The night sky surrounding Mauna Loa in Hawaii is often characterized by a faint, pervasive glow. This phenomenon is particularly noticeable due to the mountain’s high altitude, which places observers above much of the lower atmosphere and many sources of light interference. The constant illumination results from a complex interplay of natural atmospheric processes, localized volcanic byproducts, and the scattering of ground-based light. The Big Island’s unique geographical position, dominated by an active volcano, creates the specific conditions that amplify this sky glow. The causes are layered, beginning with a continuous, global atmospheric emission that is then amplified by local environmental factors.
The Mechanism of Atmospheric Airglow
The fundamental source of the natural, widespread glow in the night sky is airglow, a form of chemiluminescence occurring in the upper atmosphere. Unlike the aurora, airglow is powered by ordinary solar radiation. During the day, atoms and molecules in the upper atmospheric layers absorb energy from the sun’s extreme ultraviolet radiation.
As night falls, these excited components return to a stable state by releasing stored energy. They do this by recombining or by having electrons drop to a lower energy level, emitting a photon of light. This emission, or nightglow, occurs at altitudes ranging from roughly 80 to 400 kilometers above the surface.
Different molecules and atoms emit light at specific wavelengths, producing various colors. Atomic oxygen is responsible for a prominent green emission, and a higher-altitude layer of oxygen atoms produces a fainter red light. Reactions involving hydroxyl (\(\text{OH}\)) molecules also contribute to the airglow.
The glow is present globally but is often more intense in tropical and equatorial regions due to the dynamics of the upper atmosphere’s circulation. This makes the airglow above Hawaii naturally brighter compared to mid-latitudes, setting a higher baseline for illumination.
Influence of Volcanic Emissions and Vog
A significant factor contributing to the enhanced glow around Mauna Loa is volcanic smog, or Vog, a byproduct of the volcano’s outgassing. Mauna Loa emits substantial amounts of volcanic gases, primarily sulfur dioxide (\(\text{SO}_2\)), which is the precursor to the dense atmospheric haze that often blankets the area.
Once in the atmosphere, sulfur dioxide undergoes complex chemical reactions involving moisture, oxygen, and sunlight. This process converts the \(\text{SO}_2\) gas into fine solid particles, predominantly sulfate aerosols. These minuscule particles are highly effective at scattering light, a process far more pronounced than scattering caused by normal atmospheric gases.
The Vog layer acts as a diffusing screen, trapping and scattering light from both the natural airglow above and artificial light from the ground below. This dense aerosol layer enhances the perceived brightness of the sky glow by increasing the amount of scattered light directed toward the observer.
The trapping of Vog close to the ground is often facilitated by a nocturnal temperature inversion, where cooler air near the surface is capped by warmer air. This inversion prevents the volcanic plume from dispersing, concentrating the light-scattering aerosols on the mountain slopes and intensifying the localized sky glow.
How Ground Light and Observation Conditions Affect Visibility
Man-made light significantly contributes to the overall intensity of the glow, especially when viewed from a high-altitude vantage point. Light pollution originating from urban centers along the coast, such as Hilo, radiates upward into the night sky. This artificial light is then intercepted and scattered by atmospheric particles, including the concentrated Vog layer.
The scattering of this ground light by the volcanic haze projects a dome of illumination visible for many kilometers. The observer’s altitude is a determining factor, as the high vantage point looks down upon the scattering layer, making the light dome appear brighter and more defined. Low-lying cloud cover can also magnify this effect by providing a highly reflective surface for the ground light to scatter off of, further intensifying the perceived glow.
Incidental light sources on the mountain itself also temporarily contribute to the scattered light, which is diffused by the ever-present aerosols. The pronounced glow around Mauna Loa is ultimately the product of three factors working in concert: constant natural airglow, efficient scattering by volcanic Vog, and enhancement from artificial lights on the ground.