SPACECRAFT BASED STUDIES OF TRANSIENT LUMINOUS EVENTS

Abstract

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) is a scien- tific payload on Taiwan's FORMOSAT-2 (previously known as ROCSAT-2) that provides new observations of transient luminous events (TLEs) from space. The ISUAL project is an international collaboration between the National Cheng Kung University, Taiwan, Tohoku University, Japan and the instrument devel- opment team from the University of California, Berkeley. The project was sup- ported by the National Space Program Office in Taiwan. The ISUAL payload includes a visible wavelength intensified CCD imager, a boresighted six wave- length 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 6 th open position. The spec- trophotometer 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 instru- ments will provide the first comprehensive global latitude and longitude survey of TLEs near the midnight local time region. One of the great advantages of spaceborne observations is the lack of the intervening atmosphere between the TLEs 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 vari- able atmospheric extinction. The channels of the spectrophotometer 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 emis- sions is highly variable during the lightning and the associated TLEs. The data is qualitatively consistent with harder characteristic energy electron production