Infrasonic detection of a Leonid bolide: 1998 November 17

During the early morning hours of the night of the peak of the annual Leoni d meteor shower on 1998 November 17, a bright fireball (approximately -12 t o -14 visual magnitude at 100 km in the zenith) was observed over northern New Mexico with visual sightings as far away from Los Alamos as Albuquerque (similar to 150 km to the south of Los Alamos), including direct persisten t trail observations at the U. S. A. F. Starfire Optical Range (SOR), which is also near Albuqerque. This event did not produce any sonic boom reports , presumably because of its high altitude. It was also detected locally by an infrared radiometer at Sandia National Laboratory and by an intensified charge-coupled device (CCD) camera located in Placitas, New Mexico. Subsequ ent investigations of the data from the six infrasound arrays used by Los A lamos National Laboratory (LANL) and operated for the Department of Energy as a part of the Comprehensive Test Ban Treaty (CTBT) Research and Developm ent program for the International Monitoring System (IMS) showed the presen ce of an infrasonic signal from the proper direction at the correct time fo r this bolide from two of our six arrays (both located in Los Alamos). The infrasound recordings (ie., the wave amplitude and period data) indicated t hat an explosion occurred in the atmosphere at a source height of similar t o 93.5 km (with respect to sea level) or similar to 90 km with respect to t he altitude of Los Alamos, having its origins slightly to the north and wes t of Los Alamos. Purely geometric solutions from the ground observers repor ts combined with direct measurements from the CCD camera at Placitas produc ed a source height of 91 +/- 7 km. The signal characteristics analyzed from 0.5 to 3.0 Hz include a total duration of about 3-4 s for a source directe d from Los Alamos toward 353.6 +/- 0.4 degrees measured from true north at a maximum elevation arrival angle of similar to 72.7 degrees. The latter wa s deduced on the basis of the observed signal trace velocities (for the par t of the recording with the highest cross-correlation) and ranged from a co nstant value of about 920-1150 m/s (depending on the window length used in the analysis) for a pay trajectory along a direct refractive path between t he source and the Los Alamos arrays. The dominant signal frequency at maxim um amplitude at Los Alamos was similar to 0.71 Hz. These highly correlated signals had a peak to peak, maximum amplitude of similar to 2.1 microbars ( 0.21 Pa). Using several methods that incorporate various observed signal ch aracteristics, total distance traveled, etc., our analysis indicates that t he bolide probably had a source energy of similar to 1.14 t (TNT equivalent ) or 4.77 x 10(9) J. This is similar to 14.1x smaller than the source energ y estimate made using the infrasonic, empirical source energy relationship for low-altitude stationary point sources developed in the 1960s by the Air Force Technical Applications Center (AFTAC), Patrick Air Force Base, Flori da. This relation was originally developed, however, for much larger source energies and at much longer ranges.