Inverse problem in Ionospheric Science: Prediction of solar soft-X-ray spectrum from Very Low Frequency Radiosonde results

TL;DR

This paper explores whether the Earth's atmosphere can be used as a detector to invert VLF radio signals and accurately infer the solar soft-X-ray spectrum, enabling remote sensing of high-energy astrophysical events.

Contribution

It introduces a deconvolution method to retrieve the incident photon spectrum from VLF amplitude signals, demonstrating potential for atmospheric-based detection of energetic cosmic events.

Findings

Spectral inversion is feasible with certain accuracy.

Method can be extended to detect gamma-ray bursts and similar events.

Atmosphere acts as a large-scale detector for high-energy phenomena.

Abstract

X-rays and gamma-rays from astronomical sources such as solar flares are mostly absorbed by the Earth's atmosphere. Resulting electron-ion production rate as a function of height depends on the intensity and wavelength of the injected spectrum and therefore the effects vary from one source to another. In other words, the ion density vs. altitude profile has the imprint of the incident photon spectrum. In this paper, we investigate whether we can invert the problem uniquely by deconvolution of the VLF amplitude signal to obtain the details of the injected spectrum. We find that it is possible to do this up to a certain accuracy. Our method is useful to carry out a similar exercise to infer the spectra of more energetic events such as the Gamma Ray Bursts (GRBs), Soft Gamma Ray Repeaters (SGRs) etc. by probing even the lower part of the atmosphere. We thus show that to certain extent, the Earth's atmosphere could be used as a gigantic detector of relatively strong events.