Solar Conditions

At rest, however, in the middle of everything is the sun.
Nicolaus Copernicus

The sun, ultimate source of life and energy on Earth, influences all radio communication beyond ground-wave or line-of-sight ranges. Conditions vary with such obvious sun-related cycles as time of day and season of year. Since these conditions differ for appreciable changes in latitude and longitude, and everything is constantly changing as the Earth rotates, almost every communication circuit has unique features with respect to the band of frequencies that are useful and the quality of signals in portions of that band.1

Current Solar Data and the meaning of some of the most important indicators

The Solar-Terrestrial Data widget from Paul L Herrman (N0NBH) may not update timely due to web server/page caching and the like.

It is actually

SFI

Solar Flux Index

Measures the intensity of the Sun’s radio emissions at a wavelength of 10.7 cm (2,800 MHz), reported in Solar Flux Units (SFU). It acts as a proxy for the ultraviolet (UV) and extreme ultraviolet (EUV) radiation that ionizes Earth’s upper atmosphere (the F-layer), allowing long-distance radio signals to bounce further. Values typically range from around 50 (poor conditions) to 300 or higher (ideal conditions).

SN

Sunspot Number

Measures the total visible number of dark, magnetically active spots and spot groups on the surface of the Sun. Higher sunspot numbers mean more solar activity and greater ionization of the atmosphere, which generally improves long-distance radio communication on higher bands (like 15m and 10m).

Kp

K-index Planetary

A short-term measure of the disturbance in the Earth’s horizontal magnetic field. Because the scale is logarithmic, each step represents a much larger increase in magnetic disturbance than the previous one. This is a planetary average of local K-index readings throughout the world.

  • Timeframe: Recorded every 3 hours (8 times a day).
  • Scale: A quasi-logarithmic integer scale from 0 to 9 (or in-thirds like 3+ or 4- for the planetary Kp index):
    • 0 – 1: Calm / Quiet
    • 2 – 3: Unsettled (good radio propagation usually holds at ≤ 3)
    • 4: Active
    • 5 or higher: Geomagnetic storm (up to 9 for an extreme storm)

Ap

A-index Planetary

A daily average of geomagnetic activity. Because the A-index uses a linear scale rather than a logarithmic one, severe magnetic storms stand out much more sharply. This is a planetary average calculated from Kp-index readings.

  • Timeframe: Calculated daily.
  • Scale: A linear scale generally ranging from 0 to 400 (where the 3-hourly K values are first converted into a linear “equivalent amplitude” or a-index, then averaged across the eight 3-hour blocks of the day.
    • 0 – 7: Quiet
    • 8 – 15: Unsettled (ideal radio conditions are generally A ≤ 15)
    • 16 – 29: Active
    • 30 – 49: Minor storm
    • 50 – 99: Major storm
    • 100 or more: Severe storm

HF and VHF Band Conditions are based on calculations using the above parameters. For more information and other tools visit Paul’s website or refer to his document Understanding HF/VHF/UHF/SHF Propagation relative to Guided Wave, Ground Wave, Direct Wave, Ionosphere, Troposphere, Aurora, Meteor Scatter, and Earth-Moon Earth (EME or Moon Bounce)

  1. American Radio Relay League (1991). The ARRL Handbook for Radio Amateurs. ISBN 0-87259-168-9 ↩︎