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METEORS

BY M. CAMPBELL-BROWN AND P. BROWN

A meteor (a word derived from the Greek meteoros, meaning high in the air) is the light, heat, ionization, and, occasionally, sound phenomena produced when a solid body (a meteoroid) collides with molecules in Earth’s upper atmosphere. These collisions heat the surface of the object; then at a height typically between 80 km and 120 km the meteoroid begins to ablate, that is, lose mass. Typically, ablation takes the form of vaporization, although melting and quasicontinuous fragmentation may also contribute. It is these ablated meteoric atoms that collide with air molecules to produce atomic excitations, or ionization, leading to the emission of light we see in the night sky. Typical visual meteors are produced by meteoroids the size of a small pebble, although the relationship between mass–brightness and velocity is complex (Figure 1). The faintest meteors visible to the naked eye are due to meteoroids the size of the tip of a ballpoint pen; the brightest meteors are due to meteoroids whose diameters roughly match the thickness of a pen.

Larger and low-velocity meteoroids are favoured to pass through Earth’s atmosphere, although rarely as a single monolithic body. When these ponderable masses reach the ground, they are called meteorites. At the other extreme, very small meteoroids (micrometres in size) efficiently radiate heat from their surface and do not reach ablation temperatures; such particles reach Earth’s surface without having been fully ablated, although most are heated many hundreds of degrees.

Meteoroids can be divided broadly into two groups: stream and sporadic meteoroids. Stream meteoroids follow similar orbits around the Sun, many of which can be linked to a particular parent object, in most cases a comet. When Earth intersects the orbit of a stream, a meteor shower occurs. Since all the meteoroids in a stream move along nearly identical orbits, their paths in the atmosphere are parallel. This creates a perspective effect: the meteor trails on the celestial sphere appear to radiate from a fixed location, called the meteor radiant. Sporadic meteoroids, in contrast, are much more loosely associated and are not part of tightly grouped streams.

Meteor showers are named for the constellation from which they appear to radiate. When several showers have radiants in the same constellation, nearby bright stars are used in naming. The Quadrantid shower is named for the obsolete constellation Quadrans Muralis, but now has a radiant in Boötes.

The sporadic complex as seen at Earth, however, is structured and shows definite directionalities as well as annual variations in activity levels. Sporadic meteor radiants are concentrated in six major source regions throughout the sky. These sources are in fixed locations with respect to the Sun. Variations in the strengths of these sources throughout the year have been observed; it is the elevation of these sources at a particular location plus the intrinsic strength of each source at a given time of the year that determines the average background sporadic rate. Figure 2 shows the expected sporadic rate as a function of the altitude of the apex of Earth’s way throughout the year. The apex is the instantaneous direction of travel of Earth around the Sun; it is the point on the ecliptic that transits at exactly 6:00 a.m. true local time. The altitude of the apex for a given latitude, time of year, and local time can be roughly calculated as follows: A ±23.5 sin(  90)  (±lat  90) cos(LT/24  360), where  is the solar longitude, lat is the latitude of the observer, and LT is the local time on a 24-hour clock.

In general, meteoroid streams are formed when particles are ejected from comets as they approach the Sun. The parent objects of many showers have been identified from the similarity of the orbits of the object and the stream. Cometary associations include the -Aquarids and Orionids, which are derived from 1P/Halley, the Leonids, which originate from 55P/Tempel-Tuttle, and the Perseids, from 109P/Swift-Tuttle. At least one asteroid is known to be associated with a stream: 3200 Phaethon and the Geminid stream. All of these particles have orbits that coincide closely with that of the parent object, but their orbits are gradually shifted through radiative effects and planetary perturbations. Such effects lead to a broadening of the stream over time, resulting in an increased duration of the meteor shower as seen from Earth—older streams tend to be more long-lived—and eventually transform stream meteoroids into sporadic meteoroids.

The visual strength of a meteor shower is measured by its Zenithal Hourly Rate (ZHR), defined as the number of meteors a single average observer would see if the radiant were directly overhead and the sky dark and transparent with a limiting stellar magnitude of 6.5 (conditions that are rarely met in practice). A more physical measure is the flux of a meteoroid stream, measured in numbers of meteors of absolute brightness (referenced to a range of 100 km) brighter than 6.5 per square kilometre per second perpendicular to the radiant direction. While an observer will likely see the largest number of meteors by looking at the shower radiant, the most useful counts are obtained by looking some distance away from the radiant.

Most data on meteor showers are currently gathered visually by amateur observers. It is crucial to observe from a dark-sky location with a clear view of the sky, and to allow at least 20 min before the start of observations to allow dark adaptation of the eyes. One should choose an area of the sky to observe, preferably with an elevation greater than 40°. The limiting magnitude should be carefully recorded for each session, along with the UT and the centre of the field of view of the observer. The most basic observations should include an estimate of the brightness of the meteor, the time of observation, and a shower association (based on the radiant and apparent speed of the meteor). Information on collecting and reporting scientifically useful observations can be found at www.imo.net.

(Please note, figures to be added at a later date. Sorry.)

FIGURE 1 Magnitude as a function of velocity for meteoroids of different sizes. Uncertainties in the mass scale are largest at high velocities and small masses, and may be as large as an order of magnitude in mass at the extremes.

FIGURE 2 The observed visual rate of sporadic meteors as a function of various elevations of the apex of Earth’s way throughout the year. The equivalent hourly rate (like ZHR) is measured for a sky with a limiting stellar magnitude of +6.5. The position of the apex is 90º west of the Sun along the ecliptic plane. Total meteor rates on a given night may be somewhat higher due to contributions from minor showers. Data courtesy of the International Meteor Organization and R. Arlt.

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