About ISUAL?

What is ISUAL?

ISUAL is the acronym for Imager of Sprites and Upper Atmospheric Lightnings. It is a set of scientific payload instruments on the FORMOSAT-2 satellite, which is the second satellite from Taiwan, ROC. ISUAL project is an international collaboration supported by the National Space Program Office in Taiwan, with additional contributions from the National Cheng Kung University, Taiwan, the Space Science Laboratory of the University of California at Berkeley, USA, and Tohoku University, Japan.

The scientific goals of this mission are to investigate lightning induced, luminous atmospheric phenomena, polar aurora and upper atmospheric airglow. This scientific cooperation includes the design and construction of a scientific instrument package, its integration on the FORMOSAT-2 satellite, its placement into orbit and its on orbit operation for five years. This mission is scheduled to launch on 16 January, 2004.

Science background

The energy which is dissipated by each terrestrial lightning discharge can be greater than 10 GW and several thousand electrically active thunderstorms occur simultaneously on the earth surface. Thus the lightning could be a significant source of electrical power in the earth atmosphere/ionosphere system. The magnitude of the electric fields created by such lightning discharges has even greater significance because lightning can cause huge transient electric fields (in the kilovolts per meter range) in the upper atmosphere. These fields can cause particle acceleration, heating and energy dissipation. Surprisingly the effects created by these fields were not directly observed until quite recently.

Although many observers had described luminous displays flickering through the night sky above thunderstorms, the scientific community largely ignored these reports. Franz et al., [1990] were the first to a sprite on a low-light level video camera. Their achievement ushered in a large flurry of activity to document such high altitude electrical phenomena. Sprites are one of the most spectacular luminous phenomena, which occur above thunderstorms in the mesosphere [Lyons, 1994; Mende et al., 1995; Boeck et al., 1995; Lyons 1996, Winckler et al., 1996; Su et al., 2002]. Other luminous phenomena induced by the large fields include blue jets [Wescott et al., 1995; Pasko et al., 2002], elves [Lyons et al., 1994; Fukunishi et al., 1996], and gigantic jets [Su et al., 2003]. Collectively, these upper atmospheric luminous phenomena are termed transient luminous events (TLEs).


The first color image of sprites taken by Sentman et al in 1994. (From elf.gi.alaska.edu)

       TLEs have been interpreted as the luminosity which had been excited by the large electromagnetic pulses from the lightning (possible elves mechanism) [Inan et al., 1991, 1996b; Taranenko et al., 1993a,b; Milikh et al., 1995; Rowkland et al., 1995,1996, Glukhov and Inana, 1996; Femsier and Rowland, 1996; Vladivia et al., 1997] or by electrons which had been energized by the residual electric field between the ionosphere and the thundercloud after a large CG lightning discharge (possible sprite mechanism) [Pasko et al., 1995, 1996a,b, 1997 a,b, 1998; Boccippio et al., 1995; Winckler et al., 1996; Fersler and Rowland, 1996]. Some workers have invoked runaway electron processes to explain some of the very energetic electron phenomena (gamma ray flashes) which have also been observed in conj notion with large thunderstorms [Bell et al., 1995, Winckler et al., 1996; Roussel-Dupre and Gurevich, 1996; Taranenko and Roussel-Dupre, 1996; Lehtinen et al., 1996; 1997].

Mission Objectives

The ISUAL experiment on FORMOSAT 2, consisting of a boresighted imager, a 6-channel spectrophotometer and a two-channel array photometer, will make global surveys of lightning-induced TLEs from orbit. Major science objectives of the ISUAL instrument are

  1. To determine the location and timing of luminous phenomena above thunder clouds to investigate their spatial, temporal and spectral properties.
  2. To obtain a global survey of upper atmospheric optical flash transients (sprites, elves, blue jets, gigantic jets etc.).

       A low light level imager in orbit with a bore sighted limb-viewing photometer is a powerful tool in the investigation of aurora and airglow. Thus we define ISUAL experiment's secondary objective as: to investigate of the global distribution of the airglow intensity as a function of altitude.

        During the northern summer several international teams will be participating in campaign operations and will be observing thunderstorm induced optical flashes from ground aircraft and balloon. It is intended that the ISUAL payload will take advantage of these ground campaigns and make coordinated observations. During such coordinated observations several additional parameters will be measured thus providing multi dimensional view of the phenomena. Using ISUAL data alone and in conjunction with the campaign data many of the critical questions about the phenomena will be answered. Some of the questions are listed below:

What is the mechanism producing the flashes?

The ISUAL imager spectrophotometer and array photometer will measure the spectral content and the spatial and temporal intensity distribution of the optical flashes. Intensity ratios of the various emissions will be used to determine the energetics of the electron excitation processes. Coordinated observations with ground based simultaneous measurement of electric fields and VLF waves will provide a measure of the degree of ionization in the sprites and the role of the electric field in producing the ionization. During passes over the US, the ISUAL experiment can take advantage of observing facilities which can produce lightning data routinely.

Where do sprites elves and blue jets occur and why?

         Ground based observations seem to suggest that the largest frequency of sprites occurrence be over the central part of the North American continent. However the ground and aircraft based experiments cannot provide a comprehensive survey. The ISUAL payload will be able to provide the first real unbiased survey of lightning induced optical flashes over the entire globe. From such a survey we will be able to decide whether the occurrence frequency of sprites depend on factors other than the magnitude of the storm such as the prevailing tropospheric or mesospheric conditions. As an example there are many more storms.

         Space-based observation of these phenomena was reported by Vaughan et al. (Mon. Weather Rev., 120,1459, 1992 ), who examined video recordings taken by the space shuttle video cameras. Although they were able to obtain images, their results show that it is desirable to discriminate against the parent lightning. The parent cloud to ground flash precedes the sprite event by a few (3 msec; Rairden and Mende 1995) but it can be so large that a camera which is not designed specifically for the purpose would be blinded. The ISUAL instrument therefore uses three separate techniques to improve the sprite to parent lightning intensity ratios. These techniques are:

1.)     By observing the flashes near the limb or on the limb it is possible to get spatial separation between the two sources of intensity.

 2.)     By using appropriate spectral filtering it is possible maximize the sprite induced spectral bands and cut down on the spectral bands produced by cloud to ground lightning.
3.)     By using a trigger photometer and only turning on ISUAL imager at the appropriate time it is possible to remove some of the parent lightning intensity.

The ISUAL imager is a versatile instrument capable of multiple functions.

1.)     It is a survey instrument to determine the statistical properties of sprites and other high altitude flashes, such as latitude longitude and local distributions, and their altitude distributions.
 2.)     It is an instrument which enables the study of individual flashes, their altitude/time development, and spectral spatial properties.
 3.)     With the inclusion of a filter wheel ISUAL can select specific airglow wavelengths and can be used to study the altitude distributions of the airglow luminosity.

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