Indicators of HF propagation correlate with space weather

HF Propagation Indicators are essential for amateur radio operators and other communication services.
• The MUF, LUF, and ionospheric noise levels affect the HF propagation conditions.
• The SSN and SFI indicate solar activity that affect the MUF.
• X-Ray flares may correlate with potential blackouts and fadeouts.
• The geomagnetic indices, K, and A quantify geomagnetic instability that may disrupt HF propagation conditions.
• Other Space Weather indices cover a broader range of space-related measurements, including solar and geomagnetic activity.

1. Key propagation parameters indices include:
   1. MUF (maximum usable frequency) is the highest frequency possible for skywave propagation.
   2. LUF (lowest usable frequency) is the lowest frequency possible for skywave propagation.
   3. SNR (signal-to-noise ratio) indicates the quality of the received signal compared to background ionospheric noise.

   Solar indices (SSN and SFI) vs. geomagnetic indices (K or Kp and A or Ap) are often used as propagation indicators to evaluate prospective HF band propagation conditions.

    
2. Solar Indices are indicators that describe the activity of the sun, particularly in relation to its impact on the Earth's ionosphere and geomagnetic field.

   Key solar indices include:

   1. SSN (Sunspot Number) indicates the daily count of sunspots on the sun's surface over a 24-hour period. A higher SSN corresponds to greater ionization levels and a higher Maximum Usable Frequency (MUF).
   2. SFI - The Solar Flux Index measures radio emissions from the sun at a wavelength of 10.7 cm, correlating with solar activity. It’s determined by the strength of a signal received by satellite-borne microwave receivers at 2,800 MHz. Like the SSN, a higher Solar Flux Index indicates better HF conditions.
   3. 304A Index measures the solar radiation strength at 304 Ångstrom (30.4 nm), emitted primarily by ionized helium in the sun's photosphere. This parameter has two measurements: one from the Solar Dynamics Observatory's EVE instrument and the other from the SOHO satellite's SEM instrument. It accounts for about half of the ionization of the F layer in the ionosphere and loosely correlates to the Solar Flux Index (SFI). The background level is typically around 134 at solar minimums and can exceed 200 or more at solar maxima. It is updated hourly.
   4. Solar X-ray flares (1–8 Ångstrom) is measured by instruments onboard GOES satellites.
      Excessive X-ray flares can cause ionization at the D-region, leading to communication disruptions and blackouts.

---

 
3. Geomagnetic indices

   The geomagnetic indices quantify geomagnetic activity, characterized by disturbances in the magnetic field around the Earth.

   Geomagnetic indices include:

   1. The K-index measures geomagnetic activity over a 3-hour period, indicating the level of disturbance in Earth's magnetic field on a logarithmic scale from 0 to 9, with higher values indicating higher disturbances.
   2. The A-index is a daily average of the K-index values, providing a longer-term view of geomagnetic activity over a 24-hour period.
   3. The DST-index measures the intensity of the ring current around Earth, which affects geomagnetic activity.

   The "planetary indices" are Kp, AP, and Ap:

   4. The Kp-index is a geomagnetic index estimated over 3 hours with a resolution of 0.33 and a quasi-logarithmic scale. It is derived from measurements from 13 geomagnetic observatories, representing quasi-logarithmic values between 0-9.
   5. The ap-index is a geomagnetic index estimated over 3 hours, based on K index data from 11 Northern and 2 Southern Hemisphere observatories, with a linear scale of 0-400.
   6. The Ap-index is a planetary average of the A-Index, a geomagnetic index estimated over 24 hours and presented as a simple moving average of 8 values every 3 hours.

---

 
4. Space weather indices

   Space weather indices refer to various measurements that describe conditions in space, particularly those influenced by solar activity. These indices help predict and understand the effects of space weather on Earth and its technological systems. Space weather indices include:

   1. X-ray flux quantifies the intensity of solar flares, which cause radio blackouts and fadeouts at the equator and mid-latitudes.
   2. High solar proton flux enhances ionization when hitting Earth's magnetosphere, leading to increased absorption of HF radio waves, signal degradation, or complete communication blackout. This effect is similar to solar flares but is often more prolonged and affects larger areas near Earth's poles.
   3. High solar electron flux can significantly impact HF propagation. Key effects include:
      • Higher ionization in the D and E regions, causing auroral activity that leads to fading and scattering of the lower HF bands.
      • Increased noise on the lower bands, making it harder to receive weak signals.
      • Enhanced ionization in the F2 region, improving VHF signal propagation.

Summary:

Understanding these effects can help radio operators adjust their frequencies and strategies to maintain effective communication.

References:

1. Circular of Basic Indices for Ionospheric Propagation published by ITU
2. Description of propagation indices published by DXMAPS
3. HF Radio & Space Weather Dashboard published by EA4FSI
4. Geomagnetic Indices in Solar-Terrestrial Physics and Space Weather publication by M. Menvielle & A. Marchaudon
5. HF Propagation and Solar-Terrestrial Data by Paul L Herrman, N0NBH
6. Propagation related data from NOAA relayed by eHam
7. Quick Guide to HF Propagation Using Solar Indices by Doron Tal, 4X4XM
8. Real‑Time HF Propagation Space Weather
9. Solar activity published by Space Weather Live Belgium
10. Current Sunspot Regions published by Space Weather Live Belgium
11. Recent Solar X-ray data from the primary GOES satellite published by Space Weather Live Belgium
12. The Sun, the Earth, the Ionospher published by ARRL