Time sequence of TGFs and optical pulses detected by ASIM and a comparison of TGFs observed by different spacecrafts
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- Master theses 
Terrestrial gamma-ray flashes (TGFs) are short energetic bursts of photons associated with lightning activity. TGFs are believed to be produced in relation to positive intracloud (IC+) lightning, during the upward propagation of the negative lightning leader. In April 2018, the Atmospheric Space Interactions Monitor (ASIM) was launched and mounted on the Columbus module on the International Space Station (ISS), and is the first mission specifically designed for detection of TGFs from space. Using catalogs of TGFs detected by other platforms prior to the launch of ASIM, we studied the global distribution of TGFs, and how the instrumental properties of the spacecrafts affect the TGF durations and number of counts. By using both X- and γ-ray detections and optical detections by ASIM, we investigated the TGF duration and the time sequence of TGFs and optical lightning pulses. Data from both detector modules of ASIM, the Modular X- and Gamma-ray Sensor (MXGS) and the Modular Multi-spectral and Imaging Array (MMIA), were used to construct timelines of ASIM detections. Between June 2018 and March 2019, the relative timing accuracy of MXGS and MMIA was ±80 μs. Within this timespan, 95 TGFs detected by MXGS had associated optical detections by MMIA. Of these, 39 had clear optical associations. Lightning detection network data were used to ascertain that the detected optical pulses originated from lightning activity occurring within the field of view of MMIA. TGFs detected by the different platforms showed similar geographic distributions, with a clustering of TGFs around the continents in the equatorial region. The ASIM TGF durations, determined using MXGS detections, were shorter than those previously found by other platforms. The ASIM TGFs also contained more counts. The time difference between the onset of the MXGS TGF detection and the onset of the associated optical pulse in MMIA, was used to study the time sequence of TGFs and the lightning strokes. The time sequence of ASIM TGFs (with optical pre-activity, TGF detection in MXGS, and main optical pulse detection in MMIA) suggests that TGFs are produced towards the later stages of leader development, before the current pulse heats up the channel to emit an optical pulse. The relation found between TGF duration and the delay of the associated optical pulses further suggests that the optical pulse is less delayed for shorter TGFs.