Lower Ionospheric Plasma‐Chemical Evolution and VLF Signal Modulation by a Series of SGR X‐Ray Bursts: Numerical Simulation With an Ion‐Chemistry Model

Abstract

AbstractThe X‐ray and gamma ray radiation from astrophysical transient sources, like X‐ray bursts from soft gamma repeaters (SGRs) and gamma ray bursts (GRBs), can affect the plasma properties of the lower ionosphere and middle atmosphere. Multiple very low frequency (VLF) receivers in South America, with an unprecedented high time resolution of 20 ms, detected one such series of bursts from SGR J1550‐5418 on 22 January 2009. Due to lack of other suitable means of observation corresponding to the lower part of Earth's ionosphere (∼60–100 km), the VLF detection and analysis of transient ionizing events (mostly of solar origin) has emerged as an excellent method to investigate various chemical and plasma characteristics at these heights. Extragalactic events, like SGR bursts and GRBs, with sharp modulation in their radiation time profile and very high energy photon abundances provide most unique opportunities of such studies with the possibility of extending even lower heights in the atmosphere. Here, for the first time, an extensive computer model, consisting of the combination of Monte Carlo ionization rate computation, a one dimensional atmospheric chemistry module, and VLF waveguide mode calculation, for the reconstruction of VLF signal modulation produced by SGR X‐ray burst starting from the observed spectrum and lightcurve of the event is presented. We gain some valuable insight on the nature of chemical and dynamic evolution over the entire height range of the atmosphere examined from the exercise.