Instrumentation

The GPS-based New Mexico Tech (NMT) interferometer/atmospheric (sferic) data acquisition system (see Appendix A.2) was used to acquire very broadband ($<$1 Hz - 250 kHz) electric field data. A sensitive bipolar trigger setting was used on August 14, 1998 was used to acquire CG and IC sferics. The sensitive setting made it possible to unambiguously discriminate between slow tails which are associated with CGs and delayed slow field changes associated with sprites. The pre- and post-trigger lengths were set to 2 and 6 ms respectively on August 14, 1998.

The NMT sferic system was located on a mountain ridge at Langmuir Laboratory, NM. In order to determine the electric field intensification due to the ridge, sferics at $>$800 km range were compared between the NMT sferic system and an electric field sferic system located ${\simeq}$30 km away on level terrain operated by Marx Brook. This comparison revealed that the ridge intensified the electric field by a factor of 1.9${\pm}0.2$. The NMT system's electric field values were converted to flat terrain values and these were used for the ELF propagation model.