Abstract
The Imager of Sprites/Upper Atmospheric Lightning (ISUAL) is a scientific payload on Taiwan's ROCSAT-2 satellite launched on May 20, 2004, that provides new observations of TLEs from space. The ISUAL project is an international collaboration between the National Cheng Kung University, Taiwan, Tohoku University, Japan and the instrument development team from the University of California, Berkeley, and the project was supported by the National Space Program Office of Taiwan. The ISUAL payload includes a visible wavelength intensified CCD imager, a bore-sighted six wavelength channel spectrophotometer, and a two channel Array Photometer (AP) with 16 vertically spaced horizontally wide sensitive regions. The imager is equipped with 5 selectable filters on a filter wheel and a 6th open position. The spectrophotometer contains six filter photometer channels, their bandpasses ranging from the far ultraviolet to the near infrared regions. The two channel AP is fitted with broadband blue and red filters. The orbiting platform with this set of instruments will provide the first comprehensive global latitude and longitude survey of TLE-s near the midnight local time region. One of the great advantages of spaceborne observations is the lack of the intervening atmosphere between the TLE-s and the observer. Ground based observations are often adversely affected by clouds, atmospheric extinction or scattering whereas the space-borne ISUAL instrument measurements provide true emission ratios unobstructed by the variable atmospheric extinction. Spectrophotometer's channels cover the far and mid ultraviolet in addition to channels that respond to various excitation levels of the neutral and ionized nitrogen molecule atmosphere and to emissions from oxygen. The preliminary data shows that the ratio of the emissions is highly variable during the lightning and the associated TLE-s. The data is qualitatively consistent with harder characteristic energy electron production in lightning then in sprites and harder in sprites than in elve-s. The focus of the data analysis will be to solidify these conclusions and to put them on firmer statistical and quantitative basis.