Temporal and spectral characteristics of terrestrial gamma flashes

Abstract

Abstract. We have analyzed the Burst and Transient Source Experiment (BATSE) highresolution timing data for 13 terrestrial gamma flashes (TGFs) to better characterize this newly identified phenomenon, which may be related to atmospheric lightning. We find that the minimum timescale for TGF variability is -25-250/as, with 50/as near typical. In general, TGFs are spectrally much harder than cosmic gamma ray bursts (GRBs). We additionally find that as with GRBs, individual pulses within a TGF tend to peak earlier at higher energies. This time-asymmetry rules out models such as sweeping beams. We also find that different pulses can have different spectra, with spectra typically softening as a pulse progresses. Event-averaged spectra for the TGFs were examined and found to be better fit in the 25-500 keV range by a power law than by a blackbody model. However, in general, even a power law is not a perfect fit. We find correlation between minimum TGF timescale and the power law spectral index, with rapidly varying TGFs appearing softer. From empirical comparisons of timescales and structures we speculate that if TGFs are somehow related to known high-atmospheric lightning events, then they are more probably related to red sprites than to blue jets or transionospheric pulse pairs. BackgroundTerrestrial gamma flashes (TGFs) were discovered surrendipitously by Fishman et al. [1994] In this paper we report analysis of the triggered time series of all BATSE-detected TGFs that have data in the public domain. For most of these analyses we make use of the highest temporal resolution timescale recorded by BATSE which has 2 txs time resolution. We analyzed spectra and looked for differences in the light curves of these events. In the next section we describe the data and report results, and in the last section we discuss the implications of these results on the nature of TGFs.•Now at Department of Physics, Michigan Technological University, Data and Results Energy-Dependent Light CurvesWe have analyzed all Time Tagged Event (TTE) data labelled as originating from TGFs that have entered the Compton Gamma Ray Observatory Science Support Center as public domain by July 1, 1996. BATSE TTE data are recorded in 2 txs time bins and in four energy channels, with the following approximate energy boundaries: 25-50 keV, 50-100 keV, 100-300 keV, and >300 keV. TTE data are recorded by the triggered BATSE large area detectors (LADs). Here we present TGF light curves, broken into these four energy channels, in The first detected TGF was BATSE trigger number 106. Figure la illustrates the time profiles of this TGF in the four energy channels. Inspection of these light curves immediately indicate that this TGF has at least two significant peaks. We hypothesize, in analogy to cosmic gamma ray bursts (GRBs), that this TGF is therefore composed of at least two major emission events, or "pulses." Pulses are defined as components of the time series that appear to act in a coherent way in both time and energy [Norris et al., 1986]. Pulses are hypo...

A comparison between Monte Carlo simulations of runaway breakdown and terrestrial gamma‐ray flash observations

“…[21] These results appear to contradict the subset of the BATSE spectra shown by Nemiroff et al [1997], which continues to rise down to 25 keV, but the raw BATSE light curves (http://www.batse.msfc.nasa.gov/batse/tgf/) show a broad range of spectral variation, and many of the harder events may be similar to those that are found in the RHESSI data by the current analysis. As discussed by Smith et al [2005], the selection criteria for the BATSE TGFs and RHESSI TGFs were different.…”

“…[11] Also shown in Figure 1 is the average of the four quasi-deconvolved photon spectra presented by Nemiroff et al [1997], who assumed a power law. It has been normalized to match the RHESSI spectrum created under the same assumption.…”

“…Figure 1. TGF photon spectra as measured by RHESSI and BATSE [Nemiroff et al, 1997]. The upper RHESSI data have been corrected for the instrumental response, assuming a model spectrum with the source at an atmospheric depth of 50 g/cm 2 (black solid curve), corresponding to an altitude of 21 km, and the bottom RHESSI data have been corrected for the instrumental response assuming a E À1 power law (black solid line).…”

High-Energy Atmospheric Physics: Terrestrial Gamma-Ray Flashes and Related Phenomena

“…Spectral differences among TGFs were first noted by Nemiroff et al (1997). Østgaard et al (2008) noted that more distant TGFs (relative to the sub-satellite point) should be softer, and indeed found such a correlation, using the relative response of the BATSE detectors themselves to approximately calculate the sub-satellite distance.…”

“…Nemiroff et al (1997) and Feng et al (2002) performed more detailed analyses of the BATSE data, looking at the temporal and spectral variations, and noting that softer photons (below 100 keV) tend to arrive later. Nemiroff et al (1997) noted that TGFs with a softer spectrum tend to have faster characteristic timescale variations, a result that should be explored in reference to more recent data. They also noted that the spectral asymmetry in time rules out any models in which the short duration is caused by a beam sweeping across the spacecraft.…”