Abstract
The Modular X- and Gamma-ray Sensor (MXGS) is an imaging and spectral X- and Gamma-ray instrument mounted on the starboard side of the Columbus module on the International Space Station. Together with the Modular Multi-Spectral Imaging Assembly (MMIA) (Chanrion et al. this issue) MXGS constitutes the instruments of the Atmosphere-Space Interactions Monitor (ASIM) (Neubert et al. this issue). The main objectives of MXGS are to image and measure the spectrum of X- and gamma -rays from lightning discharges, known as Terrestrial Gamma-ray Flashes (TGFs), and for MMIA to image and perform high speed photometry of Transient Luminous Events (TLEs) and lightning discharges. With these two instruments specifically designed to explore the relation between electrical discharges, TLEs and TGFs, ASIM is the first mission of its kind. With an imaging system and a large detector area MXGS will, for the first time, allow estimation of the location of the source region and characterization of the energy spectrum of individual events. The sensors have fast readout electronics to minimize pileup effects, giving high time resolution of photon detection for comparison with measurements on μs-time scales of lightning processes measured by the MMIA and other sensors in space or on the ground. The detectors cover the large energy range of the relevant photon energies. In this paper we describe the scientific objectives, design, performance, imaging capabilities and operational modes of the MXGS instrument.
Highlights
Terrestrial Gamma-ray Flashes (TGFs) were first observed in 1991 by the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma-Ray Observatory and reported a few years later by Fishman et al (1994)
Energies up to 40 MeV was later measured by the Italian satellite, Astrorivelatore Gamma a Immagini LEggaro (AGILE) (Marisaldi et al 2010), and duration down to just a few tens of microseconds was determined by the Gamma Burst Monitor (GBM) on the Fermi satellite (Connaughton et al 2013)
The values for TGFs are based on modeling an average Ramaty High Energy Solar Spectroscopic Imager (RHESSI) TGF onto the Modular X- and Gamma-ray Sensor (MXGS) detector planes at ISS altitude, while the values for Lightning-Induced Electron Precipitation (LEP) are from Rodger et al (2004) and Clilverd et al (2004), the Relativistic Electron Precipitation (REP) values from Rodger et al (2007) and aurora values from Østgaard et al (2001)
Summary
TGFs were first observed in 1991 by the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma-Ray Observatory and reported a few years later by Fishman et al (1994). Having high time resolution (microsecond level) is important for comparing the γ -ray measurements with the fast optical pulses from lightning provided by MMIA as well as all measurements from other platforms or from ground of the fast microsecond processes in lightning Another lesson learned from earlier measurements of TGFs is the importance of detecting photons in the energy range from the lowest energies around 20 keV up to at least 20 MeV. A fourth driving factor is to be able to image TGFs with the highest possible resolution over a large field of view (FOV) This means to optimize the pixelated low-energy detector plane and coded mask geometry, as well as shielding this detector plane on all sides except through the coded mask. Research objectives related to Earth Observation: Explore effect of dust storms, mega cities, forest fires and volcanoes on cloud formation and electrification Study the relation of eye-wall lightning to the intensification of hurricanes
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