Long-duration geomagnetic storm effects on the D region of the ionosphere: Some case studies using VLF signal

摘要

The present work investigates the effects of long-duration geomagnetic storms on VLF signal during ionospheric sunrise time, commonly known as D Layer Preparation Time (DLPT) depth. The VLF signal at 19.8 kHz transmitted from Northwest Cape, Australia, and received at a low-latitude station, Tripura, India, is used for the present analysis. The data for the analysis are selected from November 2008 to October 2011. In the active period of the geomagnetic storms, the average DLPT depth is found to have a negative correlation coefficient of 0.91 with geomagnetic Ap index. It is also found that with each 10 unit increase of Ap index, the DLPT depth (the day and night asymmetry level) changes by 1.25 dB. The results are supported with modeled International Reference Ionosphere (IRI) electron density data and DLPT depth at 71 km height for the three positions, namely, receiver position, signal hop position, and the transmitter position along the total Great Circle Path. It is found that the receiver position electron density is the main controlling factor for DLPT depth. The correlation between IRI electron density and DLPT depth increases from -0.13 at transmitter position to -0.33 at the first hop position, to -0.46 at the receiver position, respectively. The percentage change of post storm electron density, at 71 km height, is found to increase by more than 100 at the receiver position. The results are discussed on the basis of the electron density changes over the signal propagation path, mainly caused by the geomagnetic storms.