Introduction.

The energy dissipated by each terrestrial lightning discharge is often very large, it can be greater than 10 GW. More than 1000 electrically active thunderstorms occur simultaneously on the earth surface. Thus lightning is a significant source of electrical power in the earth atmosphere. The magnitude of the electric fields created by such lightning discharges have powerful influence the stratosphere, mesosphere and lower thermosphere region due to electric field induced particle acceleration, heating and energy dissipation. Several types of interesting luminous phenomena (sprites are the best known) have been observed which testify that some of this energy is in fact dissipated in the stratosphere and mesosphere. Surprisingly these significant effects avoided detection until quite recently. The understanding of the mechanisms producing the phenomena or the degree they exert influence on the electrodynamics, chemistry or energetics of the mesosphere lower thermosphere is incomplete. In order to make a significant step towards this understanding several new experimental approaches are required. One of these approaches is the performance of a spacecraft based systematic observing campaign which will be performed on board of the ROCSAT-2 spacecraft.

Under the sponsorship of the National Space Program Office of Taiwan (NSPO) the University of California, Berkeley (UCB) will participate in scientific investigation of lightning induced atmospheric phenomena, polar aurora and upper atmospheric airglow in collaboration with National Cheng Kung University (NCKU). This scientific investigation will include the design and construction of a scientific instrument package, its integration on the ROCSAT-2 satellite, its placement into orbit and its on orbit operation for several years.

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] captured one of these enigmatic light flashes on a video camera. Their achievement ushered in a large flurry of activity to document such high altitude electrical phenomena. Sprites are the most spectacular luminous phenomena (Figure 1) which occur above thunderstorms in the mesosphere [Vaughan et al., 1992; Sentman and Wescott, 1993; Lyons, 1994; Mende et al., 1995; Rairden and Mende, 1995;Boeck et al., 1995; Lyons 1996, Winckler et al., 1996]. Other luminous phenomena induced by the large fields include blue jets [Wescott et al., 1995], and elves [Boeck et al., 1992, Fukunishi et al., 1996; Inan et al., 1997]. The observations have been interpreted as the luminosity which is produced by hot ambient electrons 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; Rowland et al., 1995,1996, Glukhov and Inan, 1996; Fernsler and Rowland, 1996; Vladivia et al., 1997] or by electrons which had been energized by the residual electric field between the ionosphere and the thunder cloud after a large 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 conjunction 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].

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment on ROCSAT 2 will make the first spacecraft based measurements of lightning induced luminous phenomena. Major Science objectives of the ISUAL instrument are

1. Determine spatial, temporal and spectral properties of lightning induced upper atmospheric optical flash transients (sprites, elves, blue jets etc.).

2. Determine the global (geographic/seasonal) distribution of upper atmospheric optical flash transients .

The ISUAL instruments consist of a limb viewing low light level imager with a set of bore sighted limb-viewing photometers. This complement of instrumentation is also a powerful tool for observing aurora and airglow. The ISUAL experiment will also make global observations of aurora and airglow from a limb viewing perspective.

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 of the spatial and temporal intensity distribution of the optical flashes uninhibited by the effects of lower atmospheric absorption. 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 U.S., the ISUAL experiment can take advantage of the extensive observing facilities which can produce lightning data routinely.

Where do sprites elves occur and blue jets occur and why? Ground based observations seem to suggest that the largest frequency of sprite occurrence is 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 and 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 storms such as the prevailing tropospheric or mesospheric conditions. As an example there are many more storms around the equatorial regions than at midlatitude, but the observation of sprites near the equator is relatively infrequent.

Figure 1: Sprite image taken in July 1995 by S. B. Mende and his team from the Lockheed Palo Alto Research Laboratories. This image was used on the Lockheed sprite poster. Note color scale was applied to a black and white video frame.

Vaughan et al [1992] reported the only space based observation of sprite phenomena. They examined video recordings taken by the space shuttle video cameras. Although they were able to obtain images of sprites, their results show that it is highly desirable to discriminate against the accompanying lightning flashes. The parent cloud to ground flash usually precedes the sprite event by a few (3 msec) [Rairden and Mende 1995] but it can illuminate the thunderstorm cloud above with very high intensity light. This intensity can be so large that a camera, which is not designed specifically for the purpose, would be blinded. The ISUAL instrument therefore uses several techniques to improve the discrimination of the sprite from the parent lightning. These techniques are the following:

In order to accomplish this the ISUAL imager is a versatile instrument capable of multiple functions.

1) It is a survey instrument designed to provide continuous coverage to determine the statistical properties of sprites and other high altitude flashes, such as latitude longitude and local time and altitude distributions. To facilitate this mode of operation the instrument is run in the "Sprite Continuous Mode".

2) The ISUAL imager can be operated to study the fast spatial/temporal profiles of individual flashes, e.g. their altitude/time development, and individual spectral spatial properties. To facilitate these studies the ISUAL can be run in the "Sprite Burst Mode". In this mode the instrument has a high time resolution (1 msec for the imager) but it is unable to provide continuous coverage.

3) With the inclusion of a filter wheel ISUAL can select specific airglow wavelengths and can be used to study the limb altitude distributions of the airglow and auroral luminosity. For these studies we have introduced the so called "Aurora and Airglow Mode" which produces an image sequence of a regular exposure repetition rate.

The requirement to analyze the time response of these fast discharge events necessitates having to take extremely fast snap shots. The ISUAL instrument uses the technique derived from the observations of Rairden and Mende [1995]. Rairden and Mende used the charge sweeping capability of the CCD to obtain the msec time resolution required. ISUAL will be designed specifically to have a fast vertical sweep with a specially masked CCD allowing to store several images under the masked area. Another stringent requirement is the necessity to have a mode in which the instrument has continuous coverage. So it is necessary to operate the CCD at a framing rate equivalent to~30 frames per second on a 512x512 equivalent size CCD.

To satisfy the above requirements the ISUAL instrument consists of a limb viewing imager and a set of six bore sighted filtered photometers and a two channel array photometer. The observing scenario is illustrated in Figure 2.

The detector is a rectangular CCD of 2048 x 1024 pixels binned to 1024 x 512 resolution elements. The images have 4 to 1 aspect ratio and it is therefore possible to stack 8 128 resolution element pictures each on the 1024x512 res. cell CCD. The stacking is illustrated at the bottom of Figure 2. The limb image is focused on the top of the detector. The rest of the detector is covered with a mask. These covered areas represent additional image storage for fast exposure sequences and associated image storage.

Table 1: Observing Parameters.

The stacking permits exposing the images in quick succession and a sequence of images can be taken which depict the time history of the sprite.

The operation of the system is as follows. Generally the system is idle with the detectors taking data but only minimal data is recorded. When a flash is detected in the selected photometer the image intensifier is commanded to turn the gate on after some (1-6 msec) delay. After the first exposure is completed the image is shifted up in the CCD and a second exposure commences. After a suitable delay the CCD stops the exposure sequence and reads out the 8 images. This way the camera can record a number of frames for each flash event. The duration and repetition rate of the exposures is programmable. It should be noted that if the image intensifier is gated on permanently this technique permits the capture of images, which are taken prior to the occurrence of the trigger photometer pulse.

The array photometer is a two wavelength channel instrument, where each takes data from sixteen separate regions located vertically above each other. This instrument has very high temporal resolution for determining the altitude distribution of the phenomena.

The ISUAL imager consists of three sensor packages, which are controlled by an Auxiliary Electronics Package (AEP). The instrument is best described by referring to Figure 3 the instrument block diagram.

In the AEP the imager control electronics board stores the images. To accommodate the high volume of ISUAL data a mass memory system is added. In the aurora or airglow mode entire 128x512 frames are stored. In the sprite observing mode rapid sequence imaging is expected to occur when passing above active storms. To reduce the data volume Digital image Signal Processor (DSP) is used to compress the image. The parameters regulating image compression will be programmable through ground commanding.

There is also memory for data storage for both photometers. The AEP also contains the DPU, which performs necessary commanding and sequencing of the data taking and maintenance of the communication through the spacecraft interface sending of data and reception of commands.

The AEP also contains the circuitry for generating low voltage regulated power from the nominal 28 V.D.C. spacecraft bus.

There is a triggering circuit using one of the photometer channels, which initiates the data collection. A multiplexer is used to select one of the photometer channels for triggering. This selection is programmed from ground command. A fast comparator performs the triggering function by comparing the photometer signal to a D.C. voltage. The comparison voltage is programmable by command through the DPU.

The system will provide several conventional features (used on other UCB instruments). A wide field of view daylight sensor reduces the output of the high voltage supply so that the imager is protected even when looking accidentally directly into the sun. All high voltage settings at the nominal full value and at the reduced daylight protection value are set up by ground command and these are accepted by the DPU on the satellite only after verifying a password prior to accepting the command. The software systems on the spacecraft or on the ground do not have the password therefore it nothing can accidentally change the high voltage settings. An additional feature of the system is that it can be made to override the daylight sensor by programming the high voltage even if the daylight sensor fails in the on condition.

The ISUAL instrument consist of four packages:

The instrument is illustrated on Figure 4. This image shows a cut away view of the instrument.

The six channel photometers use high time resolution burst mode data recording. The wavelength band selection is tabulated below.

Filter 2 -- Bandpass is from 250 - 390 nm to cover the middle ultraviolet. A Barr UG-5 glass filter is used with an EMR type 541 PMT with a Bi-alkali photocathode on a Magnesium Fluoride window. Once again there are various important lines in the ultraviolet such as the N₂ 2^nd positive and various N₂ Vegard-Kaplan emissions which might contribute strongly in sprite with somewhat moderated contribution from the low altitude cloud to cloud and cloud to ground lightning.

Filter 4 -- Band center is at 427.8 nm to cover the ultraviolet N₂ +1^st negative 0,1 band. A Barr interference filter is used with an EMR type 741 PMT with a bi-alkali photocathode on a sapphire window.

Filter 6 -- The band center is at 777.4 nm to minimize the passband and optimize the lightning detection. The photomultiplier is a ruggedized EMR type 741 PMT with a tri-alkali photocathode on a sapphire window. Tri-alkali was used for increased red enhancement.

Channel 1 may give a clear UV signature for triggering of sprite/elve and may not be blinded by the parent lightning. If this can be confirmed this channel might be used as a trigger. Channels 2, 3 and 5 would bring information about electron energy distribution in the discharge events because these emissions are excited by varying electron energy thresholds. Channel 4 is a primary trigger looking at the well documented 1P-N₂ transition. Channel 5 is a lightning monitor for possible triggering. Channel 6 is also useful in the detection of the Meinel (2,0) N₂⁺ emission and its intensity in comparison with 1P-N₂.

(4) The Auxiliary Electronic Package (AEP).

This unit contains all the large memory arrays, the fast hardware data processing electronics, and the Digital processor unit, or DPU. Most of the data is handled in hardware by programmable logic arrays because of the high speed requirement. The DPU is relatively simple, used mainly for handling the interface with the spacecraft, steering the data out of the large memories to the spacecraft for transmission and handling all the commands required to operate the instrument. The auxiliary electronic package contains power converters, which change 28 volt raw spacecraft power into the various voltages required by the instrument. We propose to use the same form of packaging what was used for the IMAGE spacecraft FUV instrument MEP. The engineering test unit (ETU) of the IMAGE prototype is presented as Figure 6.

Figure 11: Engineering Test Unit of the IMAGE Wideband Imaging Camera CCD and electronics. It is our proposal to use this type of CCD and electronics for the ISUAL imaging camera.

Figure 12: Engineering Test Unit of the Main Electronic Package (MEP) stack of the IMAGE FUV instrumentation. It is our proposal to use this type of packaging for the ISUAL electronics. One of the layers in the stack is the IMAGE FUV Data Processing Unit (DPU) which could be adapted for the ISUAL application.

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