Results of VLF campaigns in Summer and Winter in Indian Subcontinent Sandip K. Chakrabarti 2, 1 , S. Pal 1 , S. Sasmal 2 , S. K. Mondal 2 , S. Ray 2 , and T. Basak 1 1 S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-II, Salt Lake, Kolkata-700098, India, chakraba@bose.res.in, sujay@bose.res.in, tamalbasak@bose.res.in 2 Indian Centre for Space Physics, 43 Chalantika, Garia Station Road, Kolkata-700083, India, sudipta@csp.res.in, sushanta@csp.res.in, suman@csp.res.in Abstract VLF propagation effects can be understood using the earth-ionosphere waveguide. However due to emer- gence of the new fields such as VLF Astronomy where the ionosphere is treated as a giant detector for extraterrestrial energetic phenomena and the seismo-electromagnetism where the disturbances of this giant detector is influenced by terrestrial events, especially earthquakes and other seismic activities. In order to interpret observed data correctly, we need to have the basic propagation characteristics throughout India. In this paper, we concentrate on the results of the VLF campaigns we conducted using over a dozen receiving stations in Summer and in Winter. We observe the well-known asymmetry in East to West and West to East propagation. From the signal amplitude characteristics, we divide the sub-continent in four zones as far as the VTX transmitter is concerned. 1 Introduction The ionosphere is a gigantic detector which is free of cost and maintenance. High energy extra-terrestrial events can enhance ion-concentration of the ionosphere and the VLF signals disturbed by such events can be detected. It is also believed that seismic events exhibit pre-cursors in the form of disturbances in VLF signals. It is therefore of importance that we understand the propagation properties of the VLF signals in the absence of any disturbances so as to have a calibration profile. Furthermore, it is still not clear whether the wave-hop theory or the mode-theory is superior in a short distance (< 3000 km) propagation. Keeping this in mind, we carried out two campaigns, one in Summer and one in Winter, to study the nature of VLF data from about a dozen places in the Indian sub-continent. Detailed results are in Chakrabarti et al., 2010 [1] and Chakrabarti et al., 2011 [2]. We present here briefly the results of the campaigns and the conclusions drawn. 2 Campaign locations and results The summer campaign took place in the final two weeks of July, 2009 and the Winter campaign took place in the final two weeks of December, 2008. In the winter campaign, we concentrated on the VTX station (08 ◦ 23 ′ N, 77 ◦ 45 ′ E) transmitting at 18.2 kHz, while in the summer campaign we picked up data from many stations, including NWC transmitting at 19.8 kHz. Since our goal was to check the existing models, including our own based on wave hop theory, we placed the receivers scattered all over India, selecting more than a dozen places. The unique location of the Indian sub-continent where the VTX transmitter is at the southern tip and the magnetic meridian through the transmitter dividing the sub-continent vertically into east-side and west-side make it an ideal condition to study the East-West propagation effects as well. Our receiver nearest to the transmitter was placed at Bangalore, around ∼ 500 km away. The farthest one is in Srinagar, Kashmir at about 2900 km away. We used square-shaped loop antennas (3.5 ft×3.5 ft). The receiver is a Gyrator-III type with a band pass filter with a range of 5 - 30 kHz. The loop plane at each location was oriented so that its plane contains the transmitting antenna. That way, we maximized the sensitivity for VTX only. The signal from the antenna was fed to the line receiver and the output from the receiver was fed into the audio input of the computers after suitable amplification. Twice a second, the signals were automatically recorded and stored. 978-1-4244-6051-9/11/$26.00 ©2011 IEEE