|Forked Lightning Spectra
Denis Joye captured these lightning spectra during a thunderstorm over Paris. They reveal the extreme conditions inside a lightning stroke.
He used a 540 line/mm diffraction grating in front of the camera lens. A slit is usually needed to get good line spectra. Here the very narrow lightning strokes make a slit quite unnecessary. We see sharp spectrum emission lines superimposed on weaker continuum light. ©Denis Joye, shown with permission
|Lightning is a rapid series of electrical discharges between the ground and a cloud or within clouds. The detailed mechanisms of its formation are not fully understood.
What we do know is that currents of tens of thousands of amps can be involved. The immense current rapidly heats an air column just a few cm wide and up to several miles long to temperatures of 20-30,000 Celsius, several times that of the sun’s surface. At these temperatures air molecules are dissociated into atoms and ionised to a plasma. The spectrum at right shows lines from electronic transitions of nitrogen ions and of hydrogen atoms.
These spectra are different from those of the aurora and airglow where the method of excitation and the surrounding atmospheric pressure are very dissimilar.
Respect lightning immensely. Several attempting to repeat Benjamin Franklin’s alleged kite experiment found it was the last bit of science they did.