VLF PERTURBATIONS ASSOCIATED WITH SOLAR ECLIPSES OF NOVEMBER 2012 AND MAY 2013 IN THE SOUTH PACIFIC REGION

Abstract:

Sub-ionospheric VLF signals from the NWC (19.8 kHz), NPM (21.4 kHz) and NLK (24.8 kHz) MSK VLF transmitters are monitored at Suva, Fiji, with a time resolution of 0.1s using GPS based timing and SoftPAL VLF system. Here one minute averaged amplitude and phase data have been used for analysis. We present perturbations in VLF propagation and D-region changes associated with 13 November 2012 total solar eclipse (SE) and 9-10 May 2013 annular SE using VLF observations at Suva, Fiji. During 13-14 November 2012 total SE, the totality shadow intercepted NWC-Suva path and NWC signal amplitude and phase decreased by about 0.70 dB and 23^°, respectively. NPM signal amplitude during 9-10 May 2013 SE decreased by about 2.0 dB. The amplitude perturbation of ~1.8 dB on NLK signal was measured from the unperturbed level associated with 9-10 May 2013 SE. The decrease in the amplitude at the site can be understood in terms of destructive interference of modes converted at the discontinuity created by the eclipse intercepting the different Transmitter-receiver great circle paths (TRGCPs) and changes in the propagation conditions along TRGCPs. The decrease in the amplitude and phase of NWC signal for 13-14 November 2012 SE has been modeled using Long Wave Propagation Capability code to estimate the changes in D-region reflection height (H') and sharpness factor (β) which shows that H' and β were increased by 0.95 km and 0.01 km^-1, respectively. The phase changes on NWC signal associated with 9-10 May 2013 SE have been used to estimate the recombination coefficient value, for 75 km height where electron density reduction due to SE was about 40%. The changes in the D-region parameters and the electron density are due to sudden decrease of the photo-ionization creating nighttime like conditions in the D-region ionosphere.