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US Air Force Wants To Plasma Bomb The Sky To Improve Radio Communication (newscientist.com)
An anonymous reader quotes a report from New Scientist: [The U.S. Air Force has plans to improve radio communication over long distances by detonating plasma bombs in the upper atmosphere using a fleet of micro satellites. It's not the first time we've tried to improve radio communication by tinkering with the ionosphere. HAARP, the High Frequency Active Auroral Research Program in Alaska, stimulates the ionosphere with radiation from ground-based antennas to produce radio-reflecting plasma.] Now the USAF wants to do this more efficiently, with tiny satellites -- such as CubeSats -- carrying large volumes of ionized gas directly into the ionosphere. As well as increasing the range of radio signals, the USAF says it wants to smooth out the effects of solar winds, which can knock out GPS, and also investigate the possibility of blocking communication from enemy satellites. [There are at least two major challenges. One is building a plasma generator small enough to fit on a CubeSat -- roughly 10 centimeters cubed. Then there's the problem of controlling exactly how the plasma will disperse once it is released. The USAF has awarded three contracts to teams who are sketching out ways to tackle the approach. The best proposal will be selected for a second phase in which plasma generators will be tested in vacuum chambers and exploratory space flights.]
It depends on what point in the solar cycle, but the higher HF bands from ~14-15MHz up through 30MHz are far better during the day for skip due to D-layer and E-layer ionization and also more readily absorbs lower frequencies. At night, ~10MHz and lower works because there is still ionization in the F-layer which is more amenable to those frequencies, and why AM has to typically reduce power. Bear in mind that AM is technically an MF band (0.3-3MHz), which doesn't quite follow the same skywave propagation rules so strictly for a number of reasons such as auroral zone, ducting, and electron gyrofrequency that don't affect HF quite as severely.
You can still get near-vertical incidence skywave propagation during the day on the lower HF bands, but these are only good for a few hundred miles and can be subject to a higher than normal noise floor in the summer due to phenomena such as regional lightning.
I thought Solar UV deionized the skip layer during the day, which is why AM band signals travel farther at night?
No, solar UV ionizes the skip layer during the day down to lower altitudes, leading to refraction of AM band signals from those lower altitudes back to the ground closer to the transmitter than they would at night. At night, the ionized layer is higher, the refraction takes place at higher altitudes, so the signal hits the ground farther away.
There is another effect, too: The higher ionization during the day also leads to increased absorption (attenuation) of the AM band signals at even lower levels of the ionosphere (the D layer) than those at which they are refracted. The D layer disappears at sunset, so absorption by this cause goes away, increasing the received signal strength at distant locations.
The above behavior is for the AM broadcast band (~1 MHz). Above around 10-13 MHz, the situation reverses; during the day, these higher frequencies refract from layers at higher altitudes and suffer less from absorption (the absorption goes as an inverse square of the frequency), so they travel great distances, while at night, there is insufficient ionization to refract the signals back to ground, so they continue out into space and are lost. And above around 20-50 MHz, depending on the state of the sunspot cycle, there is insufficient ionization even during the day to refract the signals back to ground, so one has to resort to secondary mechanisms (e.g., ionization trails of meteors) for long-distance propagation.
Typically, typically. The above is a gross generalization: The effects of the ionosphere on radio waves depends on their frequency, their polarization, their direction and location relative to the geomagnetic equator, the time of day, the month of the year, the status of the sunspot cycle (solar wind), the magnitude of the Earth's magnetic field, the magnitude and direction of the magnetic field in interplanetary space, and eleventeen other factors. Radio propagation prediction software (e.g., VOACAP) deals in probabilities, not certainties.