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  • Communications Effects of Space Weather
  • Space weather and VHF propagation

space weather and VHF propagation

  • .R Normally, signals in the VHF range (30 to 300 MHz) penetrate the ionosphere rather than being reflected. Hence, these frequencies are mostly used for line-of-sight communications. However, there are some circumstances under which VHF can be reflected back to earth making long distance communications possible. At the peak of especially strong solar cycles, VHF signals can in fact be reflected by the ionosphere. Examples of such cycles included the peak of Cycle 19 in 1957-58, of Cycle 21 in 1980, and Cycle 22 in 1990. At times during these peaks, the monthly sunspot number rises to extremely large values and the ionosphere reflects higher frequencies than is normally the case. At peaks of a large solar cycle, VHF transmission in the lower part of the band is most likely for low latitude circuits around the local noon during the equinox periods of March and September. VHF can also be reflected from clouds of increased ionisation in the E layer of the ionosphere. These phenomenon in known as sporadic E and the clouds are generally quite localised (around 100 km in size). Sporadic E occurs at a lower height than the F layer and this tends to limit the distance over which propagation is possible in a single hop. In some cases, multi hop transmission is possible to achieve longer distance transmission. For low latitudes (e.g. IPS data for Vanimo and for Darwin) sporadic E appears throughout the year with the peak occurrence during the day in the afternoon and evening. There is evidence of this peak becoming later in the June/July period. For mid latitudes (e.g. IPS data at Townsville, Brisbane and Norfolk Island) sporadic E is most likely in summer over the local noon period and in the afternoon. This combination gives the largest chance of sporadic E for any latitude, season and time of day. Sporadic E is then considerably weaker away from the summer and the peak moves to the late afternoon. For the more southerly regions in this band, sporadic E is almost absent during the equinox periods of March and particularly September. The reflection of VHF signals can also occur during aurorae - spectacular curtains of lights arising from charged particles originating from the sun. The aurora is associated with increased ionisation in the E layer and it is from this that the signals can be reflected. Aurorae and large geomagnetic/ionospheric disturbances are associated and so VHF transmission by this means occurs at times when HF may be experiencing problems. Aurorae are also most commonly seen at polar latitudes at which location HF transmission is most likely to experience problems. The ionised trails left by meteors as they burn up in the earth's atmosphere is also a means by which VHF can be reflected. Meteors occur in "showers" - periods of a few days when meteors are most common arriving from a direction in space. Such showers are the remains of decayed comets and their dates are the same from year-to-year giving repeatable opportunities for VHF transmission. Low band microwave communications (e.g. L band) and navigation systems (e.g. GPS) may be subject to degradation due to ionospheric scintillations, particularly in the tropical areas.

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