Abstract:
The source region and propagation mechanism of low latitude whistlers (Geomag. lat. <30 ) have puzzled scientific community for
last many decades. In view of recent reports, there is consensus on the source region of low latitude whistlers in the vicinity of the conjugate
point. But the plausible conditions of ionospheric medium through which they travel are still uncertain. In addition to that, the
whistlers in daytime are never observed at geomagnetic latitudes less than 20 . Here, for the first time, we present a rare observations of
whistlers during sunlit hours from a very low-latitude station Allahabad (Geomag. Lat: 16.79 N, L = 1.08) in India on 04 February 2011.
More than 90 whistlers are recorded during 1200–1300 UT during which the whole propagation path from lightning source region to
whistler observation site is under sunlit. The favorable factors that facilitated the whistlers prior to the sunset are investigated in terms
of source lightning characteristics, geomagnetic and background ionospheric medium conditions. The whistler activity period was found
to be geomagnetically quiet. However, a significant suppression in ionospheric total electron content (TEC) compared to its quiet day
average is found. This shows that background ionospheric conditions may play a key role in low latitude whistler propagation. This
study reveals that whistlers can occur under sunlit hours at latitudes as low as L = 1.08 when the source lightning and ionospheric
medium characteristics are optimally favorable.
