15.3 Radio Practicals
213
Write up your report in the standard style, including as usual all your data and
calculations (and errors) as well as the all-important rotation curve and map.
If the orbit of the clouds around the galactic centre is Keplerian, as we would
expect, V c will scale by the square root of R. Is this what you see in your findings,
and if it isn’t, what do you think the implications of this are? You should discuss this
in your write-up.
Your map should show the HI structure of the Milky Way. You should discuss
in your write-up how your map compares to Fig. 15.2 and what you think are the
implications of any difference.
15.3.2 Radio Meteor Counting
There is a vast number of meteoroids in the solar system. Most are the size of a grain
of salt, some the size of bricks, and a very few are as big as a car or larger.
When they enter the Earth’s atmosphere, as about million kilograms of meteors
do a year, they ionize the atmosphere as they decelerate from about 20 kms
−1 . This
creates a bright streak of light in the sky at an altitude of about 80 km—a meteor. A
few of these objects survive the fiery passage through the atmosphere and are found
as meteorites.
There is a seasonality to the appearance of meteors throughout the year. Periods
may have few meteors or many, sometimes hundreds an hour, an event known as
a meteor shower. Showers are associated with comets due to the trails of dust they
produce as they orbit the Sun, with the showers occurring where the Earth’s orbit
intersects the orbit of the comet. A meteor unassociated with a shower is known as
a sporadic.
Of course, meteors arrive day and night and in all weathers, so most go unnoticed
by everyday observers. However, there is a relatively inexpensive and easy way to
detect them at all times and in all weathers.
The key to this is the ionization of the atmosphere by the incoming meteoroid.
Ionized gas is reflective to short-wavelength radio waves. So by placing such a radio
source over the horizon and pointing an antenna and suitable receiver at the source,
we find that we can detect the ionization trail. Normally, we receive no signal from
the source, since it is over the horizon. However, when a meteor event occurs between
the source and the observer, the ionized air acts as a mirror that allows us, very briefly,
to see the source. Hence, we hear a meteor in the receiver as a short blip, normally
Doppler as the meteor moves in the radial direction. This blip often sounds like a
woohoo from a fairground ride ghost train.
Fortunately, we don’t have to create our own sources. VHF television masts and
military space surveillance radars are very suitable sources, although, of course, this
was not the designers’ intention (Fig. 15.4).
213
Write up your report in the standard style, including as usual all your data and
calculations (and errors) as well as the all-important rotation curve and map.
If the orbit of the clouds around the galactic centre is Keplerian, as we would
expect, V c will scale by the square root of R. Is this what you see in your findings,
and if it isn’t, what do you think the implications of this are? You should discuss this
in your write-up.
Your map should show the HI structure of the Milky Way. You should discuss
in your write-up how your map compares to Fig. 15.2 and what you think are the
implications of any difference.
15.3.2 Radio Meteor Counting
There is a vast number of meteoroids in the solar system. Most are the size of a grain
of salt, some the size of bricks, and a very few are as big as a car or larger.
When they enter the Earth’s atmosphere, as about million kilograms of meteors
do a year, they ionize the atmosphere as they decelerate from about 20 kms
−1 . This
creates a bright streak of light in the sky at an altitude of about 80 km—a meteor. A
few of these objects survive the fiery passage through the atmosphere and are found
as meteorites.
There is a seasonality to the appearance of meteors throughout the year. Periods
may have few meteors or many, sometimes hundreds an hour, an event known as
a meteor shower. Showers are associated with comets due to the trails of dust they
produce as they orbit the Sun, with the showers occurring where the Earth’s orbit
intersects the orbit of the comet. A meteor unassociated with a shower is known as
a sporadic.
Of course, meteors arrive day and night and in all weathers, so most go unnoticed
by everyday observers. However, there is a relatively inexpensive and easy way to
detect them at all times and in all weathers.
The key to this is the ionization of the atmosphere by the incoming meteoroid.
Ionized gas is reflective to short-wavelength radio waves. So by placing such a radio
source over the horizon and pointing an antenna and suitable receiver at the source,
we find that we can detect the ionization trail. Normally, we receive no signal from
the source, since it is over the horizon. However, when a meteor event occurs between
the source and the observer, the ionized air acts as a mirror that allows us, very briefly,
to see the source. Hence, we hear a meteor in the receiver as a short blip, normally
Doppler as the meteor moves in the radial direction. This blip often sounds like a
woohoo from a fairground ride ghost train.
Fortunately, we don’t have to create our own sources. VHF television masts and
military space surveillance radars are very suitable sources, although, of course, this
was not the designers’ intention (Fig. 15.4).