Ionosphere as astrophysical detector: Extracting spectral and timing information of celestial transients using VLF studies
Abstract
The increased electron and ion densities at lower part of earth's ionosphere during astrophysical transient events, like solar flares are the combined effect of ionization by the soft X-rays and the recombination of the electrons and ions with neutrals. The rate of ionization and it's variation with altitude depend on the incident spectra. Hence the altitude variation of electron density values in lower ionosphere modulated by such sources should bear information on soft X-ray spectra of those events. We found that by analyzing the Very Low Frequency (VLF) amplitude modulation during different classes of solar flares we can uniquely determine the approximate soft X-ray spectra during the evolution of those flares. In addition, careful investigation of the peak time delay between the VLF signal amplitude and the transient lightcurve provides us important hints on the intensity and time evolution during the peaks of those events. Specifically, for solar flares uninterrupted monitoring of X-ray spectra is necessary to derive the information on time evolution of non-thermal electron acceleration, transport and interaction during flares in the solar atmosphere. Considering the limitations with space based satellite observations due to restricted window of the instruments to avoid saturation, our method of reconstruction of flare spectra from VLF data may provide us a continuous and uninterrupted mean of monitoring those events. Higher energy photons from flares (in hard X-rays) produce most of the ionization at much lower heights. The hard X-ray and gamma ray from extra-galactic transients like, SGR, GRB etc. penetrate even deeper in the atmosphere ionizing the molecules present there. With suitable means of observation of electron densities and proper extension of our deconvolution method for those heights, we should be able to extract information of hard X-ray spectra of solar flares and those of extra-galactic transients also. The outline is that the earth's atmosphere may be an excellent and ever-present gigantic detector of ionizing extra-terrestrial events with VLF and any other suitable means of detection of it's modulation in plasma properties in different parts of the atmosphere.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E2556P