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1.1 METEOR BACKGROUND

1.1.1 Introduction

Most people at some times or other seen a shooting star that is a momentary streak of light in the sky that marks the demise of interplanetary debris as it dashes through the earth's atmosphere. Meteor is the scientific name of such events. Meteors are caused by the burn of the small particles that plunges into the earth's atmosphere at high velocity. The meteor particles come in a range of sizes, from small particles of dust with masses less than 10-7 g to more substantial chunks of rocky material of masses bigger than 100 g, which causes super fireball [4].

The majority meteor particles have their origin in the solar system. They are believed to be of cometary or asteroidal origin. They tend to orbit around the sun before being trapped by the gravitational field of the earth [3].

1.1.2 Meteor Distribution

Earth is continually bombarded by meteors. One hundred billion meteors have been estimated to enter the earth's atmosphere in a 24-hour period. The distribution of meteor is often divided into two classes: Shower meteors and Sporadic meteors.

Shower meteors are collections of particles all moving at the same velocity in well-defined orbits or streams around the sun. They are believed to be remnants of comets [4,6]. When earth's orbit intersects a meteor stream a meteor shower is observed. The streams intersect the earth's orbit at the same time each year. The paths of shower meteors appear to radiate from the same point. The constellation in which this point of divergence, or radiant, seems to lie is used in naming the meteor shower. One of the most well known shower in Northern Hemisphere is the Perseids (refer to Perseus constellation), which occurs in August of each year [3,4,6].

Sporadic meteors do not have well-defined streams; instead they have random orbital distribution. The majority of orbits tend to group toward the elliptic plane (the path of earth in its orbit around the sun). Meteors tend to move in the same direction of that of the earth and the planets.

Meteor arrivals are assumed to be independent of the previous arrivals and to satisfy Poisson statistics.

1.1.3 Formation of meteor Trails:

When a meteor enters the earth’s atmosphere, friction between the particle and the increasingly dense air causes meteor particle to boil off. The particle collides with the air molecules forming trails of positive ions and free electrons used to reflect the radio waves. The reflecting process is primarily due to the effect of the electrons, since the positive ions are too massive to vibrate under the influence of an electric field [7,9].

The electron density of the ionized trail is commonly assumed to be Gaussian in the radial direction and the ionization may or may not be uniform in the axial direction. The length of the ionization region of a trial is approximately 20-40 Km from start to finish [1,7,8]. It was found that useful ionized trails occur in an altitude of about 80 to 120 Km above the earth’s surface [1,8,9].

Trails with useful electron densities for reflecting radio signals were found to be plentiful enough to provide communication in the lower portion of (VHF) bond ranging from 30 to 100 MHz over a range of roughly 2000 Km. The minimum range limitation was found to be 400 Km [4,7]. Ionized trails were found to have a lifetime of only a few tenths of a second, creating the need for rapid exchange of communication. The transmission rate had to be very fast (a burst of data) to take advantage of ionized trail. Hence the term “meteor burst” was coined [7,9].



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