THE POPULATION INDEX

In Chapter 2 it was first pointed out how important magnitude distributions were to meteor research investigations. Its importance now becomes clear as it is related to the population index of a shower. The population index in fact, is derived from these same magnitude distributions. So why is this all that important?

The raw data obtained from meteor counts may be used in such studies as hourly rate profiles, meteor colors or train percentage rates. But almost all investigations include a determination of the ZHR (zenithal hourly rate - number of shower meteors an observer would see in one hour if the radiant were in the zenith, the limiting magnitude was +6.5, and the sky was cloudless). The population index is the essential quantity to permit any further analysis, so it must be used in the ZHR calculation. But, it is sometimes difficult to understand what is meant by the population index. This does not need to be the case.

How is the population index (abbreviated simply as the letter r) defined? The following is a definition used by the International Meteor Organization:

"The population index is an estimate of the ratio of the number of meteors in subsequent magnitude classes..."

Basically, what this is saying is that r is a value indicating how many more times meteors of magnitude m+1 appear than meteors of magnitude m. The number of meteors will increase as the magnitude gets fainter. For example, let m = 4 and r = 3. Then three times as many meteors of magnitude 5 (m+1) appear than meteors of magnitude 4 (m).

The actual calculation of the population index for a shower is beyond the scope of this guide. However, it is normally assumed that for sporadic meteors, r equals approximately 3.0. For meteor showers, if no reliable data is available, then r is assumed to equal 2.5. Still, it is best to use recent values for r if possible. These values for r can then be used in the limiting magnitude correction factor for ZHR calculations. Appendix A lists recent values for several of the annual meteor showers.

Once we know a value for r, what else does it tell us? These values can tell us how many meteors we will be missing at our observing site due to worse limiting magnitudes. The higher the population index, the more meteors we will miss due to a poor site. In the above example, if our limiting magnitude is 4.5, we miss all magnitude 5 meteors which are three times as numerous as 4th magnitude ones.

Additionally, the smaller r is, the older the stream is. This results from the fact that most of the small meteoroids which cause the high values for r have left the stream due to various causes. Thus, smaller values for r indicate the average meteor we observe will be brighter.

The population index is extremely important in meteor studies. It is the fundamental quantity needed for visual studies and it can provide us with important information on the characteristics of the meteors we observe.