The main experiments were carried out with discharges in air, and the air pressure was chosen close to the pressures of high-altitude discharges. Other gases, such as argon, were chosen to better demonstrate the discoloration of the discharge.
A point that should be clarified is the choice of the gases used for the discharges. One would expect a comparison between discharges that happen in gas mixtures of similar composition. Is there any relationship between the specific choice of gases made by the authors and the composition of the atmosphere at the altitudes where the atmospheric discharges take place?