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Title:	ELF/VLF wave studies of ionosphere and magnetosphere electromagnetic phenomenon in low latitude region
Authors:	Maurya, Ajeet Kumar
Keywords:	Earth magnetosphere Earth ionosphere ELF-VLF waves D-region ionosphere D-region E-region F-region Magnetosphere electromagnetic phenomenon PhD Thesis
Issue Date:	2013
Publisher:	Indian Institute of Geomagnetism, Mumbai
Citation:	PhD Thesis, IIG, p.xviii+165, 2013.
Description:	A thesis submitted to the university of Mumbai for the Ph. D.(Science) degree in Physics under the guidance of Dr. Rajesh Singh. The present study provides the first detail description of morphological features of tweek radio atmospherics recorded at low latitude (L<1.3) Indian stations and their application in monitoring D-region or lower ionosphere during solar quiet (transition period of solar cycle 23 and 24) and eclipse conditions. This dissertation is also focused on long standing problem of propagation mechanism of low latitude whistlers. This work helps in advancing our knowledge of tweek radio atmospheric and low latitude whistlers. A brief summary of important and new finding are given below: *Seasonal variation of tweek radio atmospheric recorded at Allahabad shows that tweeks occur maximum (~63%) during summer season and have approximately same occurrence (~18%) during winter and equinox seasons. The trend of occurrence of tweeks during different seasons is somewhat similar with the number of lightnings detected by World Wide Lightning Location Network (WWLLN) in the Indian and Asia-Oceania regions. This indicates that the lightning is a necessary but not sufficient condition for tweek occurrence; rather it also depends on conductivity of boundaries of EIWG, collision frequency, local time etc. * The occurrence of tweeks with different mode varies with time of local night and seasons. The tweek with mode n=1 and 2, have maximum occurrence rate of 71%, followed by tweeks with only n =1 (~25%) and tweeks with n =1-3 (~3.5%). Tweeks having higher modes (n = 4 -6) occurred rarely throughout the year with no fifth and sixth harmonic tweek (having n =1-6) observed during the winter season. * Observations suggest that about 30-40% obscuration of solar disc can lead to the tweeks occurrence which otherwise occur only in the nighttime from 18:00 - 5:30 hrs LT. Tweek observed during eclipse time are less intense, less clear, short duration and have lower modes (n=3 or lower) compared to tweek observed in the normal nighttime. * The average distance travelled by most of tweeks varies in the range ~2000-8000 km. The WWLLN detected lightning locations within 2000-8000 km areas indicate that about 94% of tweeks observed at Allahabad were associated with lightnings in the Asia-Oceania region, which is one of the three regions of major lightning activity in the world. * The path integrated reflection height (h) of nighttime D-region ionosphere calculated by using first order cutoff frequency of tweeks varies in the range 87-95 km at both the stations. The path integrated electron density (ne) estimated using first order cutoff frequency of tweeks varies as 21-24.5 cm-3 at Allahabad and 21.5-24 cm-3 at Nainital. * The nocturnal and seasonal variability in the h′ and at Allahabad and Nainital shows that the nighttime D-region is far from static. The average values of h′ and β for both the stations are almost same (86.1-85.6 km, and 0.51-0.54 km-1) during winter and equinox seasons. The h′ is lower by 2-3 km and is higher by 0.07-0.09 km-1during summer as compared to winter and equinox seasons. * The day-to-day variability in h is about 8-9 km with temporal variability of 1-2 km in any one hour duration. The day-to-day variability in h′ is about 4-5 km and in β it is about 0.1-0.25 km-1. The ne obtained using tweek method shows lower values than those obtained using IRI-2007 model and higher during winter and equinox and lower during summer when compared with Rocket data, however, the trend of ne variation in the altitude range of 85-98 km is almost the same. The ne obtained using tweek method shows seasonal variation with higher values during summer as compared to winter and equinox seasons. * This study presents the first cases of low latitude ground whistlers unambiguously linked with their causative lightning discharges. The lightning discharges are found to be located between 200 and 450 km to the conjugate location of the receiver, providing direct evidence of inter-hemispheric propagation at low latitudes which may possibly be along low latitude ducts and open a new window into low latitude ionospheric diagnostics.
URI:	http://localhost:8080/xmlui/handle/123456789/212
Appears in Collections:	PhD_Thesis

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