History and description
The Hessdalen lights are of unknown origin. They appear both by day and by night, and seem to float through and above the valley. They are usually bright white, yellow or red and can appear above and below the horizon. The duration of the phenomenon may be a few seconds to well over an hour. Sometimes the lights move with enormous speed; at other times they seem to sway slowly back and forth. On yet other occasions, they hover in mid‑air.
Unusual lights have been reported in the region since at least the 1930s. Especially high activity occurred between December 1981 and mid-1984, during which the lights were being observed 15–20 times per week, attracting many overnight tourists who arrived in for a sighting. As of 2010[update], the number of observations has dwindled, with only 10 to 20 sightings made yearly.
Since 1983, there has been ongoing scientific research, referred to as "Project Hessdalen", initiated by UFO-Norge and UFO-Sweden. This project was active as field investigations during 1983–1985. A group of students, engineers and journalists collaborated as "The Triangle Project" in 1997–1998 and recorded the lights in a pyramid shape that bounced up and down. In 1998, the Hessdalen Automatic Measurement Station (Hessdalen AMS) was set up in the valley to register and record the appearance of lights.
Later, a programme, named EMBLA, was initiated to bring together established scientists and students into researching these lights. Leading research institutions are Østfold University College (Norway) and the Italian National Research Council.
Despite the ongoing research, there is no convincing explanation for the phenomenon. However, there are numerous working hypotheses and even more speculations.
- One possible explanation attributes the phenomenon to an incompletely understood combustion involving hydrogen, oxygen and sodium, which occurs in Hessdalen because of the large deposits of scandium there.
- 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 including oscillation, geometric structure, and light spectrum, observed in the Hessdalen lights (HL) can be explained through a 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. Computer simulations show that dust immersed in ionized gas can organize itself into double helixes like some occurrences of the Hessdalen lights; dusty plasmas may also form in this structure.
- Another hypothesis explains Hessdalen lights as a product of piezoelectricity generated under specific rock strains,[a] because many crystal rocks in Hessdalen valley include quartz grains which produce an intense charge density.
This section may be too technical for most readers to understand.(April 2016)
In a 2011 paper, based on the dusty plasma theory of Hessdalen lights, Gerson Paiva and Carlton Taft suggested that piezoelectricity of quartz cannot explain a peculiar property assumed by the Hessdalen lights phenomenon – the presence of geometrical structures in its center. Paiva and Taft have shown a mechanism of light ball cluster formation in Hessdalen lights by nonlinear interaction of ion-acoustic and dusty-acoustic waves with low frequency geoelectromagnetic waves in dusty plasmas. The theoretical velocity of ejected light balls is about 10,000 m/s (33,000 ft/s), in good agreement with the observed velocity of some ejected light balls, estimated at 20,000 m/s (66,000 ft/s).
The central ball is white, while the ejected balls that are observed are always green in colour. This is ascribed to radiation pressure produced by the interaction between very low frequency electromagnetic waves (VLF) and atmospheric ions (present in the central white-coloured ball) through ion-acoustic waves. O+
2 ions (electronic transition b4Σ−
g → a4Πu), with green emission lines, are probably the only ones transported by these waves. Electronic bands of O+
2 ions occur in auroral spectra.
The estimated temperature of Hessdalen lights is about 5,000 K (4,730 °C; 8,540 °F). At this temperature, the rate coefficients of dissociative recombination will be 10−8 cm3 s−1 for the oxygen ions, and 10−7 cm3 s−1 for the nitrogen ions.[b] Thus, in the Hessdalen lights plasma, the nitrogen ions will decompose (N+
2 + e− → N + N*) more rapidly than oxygen ions. Only ionic species are transported by ion acoustic waves. Therefore, oxygen ions will dominate in the ejected green light balls in Hessdalen lights, presenting a negative band of O+
2 with electronic transition b4Σ−
g → a4Πu after ion-acoustic wave formation.
Paiva and Taft presented a model for resolving the apparently contradictory spectrum observed in Hessdalen lights. The spectrum is nearly flat on the top with steep sides, 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. 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 batery.  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. 
- Aleya (Ghost light), Bengal
- Chir Batti
- Ball lightning
- Hessdalen AMS
- Marfa lights
- Paulding Light
- St. Elmo's fire
- Naga fireballs
- Based on 1998 research by Takaki and Ikeya.
- Using the measurements of electron–molecular ion dissociative recombination rate coefficients as functions of electron temperature and cross sections as a function of electron energy by Mehr and Biondi for N+
2 and O+
2 over the electron temperature interval 0.007–10 eV.
- Leone, Matteo (2003). "A rebuttal of the EMBLA 2002 report on the optical survey in Hessdalen" (PDF). Comitato Italiano per il Progetto Hessdalen. pp. 1–29. Archived (PDF) from the original on 2014-02-07.
- Zanotti, Ferruccio; Di Giuseppe, Massimiliano; Serra, Romano. "Hessdalen 2003: Luci Misteriose in Norvegia" (PDF) (in Italian). Comitato Italiano per il Progetto Hessdalen. pp. 4–5. Archived (PDF) from the original on 2016-01-04.
- Pāvils, Gatis (2010-10-10). "Hessdalen lights". Wondermondo. Archived from the original on 2015-07-02.
- Ballester Olmos, Vicente‑Juan; Brænne, Ole Jonny (2008). "11 October 1997". Norway in UFO Photographs: The First Catalogue. FOTOCAT. 4. Torino: UPIAR. p. 94. ISBN 9781596058774. LCCN 2010388262. OCLC 713018022. Archived (PDF) from the original on 29 December 2015.
- Olsen, Andreas, ed. (1998). "The Triangle Project". Archived from the original on 2002-10-17.
- "The EMBLA 2000 Mission in Hessdalen" (PDF). Project Hessdalen Homepage. Retrieved 27 May 2019.
- Matteo Leone. "A rebuttal of the EMBLA 2002 report on the optical survey in Hessdalen: Part Three" (PDF). Italian Committee for Project Hessdale.
- Johansen, Karl Hans (2007-07-16). "Fenomenet Hessdalen" (in Norwegian). Norsk rikskringkasting. Archived from the original on 2015-07-03.
- Hauge, Bjørn Gitle (2007). Optical spectrum analysis of the Hessdalen phenomenon (PDF) (Report). Archived from the original (PDF) on 2014-08-30. Retrieved 2008-04-24.
- Paiva, Gerson S.; Taft, Carlton A. (2010). "A hypothetical dusty plasma mechanism of Hessdalen lights". Journal of Atmospheric and Solar-Terrestrial Physics. 72 (16): 1200–1203. Bibcode:2010JASTP..72.1200P. doi:10.1016/j.jastp.2010.07.022. ISSN 1364-6826. OCLC 5902956691.
- Teodorani, Massimo (2004). "A Long-Term Scientific Survey of the Hessdalen Phenomenon" (PDF). Journal of Scientific Exploration. 18 (2): 217–251. Bibcode:2004JSE....18..217T. ISSN 0892-3310. Archived (PDF) from the original on 2015-12-28.[unreliable source?]
- Johnston, Hamish (2007-08-15). "Helices swirl in space-dust simulations". Physics World. Archived from the original on 2016-01-10.
- Takaki, Shunji; Ikeya, Motoji (15 September 1998). "A Dark Discharge Model of Earthquake Lightning". Japanese Journal of Applied Physics. 37 (9A): 5016–5020. Bibcode:1998JaJAP..37.5016T. doi:10.1143/JJAP.37.5016.
- Paiva, Gerson S.; Taft, Carlton A. (2011). "Hessdalen Lights and Piezoelectricity from Rock Strain" (PDF). Journal of Scientific Exploration. 25 (2): 265–271. ISBN 9781596058774. ISSN 0892-3310. OCLC 761916772. Archived from the original (PDF) on 2015-12-28.[unreliable source?]
- Paiva, Gerson S.; Taft, Carlton A. (2012). "Cluster formation in Hessdalen lights". Journal of Atmospheric and Solar-Terrestrial Physics. 80: 336–339. Bibcode:2012JASTP..80..336P. doi:10.1016/j.jastp.2012.02.020. ISSN 1364-6826. OCLC 4934033386.
- Paiva, Gerson S.; Taft, Carlton A. (2011). "Color Distribution of Light Balls in Hessdalen Lights Phenomenon". Journal of Scientific Exploration. 25 (4): 735–746. ISSN 0892-3310.[unreliable source?]
- Chamberlain, J.W., Physics of the Aurora and Air-glow (Academic Press Inc., New York, 1961)[ISBN missing]
- Mehr, F J; Biondi, M A (1969). "Electron temperature dependence of recombination O+
2 and N+
2 ions with electrons". Phys. Rev. 181: 264–271. doi:10.1103/physrev.181.264.
- Paiva, Gerson S.; Taft, Carlton A. (2012). "A mechanism to explain the spectrum of Hessdalen Lights phenomenon". Meteorology and Atmospheric Physics. 117 (1–2): 1–4. Bibcode:2012MAP...117....1P. doi:10.1007/s00703-012-0197-5. S2CID 119505901.
- Paiva, Gerson S.; Taft, C. A (2011). "Color Distribution of Light Balls in Hessdalen Lights Phenomenon". J. Sc. Expl. 25: 735.
- Williams, Caroline. "Norse UFOs: What are the glowing orbs of Hessdalen?". New Scientist. Retrieved 2020-10-19.
- Christensen, Arnfinn (2014-05-13). "Little valley – a giant battery?". partner.sciencenorway.no (in Norwegian). Retrieved 2020-10-19.