Here are Some Other Useful Links
Overview of A Balloon-Borne Sprite Observatory
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.
The 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.
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]. 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.
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.
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.
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.  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.
Balloon location information will be provided by an on-board GPS receiver, which will also provide accurate time tagging for the on-board recorder.
Copyright 1999, 2004, University of Houston