top of page

Solar weather effects on HF propagation

by Dan Adair N6AAD


In addition to the routine solar energy and radio noise provided by the sun, specific events affect the ionosphere and can cause radio signal disruption. There are 6 types of space weather disturbances, based on solar Energetics or ejected Particles, that affect the ionosphere, including geomagnetic storms from gusts in the solar wind by CME’s or Coronal holes, solar radiation storms caused by increases in charged energetic particles, and radio blackouts caused by X-ray emissions.


Specific events: 


1   Sunspots occur continuously due to solar surface irregularities and vary in number over the 11 year solar cycle. They provide X-radiation frequencies that energize the F layers, improving skip. More is better. Their intensity is measured using solar flux units (SFU), also called 10.7 cm-solar flux, and appears as the Solar Flux index (SFI). Higher is better. Travel time to earth is 8 minutes. Sunspots can also produce solar flares.


2   Solar flares. They develop from sunspots and provide bursts of higher X-radiation frequencies that energize the D layer, increasing lower frequency absorption. More is bad. They are listed on the R-scale and can cause radio blackouts. These are also known as short wave fadeouts, and are caused by large eruptions of electromagnetic energy from solar flares on the Sun, mainly in the X-ray and extreme ultraviolet frequencies. Conditions that cause radio blackouts are measured by how much higher frequency X-ray radiation (1to10 A) from the Sun is arriving at Earth. The intensity of solar flares is described by a sequence of letter classes A, B, C, M and X. Travel time to earth is 8 minutes.


3   CME, or Coronal Mass Ejection. They develop from intense solar flares, releasing clouds of plasma that cause magnetic field disruption in the F2 layer, leading to Geomagnetic storms, and auroras. They are measured using the Kp scale. Higher Kp increases noise and reduces MUF. Geomagnetic activity is measured by magnetic field fluctuation caused by the CME plasma cloud. Globally, magnetic field fluctuations are measured by Kp values, collected daily and listed as G scale. The 3 hour K component, listed separately, is more important than the 24 hour A component for predicting present time propagation. Higher is bad. Travel time to earth is 1/2 day or more.  


4   Coronal holes are areas on the sun where strong magnetic field lines allow solar wind particles to escape at high speeds. They can develop at any time and at any location on the Sun. They form Coronal Hole High Speed Streams (CH HHS) which are described using Solar wind measurements including Speed (km/sec), particle density (p/cm3), and by the interplanetary magnetic field total strength (Bt) and orientation (Bz). The higher solar wind speed and intensity cause geomagnetic disturbances on Earth and enhanced auroral activity. The effects of the CH HHS are measured by the Disturbance Storm index. 


 The Disturbance Storm Time (DST) index is a measure of geomagnetic activity and used to assess the severity of geomagnetic storms caused by CME or coronal holes. It measures the growth and recovery of the ring current in the earth’s magnetosphere, responsible for shielding the lower latitudes of the Earth from geomagnetic storms. During a geomagnetic storm, the number of particles in the ring current will increase, reducing the effects of magnetometer readings.The lower (more negative) the value, the more energy is stored in the magnetosphere (bad for HF) and measured by the DST index, using nanoTesla units (nT).


5   Solar radiation storm, or Proton event. A solar radiation storm (also known as a Solar Proton Event or SPE) occurs often after major flare eruptions on the Sun when high energy protons are launched at very high speeds. High energy protons can damage or interfere with electronics. Also, they energize the D layer at higher latitudes, causing partial or complete blackout of HF communication in the polar regions. The strength is measured in proton flux units (PFU), measuring high energy protons( > 10 MeV).  Listed as  S-scale for solar radiation storm. Travel time to earth is 10 minutes to hours.


6   Electron alert. Total Electron Count (TEC) is significant in determining scintillation( twinkling), or the random fluctuation in the intensity of a radio wave, caused by variations in the refractive index of the plasma. An increasing TEC increases wave interference, and increases signal intensity variation and degradation. TEC is measured using Electron Flux Units. It is caused by uneven electron densities in the ionosphere and affects the E layer primarily.

bottom of page