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Research & Reviews: Journal of Space Science & Technology ISSN: 2321-2837 (Online), ISSN: 2321-6506 V(Print) Volume 6, Issue 2 www.stmjournals.com Diurnal Variation of VLF Radio Wave Signal Strength at 19.8 and 24 kHz Received at Khatav India (16o46ʹN, 75o53ʹE) A.K. Sharma1, C.T. More2,* 1Department of Physics, Shivaji University, Kolhapur, Maharashtra, India 2Department of Physics, Miraj Mahavidyalaya, Miraj, Maharashtra, India Abstract The period from August 2009 to July 2010 was considered as a solar minimum period. In this period, solar activity like solar X-ray flares, solar wind, coronal mass ejections were at minimum level. In this research, it is focused on detailed study of diurnal behavior of VLF field strength of the waves transmitted by VLF station NWC Australia (19.8 kHz) and VLF station NAA, America (24 kHz). This research was carried out by using VLF Field strength Monitoring System located at Khatav India (16o46ʹN, 75o53ʹE) during the period August 2009 to July 2010. This study explores how the ionosphere and VLF radio waves react to the solar radiation. In case of NWC (19.8 kHz), the signal strength recording shows diurnal variation which depends on illumination of the propagation path by the sunlight. This also shows that the signal strength varies according to the solar zenith angle during daytime. In case of VLF signal transmitted by NAA at 24 kHz, the number of sunrises and sunsets are observed in VLF signal strength due to the variations of illumination of the D-region during daytime. In both the cases, the signal strength is more stable during daytime and fluctuating during nighttime due to the presence and absence of D-region during daytime and nighttime respectively. Keywords: VLF field strength, diurnal variation, ionosphere, solar zenith angle *Author for Correspondence E-mail: [email protected] INTRODUCTION the ionosphere. That’s why these rays are used The ionosphere is made up of many gases such for communication around the globe and they as nitrogen, oxygen etc. The solar ultraviolet are also used for monitoring the ionospheric radiation interacts with the atoms present at behavior. The VLF waves reflects from the this level. This ionizing radiation collides with ionosphere, and that’s why if something oxygen molecules. The ultraviolet waves are happens in the ionosphere then the VLF radio partly absorbed by the oxygen molecules and waves reflecting from the ionosphere will be so these waves ionize the oxygen molecules affected [3–5]. [1]. Simple background cosmic radiation also The ionospheric layer breaks down into many affects continuously on the ionosphere. The layers. These layers are ionized at all times energy of cosmic radiation is not strong but except the D layer. The D layer is close to the they are constantly affecting the ionosphere. earth. It disappears during night. It is due to But, the solar radiation affects diurnally as the sufficient neutral density below 200 km. well as it affects the ionosphere during solar Due to this, electrons recombine with oxygen flares and magnetic storms [6, 7]. within a few hours [2]. So, the solar flare effects are only discrete The E region ionization decreases in small events during the day time only. During the amount during the night time. The D layer night time, VLF waves travel up to 90 km to reappears only at that time when the sun rises the E layer in the ionosphere and then reflects at the D layer height. The VLF waves travel towards earth. During the daytime the D layer along the ground as well as they reflect from is partially ionized. The VLF radio waves RRJoSST (2017) 1-12 © STM Journals 2017. All Rights Reserved Page 1 Diurnal Variation of VLF Radio Wave Signal Strength at Khatav India Sharma and More travels through the D layer and so lose some THE VLF EXPERIMENT energy in the travel way. The strength of the For the study of effect of solar activities on the signal of the VLF waves is stronger at night ionosphere, Stanford University, USA, has set time than the daytime [8]. The VLF waves up a program. This University has distributed respond in different way to different solar inexpensive SID monitors to space activity. In our study, we have recorded VLF researchers and college as well as University data since 2007 and it is observed that VLF students throughout the world [10, 11]. By radio wave signal strength is affected with the using the SID monitors and VLF loop antenna, disturbances occurred in the ionosphere and it is easy to record VLF radio waves this ionospheric disturbance can be affected on transmitted by particular VLF transmitting human life [9, 10]. station. When we use VLF radio waves for The VLF field strength monitoring system at communication purposes, solar activity can Khatav, India (1646' N, 7553' E) is shown in affect the VLF signal strength. Therefore, the Figure1. Experimental set up -two channel purpose of this study was to investigate how VLF field strength monitoring system is VLF radio waves are impacted by the radiation shown in Figure 2. Figures 3 (a) and (b) shows given off by the sun at various times. loop antennas. Fig. 1: Block Diagram of VLF Field Strength Monitoring System which Consists of Loop Antennas, SID Monitor, A/D Converter and Computer. Fig. 2: Experimental set up—Two Channel VLF Field Strength Monitoring System. RRJoSST (2017) 1-12 © STM Journals 2017. All Rights Reserved Page 2 Research & Reviews: Journal of Space Science & Technology Volume 6, Issue 2 ISSN: 2321-2837 (Online), ISSN: 2321-6506 (Print) Fig. 3: Loop Antenna: (a) Hexagonal Loop Antenna (b) Square Loop Antenna. By monitoring the signal strength of the VLF level of ionization of particles in the radio waves reflecting over a period of time, it ionosphere changes, then we should be aware is possible to draw many conclusions about the of the fact that the signal strength of VLF characteristics of the ionosphere. It is because radio waves will also change. of the fact that the radio waves and the ionosphere are so closely related with each THE SOLAR BASELINE EFFECT ON other. If there is a change in the VLF signal THE IONOSPHERE OF THE EARTH strength over the daytime, then it is called as During the period 2009–10, sun was relatively sudden ionospheric disturbance (SID). During calm because there was a very low level of the SID, the numbers of ionized particles SID activities in the ionosphere of the earth. If present in the ionosphere increases or due to the solar activity, there is a change in decreases and that’s why VLF radio waves to the characteristics of ionosphere then there be reflect either more or less. The reason of will be change in the signal strength of VLF the SID can be determined by comparing SID radio waves. There is 11-year cycle for solar data to GOES satellite data. GOES satellite activity which corresponds to the reversal of directly and continuously monitors the sun and sun’s magnetic field [3]. However, it is not records solar flares [11–13]. If the SID and possible to predict when individual solar flares solar flare occurs at the same time which is are going to occur, with a high level of found by using GOES satellite data, then there accuracy. During the period 2009–10, the suns is direct correlation in between ionosphere, activity was at its lowest point of the 11-year sun and the VLF radio waves. In this way, sun cycle. Hence, there were not a big number of and solar flares affect the VLF radio waves solar flares, or SIDs observed during this which is useful because indirect observations period. Hence, it was important to take of the ionosphere as well as the sun is advantage of the lowest solar activity period to possible. The collection of the VLF data by understand the sun’s baseline effect on the using SID monitor is possible from several ionosphere of the earth as well as on the VLF different locations. Thus monitoring of VLF radio waves during sun’s minimum activity waves is very useful not only in understanding level [14]. If we are able to understand what the impact of sun on VLF radio waves which has happened in the ionosphere without or relies on the ionosphere but also the minimum solar activity then could be easy to understanding of ionosphere. It is very solve the issues of the technology accuracy. important to study VLF radio wave technology Further we will be prepared to handle the and the ionosphere because the level of technology and accuracy issues when the solar ionization of particles in the ionosphere does activity are frequent or more. Around the not remain constant for a long time. When the world, more and more scientists are recording RRJoSST (2017) 1-12 © STM Journals 2017. All Rights Reserved Page 3 Diurnal Variation of VLF Radio Wave Signal Strength at Khatav India Sharma and More SID monitor data and they noticed the effect change in radio signal strength meaning that on radio communications. For this study a SID there is a change in the level of ionization in monitors was used to record VLF radio waves ionosphere. The goal of this study was to use from US navy transmitters NAA (24 kHz) and VLF radio waves as a tool to monitor the NWC (19.8 kHz) and monitors changes in the ionosphere and to observe how radiation from ionosphere [15, 16]. The SID monitors data for the sun affects the strength of VLF radio above mentioned frequency was collected waves and how the solar flare affects on the since 2007 and analysis as well as comparison ionosphere.
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