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Instrumentation
Overview of A Balloon-Borne Sprite Observatory
Payload InstrumentationOverviewThe payload, shown in Figure 1, has seven major detection systems, along with some
minor ambient conditions sensors. The sensors include two separate three axis electric
field detectors, a high-gain low voltage system and a low-gain high voltage system. The
sensors also include both fluxgate and search coil magnetometers, with several gains and
bandwidths of associated telemetry. There is an X ray scintillation counter that is
sensitive to x rays from 25 to 250 keV in photon energy and a Geiger-Mueller tube for
detecting penetrating charged particles. There is a flash photometer looking up that will
register the presence of sprites directly above the payload. Finally, there is a vertical
air-Earth current detector. Figure 1 shows Jim Benbrook enjoying the moment of "first
byte", when the PCM generation software ran successfully for the first time. The sensors are run by an on-board computer, which controls the sampling sequence,
generates the pulse code modulated (PCM) telemetry, and manages the high speed
digitization and on-board storage of the broad-band data that will be obtained during a
Sprite event. A block diagram of the entire payload is shown in Figure 2 (click on the
image for a 300x300 version that is big but readable). Figure 3 shows a block diagram of
the digital portion of the payload. Electric Field:The most critical parameter that must be measured to address the questions posed above
is the electric field. What is required is a vector measurement with substantial dynamic
range (±100 V/m) and bandwidth (20 kHz). It must be remembered that the fields at 35-40
km will be considerably weaker than the 104-105 V/m field found near
and within the storm. The necessary measurement must be made above the originating
thunderstorm, preferably as close as possible to the light emitting volume. This
requirement points very specifically to a stratospheric balloon measurement as the
technique of choice. For a prior project, the Space Physics Group at the University of
Houston developed preamplifiers with sufficiently high dynamic range, slew rate, and input
impedance to make the necessary measurements [Byrne et al., 1988]. c:2:2. Magnetic Field:The ELF magnetic field signature of the sprites can be used to determine the amount of current being carried, at least in principle. In previous studies of the EM field above thunderstorms, we have developed and flown a set of three axis induction or search coil magnetometers with 100 kHz bandwidth. c:2:3. Conductivity:The standard University of Houston electric field experiment measures the conductivity owing to both positive and negative ions at 4 minute intervals [Bering et al., 1980b, 1991; Byrne et al., 1988, 1990, 1991]. Two methods are used, the relaxation method [Benbrook et al., 1974; Hu et al., 1989; Holzworth, 1991; Hu, 1994] and a version of the blunt probe method. Since it is somewhat unlikely that we will fly directly through a sprite, this time resolution should be sufficient to monitor any large scale conductivity changes in the stratosphere. c:2:4. Current:A balloon-borne version of the air-Earth current sensor used in our measurements at South Pole station [Byrne et al., 1993] has been developed and successfully test flown. We will including this measurement in the balloon payload, which means that we will measure all three terms in Ohm's Law simultaneously. c:2:4. X Ray Counting Rate:The University of Houston Space Physics Group has extensive experience in measurements of auroral X rays [Bering et al., 1980a, 1988; Matthews et al., 1988]. Bremsstrahlung X rays and gamma rays of the energies reported by Fishman et al. [1994] can penetrate to a depth of 5 gm/cm2 with relatively little attenuation [Berger and Seltzer, 1972]. Thus, a balloon borne X ray counter will be capable of monitoring X ray and gamma ray production by sprites. We plan on flying a NaI scintillation counter with a 125 mm diameter crystal and a 16 channel pulse height analyzer that will be read out to telemetry every 100 ms. Following event triggers, higher sampling rate data will be stored in the on-board recorder. A set of particle detectors including a Geiger-Muller counter and two solid state MeV electron counters will be included to look for in situ acceleration processes. It must be noted that the ambient air pressure at balloon altitude will probably make it impossible to devise a particle detector that can directly observe the ~10 eV electrons that appear to be responsible for exciting the visible emissions from sprites [Mende et al., 1995; Hampton et al., 1996]. c:2:5. Optical Instruments:The payload that we presently envision will have enough available volume, power and telemetry to accommodate several photometers. We will install two broadband, rapid response all sky photometers to provide one source of event triggers for the high sampling rate on-board data recording system. They will also monitor the temporal relationship between tropospheric lightning and sprite emissions. c:2:6. Navigation:Balloon location information will be provided by an on-board GPS receiver, which will also provide accurate time tagging for the on-board recorder.
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The reason that we have chosen high altitude balloons as the
vehicles for these proposed experiments is that the known drawbacks have been shown to be
relatively minor problems. First, it has been shown that sprites are predictable enough to
make long duration of balloon launches tolerable. The discovery that sprites are generated
with some regularity by large (2.0 x 105 km2) mesoscale convection complexes means that
the occurrence of sprites can be predicted several hours in advance. The forecasting
record of the staff of FMA Research is impressive. With one to 3 hours notice, they hit on
100% of their forecasts for sprites from mesoscale convective systrems during 1995 and
1996. That is for more than 50 cases. Many sprite storms last in excess of three hours.
This predictability opens up the possibility of studying sprites with balloon-borne
payloads flying at a constant altitude (> 35 km) in the stratosphere. The use of such
payloads to study the electrical environment above thunderstorms has been very fruitful
historically [Benbrook et al., 1974; Bering et al., 1980b; Holzworth et
al., 1986; Pinto et al., 1988]. The vector electric field detectors that will
be flown on these payloads will be built with enough dynamic range to detect perturbations
similar to observed sprite related perturbations [Marshall et al., 1996] at
distances from 10's to 1000's of km [Bering and Benbrook, 1995]. In midsummer, when
a Sprite Campaign might be conducted, the winds at 3-5 mbar above the great Plains will be
easterlies. Thus, it should be possible to select a centrally located site such as
Ottumwa, Iowa from which to launch suitably equipped payloads carried by stratospheric
balloons. A dusk or early evening launch on a promising night will then position the
balloon over the heart of the region viewed from the Colorado and Wyoming video
observatories during the best observing hours. Such a launch strategy will guarantee that
the payload will be well within 300-500 km of the sprite activity on perhaps half of the
flights in a six flight campaign.
Unfortunately, the combination of wide dynamic range and high slew rate
means that the input amplifiers will be relatively power hungry, which precludes the use
of a sub 6 kg radiosonde type of payload. Therefore, we have built a dual three axis
double probe instrument on a medium sized (~100kg) payload with a complement of other
instruments. The high gain, low voltage 3 axis double probe will have two gain states,
with dynamic ranges of ±5V/m and ±2 V/m shown in Figure 4. The low gain, high voltage
three axis double probe will have a dynamic range of ±100 V/m. The input circuit is shown
in Figure 5. ( A large but readable version can be obtained by clicking on the figure.) A
combination of three telemetry approaches will be used. First, analog waveforms from two
of the axes of the low gain instrument will be transmitted in real time using high
band-width (4 kHz). Second, a relatively slow 1 kHz sample rate PCM will be transmitted in
real time via a radio telemetry link. Third, an event trigger will be used to capture
bursts of 50 kHz sampling rate data of all six components of the E-M field that will
be stored in an on-board recorder for post-flight recovery. 6 channels of electric field
will be sampled, 3 axes each of both lowest and highest gain.