Population index

The population index r of a meteor stream describes the brightness distribution of its meteors : it characterizes the factor by which the number of meteorites increases towards the next weaker size class . If one can observe a certain number of meteors with a certain ambient brightness, one can observe r times as many meteors with an ambient brightness lower by one magnitude. For many meteor streams, the population index is between 2 and 3.

A meteor shower consists of many meteors of different sizes. The larger a meteor, the greater its brightness when it enters the earth's atmosphere . If one assumes that in a certain size range the number of meteors of a certain size decreases exponentially with the size, then this can be quantified in a formula. The population index quantifies this exponential relationship between the number of meteors and their brightness.

The ZHR of a meteor shower is given by definition for a limit magnitude of 6.5  mag . However, since the actual limit value often deviates from this, the ZHR can be determined from the number of observed meteors with the help of the population index. Conversely, you can use the ZHR and the limit size to determine how many meteors you should be able to observe per hour, provided the sky is free of clouds: ${\displaystyle m_{\text{gr}}}$

${\displaystyle {\frac {N}{t}}\approx {\frac {\sin h}{r^{6,5-m_{\text{gr}}}}}\cdot ZHR}$

With

• the actually observed rate ${\displaystyle {\frac {N}{t}}}$
• the elevation angle  h of the radian of the observed meteor shower above the horizon , measured at the middle of the observation period.

Since the current limit value will often be below the very good 6.5 magnitudes that apply to the ZHR itself, the above formula results in an exponent > 0 for the population index in the denominator .

Web links

• www.namnmeteors.org