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Seasonal Results of Aspect Sensitivity from an MF Radar over Kunming (25.6oN, 103.8oE), China

Jin-songCHEN#%*1, Ji-yao XU#2, Zhen-weiZHAO*3

#State Key Laboratory of Space Weather, Center for Space Sciences and Applied Research, Chinese Academy of Sciences, Beijing, 100190, China

1jscen@spaceweather.ac.cn2jyxu@spaceweather.ac.cn

%Graduate University of Chinese Academy of Science, Beijing, China *National Key Laboratory of Electromagnetic Environment,Chinese Research Institute of Radiowave Propagation,

QingdaoShandong 266107, China 3zhaozw22s@163.com

Abstract—the physical shapes and causes of these partial reflection scatterers for MF radar is, as yet, an unsolved question.The aspect sensitivityparameterized by a quantity isdefined often as the rate of decrease of power as a function of angle.Small values of imply the existence of specular scatterers.The seasonal results of aspect sensitivity from Kunming MF radar have been studied based on the first 12 months data, which shows a clear seasonal dependence with the bigger values in winter and the smaller values in summer. Aspect sensitivity also presents an evident variation in height, increasing between 80 and 90 km while decreasing from 90 to 100 km. Turbulence velocity measured during the same period supports the proposal that turbulences play an important role in causing specular scatterers which produces aspect sensitivity in D-region for MF radar.

I. INTRODUCTION

Radar partial reflections are caused by refractive index irregularities which are arise due to pressure, temperature and humidity fluctuations in the troposphere and stratosphere, and electron density fluctuations in the mesosphere and lower thermosphere[1]. Mesospheric and lower thermospheric (60-100km) electron density irregularities can be caused by several different mechanisms [2], the main ones being turbulent scatter and Fresnel partial reflection. Turbulent scatter arises from spatial variations in the refractive index of half the probing wavelength in the direction of propagation, and occurs as a result of a turbulent process. Fresnel partial reflection occurs from horizontally stratified and stable regions which cover at least one Fresnel reflection zone

in horizontal extent, and with a vertical extent no greater than where is the wavelength of the radar, and z is the height of the scatter.

The shapes of the scatter which are responsible for vertically pointing radar beams are still not clear. A better understanding of them will not only improve our ability to make and have faith in routine measurements such as the wind velocity, but it will aid in the understanding of the dynamical processes which cause them in the first place. A fairly common occurrence in atmospheric observations, specular

reflectors have been found numerous other studies [3, 4].The partial radar reflections from these scatters have shown that the back-scattered power decreases as a function of the off-zenith angle.One distinguishing feature of these scatterers is that they are aspect sensitive. This implies that their structure is not isotropic. The degree of aspect sensitivity, often denoted by , is a gauge of how quickly the back-scattered power falls off as a function of the zenith angle. A of close to 90o

indicates highly isotropic scatter, while a close to 0o

indicates scatter mainly from the vertical, or highly aspect sensitive scatter.Since aspect sensitivity is a clear measure of the stratification of the scatterers, it is obviously desirable to try and physically measure or quantify it.

In this study, the data from Kunming MF radar between August 2008 and July are used to study the aspect sensitivity of scatterers in the height region of 80-100 km. The rest of this paper is organized as follows: Section describesthe method to measure aspect sensitivity from the mf radar data. Section presents seasonal results of aspect sensitivity from August 2008 to July 2009 measured by Kunming MF radar. Finally discussions and summary are given in section .

II. THE METHOD TO MEASURE ASPECT SENSITIVITY FROM THE MF RADAR DATA

A.The Kunming MF radar system The Kunming MF radar is a monostatic system, which was

installed at Kunming Radio Observatory, and the block diagram is shown schematically in figure 1. The site is located in Qujing city (25.6oN, 103.8oE), Yunnan province, China.The transmitting and receiving array is arranged in an equilateral triangle whose basic space is about 180 m, and consists of four tween fed half wave dipoles, approximately 70 m in length, and suspended between stage telescopic towers at a height of about 30 m above ground level. The antennas are situated at the vertices of the triangle with one at the centre.The heart of the system consists of the transmitter, the receiving and data acquisition system, and the computer which ultimately controls the system [6]. ______________________________________

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The Kunming MF radar was installed and successfully commissioned in July 2008, and has collected data ever since. Operating in the spaced-antenna mode with full correlation analysis, horizontal wind velocities and electron density are obtained every 2minutes and 1 minute respectively, at 2 km height intervals. A total of 26 heights are sampled, extending over the height range of 50 to 100 km.

The data used in this work spans over about 12 months, from August 2008 to July 2009, and was of an excellent temporal resolution. The full correlation analysis rejected data for various reasons, and all these data were accepted by the full correlation analysis, having passed all the relevant rejection criteria. Clearly evidence is the low data rates which are experienced below 80 km. Whilst lower data rates are also evident above 92 km commonly, they are still approximately double those below 80 km. Due to the data rates only the data between 80 km and 100 km are analysed in this study.

Fig.1The block diagram of the Kunming MF radar system

B. Spatial correlation method There are two methods which are commonly used to derive

the aspect sensitivity from the MF radar data. One is the spatial correlation method, which uses the spatial correlation size, and the other is spectral width method, which uses the fading time of the signal. In this study, the first method is chosen to calculate the aspect sensitivity of scatterers, so only the primary principle of the spatial correlation method is given below.

A volume of scatterers which is probed with a radar beam will partially scatter the signal back to ground level. The back-scatter contributions from each of these scatterers will interfere and produce an electric field “shadow” or diffraction pattern on the ground. If the ground diffraction pattern is symmetrical, one need only use a single antenna which measures a cross-section of the pattern as it moves. This idea has been used in the past to measure aspect sensitivity. Unfortunately the assumption of symmetry is unlike to be true.In that case a useful approach is to parameterize the form

of the spatial correlation function. Although the full correlation analysis is used primarily to determine the wind, as a by-product it does determine the statistical properties of an elliptical correlation function, including the orientation of the major axis, its length and the axial ratio. The mean of the semi-major and the semi-minor axis was used to calculate a mean scale, from which was determined. Since this ellipse is defined as the scale at which the modulus of the complex auto-correlation function falls to 0.5, we have the simple relationship [5]

Where is e-1 the width of the effective polar diagram, is the radar wavelength, a and b are the lengths of the major and minor axis respectively. Finally can be derived from through the below relationship

where is the half-power half-width of the polar diagram of the radar beam and is about 30 degrees for Kunming MF radar. In the special case where a wide radar beam is used, just as the beam of the Kunming MF radar we can make the further approximation that , since . Generally, the spatial correlation method uses the geometrical mean of the major and minor semi-axes of the correlation ellipse, and is not dependent on the orientation of the ellipse with respect to the various parameters like wind and geomagnetism.

Figure 1 show one day results of aspect sensitivity presented on the bottom calculated from the length of the major axis on the top of the figureand the ratio of the major to minor axes ofthe characteristic ellipse on the middle measured by the Kunming MF radar.From this figure we can see the biggest arise in the middle height about 90 km and the smaller ones appear in the top and bottom of the height range, and this feature will also be seen in the following section.

Fig.2 One day results of aspect sensitivity calculated from the length of the major axis and the ratio of the major to minor axes ofthe characteristic ellipse measured by the Kunming MF radar. (a) the length of the major axis, (b) the ratio of the major to minor axes ofthe characteristic ellipse, (c) aspect sensitivity .

III. SEASONAL RESULTS OF ASPECT SENSITIVITY

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It is shown in the last section how these various parameters which are deduced as by-products of the full correlation analysis could be used to make qualitative calculations of aspect sensitivity, , which can be calculated by using either the fading time of the signal, or the spatial correlation size. Here we discuss the seasonal and height behaviour of aspect sensitivity calculated at Kunming site using the first 12 months of data.

Figure 3 is an image contour plot of the daily-mean aspect sensitivity calculated by the spatial correlation method. It is shown that the general behaviour of is to vary both with height and on a daily basis. The valuesof range from 5 to 15 degrees.Looking down from 80 to 84 km, one can see that while there is no evident seasonal variations in , whilst a clear seasonal variations exists at heights above 86 km, which is more evident around the height of 90 km. At the height of 90 km is more than 10o during the winter months, which extends up to 100 km, decreasing by several degrees during the summer months, while the peak value about 12o tends to emerge during April and May in 2009. In addition, contours progress upwards during autumn and downwards during spring at a rate of several kilometres per month.

Fig.3Seasonal results of daily-mean aspect sensitivity in the height range of 80-100 km at Kunming site between August 2008 and July 2009.

Fig.4 Monthly-mean profile of aspect sensitivity as the function of height (80-100 km) from August 2008 to July 2009.

In order to reveal the height variation of aspect sensitivity, figure 4 presents the monthly-mean profile of aspect sensitivity with the function of height (80-100 km) from August 2008 to July 2009. In figure 4 all months show the similar height variations. The aspect sensitivity increases continuously from 80 km upwards to 90 km, and this increase almost keep constant between 82 and 90 km, while the values decrease gradually from 90 upwards to 100 km. The increase in suggests that the structures which cause the partial radar reflections are more isotropic with increasing height.

Overall, the scattering structures are seen to be more isotropic during winter around 90 km, and more specular at other times and heights.

IV. DISCUSSIONS AND SUMMARY

Various modes, ranging from stratified mirror-like layers, to individual scattering “disks”, have been proposed to explain the experimental observations of partial reflection scatter. The first type of model proposes individual scatters which range in their length-to-depth ratio as a function of scale. The most highly aspect sensitive scattering is due to these specular reflectors, claimed by some authors to be anisotropic turbulence [7].Others argue that these types of scatterers cannot explain the sharp fall-off in power with respect to zenith angle, and that at other times the fading is too slow to be due to anisotropic turbulence alone. They propose a second type of model which consists of stratified horizontal layers, or steps, in which the refractive index varied vertically. Three mechanisms which could produce the stratified reflecting layers have been put forward, they could be either the edges of a strongly turbulent layer, damped gravity wave with short vertical wavelengths, or viscosity waves [8].

Since neither type of model can explain all of the observational features, models which combine the two types of structures have also been proposed [9]. They envisaged atmospheric layerswhich were so strongly turbulent in some cases that they have sharp refractive index edges at the top and bottom of the layer. These layers then cause the specular reflections.

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Fig.5Weekly-mean contour of turbulence velocity as the function of height (80-100 km) and time from August 2008 to July 2009 measured by Kunming MF radar.

Another model proposes a scenario where thin anisotropic turbulence exists at the edges of a more turbulent layer. The thin anisotropic turbulence causes radar partial reflections for a vertically pointing beam, and explains the observed aspect sensitivity, while the more isotropic turbulence contained within the layer is responsible for partial radar reflections from an off-zenith pointing beam.This type of model is more realistic, since turbulence is often more stable near the edges of the layer than within. Another proposal argues that some of the observations cannot be explained by either anisotropic turbulence, or stratified layers as discussed above, and the observed specularitiesare due to highly damped waves.

In attempt to show the effect of turbulence in causing the aspect sensitivity observed here, we have presented the turbulence velocity which is derived from the fading time, or mean auto-correlation functionhalf-width, during the same time in figure 5. It is evident that the turbulence velocity is correlated highly with the aspect sensitivity in figure 3, and they tend to show almost similar variations in time and height, which indicates that turbulences contribute mainly to the feature of aspect sensitivity rather than waves.

In summary, the seasonal results of aspect sensitivity from Kunming MF radar have been studied based on 12 months data, which is seemed to show a clear seasonal dependence with the bigger values in winter and the smaller in summer. Aspect sensitivity also presents an evident variation in height, increasing upwards between 80 and 90 km and decreasing from 90 to 100 km. Turbulence velocity measured during the same period supports the proposal that turbulences play an important role in causing specular scatterers which produces aspect sensitivity in D-region for MF radar.

ACKNOWLEDGEMENTSKARF belongs to and is operated by China Research

Institute of Radiowave Propagation (CRIRP). This work was supported in part bythe foundation of National Key Laboratory of Electromagnetic Environment under Grants 9140C0803020905 and 9140C080105100C08.

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