Statistical study of large-scale traveling ionospheric disturbances generated by the solar terminator over China

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL. 118, , doi: /jgra.50423, 2013 Statistical study of large-scale traveling ionospheric disturbances generated by the solar terminator over China Qian Song, 1,2,3 Feng Ding, 1,2 Weixing Wan, 1,2 Baiqi Ning, 1,2 Libo Liu, 1,2 Biqiang Zhao, 1,2 Qiang Li, 4 and Rui Zhang 4 Received 27 January 2013; revised 23 June 2013; accepted 24 June 2013; published 22 July [1] This paper presents a statistical result of large-scale traveling ionospheric disturbances (LSTIDs) associated with moving solar terminator in China during 12 months from February 2011 to January The LSTIDs are identified by the two-dimensional total electron content (TEC) perturbation maps, which are built based on the observations of GPS network data from China. The GPS observations are combined with the observations from an ionosonde chain established by the Institute of Geology and Geophysics, Chinese Academy of Sciences. A total of 135 LSTID events are identified at dawn, while there is indiscernible LSTID at dusk. Meanwhile, these LSTIDs are captured by the ionosonde chain which shows that there are perturbations in the virtual heights during the passage of LSTIDs at the height between 200 and 700 km. The occurrence rate of LSTIDs shows a maximum in winter and a minimum in summer. The LSTIDs propagate across China with phase front widths larger than 1500 km. The propagation direction of LSTIDs is northwestward in winter, southwestward in summer, and quasi-westward in equinoxes, respectively. The average period, horizontal phase velocity, and horizontal wavelength of LSTIDs are 79 ± 12 min, 288 ± 43 m/s, and 1503 ± 205 km, respectively. The relative TEC perturbations of the LSTIDs attenuate as the LSTIDs travel across China. Citation: Song, Q., F. Ding, W. Wan, B. Ning, L. Liu, B. Zhao, Q. Li, and R. Zhang (2013), Statistical study of large-scale traveling ionospheric disturbances generated by the solar terminator over China, J. Geophys. Res. Space Physics, 118, , doi: /jgra Introduction [2] Large-scale traveling ionospheric disturbances (LSTIDs) are wave-like perturbations of the ionospheric plasma that are created by the upward propagation of atmospheric gravity waves (AGWs) [Hines, 1960; Francis, 1975; Crowley, 1991; Hocke and Schlegel, 1996; Kirchengast et al., 1996]. Earlier studies have classified LSTIDs as having horizontal wavelengths greater than 1000 km and periods larger than 30 min [Hunsucker, 1982; Hajkowicz, 1990]. [3] The major generation mechanism of LSTIDs is thought to be the energy input from the magnetosphere to the auroral ionosphere, such as Joule heating and particle precipitation during auroral substorms [Crowley and Jones, 1987; Hajkowicz and Hunsucker, 1987; Bowman, 1990; Hajkowicz, 1990, 1991, 1992; Ma et al., 1998; Afraimovich et al., 2000; Afraimovich 1 Key Laboratory of Ionospheric Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 2 Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 3 University of Chinese Academy of Sciences, Institute of Geology and Geophysics, Beijing, China. 4 National Earthquake Infrastructure Service, China Earthquake Administration, Beijing, China. Corresponding author: F. Ding, Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing , China. (dingf@mail.iggcas.ac.cn) American Geophysical Union. All Rights Reserved /13/ /jgra et al., 2006; Afraimovich, 2008; Saito et al., 2001; Shiokawa et al., 2002; Tsugawa et al., 2003; Tsugawa et al., 2004; Tsugawa et al., 2006; Hawlitschka, 2006; Ding et al., 2007; Ding et al., 2008; Ding et al., 2012; Ogawa et al., 2009; Crowley and Rodrigues, 2012; Nicolls et al., 2012; Perevalova et al., 2008]. Other possible excitation mechanisms have also been discussed by some authors. Chimonas [1970] pointed that the equatorial electrojet could launch the low-frequency TIDs through the Lorentz coupling of the electrojet to the neutral atmosphere even though this coupling is much weaker in the equatorial region than that in the auroral region. Müller-Wodarg et al. [1998] modeled the thermospheric and ionospheric responses to the solar eclipse of 11 August 1999, and they found that temperature and electron disturbances could be generated in situ in the thermosphere and ionosphere, respectively, and such disturbances would propagate as a wave from the path of totality into the equatorial regions. Vadas and Liu [2009] showed through modeling that the body force accompanying the dissipation and breaking in the thermosphere of convective gravity waves is an important mechanism for generating large-scale second gravity waves in the thermosphere. They also showed that these second AGWs created total electron content (TEC) perturbation. Later, Vadas and Crowley [2010] detected LSTIDs which were identified as manifestations of secondary AGWs from deep convection. Beer [1973] predicted that the moving solar terminator might serve as a source of gravity waves similar to the solar eclipse. This prediction was confirmed by Raitt and Clark [1973]. 4583

2 [4] Unlike other sources, the solar terminator is a regular, well-defined, and predictable phenomenon. As such, it attracts many researchers attention. Somsikov and Troitskii [1975] theoretically proved that the solar terminator can cause AGWs during its supersonic and subsonic motion. By taking into account the energy exchange between atmospheric gases and solar radiation in the fluid dynamic equations along with the energy and continuity equations, Somsikov [1987, 1992, 1995] mathematically analyzed the regular and irregular disturbances caused by the solar terminator. He suggested that regular waves (such as AGWs) could be generated by several mechanisms such as the moving gradients of atmospheric parameters created by the solar terminator, the gradient-radiation instability, and the gradient-drift instability in the solar terminator region. As these waves propagated within the thermosphere, they could cause the oscillation of the ionosphere such as TIDs. This theoretical conclusion was confirmed by earlier experimental studies [Ivanov and Terekhov, 1983; Chernysheva et al., 1985; Ovezgeldyev et al., 1987; Gokov and Gritchin, 1994; Scotto et al., 1994; Hocke and Igarashi, 2002; Boška et al., 2003; Afraimovich, 2008; Liu et al., 2009; MacDougall et al., 2009; MacDougall and Jayachandran, 2011]. In consideration of the stable and repetitive features of the solar terminator, one can use this natural laboratory to investigate the generation and propagation of the AGWs, as well as the ionospheric manifestation of AGWs, such as LSTIDs. [5] However, little attention has been paid to the statistical study of solar terminator-related LSTIDs in the region of China. Mainland China has latitudes range from 18 to 53 N, extending to lower latitudes than North America, Europe, and Japan. Thus, this particular latitudinal range of China provides us a good opportunity to study the characteristics of LSTIDs at lower latitudes. In this paper, we present the 12 months statistical study of LSTIDs generated by moving solar terminator from February 2011 to January 2012 using the GPS network data in China, combined with the observations of an ionosonde chain established by the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The propagation, attenuation, and generation properties of the solar terminator-related LSTIDs are discussed based on the observation data. Our study will supplement the previous studies of LSTIDs related to the moving solar terminator. 2. Data and Methods 2.1. Data [6] The Crustal Movement Observation Network of China (CMONOC) is a GPS network in China completed in 2010, which consists of more than 200 GPS receivers and provides GPS data with a temporal resolution of 30s. Most of the receivers locate in the middle and east of China. The GPS TEC data used in this paper come from 247 GPS receivers in and around China, among which 188 receivers belong to the CMONOC, and 59 receivers belong to the International GNSS service (IGS) network. The geodetic locations of these GPS receivers are plotted as black dots in Figure 1. Furthermore, we use an ionosonde chain to monitor the propagation of the LSTIDs over China. This chain is established by the IGGCAS and locates from the north to south of Figure 1. Locations of GPS stations (black dots) and ionosondes (blue triangles) in China. China along 120 E meridian, including four ionosondes (Digital Portable Sounder-DPS-4) located at Mohe (52.5 N, E), Beijing (40.4 N, E), Wuhan (30.5 N, E), and Sanya (18.3 N, E), respectively (blue triangles in Figure 1). The ionograms used are spaced at 15 min intervals for Mohe, Beijing, and Wuhan, and at 5 min intervals for Sanya, respectively Processing Methods [7] The GPS receiver generates RINEX files recording carrier phase delays and group delays in two L-band frequencies (f 1 = and f 2 = GHz) every 30 s. The slant TEC (STEC) is calculated from carrier phase delay by equation [Mannucci et al., 1999] STEC ¼ 1 f 2 1 f :3 f 2 1 f 2 2 cφ 1 f 1 cφ 2 f 2 ðλ 1 n 1 λ 2 n 2 ÞþB s r where Φ 1 and Φ 2 are the recorded carrier phases of signal; c is the light speed; λ 1 and λ 2 are wavelengths of signal; n 1 and n 2 are integer cycle ambiguities; and B r s is the instrumental receiver and satellite biases. The integer cycle ambiguities n 1 and n 2 are estimated by smoothed pseudo-range measurement. B r s is the main error source of the ionospheric TEC deriving from GPS observation and could reach up to several units of TEC (TECU, 1 TECU = electrons m 2 ) or even larger than the real STEC [Sardón and Zarraoa, 1997; Zhang et al., 2010]; thus, the effect of this biases must be removed to get the real STEC. Here, we corrected this biases by adjusting the data with those of Global Ionospheric Maps from the Crustal Dynamics Data Information System [Noll, 2012], which are interpolated in both space and time [Ding et al., 2012; Song et al., 2012]. [8] By the above procedure, we can get the time sequence of STEC from the GPS data. However, in this paper, we are interested in the perturbation components of the vertical TEC which is obtained by using the filtering method from the following equation [Ding et al., 2012] DTEC ¼ STEC SF C 1 ðlat lat 0 Þ C 2 ðlat lat 0 Þ 2 C 3 ðlt LT 0 Þ (2) where DTEC is the TEC perturbation time sequence of a GPS receiver-satellite pair; SF is the slant factor; lat and LT are the (1) 4584

3 (a) UT=21:00: (b) UT=21:10: (c) UT=21:20: (d) UT=21:30: (e) UT=21:40: (f) UT=21:50: (g) UT=22:00: (h) UT=22:10: (i) UT=22:20: (j) UT=22:30: (k) UT=22:40: (l) UT=22:50: DTEC (TECU) Figure 2. Two-dimensional maps of the TEC perturbation between 2100 UT and 2250 UT on 22 April The dash curve on each map denotes the location of the sunrise terminator at corresponding time, and the left side of the dash curve indicates the night sector. geographical latitude and local time (LT) of the ionospheric pierce points (the point where the line of sight between the satellite and ground receiver intersects the ionosphere at 300 km, which is the average height of F region peak predicted by the IRI model), respectively; lat 0 and LT 0 are the latitude and LT when the satellite elevation reaches its maximum, respectively; C 1,C 2, and C 3 are the fitting coefficients. We use the STEC data with satellite elevation larger than 30 to compensate for the scarcity of the GPS receiver distribution, and to minimize the errors caused by converting STEC into vertical TEC and the multipath effects. [9] By applying this filtering method in the region of N and E, we can obtain the two-dimensional TEC perturbation maps of this region. In this paper, we divide this region into small pixels with the spatial resolution of (latitude longitude) and temporal resolution of 150 s. The TEC perturbation value at each pixel at the LT is an average of DTEC for all the GPS satellite-receiver paths (with satellite elevation larger than 30 ) whose ionospheric pierce points cross this pixel during the time between LT 60 s and LT + 60 s at the height of 300 km. [10] We check the TEC perturbation maps at every 10 min to see whether there are regularly moving band-like structures representing the existence of LSTIDs. Figure 2 shows an example of two-dimensional TEC perturbation maps over China between 2100 and 2250 UT on 22 April 2011, and the 4585

4 Figure 3. (a) A copy plot of Figure 2e. The black line is the least squares fitted line of the negative phase front of the TEC perturbation. The propagation direction is perpendicular to the phase front of the TEC perturbation. (b) Temporal variations of the locations of the phase fronts of the TEC perturbation as shown in Figure 2 along the propagation direction from the reference point (35 N, 120 E). dash curve on each map denotes the location of solar terminator at dawn. As illustrated in Figure 2, clear wave-like structures are seen. A negative TEC perturbation front is located in the east of China at 2100 UT, while a positive TEC perturbation front is located simultaneously in the west of the negative region (Figure 2a). These two TEC perturbation fronts are defined as the negative and positive phase fronts, respectively. Analysis of the sequence of maps in Figure 2 shows the phase front of the TEC perturbation approximately moves across China. Figure 2 reveals that the maximum amplitude of the TEC perturbation is about 1 TECU. The phase fronts of the TEC perturbation are nearly aligned along the direction of the solar terminator and have a latitudinal range of more than 15. These are in agreement with the results of Afraimovich [2008], who first showed the GPS-TEC evidence for the large-scale wave structure caused by solar terminator with amplitude of about TECU and a phase front exceeding 1600 km. [11] The horizontal phase velocity of TEC perturbation can be derived by tracking the locations of the phase fronts of TEC perturbation from the reference point (35 N, 120 E) along the propagation direction with time. The propagation direction of an AGW is perpendicular to its phase front [Hines, 1960]. Therefore, we define the propagation direction of the LSTID to be perpendicular to the phase front of the TEC perturbation as shown in Figure 3a, and the propagation azimuth of the TEC perturbation is measured clockwise from north. The black line in Figure 3a is the least squares fitted line of the negative phase front of TEC perturbation. Figure 3b shows the temporal variations of the locations of the negative (triangles) and positive (asterisks) phase fronts of the TEC perturbation illustrated in Figure 2 from the reference point along the propagation direction. The horizontal phase velocity V ph is the average slope of the least squares fitted lines of the locations of the negative and positive phase fronts. The period T is twice average of the time lag between the negative and positive phase fronts. The horizontal wavelength of the TEC perturbations is obtained from λ h =T V ph. For the case of the TEC perturbation in Figure 3b, the propagation azimuth, horizontal phase velocity, period, and wavelength are 258, 365 m/s, 81 min, and 1774 km, respectively. Judging from these parameters, this TEC perturbation structure is identified as LSTID [Hunsucker, 1982; Hocke and Schlegel, 1996]. [12] It should be noted that some of the phase fronts curved during the propagation of TEC perturbation, such as the negative phase front shown in Figures 2g 2i. In these cases, we determine the propagation azimuth of the TEC perturbation every 10 min from the two-dimensional TEC perturbation maps and then define the average of these azimuths to be the propagation direction of the TEC perturbation. [13] The LSTID event illustrated in Figure 2 is detected simultaneously by the ionosonde chain in China. Figures 4a 4d illustrates the temporal variations of the virtual height at different detection frequencies (isofrequency lines), according to the F-trace in the ionograms registered by ionosondes located at Mohe, Beijing, Wuhan, and Sanya, respectively. The frequencies range from 2 to 7.5 MHz, with a step of 0.5 MHz. The virtual height [Helliwell, 1949] starts increasing gradually after sunrise at Mohe (Figure 4a) and peaks at around 0400 UT. The earlier appearances of the virtual height peaks at higher frequencies (see the red dash line) imply a downward vertical phase velocity, which is a typical feature of upward-propagating AGWs observed by ionosondes in many previous studies [Hajkowicz and Hunsucker, 1987; Hajkowicz, 1990; Liu et al., 1998; Becker-Guedes et al., 2007; Dashora et al., 2009; Klausner et al., 2009; Bowman and Mortimer, 2010]. Similar variations of virtual heights are observed by ionosondes located at Beijing, Wuhan, and Sanya (Figures 4b 4d), respectively. 3. Results and Discussion 3.1. Occurrence Rate of LSTIDs [14] We require two criteria in order to identify an event as a LSTID event. (1) The amplitude of TEC perturbation should exceed 0.5 TECU. This criterion is based on previous research. Namely, Tsugawa et al. [2006] defined LSTIDs as TEC enhancements larger than 0.5 TECU, and Ding et al. [2008] found the maximum amplitude of TEC perturbations 4586

5 Figure 4. Temporal variations in virtual height, at detection frequencies ranging from 2 to 7.5 MHz, and with a step of 0.5 MHz. The variations of virtual heights were read from ionograms observed by the ionosonde chain in China on 22 April Frequencies are shown on each curve. The frequencies and virtual heights are read from the F-layer trace in the ionograms recorded by ionosondes at Mohe, Beijing, Wuhan, and Sanya. The time resolution is 5 min for Sanya and 15 min for the rest. The black vertical dash lines in each plot mark the sunrise, and the red dash lines connect the peaks of variation at different frequencies. caused by LSTIDs were mostly ~1.1 TECU. (2) The propagation time duration of the phase front of the TEC perturbation should be larger than 20 min. The second criterion is set to ensure that we observe a moving wavefield, not a transient TEC increment or decrement area. Only if both of these criteria are satisfied do we identify an event as a LSTID event. Based on these two criteria 135 solar terminator-related LSTIDs are observed by the GPS network in China from February 2011 to January According to the sequences of the two-dimensional TEC perturbation maps of these 135 LSTIDs, the time duration of the LSTIDs varies from 1 to 3.3 h. To investigate seasonal variations of the occurrence rate of LSTIDs, we divide this observational interval into three segments: winter (February, November, and December in 2011, and January in 2012), summer (May August), and equinoxes (the rest months of the interval). [15] The seasonal dependency of the occurrence rate of LSTIDs is shown in Figure 5. The occurrence rate is defined as the ratio of the sum of LSTIDs durations to the sum of the observation time within each month. The occurrence rate shows a strong seasonal dependency and varies between 21.2% in January and 0.2% in June. As can be seen from the histogram in Figure 5, the LSTIDs generated by the solar terminator occur much more frequently in winter than in summer, and the occurrence rate of LSTIDs in equinoxes is between those in winter and summer. This seasonal dependency of TIDs activity has been noted by some previous studies. Using the data of the VHF radio beacons of the year 1993, Jacobson et al. [1995] had found a major occurrence rate peak in winter for TIDs in North America. Similarly, Ding et al. [2011] had shown a maximum occurrence rate of TIDs in winter in the center of China on the basis of GPS data during 15 months from January 2009 to March The seasonal dependency of the occurrence of TIDs in these two previous studies may be related with the temperature gradient filtering on AGWs in the mesosphere region, since the TIDs they detected had periods less than 30 min and horizontal phase velocity less than 200 m/s. However, the TIDs we detect in this paper have periods larger than 50 min, and most of these TIDs have the horizontal phase velocity larger than 250 m/s. According to Hines [1960], the mesosphere can only support the propagation of AGWs with group velocities less than the sound speed in the mesosphere. Note that for most AGWs, the group velocity is approximately equal to its horizontal phase velocity. Furthermore, only the AGWs with periods close to the buoyancy frequency (less than min) are likely to be reflected if the buoyancy frequency decreases from the changing temperature gradient. Thus, the seasonal variation of the occurrence of LSTIDs is not likely caused by the temperature gradient filtering effect 4587

6 summer winter equinoxes 16 Occurrence Rate (%) Month Figure 5. Seasonal variations of the solar terminator-related LSTIDs occurrence rate in China from February 2011 to January The occurrence rate is defined as the ratio of the sum of the duration of LSTIDs to monthly observational intervals. in mesosphere. Further investigation is needed to explain the seasonal variation of the occurrence of the solar terminatorrelated LSTIDs. [16] It is worth noting that no similar LSTIDs structures are detectable after sunset in this study. Similar results have been reported in earlier studies. For example, Raitt and Clark [1973] analyzed the electron temperature data from ESRO-1A; they found that the amplitudes of electron temperature oscillations were much larger after sunrise versus sunset. Based on the spectral and cross-correlation analysis of the ionospheric density observations of the Millstone Hill incoherent scatter radar, Galushko et al. [1998] found that the AGWs/TIDs were more pronounced after sunrise. Using ionosondes of Pruhonice (49.9 N, 14.5 E) and Ebro (40.8 N, 0.5 E), Boška et al. [2003] evaluated the short-term variability of the electron density of the F region, and observed higher occurrence rate of AGW oscillations during sunrise than that during sunset. Afraimovich [2008] used the TEC measurements from the global network of GPS receivers to investigate the wave structure excited by the solar terminator; they found that the solar terminator-generated wave packets are more pronounced after sunrise. MacDougall and Jayachandran [2011] analyzed the f 0 F 2 measured from standard ionograms recorded by an ionosonde near London (43.0 N, 81.2 W); they found that the TIDs were more frequently around sunrise. These previous observations indicate that the sunrise solar terminator is more effective in generating TIDs in the ionosphere than the sunset solar terminator. According to the work of Somsikov [2011], the characteristic width of the solar terminator of sunrise region is much smaller than that of the sunset region, due to the powerful processes of photoionization and atmospheric heating proceed during sunrise. This makes the sunrise solar terminator generate the AGWs more efficiently Horizontal Phase Velocities, Periods, and Propagation Azimuths [17] Figure 6 shows the horizontal phase velocities (V ph )of the 135 solar terminator-related LSTIDs from February 2011 to January 2012 with respect to (a) period (T), (b) horizontal wavelengths (λ h ), and (c) azimuths. Figure 6(a) illustrates that the phase velocities of LSTIDs are distributed between 209 and 390 m/s. The average velocities of LSTIDs in winter, summer, and equinoxes are 293 ± 44, 292 ± 33, and 281 ± 42 m/s, respectively. The periods of the LSTIDs are long and distributed between 58 and 116 min. The average periods of LSTIDs in winter, summer, and equinoxes are 88 ± 13, 84 ± 11, and 94 ± 15 min, respectively. This is in reasonable agreement with the study of Galushko et al. [1998], while the period in this paper is longer than that obtained by Afraimovich [2008]. The difference may be because Afraimovich limited the time window in the range of 2 90 min during the filter process. [18] It can be seen from Figure 6b, the LSTIDs with longer horizontal wavelengths tend to propagate faster. The horizontal wavelengths of the LSTIDs in winter, summer, and equinoxes are 1458 ± 204, 1467 ± 197, and 1563 ± 299 km, respectively. The average horizontal wavelength is 1503 ± 205 km. Forbes et al. [2008] discovered a solar terminator wave with a horizontal wavelength of order 3000 km in neutral thermosphere densities by using the data of CHAMP satellite. Liu et al. [2009] revealed that the solar terminator waves also existed in thermospheric wind with the wavelengths ranging between 3000 and 5000 km. In contrast with these previous researches, the wavelengths of LSTIDs in this study are smaller. This difference may be due to the fact that the previous studies were mainly focused on the neutral terminator waves prominent in the postdusk sector at an altitude of about 400 km. Furthermore, Somsikov and 4588

7 (a) (b) (c) V ph (m/s) T (min) λ h (km) Azimuth ( ) Figure 6. The horizontal phase velocities (V ph ) of the 135 solar terminator-related LSTIDs from February 2011 to January 2012 with respect to (a) periods (T), (b) horizontal wavelengths (λ h ), and (c) azimuths. The solid lines in these three panels represent the mean wavelength (1503 ± 205 km), the mean period (79 ± 12 min), and the westward direction (270 ), respectively. Ganguly [1995] suggested that the plasma instability processes might play a part in producing the terminator wave-like structures in electron densities. Therefore, the plasma and neutral terminator waves may be different in some respects. [19] From Figure 6c, we see that there is no significant difference among the horizontal phase velocities for the LSTIDs with different propagation directions. However, it is obvious that most of the propagation azimuths of the LSTIDs are scattered between 260 and 300, while only a few LSTIDs propagate with azimuths less than 250. Following, we discuss more details of the propagation azimuths of the LSTIDs. [20] Figure 7 shows the seasonal behavior of the propagation azimuth for solar terminator-related LSTIDs. Polar plots are shown of the number of the LSTIDs as a function of azimuth in winter (a), summer (b), and equinoxes (c). There are obvious differences in the preference directions of the LSTIDs in these three seasons. Most of the LSTIDs in winter propagate northwestward with the average azimuth of 286 ± 10 (Figure 7a), whereas in summer, the LSTIDs prefer to propagate southwestward with the average azimuth of 235 ± 11 (Figure 7b). In equinoxes, the quasi-westward (average of 269 ± 7 ) propagating LSTIDs are frequently observed. The seasonal variations of the propagation directions of the LSTIDs are likely caused by the seasonal variations of the geographic locations of the solar terminator at dawn. In China, the solar terminator at dawn lies in the direction of northeast-southwestward (291.2 ± 5 ) in winter, northwest-southeastward (239 ± 7 ) in summer, and quasinorth-southward (273.5 ± 4 ) in equinoxes, respectively Wave Attenuation [21] By tracking the relative amplitude of the LSTIDs along the propagation direction from the reference point (35 N, 120 E), we can investigate the dissipation of the LSTIDs during their propagation. The relative amplitude of the LSTIDs is defined as the ratio of the perturbation component of TEC (DTEC) to the background TEC (TEC 0 ). Figure 8 is an example of the spatial variations of the relative amplitude (DTEC/TEC 0 ) of the LSTID illustrated in Figure 2 along the propagation direction from the reference point (35 N, 120 E). The black line in Figure 8 is the least squares fitted line. The DTEC is averaged along the phase front under the consideration that the TEC perturbation caused by LSTIDs along the phase front is isotropic. From Figure 8, we can see that the relative amplitude of LSTID tends to decrease as the LSTID propagates Figure 7. Occurrence histogram of propagation azimuths of the solar terminator-related LSTIDs in (a) winter, (b) summer, and (c) equinoxes. The azimuths are measured clockwise from north. 4589

8 Horizontal distances (km) DTEC / TEC 0 (%) Figure 8. Spatial variations of the relative amplitude of the LSTID illustrated in Figure 2 along the propagation direction from the reference point (35 N, 120 E). The black line in the plot is the least squares fitted line. away from the reference point. The statistical result shows there are no distinctive differences in wave attenuation of LSTIDs in these three seasons. [22] Attenuation of AGWs in the thermosphere is caused by many factors. Richmond [1978] showed that large-scale AGWs attenuate as they propagate away from a source. Vadas [2007] derived an anelastic AGW dispersion relation which includes the damping effect of molecular viscosity and thermal diffusivity. They investigated this dispersion relation for a wide range of upward propagating AGWs originated at several different altitudes in the thermosphere for four different temperature profiles from extreme solar minimum to very active solar conditions. They found that the attenuation of AGWs caused by molecular viscosity and thermal diffusivity depends on many factors, such as horizontal wavelength, vertical wavelength, horizontal phase speed, and intrinsic frequency. They calculated the horizontal propagation distances for the AGWs and found that upward propagating AGWs generated in the thermosphere will propagate to distance of about a wave cycle or two before attenuation. Our results in this paper confirm their conclusion, since all of the LSTIDs observed in this paper propagate to the distance less than two horizontal wavelengths before dissipating. [23] It is generally known that ions are constrained to move along the geomagnetic field line and have difficulty in crossing it. Thus, the AGW-induced ionic velocity is along the geomagnetic field line. This difference between the mobility of neutral particles and that of ions restricts the motion of neutral particles, which is called ion drag. The result of Liu and Yeh [1969] shows that, the damping of AGWs caused by ion drag is not only dependent on the inclination of the geomagnetic field line but also dependent on the declination of geomagnetic line. Their theory predicts that the ion drag effect will not contribute significantly the attenuation of AGWs for those waves that move along the geomagnetic field line and vice versa. Considering the small declination of the geomagnetic field in China, the AGWs induced by solar terminator almost move perpendicularly to the geomagnetic field line indicating that ion drag effect also contributes to the wave attenuation. Furthermore, according to the discussion of Vadas and Liu [2009], the amplitude of an AGW decreases from geometric attenuation as it propagates away from a source Generation Mechanism [24] Some previous studies have pointed that the gravity wave-like oscillation in the ionosphere during the solar terminator or solar eclipses mainly generated in the altitude range from 150 to 250 km, which is the transition region between F1 and F2 layers [Liu et al., 1998; Altadill et al., 2001; Šauli and Boška, 2001; Boška et al., 2003; Laštovička, 2006; Šauli et al., 2006; Klausner et al., 2009]. As stated by Šauli et al. [2006], in the transition region between F1 and F2 layers, there exist sharp gradients in electron and ion concentrations due to the different response of F1 and F2 regions to the radiation flux changes induced by the solar terminators and solar eclipses. Somsikov [1991, 2011] have confirmed that the narrow region of sharp gradients of temperature, pressure, and density between two states of atmosphere corresponding to the coolings and heatings processes linked with the solar terminators can be the source of AGWs. The scales of AGWs excited by heatings and coolings depend on the scales of the heatings and coolings regions, as well as the time duration of the heatings and coolings processes [Vadas and Fritts, 2001; Vadas, 2013]. Therefore, the transition region between F1 and F2 layers would likely act as a AGWs generator. [25] Furthermore, some other researchers proposed that some of the AGWs might have their origins in the lower atmosphere due to the direct solar heating [Chimonas, 1970; Fritts and Luo, 1993; Müller-Wodarg et al., 1998; Rishbeth, 2006]. There are two reasonably distinct heating regions in the lower atmosphere caused by the solar terminators or solar eclipses. One is the region around 45 km altitude linked with the UV heating of ozone, the other is the region of about 90 km altitude related with the molecular oxygen heating [Chimonas, 1970]. Fritts and Luo [1993] suggested that the disturbances result from the ozone heating/cooling during the solar terminators or solar eclipses can propagate upward reaching the thermosphere and ionosphere, and cause the disturbance in these regions in the forms of AGWs and TIDs, respectively. [26] The isofrequency lines recorded by the ionosonde chain in China reveal that the solar terminator-related AGWs observed in this paper have downward propagating phase velocities in the altitude range from about 200 to 700 km, indicating the AGWs (or energy of the AGWs) propagate upward [Hines, 1960]. Hence, the AGWs would be originated below the height of about 200 km. Since an AGW cannot propagate in an atmosphere with a group velocity larger than the sound speed [Hines, 1960], and because an AGW s group velocity is approximately equal to its horizontal phase velocity, we can estimate the approximate height range the AGWs are originated from by comparing the horizontal phase velocities of the AGWs with the sound 4590

9 Height (km) Equinoxes Summer Winter C s (m/s) Figure 9. Sound speed profiles in winter (green line), summer (red line), and equinoxes (black line) of Beijing (40.4 N, E), at 2200 UT. speed profile. Figure 9 is a plot of the sound speed (c s ) profiles from MSIS model over Beijing (40.4 N, E) at 2200 UT in winter (green), summer (red), and equinoxes (black), respectively. The profiles show that the c s is less than 330 m/s between the height of 45 and 110 km, thus only the AGWs with horizontal phase velocities less than about 300 m/s have the possibility to be originated at the lower atmosphere, while the others with horizontal phase velocities larger than 300 m/s may be originated at the height between 110 and 200 km. 4. Summary [27] Solar terminator-related LSTIDs are statistically studied using the GPS data covering 12 months from February 2011 to January 2012 from 247 GPS receivers of CMONOC and IGS. The GPS observations are combined with data from the ionosonde chain including four DPS-4 ionosondes located at Mohe (52.5 N, E), Beijing (40.4 N, E), Wuhan (30.5 N, E), and Sanya (18.3 N, E). One hundred and thirty-five LSTIDs are detected after sunrise using time sequences of twodimensional TEC perturbation maps. The seasonal behaviors, propagation, attenuation, and the possible source altitudes of these LSTIDs are analyzed, and the major findings are summarized as follows yielding the following results. [28] The occurrence rate of LSTIDs shows apparently seasonal dependency; it is much smaller in summer comparing with those in winter and equinoxes. The propagation direction of the LSTIDs also shows seasonal differences. The seasonal average propagation directions of the LSTIDs in winter, summer, and equinoxes are 286 ± 10, 235 ± 11, and 269 ± 7, respectively. This seasonal variation of the propagation direction is very likely due to the different seasonal geographic locations of the solar terminator at dawn. The solar terminator at dawn lies in the direction of northeast-southwestward (291.2 ± 5 ) in winter, northwestsoutheastward (239 ± 7 ) in summer, and quasi-north-southward (273.5 ± 4 ) in equinoxes, respectively. There are no significant seasonal differences among the average propagation periods, horizontal phase velocities, and horizontal wavelengths. The mean period, horizontal phase velocity, and horizontal wavelength of all LSTIDs are 79 ± 12 min, 288 ± 43 m/s, and 1503 ± 205 km, respectively. The relative TEC perturbations of LSTIDs tend to decrease during LSTID propagation. There are many factors which can cause AGWs to dissipate, including molecular viscosity, thermal diffusivity, and ion drag. Additionally, geometric attenuation causes the amplitude of an AGW to decrease with distance and time. 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