کاربرد پایدارسازی بروش هیبرید در بازسازی توموگرافی چگالی الکترونی یونوسفر در ایران

توموگرافی یونوسفر یک روش بسیار موثر جهت بررسی ویژگیهای فیزیکی این لایه از جو می باشد. بازسازی توموگرافیک چگالی الکترونی یونوسفر بدلیل کمبود مشاهدات ورودی و نیز عدم توزیع یکنواخت آنها یک مساله معکوس بدوضع محسوب می شود. در این مقاله از یک روش جدید پایدارسازی بنام هیبرید جهت حل این مساله استفاده شده است. این روش ترکیبی از روشهای پایدارسازی تیخونوف و تغییرات کلی (tv) می باشد. مزیت این روش در کمتر شدن میزان بایاس ایجاد شده در نتایج و نیز ایجاد تعامل بهینه مابین مقدار بایاس و دقت نتایج حاصل است. بدلیل وجود اندازه گیریهای مستقیم یونوسفر (یونوسوند phi;=35.73820, lambda;=51.38510) در سال 2007، کارایی روش پیشنهادی این مقاله توسط داده های شبکه ژئودینامیک کشور ایران در سه روز مختلف بتاریخهای 03/01/2007 (فصل زمستان)، 03/04/2007 (فصل بهار) و 13/07/2007 (فصل تابستان) مورد تست و ارزیابی قرار گرفته است. همچنین نتایج الگوریتم پیشنهادی در این مقاله با نتایج حاصل از روش تیخونوف مرتبه صفرم، چگالی الکترونی بدست آمده از مدل مرجع جهانی یونوسفر 2012 (iri2012) و چگالی الکترونی حاصل از مدل nequick موسسه عبدالسلام ایتالیا مورد مقایسه قرار گرفته است. منطقه بازسازی شده دارای عرض 22 تا 40 درجه و طول 44 تا 64 درجه می باشد. آنالیز انجام گرفته نشاندهنده کمینه خطای نسبی 55/1 درصد و بیشینه 52/19 درصد می باشد. همچنین مقادیر کمینه و بیشینه خطای مطلق بترتیب برابر با 1011 times;32/1 (ele/m3) و 1011 times;67/6 (ele/m3) بدست آمده است.

Application of Hybrid Regularization Method in Ionospheric Tomographic Modeling in Iran

When the molecules and atoms of the atmosphere receive enough external energy, one or more electrons are dissociated from the molecules or atoms. This process is called ionization. The solar ultraviolet (EUV) radiation and particle precipitation are the two primary energy sources in the ionization. Also cosmic radiation contributes to this ionization. This layer of atmosphere is called ionosphere. The ionosphere is that part of the atmosphere in which the number of free electrons is so high that, it significantly affects the propagation of radio waves. Ionospheric refraction is one of the main error sources on GPS signals. This effect is proportional to the total electron content (TEC). TEC is a projection of electron density along signal path extending from the satellite to the receiver on the ground. The unit of TEC is TECU and 1 TECU equals 1016 electrons/m2. Production of free electrons in the ionosphere depends on many factors, such as solar, geomagnetic, gravitational and seismic activity period.There are many methods to obtain electron density or TEC profiles and predictions. In early time, direct measurements such as ionosonde was a kind of effective instrument to achieve this purpose. Later, some empirical and mathematical models were developed. For example, IRI (international reference ionosphere) model, PIM (the parameterized ionospheric model) are empirical models. Mathematical models divided to two categories: singlelayer (2D) and multilayer (3D 4D). The existing 2D methods of modeling the electron density can be classified to nongrid based and grid based techniques. The former modeling techniques are based on the least squares estimation of a functional model for certain types of observables derived from the GPS carrier phase and code measurements. Polynomials and spherical harmonics are some of the base functions that are commonly in use. In grid based modeling, the spherical shell of free electrons is developed into a grid of rectangular elements. Special reconstruction algorithms are then used for estimating the electron density within the every element of the shell. Neglecting the vertical gradient of the electron density is the main deficiency of the two dimensional modeling techniques.To study the physical properties of the ionosphere, computerized tomography (CT) demonstrated an efficient and effective manner. Due to the sparse distribution of GPS stations and viewing angle limitations, ionospheric electron density (IED) reconstruction is an illposed inverse problem. Usually, iterative or non iterative algorithm used for electron density reconstruction. Non iterative algorithms are known regularization methods. Using these methods to solve the ill posed problems will produce bias in final results. In this paper, we used hybrid regularization algorithm for solving ionosphere tomography. This method is a combination of two regularizations methods: Tikhonov regularization and total variation (TV). Tikhonov regularization is a classical method for solving illposed inverse problem and total variation effectively resists noise in results. To apply the method for constructing a 3Dimage of the electron density, GPS measurements of the Iranian permanent GPS network (at 3day in 2007) have been used. The modeling region is between 240 to 400 N and 440 to 640 W. The result of hybrid regularization method has been compared to that of the zero order Tikhonov regularization method and NeQuick model outputs. The minimum relative error for hybrid method is 1.55% and the maximum relative error is 19.52%. Also, maximum and minimum absolute error is computed 1.32 times;1011 (ele/m3) and 6.67 times;1011 (ele/m3), respectively. Experiments demonstrate the effectiveness, and illustrate the validity and reliability of the proposed method.