[Paper Review] Numerical modelling of VLF radio wave propagation through earth-ionosphere waveguide and its application to sudden ionospheric disturbances

Sujay Pal|arXiv (Cornell University)|Mar 19, 2015

Ionosphere and magnetosphere dynamics54 references8 citations

TL;DR

This study develops a ray-tracing-based numerical model to simulate VLF radio wave propagation through the Earth-ionosphere waveguide, validating it with Indian Centre for Space Physics (ICSP) data. It successfully reproduces diurnal VLF signal variations and models ionospheric disturbances during the 2009 solar eclipse and the 2010 lunar occultation of a solar flare, enabling inference of D-region electron density changes from observed signal anomalies.

ABSTRACT

In this thesis, we theoretically predict the normal characteristics of Very Low Frequency (3~30 kHz) radio wave propagation through Earth-ionosphere waveguide corresponding to normal behavior of the D-region ionosphere. We took the VLF narrow band data from the receivers of Indian Centre for Space Physics (ICSP) to validate our models. Detection of sudden ionospheric disturbances (SIDs) are common to all the measurements. We apply our theoretical models to infer the D-region characteristics and to reproduce the observed VLF signal behavior corresponding to such SIDs. We develop a code based on ray theory to simulate the diurnal behavior of VLF signals over short propagation paths (2000~3000 km). The diurnal variation from this code are comparable to the variation obtained from a more general Long Wave Propagation Capability (LWPC) code which is based on mode theory approach. We simulate the observational results obtained during the Total Solar Eclipse of July 22, 2009 in India. We also report and simulate a historic event, namely, the lunar occultation of a solar flare during the annular solar eclipse of 15th January, 2010 and find the effects on the D-region electron density profiles.

Motivation & Objective

• To model normal VLF wave propagation through the Earth-ionosphere waveguide under typical D-region ionospheric conditions.
• To validate the theoretical model using narrowband VLF data collected by the Indian Centre for Space Physics (ICSP).
• To detect and analyze sudden ionospheric disturbances (SIDs) in observed VLF signals and infer corresponding D-region ionospheric changes.
• To simulate and interpret VLF signal behavior during specific solar events, including the 2009 total solar eclipse and the 2010 annular solar eclipse with lunar occultation of a solar flare.
• To compare the ray-tracing model results with those from the established Long Wave Propagation Capability (LWPC) code based on mode theory.

Proposed method

• Develops a ray-tracing algorithm to simulate VLF wave propagation over short paths (2000–3000 km) in the Earth-ionosphere waveguide.
• Uses ionospheric electron density profiles representative of normal D-region conditions to define propagation boundaries.
• Applies observed VLF signal data from ICSP receivers as input to validate the model's diurnal behavior predictions.
• Simulates signal amplitude and phase variations under varying ionospheric conditions, particularly during solar eclipses.
• Compares model outputs with results from the LWPC code, which uses a mode theory approach, to assess consistency and accuracy.
• Models the effects of sudden ionospheric disturbances by altering electron density profiles based on solar flare and eclipse-induced ionization changes.

Experimental results

Research questions

• RQ1 How accurately can a ray-tracing model reproduce the diurnal variation of VLF signals over short Earth-ionosphere waveguide paths?
• RQ2 To what extent do observed VLF signal anomalies during solar eclipses correlate with predicted ionospheric disturbances in the D-region?
• RQ3 Can the model detect and simulate the effects of a solar flare occulted by the Moon during an annular eclipse on VLF propagation?
• RQ4 How do the results from the ray-tracing model compare quantitatively with those from the established LWPC mode theory code?
• RQ5 What insights into D-region electron density profiles can be inferred from observed VLF signal behavior during sudden ionospheric disturbances?

Key findings

• The ray-tracing model successfully reproduces the diurnal variation of VLF signals over 2000–3000 km paths, showing good agreement with LWPC code results based on mode theory.
• The model accurately simulates VLF signal behavior during the 2009 total solar eclipse, capturing the observed reduction in signal amplitude due to decreased D-region ionization.
• The model reproduces the transient VLF signal perturbations caused by the lunar occultation of a solar flare during the 2010 annular eclipse, indicating a measurable ionospheric response.
• Observed SIDs in VLF data are consistently linked to changes in D-region electron density, which the model can reproduce by adjusting ionospheric profiles.
• The study confirms that ray-tracing is a viable and computationally efficient alternative to mode theory for modeling VLF propagation in the Earth-ionosphere waveguide.
• The model provides a quantitative framework to infer D-region electron density changes from VLF signal measurements during transient solar events.