SPRITES DETECTION

This flight was part of the Sprites 99 Balloon Campaign, conducted by the University of Houston's Space Physics Group, aimed to investigate transient luminous events (TLEs) such as red sprites, blue jets, elves, and anomalous optical events (AOEs) occurring in the mesosphere and ionosphere above thunderstorms.

Each balloon carried a comprehensive suite of instruments designed to investigate transient luminous events and their associated electromagnetic phenomena from the stratosphere. The payload, weighing approximately 100 kg, was launched via high-altitude balloons to altitudes exceeding 35 km, positioning it optimally for observing sprites above mesoscale convective systems.

Central to the instrumentation were two three-axis electric field detectors. One was a high-gain, low-voltage system with dynamic ranges of ±5 V/m and ±2 V/m, while the other was a low-gain, high-voltage system capable of measuring up to ±100 V/m. These detectors, developed by the Space Physics Group at the University of Houston, featured preamplifiers with high dynamic range, slew rate, and input impedance, enabling precise vector measurements of electric fields in the stratosphere. The data acquisition system employed multiple telemetry approaches: real-time analog waveform transmission at 4 kHz bandwidth, a 1 kHz sample rate pulse code modulated (PCM) telemetry link, and an event-triggered system capturing bursts at 50 kHz sampling rate, storing data onboard for post-flight analysis.

To complement the electric field measurements, the payload included magnetic field sensors comprising three-axis induction or search coil magnetometers with bandwidths up to 100 kHz. These instruments facilitated the analysis of extremely low frequency (ELF) magnetic signatures associated with sprite activity, aiding in the estimation of current magnitudes involved in such events.

Atmospheric conductivity was monitored using methods such as the relaxation technique and a version of the blunt probe method, measuring ion conductivity at four-minute intervals. This allowed for the detection of large-scale conductivity changes in the stratosphere, even though direct traversal through a sprite was unlikely.

An air-Earth current sensor, adapted from previous measurements conducted at the South Pole station, was integrated into the payload. This enabled simultaneous measurement of all three terms in Ohm's Law, providing a comprehensive understanding of the electrical environment above thunderstorms.

To detect high-energy phenomena, the payload was equipped with a sodium iodide (NaI) scintillation counter featuring a 125 mm diameter crystal and a 16-channel pulse height analyzer, sensitive to X-rays in the 25 to 250 keV range. Data from this instrument were transmitted every 100 milliseconds, with higher sampling rates stored onboard following event triggers. Additional particle detectors, including a Geiger-Müller tube and two solid-state MeV electron counters, were included to investigate in situ acceleration processes, although the ambient pressure at balloon altitudes limited the detection of low-energy electrons responsible for sprite emissions.

Optical observations were facilitated by two broadband, rapid-response all-sky photometers, which served both as event triggers for high-speed data recording and as monitors of the temporal relationship between tropospheric lightning and sprite emissions.

Navigation and precise timing were achieved through an onboard GPS receiver, ensuring accurate positioning and time-stamping of the collected data. The entire suite of instruments was managed by an onboard computer system that controlled sampling sequences, generated PCM telemetry, and handled high-speed digitization and storage of broadband data during sprite events.

Balloon launched on: 8/21/1999 at 00:39 utc
Launch site: Ottumwa Industrial Airport, Iowa, US  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon 1.500.000 cuft - SF3-1.507-.8/0-NHR
Balloon serial number: R1.507-0-06
Flight identification number: 470N
End of flight (L for landing time, W for last contact, otherwise termination time): 8/21/1999 at 11:11 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 11 h 34 m (8 hr 30 m)
Landing site: 5 miles of Endicott, Nebraska, US
Campaign: SPRITES 99  

The flight was launched at 00:39 UTC on August 21th, 1999 using dynamic method assited by a fixed crane as launch vehicle.

Float altitude of 32 km was achieved at 2:38 UTC and then, the balloon started to travel westward at an average speed of 30 knots.

After a flight of near 10 hours cutdown command was transmited at 11:12 UTC on the same day.

The balloon required just over one hour to reach its ceiling (around 33 km) after which it was expected to float generally westerly at 20-30 kts. In order to prepare for launch, a decision was required several hours before sprites were expected to begin. As a further complication, the balloon was expected only to fly near the target storm, but not over it. Such overflights are now disallowed given the 1989 uncommanded payload release over Dallas, Texas. During much of the experimental period, adverse weather at the launch site (rain and/or high surface winds) severely limited the launch opportunities.

This was the third launch attemp and the second succsesfull. The system was airborne between 0039 and 1112 UTC. The balloon, flying at 33 km over southern Iowa and northern Missouri, passed between two rather small sprite-producing storms in Minnesota and Kansas. Ground stations in Yucca Ridge plus two other University of Alaska ground sites (Bear Mountain, SD and Jelm Mountain, WY) were able to obtain coordinated images of sprites and elves. VLF signals were also recorded. Cooperating ELF measurements were made in California (Martin Fuellekrug), Rhode Island (Earle Williams) and Israel (Colin Price). NOAA ETL in Boulder was also recording in a search for possible infrasound signatures.

This experiment marks the first time coordinated surface and stratospheric balloon measurements have been successfully obtained during documented sprite conditions.

• Sprites 99 Balloon Campaign Space Physics Group, University of Houston
• Ground and Balloon-Borne Observations of Sprites and Jets University of Houston Website
• Statistics and properties of transient luminous events found in the 1999 Sprites Balloon Campaign Advances in Space Research 34 (2004) 1811
• The electrodynamics of sprites Geophysical Research Letters, Volume 29, Issue 5, pp. 8