Multi-instrument coincident detection of sprites

Phys. Chem. Earth (B), Vol. 25, No. 5-6, pp. 417-422, 2000 0 2000 Elsevier Science Ltd All rights reserved 1464-1909/00/$ - see front matter

Pergamon

PII: Sl464-1909(00)00036-8

Multi-Instrument

Coincident

Detection

of Sprites

J. L. Biihr, J. B. Brundell, S. F. Hardman and R. L. Dowden Department Received

of Physics, 1 December

University

of Otago, Box 56, Dunedin,

1999; accepted

25 February

New Zealand

2000

Abstract. This paper describes the instrumentation

Red sprites (Figure 1) are large but weak luminous flashes that appear above an active thunderstorm system, and are usually correlated with large positive cloud-to-ground lightning strikes. They frequently consist of vertical columns of luminosity approximately 1 km in diameter and 20-50 km high (Wescott et al., 1998, Hardman et al., 1998). Measurements (Dowden et al., 1996, Rodger et al., 1998), have shown that these luminous discharge columns can be highly conducting at Very Low Frequencies (3 kHz - 30 kHz), and they always scatter VLF waves in the forward direction as well as other directions. For a review of sprites and VLF scattering see Rodger (1999).

used for sprite detection during two campaigns near Darwin, Australia, in 1997 and 1998. The sprites (large electrical luminous discharges above thunderstorms) are detected by both ‘direct’ (optical) and ‘indirect’ (radio frequency) methods. The optical detectors consist of (i) two intensified video capture systems that are capable of capturing and storing only video frames containing sprites, and (ii) up to four photometers whose outputs are monitored so that only The radioevents defined as significant are captured. frequency detectors consist of (i) four-channel sferic detectors operating in the VLF region and (ii) one or more AbsPAL VLF phase and amplitude loggers. All detectors use GPS receivers for timing reference, which allows postcapture comparison of all events with high accuracy. 0 2000 Elsevier Science Ltd. All rights reserved.

-10

1 Introduction -20 .90 km -

d=% Visible to the naked eye Red Coloured Duration: 5 - 200 tns Columns (spritelets) can be < 2 km wide

,40 km -

-40

Typical onset time (after the CG) is 1.5 -4 ms

~~

Not necessarily 20km

-30

-50

directly over the +CG discharge

100

-

Fig. 2. Location of Darwin Experiments

160

190

(Nov-Dee 97/98)

The authors observed sprites during the ‘wet season’, near Darwin, Australia (Figure 2), during two campaigns of about 4 weeks duration each, in November/December of 1997 and 1998. Meteorological data for the region shows a maximum likelihood for lightning during this period. The results of the 1997 campaign have been reported previously (Dowden et al., 1997b, Hardman et al., 1998, 1999).

Fig. 1. Sprite Characteristics - red sprites are large but weak luminous flashes that appear above an active thunderstorm system, and are usually correlated with large positive cloud-to-ground lightning strikes. They frequently consist of vertical columns of luminosity approximately lkm in diameter and 20-50 km high.

Correspondence

130

,B .\@

to: John L Bahr 417

J. L. Biihr et al.: Multi-Instrument Coincident Detection of Sprites

418

GPS Receiver

Y,

Serial ‘PPS ADSP2181 yy/mm/dd hh:mm:ss.sss

v

Photo-Event Detector

GPS Receiver

Serial

1PP

Video-Text Inserter

I * JPPs

Video Time Generator

In

VSYNCH

Fig. 4. The Video Date/Time Inserter. The unit uses a Digital Signal processor (DSP - ADSP218 1) to combine the GPS serial time information with the 1 pps (pulse per second) to create a time-string of the fotm yy/mm/dd hh:mm:ss.sss.

2 Instrumentation 2.1 Overall

Fig. 3. The Overall Instrumental Configuration. Note that the various components need not be in the same physical location.

The experiments use (a> three ELF/VLF sites (20-30 km apart), each with a Trimpi (phase and amplitude perturbation) detector for observing signals from the US Navy 1 MW VLF transmitter at North West Cape (NWC), Australia (see Figure 2), and a sferic transient detector (0 - 15 kHz) where the electric and magnetic field components Ez, Bx, BY of the sferic are logged for Poynting-vector direction finding of the lightning and (possibly) the sprite, and calculation of sprite current (when location is known); @I one optical site with a four channel photometer (fov 15’-20”) for accurate time measurement of the optical signal and two intensified video systems (fov 40’) for sprite images. The two video units, the photometers, a sferic recorder and an AbsPAL VLF phase and amplitude logger are normally used at a site (12” 39’ S, 131’ 19’ E) approximately 65 km SW of Darwin. Three or four sferic recorders and AbsPAL loggers are also stationed at various points around Darwin, and also in Dunedin. NZ.

The instrumentation used to observe red sprites has evolved over the two campaign periods, and that described here is the most recently used arrangement. Two forms of optical detection and two methods of radio-frequency detection are used, with all recording being synchronised via GPS receivers. Figure 3 shows the overall configuration. The various components need not be in the same physical location. For example, the two optical systems are often situated some 10 km from one or both of the RF systems. In this case a GPS receiver is used at each location. Each component is discussed separately below. The two main features of the instrumentation that are the subject of this paper are (a) the ability of the loggers to only record ‘significant’ data - in the simplest case a threshold detector, and (b) a time precision of at least 1 ms on all data. 2.2 Optical The outical detectors consist of (a) two intensified video capture systems that are capable of capturing and storing only video frames containing short-time bursts of light (eg sprites and/or lightning), and (b) up to four photometers whose outputs are monitored so that only events defined as significant are captured.

J. L. Blhr et al.: Multi-Instrument

Coincident

Detection

419

of Sprites

2.2.1 Video Capture The intensified video systems use a Hamamatsu C3100 image intensifier with a C3077 CCD camera, and an ITT CAM300 intensifier coupled to a Panasonic PM200 CCD camera. Video capture is achieved using a commercially available (and cheap) video card that runs under LINUX and can capture frames in either 25 frame per second or 30 frame per second formats. The card, which is based on the common Bt848A chip, is programmed to write each video frame into a RAM-disk buffer, while a separate program reads each frame-file and determines whether it should be kept or not. If the decision is to keep the frame it is written to hard disk. The simplest algorithm, threshold detection on the integration of the signal over the entire frame, works very well because the background’ image (stars in a dark sky) contrasts strongly with a sprite or lightning.

1PMT3 Trigger

I I <> I ADSP2181 (DSP Event Detection and Logging Software)

+

Serial

LINUX

Serial

PC r-----------r

; Event Logger ; ; Software ; L----**---_-. l------------1NTP Software : :___________I

A method was needed to ‘time-stamp’ each video frame with UTC to 1 ms precision. A custom-built unit (Figure 4), based on a similar system (Polczynski, 1996), does this by inserting UTC from a GPS receiver onto each video frame. The unit uses a Digital Signal Processor (DSP ADSP2181) to combine the GPS serial time information with the 1 pps (pulse per second) to create a time-string of the form yylmmldd hh:mm:ss.sss. This is sent to a video text inserter chip (UPD6451A) which adds the string to the current video frame. The combined video signal is then sent to the video card in the PC for processing.

IEvent Waveform\

Fig. 5. Automatic Event Capture. For sferics, the photometers replaced by antennas and different amplifiers.

(PMTx) are

b

/

1

2

3

:19s

I

N

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2

3

time

I

/ k

>

N

:21s

Fig. 6. The timing of events - the time-line for the ADC sampling. The thin lines represent the sample times for the ADC and the thick lines the GPS lpps. From the values of to, 11and the number of samples taken during the last second. N, the exact sampling rate, qw, and the offset from UTC are determined.

2.2.2 PhotometerKferic

Event Capture

For accurate optical timing, up to four photometers are used with suitable optical filters (Corning glass filters: CS 3-68 (red), CS 7-59, CS 7-62, CS 7-51 (blue)). Continuously recording the signals from four photometers (or sferic antennas) would create a large amount of data, most of which are unwanted, as we are only interested in data that occur just before and for some time after a sprite occurrence. We have developed an ‘event detection’ system that looks at the signals in real time, but only keeps those data that fall within certain criteria. At present we operate the detection system much like a triggered (digital) oscilloscope; the system scans one or all signals and if the data are greater than a preset threshold the last 127 pretrigger samples, and 1024 samples in total (representing 34.16 ms in time) of each channel are recorded and sent to the PC. The core of the system is shown in Figure 5.

Because we wish to preserve the phase of the signals in ADC which incorporates four digitising, a 12-bit simultaneous sample-and-holds is used (AD7874), and samples at -3OkHz. Sampling trigger pulses are provided by the DSP, which is also interrupted at one-second intervals by the leading edge of the GPS 1 pps. The algorithm for extracting the time for an event is discussed below. The hardware involved in the event detection (Figure 5) consists of 1. a desktop PC running LINUX, 2. a GPS receiver to provide UTC and 1 pps, 3. a four-channel simultaneous ADC for sampling the signals, and 4. a DSP for detecting the events and communicating with the PC. The software used is: 1. DSP code for detecting events, recording the

J. L. BIhr er al.: Multi-Instrument

420

waveforms and transmitting them to the PC, NTP (Network Time Protocol) software for synchronising the PC to UTC, and 3. software for PCYDSP communication. The software performs the following tasks: 1. trigger the ADC and read resulting samples into a circular buffer in the DSP, 2. process each sample in the buffer - look for events, 3. store detected events in DSP buffer memory (three complete records can be stored), 4. keep track of the precise UTC time and assign an accurate timestamp to each event (seconds, minutes, hours and days are loaded to the DSP from GPS at the start and maintained in software), and 5. transmit 4x1024 waveform samples plus timing information to the PC.

Coincident

Detection

of Sprites

2.

VLF Sferics 97:330 4000 2000 -

0 --

-2000 -4000 -

-6000 -

1

-8000 ’

r2

r-2

?

3

E

2

?

2

E

Time in ms after 44439 s UTC

2.2.3 Precision

UTC Timing

(a) Sferic Data

The leading edge of the Lassen-SK8 GPS 1 pps (generated at the start of a UTC second) is accurate to +500 ns of UTC. The DSP clock period is -30 ns and a DSP counter (TCOUNT) is programmed to count down to zero and interrupt (and hence trigger the ADC) every 1112 cycles, or approximately every 33.36 p.s, giving a sampling frequency of -29980 Hz. The GPS 1 pps is used to interrupt the DSP and initiate a read of TCOUNT, whose value is stored in variable tl (the previous tl value is stored in variable to). The time-line for the ADC sampling is shown in Figure 6, where the thin lines represent the sample times for the ADC and the thick lines the GPS 1 pps. From the two values of TCOUNT (to and t,) and the number of samples taken during the last second, N, the exact sampling rate, sps, and the offset from UTC are determined. When an event is detected at the k* sample, as indicated in Figure 6, the event “time-stamp” is calculated in the following way. The actual DSP clock period, tclk, is given

5000

Photometers

1

97:330

Time in ms after 44439 s UTC

(b) Photometer

Data

by 1 f 1x10”s t,, =

t, -t, +1112sps

and so the exact ADC sam ling period is 1112*t,rk. If the % event is detected at the k sample after the start of the second, the time, tk, since the start of the UTC second is t, = 1112(k - l)t,, + [(t, + lO)t,, f 500ns] where the 10 added to tl accounts for the interrupt latency in the DSP. Thus the current values of variables: tl, to, sps, k, seconds, minutes, hours and days are recorded with the waveform data, allowing reconstruction of the event time. Figure 7 depicts the capture of the same event by three of the instruments described here. The top panel (7a) shows three channels of the sferic detector (B,, B, and E,), the centre panel (7b) the output of four photometers (three with filters centred on 360 nm, 390 nm and 400 nm and the other with a high-pass filter from 540 nm) and the bottom panel (7~) the video frame containing the sprite (the GPS-based video text-inserter was not operating at the time).

(c) Sprite Fig. 7. Simultaneous sferic (a), photometer (b) and sprite (c) capture for day 330 1997. The time axis in (a) and (b) is in ms after 44439s UTC.

J. L. Btihr et al.: Multi-Instrument

4500

Photometers

1

Fig. 8 Two photometer (lightning) ms events can be buffered.

2.2.4 Buffering

Detection

of Sprites

The four-channel event detectors are capable of buffering up to three captured events, so that even with the relatively slow downloading time to the PC via a serial connection (at 38400 baud), events are rarely lost. Figure 8 shows the capture of two 34 ms photometer events (lightning in this case), the end and start of which are only separated by -5 ms.

421

97:327

events for day 327 of 1997 showing the ability of the system to catch events that are almost continuous.

Events

2.3 Radio Frequency

Coincident

Up to three 34

onset to ultimate decay (and transforming these to scatter phase and amplitude) can be used to identify the plasma formed by a sprite which exhibits amplitude decay with the logarithm of time (Dowden et al., 1998). Figure 10 shows an example of a Trimpi event recorded in Darwin on the 1 MW signal transmitted at 19.8 kHz from the US Navy transmitter NWC (21” 48’ S, 114” 9’ E), some 2,000 km from Darwin. The amplitude and phase perturbations are in B, (top) and B, (bottom).

Detection

2.3.1 Sferic Detector The sferic event detector uses the same DSP electronics as the light event detector, except that the input signals are from RF antennas and are bipolar (see Figure 7(a)). Sferic detectors are run in four locations near Darwin and also in Dunedin. 2.3.2 VLF Phase and Amplitude

Logger - AbsPAL

The holute Phase and Amplitude Logger is a digital VLFILF receiver consisting of a DSP (ADSP’z.105) card It can log up to six VLF (Figure 9) and software. transmitters at a time, logging phase and amplitude with time resolutions of 50 ms to 60 s. Measurement of phase and amplitude perturbations (Trimpis), see Dowden et al. (1997a) enables calculation of the phase and amplitude of Measurement of the the sprite-plasma diffracted wave. phase and amplitude perturbations at all time points from

Fig. 9. The AbsPAL receiver is implemented entirely in software. The above diagram indicates the workings of one of six or more (depending on DSP and PC speed) simultaneous frequency channels. The block labelled “transmitter frequency” generates the exact frequency (or centre frequency in fhe case of MSK modulation) of the transmitter being logged. The large circle is an oscilloscope representation of the conversion from Cartesian (in-phase and quadrature-phase) output to polar (Phase and Amplitude) output.

422

J. L. Bahr et al.: Multi-Instrument

NWC (19.8kHz) 97:330 Bx

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Coincident

Detection

of Sprites

the frames. The GPS synchronisation allows post-capture comparison of all events with high accuracy. This ability will allow us, in future campaigns, to investigate some of the outstanding questions regarding red sprites, such as their initial formulation (in which direction and how fast do they grow), and their fine structure (recent work in this area has been done by Wescott et al. (1998)). A more detailed account of the state of sprite investigations can be found in Rodger C.J. (1999).

139.0 66.5

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Acknowledgments: This work was supported by New Zealand Marsden Research Fund contract UO0519. The authors would also like to thank Tim Dawe, Dave Hardisty, and Peter Simpson, of the University of Otago, for their work on the electronics, and Zen-Ichiro Kawasaki of Osako University, Japan for the use of an image intensified video system.

in Bx References NWC (19.8kHz) 97:330 By

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Fig. 10. Amplitude (thick line) and Phase (thin line) perturbations on the two magnetic field components of the VLF signal (19.8 kHz) from the NWC 1 MW transmitter.

We are also able to capture video frames that meet a prescribed criterion and insert accurate time information on

Dowden R.L., Brundell J.B ., Rodger C.J., Molchanov O.A., Lyons W.A., and Nelson T. The Structure of Red Sprites Determined by VLF Scattering. IEEE Antennas and Propagation Magazine, 38,7-15, 1996. Dowden R.L., Brundell J.B., and Rodger C.J. Temporal evolution of very strong Trimpis observed at Darwin, Australia. J. Grophys. Rex 24, 24 19-2422, 1997a. Dowden R.L., Hardman SF., Brundell J.B., Bahr J.L., and Kawasaki Z., Nomura K. Red Sprites Observed in Australia. IEEE Antennas and Propagation Magazine, 39, 106, 1997b. Dowden R.L., Hardman S.F., Rodger C.J., and Brundell J.B. Logarithmic decay and Doppler shift of plasma associated with sprites. J. Atmos. Sol.Terr. Phys. 60, 741-753, 1998. Hardman S.F., Rodger C.J., Dowden R.L., and Brundell J.B. Measurement of the VLF Scattering Pattern of the Structured Plasma of Red Sprites. IEEE Antennas and Propagation Magazine, 40,29-38, 1998. Hardman S.F., Dowden R.L., Brundell J.B., Bohr J.L., Zen-ichiro Kawasaki, and Rodger C.J. Sprite Observations in the Northern Territory of Australia. J Geophys. Res. - in press , 1999. Polczynski I. Video Inserter. Electronics World, 102, 650-653, 1996a. Polczynski I. Video Inserter. Electronics World, 102,773-774, 1996b. Rodger C.J., Wait J.R., and Dowden R.L. Scattering of VLF from an experimentally described sprite. J. Atmos. Sol.-Terr. Phys. 60, 765-769, 1998. Rodger C.J. Red sprites, upward lightning and VLF perturbations. Rev. Geophys., 37, 317-336, 1999. Wescott E.M., Sentman D.D., Heavner M.J., Hampton D.L., and Lyons W.A. Columniform Sprites: A different variety of mesospheric optical flashes. J. Atmos. Sol. Terr. Phys., 60,733-740, 1998