Abstract:
The present study delineates on the observations and
modelling of low latitude D-region ionosphere perturbations
caused by strongest solar flare (X6.9) of solar
cycle 24. An extreme space weather event occurred on
9 August 2011. To understand the severity of X-class
flare on ionosphere, a comparative study was made
with a low intensity C-class flare of 6 August 2011.
Both flares originated from the same sunspot
AR#1263. Very low frequency (VLF) waves propagating
in the Earth’s ionosphere wave guide (EIWG)
measured from VLF transmitter NWC (19.8 kHz) located
in Australia, and recorded at Allahabad (India)
were used. The recorded VLF amplitude and phase
were modelled with long wavelength propagation
capability code to understand solar flare-induced
ionospheric variation. Modelling results revealed that
the lower boundary of D-region ionosphere is lowered
by 10 km during X-class and 1.0 km for C-class flare.
This implies change in the properties of EIWG, and
hence becomes important to observe our ionosphere
on continuous basis for space weather events since
ionosphere is the key medium of propagation for radio
waves.
