How to read solar data for radio propagation [English version]

[ Este artículo está también disponible en Español ]

When you look at a solar-terrestrial report, you’ll likely come across various key values like the A-Index and K-Index, among others. These numbers help us understand whether the Earth’s magnetic field is calm or being affected by solar storms.

Quick summary for people in a hurry 🙂

• If you see high values in SFI, SN, and 304A, and low values in A, K, Bz, and Solar Wind, it’s a great day for operating on HF!
• On the other hand, if there are solar storms, high solar wind speeds, or M/X-class flares, propagation may become difficult.

Full Interpretation of each solar report value

K-Index

Measures rapid changes in Earth’s magnetic field

• Range: 0 to 9
• Low K (0–2): Very quiet geomagnetic field. Best time for HF operation, especially on high bands like 15 m, 12 m, and 10 m. Signals are stable and conditions are ideal.
• Moderate K (3–4): Slight disturbances, but still good propagation. Some noise or fading may occur.
• High K (5+): Geomagnetic storm. May cause temporary blackouts, lots of noise, and erratic propagation. High bands suffer more, but low bands (80 m, 40 m) may still work fine.

A-Index

Represents overall magnetic field stability over the past 24 hours

• Scale: 0 to 400
• Low A (0–7): Stable magnetic field, usually aligned with excellent propagation.
• Medium A (8–15): Active but still usable conditions. A good time to observe how bands behave.
• High A (16+): Indicates major storms or unstable conditions. Propagation becomes unpredictable.

SFI (Solar Flux Index)

Measures solar radiation at a specific frequency

• Above 100: Good conditions on high bands (20 m, 17 m, 15 m, 12 m, 10 m)
• Below 70: High bands likely closed or weak
  → Higher is better for DX on high bands

SN (Sunspot Number)

Visible sunspot count. Sunspots help ionize the atmosphere, improving propagation

• Above 50: Very good signal for HF propagation
• Low or 0: Likely poor propagation
  → More sunspots = better conditions

X-Ray (Solar Flares)

Indicates solar flare strength

• Class levels: A, B, C, M, X (from weakest to strongest)
• C-Class: Moderate flare
• M or X-Class: Can cause HF blackouts on Earth’s sunlit side
  → Watch out for M or X: risk of temporary HF shutdowns

304A (304 Angstrom Flux)

Measures extreme ultraviolet radiation affecting the ionosphere

• Above 100: Good ionization, great for long-distance (DX) propagation
  → High value = excellent conditions for skip propagation

Proton Flux (Ptn Flx)

Number of protons arriving from the sun

• High levels signal solar storms that can interfere with HF and damage satellites
  → If high, propagation may become erratic

Electron Flux (Elc Flx)

Measures energetic electrons in the magnetosphere

• Moderate-high: Helps high-band propagation
• Too high: Can cause noise or signal absorption
  → Good for propagation if it doesn’t spike too much

Aurora

Measures auroral activity

• Below 5: Normal conditions
• Above 5: Active auroras can negatively affect HF, especially at high latitudes
  → More aurora = worse HF propagation

Bz (Interplanetary Magnetic Field Component)

Shows how the solar magnetic field is interacting with Earth’s

• Negative (–): Can allow solar particles in, triggering auroras or disruptions
• Near 0 or positive (+): Calm conditions
  → Strongly negative Bz = potential geomagnetic storm

Solar Wind

Measures the speed of solar wind impacting Earth

• Below 400 km/s: Calm
• 400–600 km/s: Normal but a bit unsettled
• Above 600 km/s: Potential geomagnetic storm
  → Faster wind = more chance of instability

GeoMag Field

Overall state of Earth’s magnetic field
Can be labeled as: Quiet, Unsettled, Active, Storm, etc.
→ Quiet = ideal radio conditions

Sig Noise Level (Atmospheric Noise)

Baseline noise in S-units visible on your receiver

• S1–S2: Almost no noise, perfect conditions
• S5 or more: Lots of noise, harder to hear weak signals
  → Lower is better for reception