The Hessdalen lights are unknown lights that have been seen in a 12-kilometer-long (about 7.5-mile) area of the Hessdalen valley in rural central Norway. These lights have been observed from time to time since the 1930s.

Background

The Hessdalen lights are seen during both the day and night. They appear to float above and through the valley. These lights are often bright white, yellow, or red and can be seen above and below the horizon. The lights can last from a few seconds to more than an hour. Sometimes they move very quickly, while at other times they move slowly back and forth. On other occasions, they remain still in the air.

Unusual lights have been reported in the area since at least the 1930s. A period of high activity happened between December 1981 and mid-1984, when the lights were seen 15 to 20 times each week. This attracted many visitors who stayed overnight. By 2010, the number of sightings had decreased to about 10 to 20 each year.

In 1983, a project called "Project Hessdalen" was started by UFO-Norge and UFO-Sverige to study the lights. This project was active from 1983 to 1985. In 1997–1998, a group of students, engineers, and journalists worked together on "The Triangle Project." They recorded the lights appearing in a pyramid shape that moved up and down. In 1998, the Hessdalen Automatic Measurement Station (Hessdalen AMS) was built in the valley to record the lights.

Later, a program named EMBLA was started to bring scientists and students together to research the lights. Major research groups involved include Østfold University College in Norway and the Italian National Research Council.

Hypotheses

Despite continued research, scientists have not reached an agreement on why the Hessdalen lights occur. Many possible explanations have been proposed.

Some sightings have been confirmed as misidentifications of objects such as stars, planes, car lights, or mirages.

One theory suggests that the lights may result from the incomplete burning of dust in the air from mining activities. Analysis found hydrogen, oxygen, and other elements like titanium in the dust. Scientists believe this may happen in Hessdalen because of the large amounts of scandium found there. When this research was published, Norwegian newspapers claimed the mystery of Hessdalen had been solved.

A 2010 hypothesis proposed that the lights form from clusters of macroscopic Coulomb crystals in a plasma created when alpha particles from radon decay ionize air and dust. Physical features such as light patterns and shapes seen in the Hessdalen lights might be explained by a model involving dust in plasma. Radon decay produces alpha particles and radioactive elements like polonium. In 2004, physicist Massimo Teodorani found higher radioactivity in rocks near a location where a large light ball was reported. Computer models show that dust in ionized gas can form double helix structures, similar to some Hessdalen lights.

Another theory suggests the lights are caused by piezoelectricity, which occurs when certain rocks, like those containing quartz, are subjected to pressure. Quartz in the Hessdalen valley can create strong electrical charges.

In a 2011 study, Gerson Paiva and Carlton Taft, using the dusty plasma theory, argued that piezoelectricity from quartz cannot explain the geometric shapes seen in the center of the lights. They proposed that the formation of light ball clusters results from interactions between ion-acoustic and dusty-acoustic waves and low-frequency geoelectromagnetic waves in dusty plasmas. Their model predicted ejected light balls could move at about 10,000 m/s (33,000 ft/s), which matches observed speeds of around 20,000 m/s (66,000 ft/s).

The central light ball is white, while the ejected balls are always green. This is believed to happen because low-frequency electromagnetic waves interact with ions in the white central ball, producing radiation pressure that moves oxygen ions (which emit green light) outward. Oxygen ions have electronic transitions that create green light, similar to those seen in auroras.

The estimated temperature of the Hessdalen lights is about 5,000 K (4,730°C; 8,540°F). At this temperature, nitrogen ions break apart faster than oxygen ions in the plasma. Only ions, not neutral atoms, are transported by ion-acoustic waves. This explains why oxygen ions dominate in the green light balls, creating a specific light pattern.

Paiva and Taft explained the flat, steep-sided spectrum of the Hessdalen lights as a result of optical thickness affecting the bremsstrahlung spectrum. At low frequencies, self-absorption makes the spectrum follow the Rayleigh–Jeans part of the blackbody curve, typical of dense ionized gas. The spectrum remains flat until a cutoff frequency, then decreases sharply, matching observations when the sky is clear. They also proposed that electrons accelerated by electric fields from piezoelectric rock fractures under the ground create the spatial color patterns of the lights.

In 2014, Jader Monari proposed a new model involving a geological battery. The two sides of the Hessdalen valley could act as electrodes, with the river Hesja functioning as an electrolyte. Gas bubbles rising from the river may become charged, producing light through gas luminescence and creating the Hessdalen lights.