University of Alaska Fairbanks
June 11, 1997
The current analysis of spectral observations of sprites is presented. The data are collected using a television rate (30 frames per second) slit spectrograph system. The spectral range of 450 nm to 900 nm has been analyzed, including system response corrections. The optical sprite energy is most promintely the molecular Nitrogen First Positive emissions.
Proposed theories of sprites predict other molecular Nitrogen emissions including Meinel, Second Positive, and First Negative bands at intensities of 1 to 4 orders of magnitude less than the First Positive emissions. Current analysis has focused on observations of these other molecular Nitrogen bands.
The data presented are three sprite images from a co-aligned black and white, intensified CCD camera. The slit on the images shows the exact location of the slit spectrograph. The observed spectral emissions from sprites are plotted.
The spectra reveal only the First Positive emission bands of molecular Nitrogen. The Meinel emissions at 805 nm is not present. There is no indication of First Negative emissions at 450 nm.
The map shows the relative positions of the sprites and the Wyoming Infrared Observatory, where the observations were made.
As shown in the Grotrian Diagram, the molecular Nitrogen First Positive transitions require lower energy than the other Nitrogen transitions that might appear in optical sprite observations. The predominance of First Positive emissions would seem to indicate sprites are a low energy phenomena (when integrated over their entire volume).
The lifetimes and quenching altitudes of the various transitions are presented in the table below. The quenching altitudes indicate that Meinel emissions would most likely occur in the upper region of a sprite. The color images of sprites previously presented, showing blue lower portions of the sprite, indicates that Second Positive or First Negative emissions (blue) would most likely occur in the lower regions of the sprites.
|Transition||Lifetime (ms)||Quenching Altitude (km)|
One possible explanation for the lack of observation of the blue (First Negative and Second Positive) emissions, both with a spectrograph and with scene cameras can be found in the plots showing atmospheric transmission at various observer altitudes, for 100 km and 500 km path lengths, with a synthetic First Positive Nitrogen spectra plotted for reference*. At 500 km path length, from 5 km altitude, the atmospheric transmission of 427.8 nm (N2+ (1N)) is less than 5%.
* (Thanks to Jeff Morrill of NRL for providing the N2 1PG synthetic code output for these plots.)