December 9, 2021 | Tracers – Clouds and Trails | NASA
“First used with sounding rockets flown in the 1950’s, scientific research with experiments which inject vapor tracers in the upper atmosphere have greatly aided our understanding of our planet’s near-space environment. These materials make visible the naturally occurring flows of ionized and neutral particles either by luminescing at distinct wavelengths in the visible and infrared part of the spectrum or by scattering sunlight.
The type of vapor selected to create these colorful clouds and trails depends on the purpose of the investigation, the local time, and the altitude under study. Commonly used vapors that are released in space are:
- Tri-methyl aluminum (TMA),
- Lithium, and
Tri-methyl aluminum (TMA)
Tri-methyl aluminum reacts with oxygen and produces chemi-luminescence when exposed to the atmosphere. The products of the reaction are aluminum oxide, carbon dioxide, and water vapor, which also occur naturally in the atmosphere. TMA releases are most often used to study the neutral winds in the lower ionosphere at night at altitudes of 100 miles (160 kilometers) or less.
Lithium vapor is also used to study neutral winds in the upper atmosphere. Lithium gas has an unusually bright narrow-band emission at 670.7 nanometers, a wavelength in the infrared range, which enables it to be visible in the daytime with cameras with infrared filters. In fact, Lithium is the only vapor that can be imaged during the day and is also one of the few vapors that can be used at high altitudes (> 124 miles or 200 kilometers) at night. At night, its color is bright red.
Barium is used to study the motion of both ions and neutrals in space. A fraction of a barium cloud ionizes quickly when exposed to sunlight and has a purple-red color. Its motions can be used to track the motion of the charged particles in the ionosphere. The remainder of the barium release is neutral, having a different color, and can be used to track the motion of the neutral particles in the upper atmosphere. A small quantity of strontium or lithium is sometimes added to the barium mixture to enhance the neutral barium emissions, making it easier to track the neutral cloud. Since the observer must be in darkness while the barium cloud is in sunlight, the technique is limited to local time observations near sunset or sunrise.”
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