بررسي تحريك و انتشار امواج الكترومغناطيس باند فركانسي بسيار پايين در ناحيه نزديك به سطح زمين

New Approach of Low-Frequency Electromagnetic Wave Generation in the Near-Earth Environment

تحريك و انتشار امواج الكترومغناطيسي در باند هاي فركانسي بسيار پايين در جو زمين، كاربردهاي بسياري در زمينه هاي مختلف، از جمله فيزيك فضا، ارتباطات راديويي و پيش نشانگري زلزله دارد. لذا اين طيف فركانسي همواره مورد توجه پژوهشگران بوده است. با توجه به پيچيدگي هاي بسيار زياد، مطالعه و بررسي تحريك اين نوع امواج الكترومغناطيسي همواره مشكل بوده است. هدف از اين پژوهش بررسي تحريك امواج راديويي در طيف فركانسي بي نهايت پايين (ULF ) Ultra Low Frequency (300 Hz to 3 kHz)) و ELF (Extremely Low Frequency )3 to 30 Hz) در بازه فركانسي 3 هرتز تا 3 كيلوهرتز، و طيف فركانسي خيلي پايين (VLF (Very Low Frequency )3 to 30 kHz) در بازه فركانسي 3 تا 30 كيلو هرتز در لايه E يون سپهر، در ارتفاع 80 تا 120 كيلومتري از سطح زمين مي باشد. در اين تحقيق از روش هاي عددي، براي تحريك جريان الكتريكي، و ايجاد آنتن مجازي در لايه پلاسما براي توليد امواج VLF استفاده شده است. همچنين تحريك لايه پلاسما و تغيير ميزان ضرايب هدايت با استفاده از امواج الكترومغناطيسي باند فركانسي بالا HF ) High Frequency) مورد بحث قرار گرفته است. مدل عددي مورد استفاده در اين تحقيق شامل قوانين ماكسول و ضرايب رسانايي لايه يون سپهر مي باشد كه در نوع خود منحصر بفرد است. اين مدل تغييرات لايه يون سپهر بر روي انتشار امواج ارسالي، و همچنين تحريك امواج ثانويه را از طريق تركيب ضرايب هدايت با قوانين ماكسول شبيه سازي مي كند، به طوري كه زمان انجام محاسبات تا حد قابل توجهي كاهش مي يابد. نتايج نشان مي دهد كه آنتن هاي زمين پايه در باند فركانسي ELF داراي بازدهي بسيار خوبي بوده و مي توانند جايگزين مناسبي براي ايجاد اين امواج توسط فرستنده هاي قوي باند HF كه به گرم كننده لايه بالاي جو معروف هستند، باشند. همچنين نتايج اين شبيه سازي نشان مي دهدكه ميزان نفوذ موج پالسي در لايه يون سپهر كمتر از10 كيلومتر در فركانس 100 هرتز بوده و امواج تحريكي داراي سرعت انتشار در حدود 1000 كيلومتر بر ثانيه مي باشند. ميزان نفوذ امواج الكترومغناطيسي در جو زمين با افزايش فركانس ارسالي به طيف فركانسي خيلي پايين VLF به ميزان بسيار زيادي كاهش مي يابد.

This work presents the study on the electromagnetic wave penetration into the ionosphere in the frequency range of 10 Hz to 3 kHz and 3 kHz to 30 kHz, corresponding to the Extremely Low Frequency (ELF) and Very Low Frequency (VLF) for telecommunication applications and earthquake prediction. The ELF-VLF waves can also be generated through natural phenomena such as lightning as well as pre-seismic activities. The ELF generation before major earthquakes has been reported in several studies. Therefore, having a complete model capable of simulating the ELF waves generation and propagation in the disturbed ionospheric conditions, associated with pre-earthquake activities can be used to save human lives by predicting the exact location of a major earthquake. This study aims at developing a computational model in order to investigate the ELF –VLF wave generation and propagation in the lower ionosphere that can be used as a precursor for seismic events. Another application of this frequency band is in the radio navigation. The VLF navigation system known as OMEGA was very popular and used for many applications such as navigation of ships, airplanes and also in the land. The system was in use until the late 1990s when it was replaced by Global Positioning Systems (GPS) due to high accuracy and low cost. Very recently, there has been an effort to renew the VLF navigation systems at a low cost. This will require a new approach for VLF wave generation in the ionosphere at a lower cost in comparison with regular transmitters. The efficiency of VLF wave generation in the lower ionosphere using a ground-based dipole antenna in the equatorial region is examined in this study. In this study, we have shown that transmitted signal from the ground into the ionosphere can generate a current in the lower ionosphere, which may expand up to a few kilometers depending on the ionospheric conductivities and the frequency or modulation of the transmitted signal from the ground. This study investigates the generation of secondary currents and the artificial antenna in the ionosphere in order to develop a new technique for generating these signals for navigation applications. The approache for improving the efficiency of this technique including pre-modulation of the ionosphere using high-power high frequency (HF) signal for modifying the conductivities in the ionosphere is discussed. The main idea here is to investigate the efficiency of Whistler wave generation in the E region in different ionospheric conditions. Specifically, the effect of pulse and continuous probing of the lower ionosphere with ELF-VLF signals and the generation of secondary waves and currents due to high conductivities are investigated. We have also proposed the application of this model to study the generation and propagation of ELF-VLF signal associated with the earthquake in the disturbed ionospheric conditions. This includes the variation of background ionospheric plasma and its effect on the penetration of the signal in the ionosphere and length scale of the excitied currents. This study may be critical to determine the exact location of a major earthquake using the pre-seismic activities such as generation and propagation of ELF waves. The variation of background ionospheric parameters such as electron density and ionospheric disturbances due to pre-earthquake conditions on the excitation and penetration of ELF-VLF waves into the E-region will be investigated in future studies.