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

The 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.

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

“…The above scenario is an ideal approximation of a more realistic situation, in which the effective recombination coefficient varies with time with the evolution of the concentration of ion species. The ion chemistry evolution scheme adopted in the current numerical investigation proceeds with the simultaneous solution of five coupled ordinary differential equations governing the rate of change of the electron density, negative and positive light ion densities, and negative and positive cluster ion densities (Inan et al, ; Palit et al, ). Simplification of these equations leads to a modified version of the continuity equation, given by where is the ratio of the sum of the negative light and cluster ion densities to the electron density.…”

“…The VLF amplitude modulation caused by the individual X‐ray peaks of a complex burst as observed by both receivers are demonstrated pictorially in the paper. Palit et al () investigated further in the analysis by thorough modeling or reconstructing the VLF modulation by those SGR X‐ray bursts with numerical simulation.…”

“…Detail reconstruction of VLF amplitude modulation due to a complex X‐ray burst from SGR J1550‐5418 on 22 January 2009 for both the NPM‐EACF and NPM‐ROI paths are presented and compared with the observation in Palit et al (). Figure represents the ACS/SPI, on board INTEGRAL, count rate variation with time of all the peaks of the X‐ray burst occurring at 05:17:43.6 UT.…”

“…In Figure the reference spectrum used in the simulation of VLF signal modulation due to the burst is shown. Detail of the spectrum and how it is acquired is described in Palit et al (). This reference spectrum is used in further numerical investigation with suitable normalization by the ratios of the ACS/SPI count rates of each of the peaks shown in Figure to that of the peak to which the reference spectrum actually corresponds.…”

“…We use GEANT4 (Agostinelli et al, ) Monte Carlo framework for the calculation of ionization and a simple five species ion chemistry code originally presented by Glukhov et al (). We are not going into any further detail of the simulation steps here, as they were previously described in Palit et al (, ). Outcome of the modeling efforts presented in those papers related to the VLF signal amplitude variations during solar flares and SGR X‐ray bursts is quite satisfactory.…”