HF radio interference is very dependent on the ionosphere. The ionosphere exists at a height of 50km to 500km above earth, and is formed by ionization of air atoms by incoming UV ionizing radiation from the sun. The ionosphere is a weakly ionized plasma, or gas, which effects radio wave propagation.
The atmospheric chemistry determines the structure, which is divided in to layers D, E, F1 and F2 in order of increasing height. The F layer is of key importance to HF communications. This is present both day and night, the strongest height at night reflects highest radio frequencies in HF bands. Extreme ultraviolet EUV radiation is responsible for forming and maintaining the ionosphere. This radiates from hot bright regions overlying sunspots, so this is also dependant on solar sunspot activity.
At lows in the sun solar cycle EUV radiation is weak and density of charged particles in the F region is lowest. This means only lower frequency HF signals can be reflected. At cycle peaks the EUV and ionosphere density are both large and higher frequencies in the HF band can be reflected.
Other factors for HF radio communications are the season, time of day, and the latitude. Solar flares produce a great deal of electromagnetic radiation. The x-ray component increases the D layer ionization. HF communications use the F layer above so they must transit the D layer twice during any signal skips. The increased ionization with higher density of neutral particles results in the absorption of signal in the D layer during a major solar flare.
This is termed Sudden Ionosphere Disturbance (SID) it is observed as increased attenuation of HF signals in particular at lower frequencies. This is also called SWF or short wave fadeout. SSWF for sudden and GSWF for gradual. This follows closely with solar flare patterns, and is also observed in conjunction with solar flares.
They characteristically have rapid onsets of several minutes and decline periods up to an hour or more. SWF affects lower frequencies in HF band and higher frequencies may remain unaffected or be the first to recover.
Signals in VHF range 30-300MHz penetrate the ionosphere rather than reflect off it. These frequencies are as we know, line of sight communications. There are circumstances in which VHF can reflect back giving “freak” long distance communications. During very strong solar cycles, VHF signals can be reflected back by the ionosphere. This occurred during cycle 19 in 1957/58, cycle 21 in 1980 and cycle 22 in 1990.
During these peaks, the monthly sunspot average rose to extremely high values and the ionosphere reflected higher frequencies than normal. At peaks in a large cycle, VHF transmission in the lower part of band is most likely for low latitude circuits around local noon during equinox periods of March and September.
VHF can also be reflected from clouds of increased ionization in the E layer of the Ionosphere. Reflection of VHF can also occur during auroras, those beautiful light curtains that are caused by charged particles from the sun. This is associated with increased ionization in the E layer and this is what reflects the signals. The ionized trails left by meteors burning up during entry to earth atmosphere are also a way for reflecting VHF signal. As arrival of meteor showers is also normally a periodic occurrence, predictions for these periods is also often possible. More great information at fishing and boats.