Bill Ward’s comparison of meteor shower spectra

Bill Ward has produced this very interesting comparison of the spectra of meteors from four different meteor showers during 2017. The differences between the four showers provide an insight not just into the composition of the meteors, but also how they each interact differently with the Earth’s atmosphere.

Meteor Shower Spectrum Comparisons

During 2017 four of the annual showers were observed around their peak dates.
These were the Lyrids, Perseids, Leonids and Geminids. The showers produced good spectroscopic results.
A representative spectrum was selected from each shower. Although it is impossible to have identical observing conditions and magnitudes the spectra were selected to be reasonably close in magnitude and wavelength span. The spectrum plots were produced in the now usual manner of geometric image correction of the composited video image then wavelength calibration and relative intensity normalisation.

The four spectra are shown in figures 1-4 and together in figure 5. The spectra in Figs 1-3 were obtained using Watec 910 HX/RC cameras fitted with 12mm f0.8 lenses carrying a 600 groove/mm gratings. The spectrum of figure 4 was obtained with the same camera type as the others but with an 830 grooves/mm grating and is thus of slightly higher resolution.

On inspection it can be seen that three of the four are very similar in their general form (Figs 1-3). These are meteors from cometary parent bodies.

Fig 1 is a Lyrid spectrum, associated with Comet Thatcher C/1861 G1, inclination of 80 degrees. Geocentric velocity ~48km/s.

Figure 1. Intensity plot for a 2017 Lyrid spectrum

Fig 2 is a Perseid spectrum, parent 109P Comet Swift-Tuttle, inclination of 113 degrees (retrograde). Geocentric velocity ~60km/s

Figure 2. Intensity plot for a 2017 Perseid spectrum

Fig 3 is a Leonid spectrum, parent 55/P Comet Temple-Tuttle, inclination of 162 degrees (retrograde). Geocentric velocity ~70km/s.

Figure 3. Intensity plot for a 2017 Leonid spectrum

Considering Figs 1-3, it can be seen that as the geocentric velocity increases the area under the spectrum at the red/near IR end of the spectrum graph increases. This is a reflection of the fact that at higher velocities there is more energy and the emission from atmospheric oxygen and nitrogen increases proportionally. The strong, unresolved O triplet at 7774A saturated in the Leonid spectrum. Also the metals (Ca, Mg, Cr, Si and Fe) at the blue end are seen to decrease, also proportionally. The intercept velocity is highest for the Leonids as due to the orbit of the meteoroids the Earth encounters them almost head on.

Close inspection of the Perseid and the Leonid spectra shows a feature at 5577A. This is due to the emission from a forbidden transition of Oxygen. This only occurs for fast meteors that start interacting with the atmosphere above 110km. Here the mean free path of the atmospheric atoms is sufficient to allow time for the transition to occur before the energy is robbed by collisions. (A process known as quenching.)
It is a weak feature here as the spectra were taken from the midpoint of each spectrum image. This forbidden line and the 7774A line are the only lines at the very start of the Perseid and Leonid spectra when the meteoroids first impinge on the atmosphere but have not yet started to ablate.

Figure 4 is a Geminid meteor spectrum.

Figure 4.  Intensity plot for a 2017 Geminid spectrum

What is immediately apparent is that there is much less emission from the atmosphere. The parent of the Geminids is asteroid 3200 Phaethon, orbital inclination of 22 degrees. The meteors from Phaethon intercept the earth with a geocentric velocity of ~35km/s.  Also, it is clear that the metals at the blue end are significantly stronger than for the cometary particles.

Figure 5 compares the four meteor showers in a single plot

Figure 5. A comparison of the four intensity plots

Note. To preserve the red/IR characteristics of the spectra they have not been corrected for instrumental response in any way.

An interesting comparison illustrating several fundamental differences in physical properties and effects of various meteor streams’ particles and velocities.

Bill Ward
Jan 2018