Last Updated on November 14, 2023 by John Berry
Meteor occurrence causes short-lived ionisation. Each one burns up as it descends through the Earth’s atmosphere. Of central significance is the additional ionisation as meteors descend through the E Region. Meteor scatter communications has parallels with other E Region propagation modes, such as sporadic E communications.
E Region
It all starts with the fact that the E Region at around 100km above the Earth’s surface has some electron density that could be augmented. So, it’s almost capable of supporting communications, but not quite. Importantly, it’s not space.
As we know though, anything above about 30MHz is, more often than not, lost to space. For VHF and above, with smaller wavelengths, no region has sufficient Sun-induced electron density to refract, scatter or reflect waves and return radio waves earthwards.
The graph on my page on the ionosphere shows the electron density for various atmosphere heights. Electron density varies also with year in the solar cycle (sunspot activity) and with reflection-point latitude.
For reference, you’ll note elsewhere on this site that the E Region is narrow and around 100km above the Earth. Geometry therefore dictates that skip distances for E Region reflection, refraction and scattering are centred around 1200km (1E) and around 2400km (2E).
The Sun
The Sun beats down on the ionosphere during the day. The normal electron density in the E Region sits at around 105 electrons per cubic centimetre (1011 per cubic metre) during the day. It falls to around 103 at night. Even 105 is not enough to be useful for radiocommunication – but importantly, it’s not zero.
Then enter meteors.
Somewhere between 8,000 and 12,000 meteors per day enter the Earth’s atmosphere. Some, termed sporadic meteors, are already in our solar system with Earth, orbiting the Sun. Others are in elliptical orbits crossing the Earth’s orbit. These, termed shower meteors, are the remnants of comets. The state is shown stylised below.
Sporadic meteors
The Earth and sporadic meteors are orbiting together. The Earth is also spinning on its axis, rotating once every 24 hours. In the morning the Earth’s spin, and the meteors’ orbit, are opposed, and the Earth’s atmosphere ‘scoops up’ meteors.
The concept is shown below.
The meteors that are scooped up by the opposing rotation have high closing velocity (and hence high energy). They enter the pull of the Earth’s gravity and descend.
As the meteors tear through the E Region, they ablate (have metal ions stripped off) with the heat from friction. They also interact with oxygen, nitrogen, and other gas molecules and the friction causes those molecules to ionise. The electron density rises with meteor occurrence, and this plasma is swept to the rear of the meteor as a tail. This enhanced ionisation, with free electrons and metal and gas ions, is short-lived. As soon as the plasma is formed, there’s re-combination. Energy is released and we may see this from Earth as a streak of light.
When one adds trail ionisation to that which exists normally in the E Region, this resultant can efficiently scatter, reflect, and refract radio waves. But it only does so for a short time. How efficient it is, depends on many variables: individual meteor velocity, angles to the transmit antenna, time of day, trail size, angles to the receive antenna, transmission wavelength and the rate of diffusion of the ionisation once formed.
Time of day
Time of day is significant. At night, a point on the Earth’s surface (let’s say UK) a) is not pointing at the Sun, and b) is moving with the meteors. Meteors have relatively lower energy. Conditions are not favourable for meteor trail ionisation.
As dawn breaks, the UK, comes round to see the Sun. E Region ionisation starts and the spin velocity and meteor velocity oppose, magnifying the meteor’s energy. Conditions for meteor scatter communications are then optimum.
After that, the meteor energy depletes, for the cycle to repeat again the next day.
Meteor occurrence
Shower meteors are typically of higher energy and give a longer and more useful trail than sporadic meteors. They are named after the constellation from which they appear to originate. But of course, as comet debris, they are on their own orbit.
The graph below shows the total rate of meteors per day across the months of the year.
This graph shows the typical rates of sporadic meteors, onto which the shower meteors are added. The greatest aggregate rate is to be found across the summer months. Then sporadic meteors appear at a rate of about 250 meteors per hour. The shower meteors add about 100 meteors per hour to give a total of about 350 meteors per hour.
In the autumn the sporadic meteors have a rate of about 130 meteors per hour. The shower meteors add about 50 meteors per hour to give a total of around 180 meteors per hour.
The image is simplified. The shower meteors in fact peak on certain days and their occurrence is somewhat cyclical across the occurrence period. On some days there will be few, while on others there will be a maximum.
Each meteor trail may support communications for a short time of between about 100 milliseconds and about 5 seconds. Or it may not. Whether it does support communications depends on a number of variables. The nature and probability of meteor burst communications is discussed on other adjacent pages on this site.