Forward Scatter Observations of the Perseids Meteor Shower 2005
Meteor showers owe their names to the point in the celestial sphere from where they seem to emanate. This point is known as the Radiant so most showers are named after the constellations in which the Radiant is located.
One of the most prominent showers of the year is the Perseids which start around 25 July to 20 August, maximum being about 12 August. Their Radian lies within the constellation of Perseus (The Greek hero who saved Andromeda from Cetus the sea monster) The Perseid shower normally provides fast moving meteors with persistent trains and if the conditions are right can give a brilliant visual display. These meteors are normally leaving trails at a rather narrow altitude band of between approximately 85km and 105km.
Detecting meteors using radio normally employs either Back-Scatter or Forward-Scatter of radio waves reflected off the meteors ionised plasma trail. Back Scatter (fig 1. St A to St B) is one of the commonest methods used by professionals to collect data on meteors, because this method uses a transmitter to reflect the stations signal back to its location these stations are sometimes called “meteor radar”. Amateurs as well use
Back-scatter to work stations on wavelengths and modes that under normal conditions would not be received at such close range to the transmitting station.
Back-scatter requires the transmitted signal to strike the Meteors ionised plasma trail at an angle that reflects the signal back to or near the transmitting station. This method allows astronomers to study meteors in greater detail based on size, velocity, direction and angle of entry into the earth’s atmosphere.
St A = Transmitter St B = Meteor Radar Receiver St C = Forward Scatter Receiver T = tangent Plane. Meteor moving in at steep angle less effect at ST C.
(and I include side-scatter as for all intents and purposes the data is undistinguishable
using my equipment) can also be used by professionals and amateurs to transmit
short bursts of information and can even be used as a backup to satellites.
The main difference being to increase range by reflecting the signals forward
away from the transmitting station (Fig1 St A to St C).
The advantage for the meteor observer is that any conveniently located transmitter on the right frequencies, even if not being used for meteor detection, can be utilised. I prefer to use TV carriers and use USB (Upper Side Band) as this gives me a narrow yet strong signal, which I then tune about 100khz away from. It is possible to find listings for frequencies on the web.
The equipment I use is a communications receiver (Fig 2), one that covers 30mhz to 300mhz and operates in USB, FM and AM modes, with an external speaker or Aux output. The antenna I use is a discone type that covers the bands that the receiver uses; this is omni-directional unlike if you were to use a Yagi type antenna. A PC with a sound card, most any will do, and a Audio Spectrum Analyzer program; these can be found on the web there are quite a few good ones around freeware. A lead to connect the receiver to the sound card in the PC and an atlas; this is to find the locations of the transmitting stations.
The Perseus lies to the north almost opposite to Ursa Major with Polaris between them. From my location that puts any north European stations in a good position. This is because the mean altitude of most trails is at approximately 95km. So for the meteor to create the forward scatter it must lie within a plane, which is a tangent to an ellipsoid, which has the transmitter and receiver as its foci. This plane is called the “Tangent plane” figures 1,3,4 show this and also that the transmitter is located beyond the reception of the ground wave. As it is the sky wave being reflected forward from the ionised trails that we wish to receive; and the Doppler effects as the ionised cloud of plasma is manipulated by the combinations of high ionosphere winds and the density, speed and mass of the meteor. The meteors position within the plane can also greatly affect the duration of the event as well as the distance between the transmitter and receiver locations. Meteors occurring at the centre of, or just off centre of the Tangent plane; can give many times the duration of echo (10 to 90 times) against meteors arriving at the extreme edges of the tangent plane; also dependant on the baseline off the two stations being worked.
St A = Transmitter St B = Meteor Radar Receiver St C = Forward Scatter Receiver T = tangent Plane. Meteor moving in at less steep angle increasing duration of affects
at St C.
St A = Transmitter St B = Meteor Radar Receiver St C = Forward Scatter Receiver T = tangent Plane. Meteor trail affected by high altitude winds varying reflection affects at St C.
Using the software to show the spectrum of the audio signal allows for the display of the frequency shift in relation to velocity of the meteor. The program creates a “Waterfall display, allowing the representation of the intensity of the signal by colour, the velocity by the increase or decrease in frequency Y – axis and time by the X – axis. The better programs allow for a screen shot to be automatically saved at set intervals. Therefore you can store completed runs for later analysis.
The resultant Meteorgrams (Fig5) show some interesting observations. An increase in the number of meteors as the earth rotates into the oncoming path of the shower in the early hours and mornings.
The short but wider bandwidth signal as the meteor enters the ionosphere at velocities of 11km/s to 72km/s.
Then the much slower moving ionised plasma, which is affected by the winds in the ionosphere.
Sometimes you may seem to get combinations maybe when a larger object breaks up after its initial contact releasing more material and forming secondary trails or it could be the effects of layering in the high atmospheric winds?
One interesting object, which is increasingly disruptive to visual observers, is aircraft and these aluminium tubes with wings give a very distinct pattern (Fig5a.