↑ What are the Solar Indices?

This page is part of the project "Understanding HF Propagation."
by Doron Tal, 4X4XM

Solar indices quantify observations of solar-derived processes that affect skywave propagation. These indices are used to understand and predict the behavior of the ionosphere in order to optimize terrestrial HF radio communications.

Some of the most commonly used solar indices include:

  1. Sunspot Number: This is a measure of the number of sunspots visible on the Sun's surface, which is an indicator of solar activity. Sunspots are areas of intense magnetic activity that can produce solar flares and coronal mass ejections that can affect the Earth's ionosphere and radio propagation.
  2. Solar Flux Index: This is an indicator of the solar radio energy in the 10.7 cm band, which serves as a gauge of the ionization state of the ionosphere.
    3. The solar indices: SSN, SFI
    Enhancement of HF propagation due to high Solar EUV
    ConditionsBADLowAverageGoodBetterBest
    MUF (MHz) <12<15> 21> 24> 28> 50
    SSN0255075100125150175200250
    SFI (sfu)6783102124148172196219240273

    Two additional indices, incorrectly labeled as "solar indices," are actually "geomagnetic indices," which measure Earth's magnetic field fluctuations induced by solar activity.

  3. K-Index quantifies geomagnetic activity resulting from solar activity, which can potentially disrupt radio communications through ionospheric disturbances.
  4. A-Index averages K index values over 24-hour period, provides a linearized view of the geomagnetic activity
    5. The geomagnetic indices: K, A
    Interference due to geomagnetic activity
    HF Propagation conditionsBestAveragePoorBAD
    Geomagnetic activity index (log-scale)K0123456789
    Geomagnetic activity index (linear)A 0  4  7  15 27 48 80132207400
    Reference to the conversion between A and K values🗗.
     
  5. The solar flare index: XR: X-Ray bursts may cause blackouts.
    6. Solar  Flare  Class🗗 A  B  C MX
    Radio Blackout Scale🗗R0R1R2R3R4R5
The propagation indices are typically updated in real-time or near-real-time, and are available online from various sources, including national meteorological and space weather agencies. Radio operators and scientists can use these indices to predict and optimize radio communications, and to study the effects of solar activity on the Earth's ionosphere and atmosphere.

*** QRZCQ server is down ***
Figure 11.7: The observed indices of propagation conditions
over the last 30 days, provided by QRZCQ🗗
Please note the correlation between the acronyms in the title (SF, SN, AI, KI, XR)
and the names of the relevant indices given below the graph:
SF:=Flux index; SN:=Spot number; AI:=A index; KI:=Kp index; and XR:=X-Ray index.

Read more about the solar Indices, SSN, and SF. See also an index of terms for HF Radio propagation.

The project "Understanding HF Propagation" provides a detailed overview and tutorials on HF propagation.
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