LASA is a classic electric field change meter [Krehbiel et al., 1979] with the added ability to derive accurate, absolute time tags at multiple, distant locations using Global Positioning System (GPS) receivers. The GPS receiver provides absolute event time tagging with an accuracy of better than 2 us. The multistation coincident waveforms were cross-correlated to determine timing corrections between events recorded by different stations and also to reject waveforms which originate from different sources. Smith et al. [2000] describe the operation and instrumentation of LASA in more detail.
The LASA operation began with five stations (only four of which were independently located) in New Mexico in 1998. The stations were located in Los Alamos (LO and LA), Socorro (SO), Roswell (RO), and Tucumcari (TU). In 1999 the array was expanded to eleven stations (all independently located) with the four stations in New Mexico; one in Omaha, Nebraska (CR); one in Lubbock, Texas (LB); and five in Florida: Kennedy Space Center (KC), Tampa (TA), Fort Myers (FM), Boca Raton (BR), and Gainesville (GV). For 2000 summer operations, the SO, RO, and TU NM array stations were relocated to Colorado: Colorado Springs (AF), Fort Collins (FO), and Kirk (KI), in order to make comparative observations with the New Mexico Tech Lightning Mapping Array (LMA), a VHF system described below, and other instruments associated with the STEPS campaign [Wesiman and Miller, 2000]. Fig. 1 presents maps of the LASA station locations.
The primary goal for 1998 was to support FORTE and gain experience in the remote operation of an array through the establishment of stations close to Los Alamos. The locations also allowed comparative observations with the New Mexico Tech LMA, a VHF system operated in the vicinity of Socorro.
In 1999, utilizing the two-cluster array plus the independent station in NE, high sensitivity, high location accuracy studies within and near each sub-array were possible, and the array was simultaneously able to detect and locate (with less accuracy) large-amplitude events that occurred over a large portion of the southern and central U. S. The expansion to Florida in 1999 was motivated by the following factors: (1) the Florida peninsula features the highest flash density in the Unites States [Cummins et al., 1998b]; (2) the opportunity for thunderstorm observations in a maritime environment; (3) and colocation with the LDAR system at KSC, a VHF system described below.
LASA was developed as a resource to locate, classify, and characterize lightning discharges in support of FORTE, in a manner similar to NLDN. One advantage of operating our own ground-based array is that we are able tailor operations for coordination with FORTE. A second advantage is the ability to retain all waveforms from all located events to permit further, non-real-time analysis. As we have advanced our understanding of lightning and developed new questions, the ability to reprocess these waveform data has been critical. The array has the potential to contribute to the understanding of thunderstorm discharges independent of FORTE, and has already begun to do so.
Fig. 2 shows typical waveforms recorded from a negative cloud-to-ground lightning stroke (59 kA peak current as reported by the NLDN) that occurred August 6, 1999 at 00:44:54.135999 near the New Mexico array and was recorded by the LB, TU, LA, and SO stations. The SO waveform shows leader radiation which caused the station to trigger and record the event (in this plot t=0 corresponds to the trigger time, rather than the cross-correlation corrected event time).
