Star Points for June, 2000; by Curtis Roelle
Observe Meteors During Daytime Shower June 7.

A meteor shower known as the Arietids is expected to peak around 5
p.m. EDT on Wednesday, June 7 - during daylight!  You might find it
surprising to learn that it is possible to detect meteors in the
daytime by listening for their reflections using an ordinary FM
radio.

First let's freshen up on some basic terminology.  A "meteoroid" is
a solid particle usually between the size of a sand grain and a
pebble, traveling in orbit around the sun.  When a meteoroid slams
into the earth's atmosphere it is vaporized by frictional forces. 
This process produces a visible steak called a "meteor" some 47-70
miles above the earth's surface.

Since World War II it has been known that meteors can reflect radar
signals.  Indeed, professional astronomers have used radar and
other instruments to give very precise information about the
flights of meteors.  It is possible using radar to measure a
meteor's distance, direction, altitude, direction of travel, and
velocity.  Using these techniques on numerous individual meteors
astronomers have discovered a handful of daytime meteor showers.

Unfortunately, most amateur astronomers do not own either a radar
transmitter or radar receiver.  However, most amateur astronomers
own FM receivers and it's a safe bet that all FM radio stations own
FM transmitters.
  
The scattering of radio waves off meteor trails is what permits
their detection during daytime.  This scattering is of a type known
as "specular" (mirror-like) reflection.  Imagine the meteor trail
as a long narrow mirror.  Let the transmitter be represented by a
light bulb and your eye is the receiver.  The reflection from the
bulb will be visible only at one point along the mirror and
likewise it is from a single point in the meteor trail that the
reflected signal will be collected by the radio receiver.  Another
receiver at a different location would "see" the light bulb
reflected at some other point of the mirror.

The reflection of radio waves is caused by ions in the meteor
trail.  Ions result from super heated air molecules as the meteor
plows though the atmosphere.  Immediately after the trail's
formation strong upper atmospheric wind sheer begins diffusing the
trail and the reflection deteriorates.  The entire process lasts
from a fraction of a second to as long as a minute or more.

The important point to keep in mind is that radio scatter from a
meteor is a form of "electromagnetic wave propagation."  In other
words the ionized trail of a meteor acts as a temporary reflector
for radio waves but the trail itself does not generate detectable
radio emission. 

Detecting meteors with a radio is quite simple.  Since meteor
trails occur high above the earth they can reflect signals from
stations over the horizon.  Tune your FM radio to an empty
frequency devoid of any detectible radio transmissions.  Now wait
until a meteor occurs whose trail is oriented such that it reflects
a distant broadcasting station. You'll hear a short segment of the
remote broadcast. Sometimes you may pick up several stations either
simultaneously or consecutively during the brief period of enhanced
reception.  Using this technique you can pick up broadcasts from
stations hundreds of miles away.  

The frequency band of the FM dial ranges from 87.5 to 108 MHz.  In
densely populated areas (such as Washington-Baltimore) it may be
difficult to find an empty broadcast frequency.  Try as low as
possible in the band and avoid the upper band (104-108 MHz).

This technique of observing meteors is referred to as "manual
observing."  If you wish to record your observations simply note
the time and duration of each suspected meteor reflection. 
Optionally, connecting a signal meter to your radio allows you to
estimate the power of the reflection.

Performance may be improved by connecting the radio to a
horizontally polarized antenna such as the popular "Yagi" antenna. 
Radio observations are not limited to the FM band.  VHF and
specific Ham radio frequencies have also yielded good results.  For
more information about observing meteors (visually or by radio)
including specifications for constructing a custom antenna check
out the International Meteor Organization (IMO) web site at
http://www.imo.net.  

If you attempt to listen for the Arietids on June 7 let me know how
you did by dropping an e-mail to starpointsusa@yahoo.com.