the effect of optical thickness on the bremsstrahlung spectrum. At low frequencies self-absorption modifies the spectrum to follow the Rayleigh–Jeans part of the blackbody curve. Such a spectrum is typical of dense ionized gas. Additionally, the spectrum produced in the thermal bremsstrahlung process is flat up to a cutoff frequency, νcut, and falls off exponentially at higher frequencies. This sequence of events forms the typical spectrum of Hessdalen lights phenomenon when the atmosphere is clear, with no fog. According to the model, the spatial color distribution of luminous balls commonly observed in Hessdalen lights phenomenon is produced by electrons accelerated by electric fields during rapid fracture of piezoelectric rocks under the ground. In 2014, Jader Monari published a new HL model involving a geological-like battery. Thus, two sides of the valley are the electrodes and the river Hesja can be acting as the electrolyte. Gas bubbles rise into the air and can become electrically charged producing gas luminesce and HL phenomenon. See also Aleya (Ghost light), Bengal Chir Batti Aurora Ball lightning Hessdalen Marfa lights Paulding Light Will-o'-the-wisp St. Elmo's fire Naga fireballs Brown Mountain lights Notes References External links Homepage of Project Hessdalen Project Hessdalen Bulletin, 1983–1985 Holtålen Atmospheric ghost lights Weather lore UFO-related phenomena Unsolved problems in physics