Abstract:
Observations of tweeks with higher harmonics (n > 1) at low latitude stations Allahabad
and Nainital, in the Indian sector, during the total solar eclipse on 22 July 2009, are
presented. Allahabad and Nainital stations were in 100% and 85% of the totality paths.
Observations suggest that about 30–40% obscuration of solar disc can lead to the tweeks
occurrence which otherwise occur only in nighttime. A total of 148 tweeks at Allahabad
and 20 tweeks at Nainital were recorded with some of them up to 3rd harmonics. The World
Wide Lightning Location Network data indicated that tweeks observed were generated
by lightning’s located in the partial eclipse area of Asia‐Oceania region. The changes in
D‐region ionospheric VLF reflection height and electron density (∼22–23 cm−³) during
eclipse have been estimated from the first cut‐off frequency of the tweeks. The reflection
height increased from ∼89 km from the first occurrence of tweek to about 91–92 km at
the totality and then decreased to ∼87 km at the end of the eclipse, suggesting a change of
about 5 km in the reflection height during eclipse. The reflection heights are lower by 2–3 km
as compared to normal nighttime tweek reflection heights. The above increase in the
reflection height indicate that the partial nighttime condition is created during eclipse, as
the main D‐region ionizing radiation Lyman ά is blocked but solar soft X‐ray and EUV
radiations originating from the limb solar corona are not totally blocked which produce some
of ionization in the D‐region.
