forecasting relies heavily on SSN trends. The values in our reference table (0-29 Very Low through 201+ Very High) correspond to measurable ionospheric electron density changes, with F2-layer critical frequencies varying proportionally with SSN levels.

In a Nutshell for the Radio Ham:

What is SSN? The Sunspot Number tells you how active the sun is right now. More sunspots = better HF radio conditions.

The Simple Rule: Higher numbers mean better DX opportunities on the higher HF bands.

Your Quick Reference Guide:

  • 0-29: Lousy propagation. Stick to 40m and 80m
  • 30-100: 20m is your friend. Check 17m and 15m during the day
  • 101-200: Great conditions! 15m, 12m, and 10m are open for business
  • 201+: Exceptional propagation. Even 10m stays open late

Bottom Line: When SSN is below 30, expect poor HF conditions – plan for local and regional contacts. When SSN climbs above 100, start checking the higher bands first. You’ll be surprised how well 15m and 10m perform during high sunspot periods.

Pro Tip: Don’t just look at today’s number. Watch the trend over several weeks. Rising SSN means improving conditions are coming, falling SSN means tougher times ahead for DX.

Remember: SSN directly affects how high in frequency you can communicate via skywave. Low SSN = low MUF = stick to lower bands. High SSN = high MUF = exploit those higher bands for amazing DX!