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A sudden ionospheric disturbance (SID) is an abnormally high plasma density in the ionosphere caused by an occasional sudden solar flare, which often interrupts or interferes with telecommunications systems. The SID results in a sudden increase in radiowave absorption that is most severe in the upper medium-frequency (MF) and lower high-frequency (HF) ranges. When a solar flare occurs on the Sun, a blast of ultraviolet and X-ray radiation hits the day-side of the Earth after 8 minutes. This high energy radiation is absorbed by atmospheric particles raising them to excited states and knocking electrons free in the process of photo-ionization. The low altitude ionospheric layers (D region and E region) immediately increase in density over the entire day-side. The Earth’s ionosphere reacts to the intense X-ray and ultraviolet radiation released during a solar flare and often produces shortwave radio fadeout on the day-side of the Earth as the result of enhanced X-rays from a solar flare. Shortwave radio waves (in the HF range) are absorbed by the increased particles in the low altitude ionosphere causing a complete blackout of radio communications. This is called a Short Wave Fadeout. These fadeouts last for a few minutes to a few hours and are most severe in the equatorial regions where the Sun is most directly overhead. The ionospheric disturbance enhances long wave (VLF) radio propagation. SIDs are observed and recorded by monitoring the signal strength of a distant VLF transmitter. As the fadeouts occur, reception of the station strength varies thus creating a fluctuating voltage output at the receiver which can be recorded and observed using a computer with a chart recorder program. You can build and investigate the phenomena of SIDs with a special receiver and a low cost data-logger setup. You can not only observe when solar flares are occurring but you can collect and analyze the data and display it on your computer. The SIDs receiver is a great opportunity to observe firsthand when a solar event occurs, but you can use it to predict when radio blackouts will affect radio propagation, which is extremely useful for amateur radio operators.
VLF signal propagation
Why do VLF signals strengthen at night instead of getting weaker? If propagation is basically via the waveguide effect, why doesn’t the signal drop-down at night when the waveguide disappears with the D-layer? Is there some kind of reduced absorption at night? If so, where is it taking place and why? Also, what accounts for the big fluctuations in signal strength at night, apparently more or less at random? The strength of the received signal depends on the effective reflection coefficient of the region from which the radio wave reflects in its multi-hop path between Earth and the ionosphere. In daytime the reflecting region is lower, the air density is higher, and the free electron density is controlled strongly by the solar radiation, etc. At night time the reflecting region is higher, the air density is lower, and the free electron density is controlled by variable ambient conditions as well as variable influences from electron “precipitation” from above, etc. At noon the electron density is about 10 electrons/cm3 at an altitude of 40 km, 100 electrons/cm3 at an altitude of 60 km, 1000 electrons/cm3 at an altitude of 80km and 10,000 electrons/cm3 at an altitude of 85 km. At night these figures become 10 electrons electrons/cm3 at 85 km, 100 electrons/cm3 at 88 km and 1000 electrons/cm3 at 95 km and then remains somewhat the same up to at least 140 km. At night the electron density in the lower part of the D region pretty much disappears. At 40 km the electron collision frequency is about 1,000,000,000 collisions per second whereas at 80 km the collision frequency drops to 1,000,000 collision per second. The reflection coefficient depends on (among other things) the number density of free electrons, the collision frequency, and the frequency of the radio signal. It is found by a mathematical integration throughout the entire D-region and of course the result depends on what time of the 24 hour day one performs the integration. We can think of the E-Layer propagating the signal at night. Then the prominent sunrise pattern we see is a shift from E-Layer propagation back to D-Layer as the sun rises and forms the daytime D-Layer. The sunset pattern is the reverse. An interesting feature of waveguide mode propagation was that the signal was split into two components which can form an interference pattern. You can build your own Sudden Ionospheric Disturbances (SIDs) receiver, and begin your own investigation of solar flares and their effects on radio propagation. The SIDs receiver is a simple VLF receiver designed to be used with a loop antenna which can be placed either inside or outside.
VLF signal propagation
Why do VLF signals strengthen at night instead of getting weaker? If propagation is basically via the waveguide effect, why doesn’t the signal drop-down at night when the waveguide disappears with the D-layer? Is there some kind of reduced absorption at night? If so, where is it taking place and why? Also, what accounts for the big fluctuations in signal strength at night, apparently more or less at random? The strength of the received signal depends on the effective reflection coefficient of the region from which the radio wave reflects in its multi-hop path between Earth and the ionosphere. In daytime the reflecting region is lower, the air density is higher, and the free electron density is controlled strongly by the solar radiation, etc. At night time the reflecting region is higher, the air density is lower, and the free electron density is controlled by variable ambient conditions as well as variable influences from electron “precipitation” from above, etc. At noon the electron density is about 10 electrons/cm3 at an altitude of 40 km, 100 electrons/cm3 at an altitude of 60 km, 1000 electrons/cm3 at an altitude of 80km and 10,000 electrons/cm3 at an altitude of 85 km. At night these figures become 10 electrons electrons/cm3 at 85 km, 100 electrons/cm3 at 88 km and 1000 electrons/cm3 at 95 km and then remains somewhat the same up to at least 140 km. At night the electron density in the lower part of the D region pretty much disappears. At 40 km the electron collision frequency is about 1,000,000,000 collisions per second whereas at 80 km the collision frequency drops to 1,000,000 collision per second. The reflection coefficient depends on (among other things) the number density of free electrons, the collision frequency, and the frequency of the radio signal. It is found by a mathematical integration throughout the entire D-region and of course the result depends on what time of the 24 hour day one performs the integration. We can think of the E-Layer propagating the signal at night. Then the prominent sunrise pattern we see is a shift from E-Layer propagation back to D-Layer as the sun rises and forms the daytime D-Layer. The sunset pattern is the reverse. An interesting feature of waveguide mode propagation was that the signal was split into two components which can form an interference pattern. You can build your own Sudden Ionospheric Disturbances (SIDs) receiver, and begin your own investigation of solar flares and their effects on radio propagation. The SIDs receiver is a simple VLF receiver designed to be used with a loop antenna which can be placed either inside or outside.
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| Sudden Ionospheric Disturbance Receiver |
SIDs research opportunities
You can join the foremost group involved with SIDs research. The AAVSO or American Association of Variable Start Observers SID Program consists of solar observers who monitor very low frequency (VLF) radio stations for sudden enhancements of their signals. Earth’s ionosphere reacts to the intense X-ray and ultraviolet radiation released during a solar flare. The ionospheric disturbance enhances VLF radio propagation. By monitoring the signal strength of a distant VLF transmitter, sudden ionospheric disturbances (SIDs) are recorded and indicate a recent solar flare event. All SID monitoring stations are home built by the observers. The SID station operates unattended until the end of each month. Recordings are then analyzed for the beginning, end, and duration of SID events. A simple A/D converter design for specific use with the VLF receivers is available by contacting the chairman. SID observers submit strip-charts or computer plots to the SID coordinator for visual inspection at the end of each month. Many observers analyze their own stripcharts and computer plots. Analyzed results are submitted via e-mail to the SID Analyst for correlation with other observers’ results. The final SID report combines individual observers’ reports with the SID Coordinator visual analysis. SID event results are sent monthly to the National Geophysical Data Center (NGDC) for publication in the Solar-Geophysical Data Report where they are accessed by researchers worldwide. The reduced SIDs data and particularly interesting plots are reproduced in the monthly AAVSO Solar Bulletin mailed to all contributing members.
You can join the foremost group involved with SIDs research. The AAVSO or American Association of Variable Start Observers SID Program consists of solar observers who monitor very low frequency (VLF) radio stations for sudden enhancements of their signals. Earth’s ionosphere reacts to the intense X-ray and ultraviolet radiation released during a solar flare. The ionospheric disturbance enhances VLF radio propagation. By monitoring the signal strength of a distant VLF transmitter, sudden ionospheric disturbances (SIDs) are recorded and indicate a recent solar flare event. All SID monitoring stations are home built by the observers. The SID station operates unattended until the end of each month. Recordings are then analyzed for the beginning, end, and duration of SID events. A simple A/D converter design for specific use with the VLF receivers is available by contacting the chairman. SID observers submit strip-charts or computer plots to the SID coordinator for visual inspection at the end of each month. Many observers analyze their own stripcharts and computer plots. Analyzed results are submitted via e-mail to the SID Analyst for correlation with other observers’ results. The final SID report combines individual observers’ reports with the SID Coordinator visual analysis. SID event results are sent monthly to the National Geophysical Data Center (NGDC) for publication in the Solar-Geophysical Data Report where they are accessed by researchers worldwide. The reduced SIDs data and particularly interesting plots are reproduced in the monthly AAVSO Solar Bulletin mailed to all contributing members.