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Spectroscopic Atlas for Amateur Astronomers 192

32.3 Spectra of Terrestrial Lightning Discharges

Already since the beginning of the Spectroscopy in 19th century it was attempted to gain
spectra of lightning discharges. At the beginning of the 20th Century, also well-known as-
tronomers have been involved, like Pickering and Slipher [708]. Further information and
references see also [33].

The following figure shows the spectrum of a lightning that has hit the ground in a distance
of approximately 220 m from the observer. Martin Huwiler filmed this event through the
closed window pane with a Canon G1X and a 300L mm-1 transmission grating, mounted in
front of the camera lens.

Table 111: Lightning spectrum recorded via cloud reflection

For this, the C8 telescope with the DADOS spectrograph and the Atik 314L+ was built up at
night in the living room. It pointed through the closed window on the approaching thunder-
storm at the western horizon. Three shots of each 180 seconds in the 2x2 binning mode
have been processed. Per image the integrated light of some 5-10 lightning discharges
could be recorded. Since the cloud base was very low, on all shots the light pollution had to
be subtracted.

The idea to gain lightning spectra this way, originates from none other than Vesto M.
Slipher. With the same intention, he directed on the evening of July 24 1917 his spectro-
graph at the Lowell Observatory in Flagstaff to a thunderstorm, which raged in a distance of
about 10 km above the south slopes of the San Francisco Peaks [708].

Striking here is the very intense CN emission at approximately λ 3900. According to [707]
this is a characteristic feature for discharges with relatively long-lasting currents, generated
mainly by the type of "Cloud-to-Ground Lightning and causing a high fire risk. Therefore in
the 1980ies, it was even discussed in the U.S. to detect this spectral feature with satellites,
as an early warning criterion for possible forest fires [707]. Anyway on all my shots, with
the integrated light of several lightning strikes, this CN emission appeared in comparable
intensity.

Otherwise, most of the lines of the lightning spectrum are rather complex, broad blends of
OII, NII, OI, NI as well as emissions of the H-Balmer series. The raw profile is wavelength
calibrated only.