Parameters of the lower ionosphere inferred from night-time VLF atmospherics

Abstract

Abstroct - Regular variations of the lower ionosphere height were investigated using night-time VLF atmospherics. TO infer the source range and the ionosphere height we employed a technique based on the modal interference properties of atmospherics. An ensemble of three-component records (vertical electric and two orthogonal horizontal magnetic) obtained from board of a scientific vessel at different points of Atlantic and Indian oceans and during different moments of local night was used for analysis. Results of analysis show gradual changing of the ionosphere height during night time on a few kilometers with maximum reached at a vicinity ofthe local midnight. I. INmODUCTlON Different phenomena of the space, atmospheric, and terrestrial origin such as charged particle precipitations, lightning effects in the upper atmosphere, intemal gravity and planetary waves etc. develop at the lower ionosphere heights. A possibility of remote detection and monitoring of perturbations in the lower ionosphere from distant ground-based observatories is provided by traditional approach based on observation of transmitted by navigational stations VLF electromagnetic waves, which reflect from lower ionospheric layer with so small plasma density (1-100 cm-3). Lighhling discharges play a role of natural sources of a wideband electromagnetic radiation - atmospherics with the main portion of spectral density in the frequency range from ELF to VLF. The most informative ones are atmosferics propagating under the night time ionosphere. During night time, losses in the lower ionosphere decrease and so-called hveek-atmospherics (or ‘tweeks’) are observed. Passing through the whole range practically from sliding to orthogonal to the boundaries incident angles the waves radiated by lighming discharge form a dispersive waveform of 10-150 ms length at a distant receiving point. Due to waves propagating under steep angles to the waveguide boundaries the spectral density of a signal increases in the vicinity of the waveguide cut-off frequencies, and, thus, there appears a possibility of direct determination of the effective ionospheric height by measuring the tail part of tweeks. Consideration for propagation models with accounting for geomagnetic field, losses, and electron density profile allows us to obtain more detail information on the ionosphere properties [1]-[41. In this paper we demonstrate a possibility of monitoring of regular changes of the lower ionosphere height using tweeks that is important for the fiuther development of techniques to study ionospheric perturbations.