Meteorol Atmos Phys

Magazine Overview

Title: Meteorol Atmos Phys
Issue: 117:1-4
Date: Published online: 26 May 2012
Publisher: Springer-Verlag
Country: Brazil
Language: English
Document Type: Review Article

This review article, authored by G. S. Paiva and C. A. Taft, published in Meteorol Atmos Phys, presents a model to explain the spectrum of the Hessdalen Lights (HL) phenomenon. The authors propose that the observed spectrum, characterized by a nearly flat top with steep sides, is a result of optical thickness effects on the bremsstrahlung spectrum, modified by self-absorption at low frequencies, which is typical of dense ionized gas.

Abstract

The abstract states that the model explains the apparently contradictory spectrum of the Hessdalen Lights (HL) phenomenon. The nearly flat spectrum on top with steep sides is attributed 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, characteristic of dense ionized gas. The thermal bremsstrahlung process produces a spectrum that is flat up to a cutoff frequency (veut) and falls off exponentially at higher frequencies. This sequence forms the typical HL spectrum when the atmosphere is clear and without fog.

1 Introduction

The introduction contextualizes the Hessdalen Lights (HL) within a broader category of rare and unexplained atmospheric light phenomena, such as ball lightning, blue jets, red sprites, and terrestrial gamma ray flashes (TGFs). HL are specifically described as unexplained lights observed in the valley of Hessdalen, Norway. These phenomena appear as free-floating light balls, ranging from decimeters to 30 meters in size, and are often associated with strong pulsating magnetic perturbations of about 5 Hz and short-duration pulsating "spikes" in HF and VLF radio ranges, sometimes exhibiting Doppler features. The HL phenomenon is also noted for visually displaying "satellite spheres" around a central luminous core.

The article highlights that no existing theory or model fully accounts for all observations of HL. Previous explanations have included incompletely understood combustion processes involving dust, misperceptions of astronomical bodies or aircraft, and piezoelectricity generated under rock strain. A more recent hypothesis suggests they are formed by macroscopic Coulomb crystals in a plasma produced by alpha particle ionization from radon decay. The luminosity of these light balls has been estimated at approximately 19 kW.

The spectrum of the HL phenomenon is described as a continuum with no resolved lines. Photometric and spectroscopic analysis indicates it does not possess the characteristics of a classic plasma of free electrons and ions. When atmospheric transparency was low, the intensity distribution (ID) profile resembled a heated, glowing plasma with Gaussian shape and exponential wings. However, when the atmosphere was clear, the ID profile was nearly flat on top with steep sides.

2 The Model

According to Teodorani, the spectrum of Hessdalen lights is a continuum without resolved lines. Atmospheric turbulence or fog can smooth the spectrum and induce exponential wings, similar to the "seeing disk" effect observed with stars through atmospheric layers. The three-dimensional analysis of the intensity distribution suggests the radiant power originates from a heated substance. However, the phenomenon does not exhibit characteristics of a classic plasma of free electrons and ions. The intensity distribution shape is comparable to that of an approximately spherical, solid-like object with uniform luminosity, such as the moon.

Bremsstrahlung is defined as electromagnetic radiation produced by the deceleration of charged particles, indicating the presence of ionized gas or plasma. Astrophysical examples include thin plasmas in stellar atmospheres and hot, dense plasmas in active galactic nuclei. In an optically thin medium, internally generated radiation escapes freely. In an optically thick medium, radiation is absorbed and re-emitted, constraining the spectrum to be no more efficient than a black body, with a turnover at low frequencies and a power-law dependence similar to the Rayleigh-Jeans part of the blackbody spectrum.

The authors then generalize these results to a population of electrons with a specific velocity and density distribution, defining total emission as thermal bremsstrahlung. They calculate the temperature of HL using its estimated luminous intensity of 19 kW. The velocity distribution of particles in the ionized cloud is given by the Maxwell distribution. The typical impact parameter between electrons and positive ions is set by the number density of electrons (ne) and ions (ni). The formula for intensity (in the flat part of the spectrum) is provided, along with the total emitted power per unit volume, which includes a Gaunt factor (gff). This expression cuts off at approximately hv/kT, allowing for temperature determination. The formula for emitted power in cgs units is also given, corrected for sub-relativistic temperatures.

Considering a mean electron density of 10^11 cm^-3 and assuming a spherical light ball with a diameter of 10 m and a maximum optical power of 19 kW, the authors estimate the temperature. They note that if the assumption of an isotropic radiator is dropped, the luminous intensity estimate aligns with vehicle headlights. Under these conditions, they found T ~600 K, which is lower than Teodorani's estimate of 5,000 K obtained by considering the phenomenon as black body radiation.

3 Conclusion

The authors conclude that the Hessdalen Lights (HL) are likely a cold plasma, which explains the absence of combustion or fire when observed among trees. They acknowledge that this prediction could also be a consequence of more mundane explanations, such as vehicle headlights. However, they maintain that Teodorani's spectrum (Fig. 1b) is accurate and not an artifact of instrumentation. They assert that the bremsstrahlung spectrum (Fig. 2) is similar to the observed HL spectrum (Fig. 1b) when the atmosphere is clear. They propose that the HL spectrum is caused by high-energy electrons in thick atmospheric plasmas accelerated upward by electric fields from rocks, forming a thermal bremsstrahlung spectrum with a flat top and steep sides, characteristic of an optically thick plasma that simulates an illuminated solid body. The authors emphasize their belief in the genuineness of Teodorani's spectrum due to its similarity with a bremsstrahlung spectrum, though they note that a comparison with other conventional spectra was not performed.

Acknowledgments

Financial support was acknowledged from CNPq and Faperj (Brazil).

References

The references section lists numerous scientific articles and reports related to ball lightning, blue jets, red sprites, terrestrial gamma ray flashes, Hessdalen Lights, plasma physics, and spectroscopy, citing authors such as Adams, Bjorn, Dawson, Leone, Luo, Narayana, Paiva, Pasko, Rybicki, Takaki, and Teodorani.

Recurring Themes and Editorial Stance

The recurring theme throughout the article is the scientific investigation and explanation of anomalous atmospheric light phenomena, specifically the Hessdalen Lights. The authors adopt a theoretical and modeling approach, attempting to reconcile observational data (particularly spectral characteristics) with physical processes. The editorial stance, as reflected in the publication in a scientific journal like Meteorol Atmos Phys, is one of rigorous scientific inquiry, seeking to provide plausible physical mechanisms for unexplained phenomena, while also acknowledging alternative explanations and the need for further research and validation.