Very low frequency (VLF) waves as a probing tool to study the simultaneous effect of Solar Flare and Geomagnetic Storm (occurred on 9 March 2012) on D-region ionosphere

Abstract:

We examine the effects on the low-latitude D-region ionosphere of two peculiar events occurred on 9 March 2012, the solar flare and the geomagnetic storm, by means of the associated perturbations of several subionospheric VLF/LF signals. We use VLF/LF daytime data recorded at our low latitude station Varanasi (L = 1.07). On 09 March 2012, a ~ 2.5 dB enhancement in VLF amplitudes was recorded at the peak of the solar flare event. Strong fluctuations in the amplitude of the VLF signals were again observed in the recovery phase of the flare event, which is due to geomagnetic storm and persisted through the end of the data-recording period. We suggest that both the signal enhancement and subsequent fluctuations were associated with variations in the precipitation flux of energetic electrons onto the upper atmosphere. Farther down, in the lower ionosphere, a strong increase of the electron density is observed as a consequence of a very strong enhancement of particle precipitation. Quantitative modeling of subionospheric VLF wave propagation incorporating energetic electron flux measurements (and the associated altitude profiles of secondary ionization produced) yield results consistent with the variations in the VLF signal amplitude observed.