Distribution Characteristics of Meteor Angle of Arrival in Mohe and Wuhan, China

Abstract

Meteor radar is one of the key tools for studying the atmospheric dynamics in the mesosphere and lower thermosphere. The physical parameters obtained by meteor radar inversion can provide important statistical information for research. The daily and annual variations in meteor azimuth distribution detected by meteor radars contain information about meteor source regions and patterns related to the rotation and revolution of the Earth. Using the meteor parameters from two meteor radars located in Mohe (53.5° N, 122.3° E) and Wuhan (30.6° N, 114.4° E), this study calculates the variation patterns in the meteor azimuth distribution over the two sites over 1 year. Additionally, this study introduces the variable, Max_Azi, to describe the position of the peak of azimuth distribution. The peak value of azimuth distribution is calculated by Gaussian fitting to quantify the variation patterns in azimuth distribution. This study provides complementary information on the azimuth distribution in high and middle latitudes. The results indicated that the azimuth distribution variation for the Mohe meteor radar is consistent with the Earth’s revolution model.

1. Introduction

The mesosphere and lower thermosphere (MLT), located between ~60 and ~110 km in the atmosphere, is the transition zone from the neutral atmosphere to the ionized atmosphere [1,2,3,4,5]. Various atmospheric fluctuations in the near-Earth lower atmosphere increase during upward transmission. As a result, energy is transmitted to the MLT, leading to changes in the background neutral atmospheric wind field, density, and temperature parameters in the MLT [2,4]. Therefore, the MLT has gradually become a research hotspot in recent years. Experimental and theoretical studies in the past few decades have established a theoretical framework for the atmospheric dynamics of MLT. The study of MLT atmospheric fluctuations plays a crucial role in understanding global energy transfer and the coupling of the neutral atmosphere to the upper atmosphere and the ionosphere [2].

To study the variation in and structure of MLT atmospheric fluctuations, ground-based detection devices (such as meteor radar, lidar, mesosphere–stratosphere–troposphere (MST) radar, and medium frequency (MF) radar) [6,7,8,9,10,11,12,13] and space-based detection devices (such as satellite remote sensing) are used extensively to detect atmospheric wind fields in the MLT. Lidar emits laser pulses into the atmosphere and then collects and samples the backscattered signal. Winds are retrieved by measuring the Doppler shift of the received signal. Lidar can obtain three-dimensional wind speed at different heights by volume scanning, which has the characteristics of high measurement accuracy, high resolution, and rapid response. Nevertheless, limited by adverse weather conditions, lidar obtains fine measurements only under fair weather. MST radar is a large array-phased array radar for observing three-dimensional wind fields in the mesosphere, stratosphere, and troposphere. Operating in the very-high-frequency band, MST radar mainly uses the echo mechanisms of turbulent scattering, Fresnel reflection, and thermal scattering to obtain atmospheric three-dimensional wind profiles and characteristic turbulence parameters for different height intervals. Moreover, MST radar mainly uses the echo of clear air to obtain information on the atmospheric structure. MST radar has the limitations of producing wind bias by aspect sensitivity, gravity wave effects, scatterer intermittency, and expensive maintenance [14]. Although satellite remote sensing can provide global coverage, it can only obtain a small amount of data per day for a certain region, which is not suitable for collecting and analyzing the long-term wind field observation data for a region. Meteor radar equipment is designed to be reliable with the ability to have 24-h uninterrupted monitoring. With a detection range of approximately 300 km, meteor radars can detect the two-dimensional wind field information for 70–110 km, which has obvious advantages when compared with other remote sensing techniques [5,15].

Meteoroids enter the atmosphere at high speed, collide with atmospheric molecules, and are ionized to form plasma columns on their path, which are often referred to as meteor trails [3]. Meteor radars periodically emit pulsed electromagnetic waves. When meteor trails are detected, meteor radars receive backscattered echoes through the interferometric antenna array. The angle of arrival of meteor trails is calculated using phase interferometry. Meteor trails are affected by the background neutral atmospheric wind field, resulting in displacement. Hence, for meteor radar, a method has been developed to calculate the background neutral atmospheric mean wind field using the angle of arrival and radial velocity of undersense meteor trails combined with the least square method [15].

The physical parameters retrieved by meteor radars are mainly two-dimensional wind speed, including meridional and zonal wind speeds. Because of the limitation of the equipment itself, vertical wind speed is generally disregarded. Meanwhile, the distributions of meteor heights and dual-polarization diffusion coefficients are obtained from the statistical information on meteor trail echoes, which can be used to deduce the temperature and density parameters of the middle and upper atmosphere at different heights. By analyzing the parameter information obtained from meteor radar inversion, the parameters of different study areas can be obtained in the region.

The statistical characteristics of different meteor radar parameters have been examined in previous studies, such as estimated atmospheric temperature and height distribution [16,17]. Previous statistical results often include the angle of arrival distribution of meteor radar [18,19]. However, the variation in meteor azimuth distribution detected by meteor radars has seldom been studied. Jones and Brown, in 1993, described six possible source regions of meteoroids based on 10 years of orbital data. They studied the size and meteoroid origin of different source regions. The source regions of the six possible meteoroids are Hellion, Anthelion, South Apex, North Apex, South Toroidal, and North Toroidal [20]. In their 2004 study of the daily and seasonal variations of meteor flux of the Antarctic COBRA meteor radar, Janches et al. also discovered that most meteor activities occur during summer in Antarctica and revolve around a sky region with a high concentration of altitude and azimuth. The possible meteor source regions proposed in the study are consistent with those of Jones and Brown [18]. In 2006, Lau proposed the Earth’s revolution model to explain meteor source regions in the data statistics of the azimuth distribution of the Antarctic COBRA meteor radar [19]. As the Earth revolves around the Sun, it skims more high-energy meteoroids in the direction of the Earth’s translation movement. This is because these meteoroids have an additional velocity, resulting from the Earth’s translation movement as they enter the atmosphere [19].

This study uses the data from two meteor radars located in Mohe (53.5° N, 122.3° E) and Wuhan (30.6° N, 114.4° E) to calculate the variation in meteor azimuth distribution at the two sites within 1 year. This study also introduces the maximum azimuth variable to quantify the variation patterns in azimuth distribution. This study provides complementary information on the azimuth distribution in high and middle latitudes. The results indicate that the azimuth distribution of meteors at Mohe in this study supports the Earth’s revolution model, whereas the results for the meteor radar at Wuhan do not completely agree with the conclusions.

2. Data

Produced by ATRAD company, the first domestic meteor radar was set up in Wuhan in 2002, which has a similar configuration to the Buckland Park meteor radar and SkiYMet meteor radar system [5]. Later, the Wuhan meteor radar was upgraded and placed under the management of the Chinese Meridian Project. At the same time, the Chinese Meridian Project also set up a meteor radar with the same configuration in Mohe. The operating frequency of the meteor radar is 38.9 MHz. The pulse repetition frequency is 430 Hz. Meteor radars transmit pulsed electromagnetic waves periodically and receive the backscattered echoes of the underdense meteor trails through the interferometric antenna array. The interferometric antenna array consists of five two-element Yagi antennas to form the classic Jone’s array, also known as a crossed antenna array. The transmitting antenna consists of a folded two-element Yagi antenna.

Figure 1 presents the statistical chart presenting the detection results of the Mohe meteor radar on 1 August 2019. Figure 1a illustrates the time variation in the number of meteors detected by the Mohe meteor radar on that day, which is the same as the local time variation in meteor appearances observed in previous study data [18,19,20]. Figure 1b,c, respectively, illustrate the distribution of the azimuth and zenith of the meteors detected by the Mohe meteor radar on that day. The azimuth refers to the horizontal angle of a meteor trail’s projection on the x–y plane, measured relative to a reference direction of true north (azimuth definition: North is 0°, increasing clockwise). The zenith refers to an angle measured from the z-axis in spherical coordinates. The antenna pattern is one of the main factors affecting the zenith distribution of meteors, which is mainly distributed in the range of 30°–70° and is in accordance with the antenna design index. Other minor factors that can also affect the zenith distribution of meteors include atmospheric conditions, meteoroid properties, and the location of the radar. Figure 1d illustrates the variation in meteor height detected by the Mohe meteor radar on that day. The height distribution of meteor echoes is consistent with the Gaussian function. The detected meteor counts are significantly attenuated beyond ~100 km, which is known as the ceiling effect [17,21,22], and refers to the fact that the backscattered signals of meteor trails received by meteor radar beyond the ~100 km range are detected with significant attenuation. Figure 1e illustrates the all-sky distribution of meteors detected by the Mohe meteor radar on that day.

Figure 2 presents the statistical chart of the Wuhan meteor radar detection results on 1 August 2019. Figure 2a illustrates the time variation in the number of meteors detected by the Wuhan meteor radar on that day. Figure 2b,c, respectively, illustrate the distribution of the azimuth and zenith of the meteors detected by the Wuhan meteor radar on that day. Figure 2d illustrates the variation in meteor height detected by the Wuhan meteor radar on that day. Figure 2e displays the all-sky distribution of meteors detected by the Wuhan meteor radar on that day. Overall, the detection number of the Wuhan meteor radar was smaller than that of the Mohe meteor radar, which was caused by the differences between the Wuhan and Mohe meteor radar sites for radar specification, electromagnetic environment, and mid-upper atmosphere density.

3. Results: Analysis and Discussion

Figure 3 presents daily variations in meteor azimuths at different times of the day of the Mohe meteor radar on 1 January 2019 in LT hours. The red point refers to Max_Azi where the peak of the azimuth distribution occurs. After the meteors with a zenith greater than 65° were removed, we divided the meteors according to the azimuthal range. The number of meteors in each azimuthal range was counted and normalized to obtain its probability distribution. The results in Figure 3 indicated that the position of the peak azimuth distribution of the meteor azimuth distribution changes over time during the day. It can be all observed that the position of the peak azimuth distribution rotates clockwise with time, which is similar to the results observed in the studies of Janches and Lau [18,19]. This is related to the rotation of the Earth. It is plausible that if the meteor source region is in a fixed direction, the direction of the meteor radar antenna array toward the meteor source region continues changing as the Earth rotates, resulting in a change in the position of the peak azimuth distribution.

For a comprehensive study, the variable of maximum azimuth position was introduced to quantitatively illustrate the variation in meteor azimuth. The position of the maximum azimuth distribution is defined as Max_Azi. First, histogram statistics were obtained for the array containing n azimuthal data. Figure 4a illustrates that the maximum value of azimuth histogram distribution is challenging to obtain directly. Because the angle was flipped from 0° to 360°, this paper spliced the array head to tail and modified the display angle range from [−180°, 180°] to [−540°, 540°]. As shown in Figure 4b, the peak azimuth position can be better observed, as in the azimuth distribution data after the modified range. For the value of Max_Azi, the more accurate peak azimuth position was obtained using a Gaussian fitting method.

By defining the position of the maximum azimuth distribution as Max_Azi, the current study quantitatively analyzed the variation in the azimuth distribution. This study statistically analyzed the meteor radar data from Wuhan and Mohe. To study the variation of meteor azimuth distribution within the time period, the time of day was divided into four time periods of six hours. The time unit is local time (LT), marked as LT0–LT6, LT6–LT12, LT12–LT18, and LT18–LT24. The variations in maximum azimuth Max_Azi of the Mohe and Wuhan meteor radars were obtained, and presented in Figure 5 and Figure 6. Because of the issue of missing data for certain years, the statistical data was collected in 2019 for the Mohe meteor radar and 2018 for the Wuhan meteor radar. In the statistical process, data preprocessing was adopted to filter out abnormal data points, preventing the interference of abnormal data. Whether the meteor azimuth distribution in the time period is abnormal data is determined by the goodness of fit of Gaussian fitting results. If the goodness of fit is less than the threshold value, it means that the significant maximum azimuth distribution position Max_Azi cannot be found. We use the R-square metric to evaluate the goodness of fit. Generally, the threshold is set empirically after repeated experiments and manual inspections. The removed data points are presented as null in the figure. Disregarding the higher harmonics, the second harmonic fitting was performed on the data to better observe the trend in azimuth distribution.

Figure 5 displays the position variations of the maximum azimuth distribution for Max_Azi in 2019 of the Mohe meteor radar. From the LT0–LT6 time period to the LT18–LT24 time period, the peak azimuth position rotates clockwise (azimuth definition: North is 0°, increasing clockwise). The harmonic fitting curve indicates that the overall peak azimuth position shares the same trend.

Figure 6 presents the position variations of the maximum azimuth distribution for Max_Azi in 2018 of the Wuhan meteor radar. Different from the relatively clear azimuth distribution of the Mohe meteor radar in Figure 5, the peak azimuth position of the Wuhan meteor radar is more complex. The blue curve of the LT0–LT6 time period crosses with the red curve of the LT6–LT12 time period in the interval of 150 to 250 days. Moreover, the harmonic fitting curve indicates that there are different trend directions for the overall peak azimuth position variations.

The Earth’s revolution model was proposed in Lau’s study in 2006 to explain the meteor source region [19]. As the Earth revolves around the Sun, meteoroids will have more energy passing by in the direction of the Earth’s translation. These meteoroids have additional velocity caused by the Earth’s revolution as they enter the atmosphere. Hence, the direction of the Earth’s rotation is considered to be the largest source region of meteors. The peak azimuth position distribution of the Mohe meteor radar is portrayed in the Earth’s revolution model. It can be observed in Figure 7 that the peak azimuth position distribution of the Mohe meteor radar changes clockwise, with the counterclockwise rotation of the Earth. This is because the direction of the Mohe meteor radar toward the largest source region of meteors continues changing with the rotation of the Earth.

Regarding the reason why the position variation of the maximum azimuth distribution of the Wuhan meteor radar in 2018 is different from that of the Mohe meteor radar, possible reasons are discussed below. In Figure 6, the curve of Max_Azi variations during the time interval of LT 6-12H has some overlaps with the curve of Max_Azi variations during LT 0-6H, that is, the variation in peak azimuth does not align with the rotation of the Earth in the same ways as the results at Mohe. The lack of data in Max_Azi variations during the time interval of LT 6-12H due to the poor fitness may lead to the overlap. Considering that Wuhan is in the middle latitudes, the effect of the Earth’s rotation on the orientation change of the Wuhan meteor radar is not very significant. Moreover, the Earth’s revolution model proposed in Lau’s study is based on COBRA radar placed at the poles [19]. In addition, the position of the Mohe meteor radar is at high latitudes. Considering that the changes in the orientation of the meteor radar antenna array at high latitudes will be more obvious at high latitudes, the statistical results of the Mohe meteor radar can support the Earth’s revolution model, which is consistent with Lau’s study. For the variation in azimuth distribution in middle latitudes, more complex conditions need to be considered, and more comprehensive studies and models are needed to provide an explanation.

4. Conclusions

Based on the data from the Mohe and Wuhan meteor radars, this study calculates the variation patterns in the meteor azimuth distribution of the two sites within one year. This study introduces the position variable Max_Azi of the maximum azimuth distribution to quantify the variation patterns in azimuth distribution. Previous studies only used meteor radar at the South pole and found that the variation in Azimuth distribution changes as the Earth rotates. Radar at the pole will rotate as the Earth rotates, but when the latitude decreases, we can imagine that the rotation will not be that obvious. Our studies use meteor radar data from Mohe (high latitude) and Wuhan (middle latitude), which provide complementary information. The results indicated that the variation in the azimuth distribution of the Mohe meteor radar supports the Earth’s revolution model, which is consistent with previous studies. The results of the Wuhan meteor radar do not completely fit the Earth’s revolution model as the curve of Max_Azi variations during the time interval of LT 6-12H has some overlap with the curve of Max_Azi variations during LT 0-6H, which means that the variation in peak azimuth does not align with the rotation of the Earth. Possible reasons are that Wuhan is in the lower latitudes and the effect of the Earth’s rotation on the orientation change in the Wuhan meteor radar is not very significant compared to the pole region. Moreover, the lack of data in Max_Azi variations during time interval of LT 6-12H due to poor fitness may also lead to the overlap of the curves.