The ISUAL instruments

    The requirement to analyze the time response of fast discharge events produces an important requirement, which is the ability 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 submilisecond time resolution required.

        ISUAL will be designed specifically to have a fast vertical sweep with a specially masked CCD allowing storage of 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 fast framing rate (-30 frames per second on a 512 x 512 CCD). To satisfy the above requirements the ISUAL instrument consists of a limb viewing imager, a set of six bore sighted filtered spectro photometers, and an array photometer.

        Assuming a satellite orbit of 891 km, the limb is about 2600 km from the satellite. In the vertical dimension the field of view is relatively small (6 degree) covering an altitude range on the limb of only about 270 km

        The 6:1 aspect ratio permits taking several images on a square format CCD by stacking them in the vertical direction. This way images can be taken in quick succession providing a sequence of images depicting the time history of the phenomena. One of the boresighted photometer channels is used to detect optical flashes. When a flash is detected the photometer signal is used to command the image intensifier to gate on after some delay. After the first exposure is completed the CCD shifts the image up and a second exposure is commenced. After a suitable delay the CCD stops the exposure sequence and reads out the last 6 images. This way the camera can record a number of frames for each flash event and the duration and repetition rate of the exposures are 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 ISUAL imager consists of three sensor packages, a ICCD imager, a sin-channel spectrophotomer, and a red/blue-band array photometer, which are controlled by a main electronics package (MEP).

(1) The ISUAL Imager

        Light enters the imager through the lens. A filter wheel selects the pass band of the observation. A single stage MCP intensifier precedes the CCD. The photo cathode of the tube can be back biased so that the instrument is gated off. The photometer is programmed to control this gating function. Behind the intensifier is the CCD. The CCD has a metal mask which covers most of the active area except the lower region corresponding to a narrow strip of l/6th of the image. This is exposed to the incoming light. When the exposure time is finished the image (images) is (are) transferred upward into the upper part of the CCD where the storage area is behind the metal mask and therefore not light sensitive. This way a fast sequence of images is collected on the CCD. When the photometers detect a flash, a programmable delay will start, at the end of which the intensifier is gated on. Exposures are taken and the upward transfers are initiated until five exposures are stored. The CCD will be shielded against penetrating radiation so that trapped protons in the <50MeV range are excluded.
 

(2) The Spectrophotometer

        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 4 would bring information about the electron energy distribution in the discharge events because they are excited by varying electron energy thresholds. Channel 5 is a primary trigger looking at the well established IPN₂ transition. Channel 6 is a lightning monitor for triggering.
 

(3) The Array photometer