As part of a program of work on low-frequency oscillations and fluctuations in mercury-vapor discharges at a pressure of about 1 µ Hg, observations have been made of the light output from a number of discharge tubes. Qualitative considerations suggested that the fixed or moving striations commonly encountered at rather higher pressures should not be present in our range of discharge parameters. Some independent experimental evidence, however, had indicated the presence of a series of moving striations. Under similar conditions we have been unable to observe this phenomenon which we ascribe to the unsuspected presence of impurities. These are known to have a profound influence on the production of striations. Some strong low-frequency fluctuation components were found to be excited, but these can be explained as electrostatic sound-wave modes. Further evidence on the unlikelihood of striations being present is adduced by observations of the decay of non-Maxwellian electron velocity groups generated at points in the discharge.
F. W. CRAWFORD and H. R. PAGELS, "Light Output Observations on Low-Pressure Mercury-Vapor Discharges," J. Opt. Soc. Am. 53, 734-740 (1963)