History and description 
Unusual lights have been reported here since 1940s or earlier. Especially high activity of Hessdalen lights took place from December 1981 until the summer of 1984 when lights were observed 15 to 20 times per week. The frequency of the lights caused a gathering of numerous tourists staying there overnight to see the phenomenon. Since then, the activity has decreased and now the lights are observed some 10 - 20 times per year.
The Hessdalen light most often is a bright, white or yellow light of unknown origin standing or floating above the ground level. Sometimes the light can be seen for more than one hour. There are several other types of unexplained lights observed in the Hessdalen valley.
Since 1983 there has been ongoing scientific research often nicknamed "Project Hessdalen", initiated by UFO-Norge and UFO-Sweden. The project was active as field investigations during 1983-1985. In 1998, the Hessdalen AMS automated scientific research station was built in the valley. It registers and records the appearance of lights.
Later, the EMBLA program was initiated. It brings together established scientists and students into researching these lights. Leading research institutions are Østfold University College (Norway) and the Italian National Research Council.
Possible explanations 
In spite of on-going research there is no convincing explanation to the origin of these lights. However, there are numerous working hypotheses.
- One explanation attributes the phenomenon to an incompletely understood combustion process in the air involving clouds of dust from the valley floor containing scandium. Some sightings though, have been identified as misperceptions of astronomical bodies, aircraft, car headlights, and mirages.
- One recent hypothesis suggests that the lights are formed by a cluster of macroscopic Coulomb crystals in a plasma produced by the ionization of air and dust by Alpha particles during radon decay in the dusty atmosphere. Several physical properties (oscillation, geometric structure, and light spectrum) observed in Hessdalen lights phenomenon can be explained through the dust plasma model. Radon decay produces alpha particles (responsible by helium emissions in HL spectrum) and radioactive elements such as polonium. In 2004, Teodorani showed an occurrence where a higher level of radioactivity on rocks was detected near the area where a large light ball was reported. In fact, when radon is released into air, its solid decay products readily attach to airborne dust. A new computer simulation shows that dust immersed in ionized gas (i.e., dusty plasmas) can organize itself into double helixes. The simulations suggested that under conditions commonly found in space, the dust particles first form a cylindrical structure that sometimes evolved into helical structures. Along some spirals, the radius of the helix was seen to change abruptly from one value to another and then back again, providing a mechanism for storing information in terms of the length and radius of a section of a spiral. Hessdalen Lights may take the helical structure. Surprisingly, dusty plasmas may also assume this structure.
- Another hypothesis explains HL as a product of piezoelectricity generated under specific rock strains (Takaki and Ikeya, 1998) because many crystal rocks include quartz grains which produce an intense charge density. In a recent paper, based in the dusty plasma theory of HL, it is suggested that piezoelectricity of quartz cannot explain a peculiar property assumed by the HL phenomenon – the presence of geometrical structures in its center. Paiva and Taft have shown a mechanism of light ball cluster formation in Hessdalenlights (HL) by the nonlinear interaction of ion-acoustic and dusty-acoustic waves with low frequency geoelectromagnetic waves in dusty plasmas. The theoretical model shows that the velocity of ejected light balls by HL cluster is of about 10,000 m s−1 in a good agreement with the observed velocity of some ejected light balls, which is estimated as 20,000 m s−1. Why the ejected ball is always green-colored? Ejection of small green light ball from HL is due to radiation pressure produced by the interaction between very low frequency electromagnetic waves (VLF) and atmospheric ions (present in the central white-colored ball) through ion-acoustic waves (IAW). Probably only O2+ ions (electronic transition (b4Σg- → a4Πu)), with green emission lines, is transported by IAW. Electronic bands of O2+ ion occur in auroral spectra. Electron-molecular-ion dissociative recombination coefficient rate α as functions of electron temperature Te and cross sections σ as a function of electron energy E have been have measured by Mehr and Biondi  for N2+ and O2+ over the electron temperature interval 0.007-10 eV. The estimated temperature of HL is of about 5,000 K. In this temperature, the rate coefficient of dissociative recombination will be respectively α(Te)O2+ ~ 10-8 cm3 s-1, and α(Te)N2+ ~ 10-7 cm3 s-1. Thus, the nitrogen ions will be decomposed in N2+ + e- → N + N* more rapidly than oxygen ions in the HL plasma. Only ionic-species are transported by IAW. Therefore, only oxygen ions will be predominant ejected green light balls from a central white ball in HL, presenting negative band of O2+ with electronic transition b4Σg- → a4Πu after an IAW formation. Paiva and Taft  presented a model for resolving the apparently contradictory spectrum observed in Hessdalen Lights (HL) phenomenon. Thus, its nearly flat spectrum on the top with steep sides is due to 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. This spectrum is typical of dense ionized gas. Additionally, spectrum produced in the thermal bremsstrahlung process is flat up to a cutoff frequency, ν cut, and falls off exponentially at higher frequencies. This sequence of events forms the typical spectrum of HL phenomenon when the atmosphere is clear, with no fog. According to the model, spatial color distribution of luminous balls commonly observed in HL phenomenon are produced by electrons accelerated by electric fields during rapid fracture of piezoelectric rocks under the ground.
See also 
- "Hessdalen lights". Wondermondo.
- Teodorani, M (2004). "A Long-Term Scientific Survey of the Hessdalen Phenomenon". Journal of Scientific Exploration 18 (2): 217–251. Archived from the original on January 13, 2008.
- Paiva, G. S.; Taft, C. A. (4 August 2010). "A hypothetical dusty-plasma mechanism of Hessdalen Lights". Journal of Atmospheric and Solar-Terrestrial Physics.
- Chamberlain, J.W., Physics of the Aurora and Air-glow (Academic Press Inc. , New York, 1961)
- Mehr, F J and Biondi, M A, Electron temperature dependence of recombination O2+ and N2+ ions with electrons, Phys. Rev. 181, 264-71 (1969)
- Teodorani, M. A., Long-term scientific survey of the Hessdalen phenomenon. Journal of Scientific Exploration, 18, 217–251 (2004)
- Paiva, G. S.and Taft, C. A, A mechanism to explain the spectrum of Hessdalen Lights phenomenon, Met. Atm. Phys. 117, 1-4 (2012)
- Paiva, G. S.and Taft, C. A, Color Distribution of Light Balls in Hessdalen Lights Phenomenon, J. Sc. Expl. 25, 735 (2011)
- Homepage of Project Hessdalen
- Homepage of the Italian Committee for Project Hessdalen
- Hessdalen 2003: luci misteriose in Norvegia by Massimiliano Di Giuseppe (Italian)
- Hessdalen 1998: Triangle Project
- Project Hessdalen Bulletin, 1983-1985