Response of Earth's Upper Atmosphere and VLF Propagation to Celestial X-Ray Ionization: Investigation With Monte Carlo Simulation and Long Wave Propagation Capability code

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Journal of Geophysical Research: Space Physics
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Palit S.
Raulin J.-P.
Szpigel S.
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©2018. American Geophysical Union. All Rights Reserved.Ionization of Earth's upper atmosphere by celestial and terrestrial sources of radiation is the driver of most of the significant physical and chemical evolution distributed over a large range of timescales. Part of the atmosphere, called ionosphere, owes its existence to the ionization of neutral molecules by cosmic rays and solar ultraviolet radiation. Solar flares, with extreme ultraviolet and soft X-rays, modulate the lower part of the ionosphere for few minutes up to few hours. Currently, plenty of signatures of hard X-ray and gamma ray ionization effects on the middle and upper atmosphere due to extragalactic sources like gamma ray bursts and soft gamma repeaters have been observed. Most of such events, which look like spikes of X-ray radiation, are manifested in impulsive ionization and gradual decay due to atmospheric recombination processes. Such ionization source profile in form of delta function in time or a spike is the most ideal and simplest candidate to investigate the basics of X-ray ionization influences on atmospheric chemistry and plasma characteristics. Here with a Monte Carlo simulation and a simple ion chemistry model we investigate numerically the effect of such X-ray ionization impulses, with different source characteristics, on various layers of the Earth's atmosphere and those on very low frequency (VLF) radio wave propagation. We also justify our assumption that in terms of their effect on atmosphere and VLF, any actual source of ionization may be considered as a bunch of consecutive ionization spikes of varying intensity and spectral characteristics.
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