Ionospheric F 2 region perturbed by the 25 April 2015 Nepal earthquake

Transcription

1 PUBLICATIONS Journal of Geophysical Research: Space Physics RESEARCH ARTICLE Key Points: Coseismic ionospheric disturbance observed by FORMOSAT-3/COSMIC Near-supersonic uplifting of the ionospheric F 2 peak caused by the Nepal earthquake Vertical scale of the coseismic disturbance is near 150 km in the F 2 region Correspondence to: Y.-Y. Sun, yysun0715@gmail.com Citation: Sun, Y.-Y., J.-Y. Liu, C.-Y. Lin, H.-F. Tsai, L. C. Chang, C.-Y. Chen, and C.-H. Chen (2016), Ionospheric F 2 region perturbed by the 25 April 2015 Nepal earthquake, J. Geophys. Res. Space Physics, 121, , doi:. Received 17 DEC 2015 Accepted 3 JUN 2016 Accepted article online 6 JUN 2016 Published online 23 JUN American Geophysical Union. All Rights Reserved. Ionospheric F 2 region perturbed by the 25 April 2015 Nepal earthquake Yang-Yi Sun 1, Jann-Yenq Liu 1,2, Chi-Yen Lin 1, Ho-Fang Tsai 3, Loren C. Chang 1, Chao-Yen Chen 1, and Chia-Hung Chen 3 1 Institute of Space Science, National Central University, Taoyuan, Taiwan, 2 Center for Space and Remote Sensing Research, National Central University, Taoyuan, Taiwan, 3 Department of Earth Science, National Cheng Kung University, Tainan, Taiwan Abstract Seismic waves can be detected in the Earth s atmosphere and ionosphere; however, their impacts on ionospheric electron density (Ne) structures near the altitude of peak Ne (h m F 2 ) have not yet been fully determined due to the lack of sufficient observations sampled in the vertical direction. Here we apply a ground-based Global Positioning System (GPS) receiving network in Asia as well as the space-based GPS occultation experiment on board the FORMOSAT-3/COSMIC (F3/C) satellite to vertically scan the ionospheric Ne structures, which were perturbed by the magnitude M w 7.8 Nepal earthquake that occurred on 25 April The F3/C altitudinal Ne profiles show that the Nepal earthquake-induced air perturbations penetrate into the ionosphere at supersonic speeds of approximately 800 m/s and change the Ne structure by 10% near h m F 2. The vertical scale of the Ne perturbation is 150 km, while the h m F 2 is uplifted by more than 30 km within 1 min. Those results reveal that the earthquake-induced ground displacement should be considered as a significant force that perturbs the vertical Ne structure of the ionosphere. 1. Introduction Sudden vertical motions of the Earth s surface, such as seismic and tsunami waves, of strong earthquakes are capable of exciting seismotraveling atmospheric disturbances, which propagate through the Earth s neutral atmosphere (from the ground to tens of kilometers in altitude) and reach the ionosphere (from roughly 60 to several thousand kilometers) resulting in electron density (Ne) fluctuations within it, which are often termed seismotraveling ionospheric disturbances (STIDs) [e.g., Blanc, 1985; Ducic et al., 2003; Artru et al., 2004; Liu et al., 2006, 2011, 2012; Liu and Sun, 2011; Tsai et al., 2011; Chen et al., 2011; Rolland et al., 2011; Saito et al., 2011; Astafyeva et al., 2011; Komjathy et al., 2012; Maruyama et al., 2011, 2012; Maruyama and Shinagawa, 2014; Garcia et al., 2013; Reddy and Seemala, 2015]. These STIDs with velocities ranging from few hundreds to thousand meters per second have been mainly attributed to the acoustic waves, the gravity waves, and their mixtures induced by the Rayleigh waves [Ducic et al., 2003; Mutschlecner and Whitaker, 2005] and tsunami [Liu et al., 2006, 2011]. The STIDs traveling along Earth s surface (horizontal direction) have previously been studied using total electron content (TEC, 1 TEC Unit (TECU) = el m 2 )[Liu et al., 1996] observed by ground-based Global Positioning System (GPS) receiving networks [e.g., Calais and Minster, 1995; Liu et al., 2011, 2012; Tsai et al., 2011; Chen et al., 2011; Rolland et al., 2011; Saito et al., 2011; Astafyeva et al., 2011; Komjathy et al., 2012; Reddy and Seemala, 2015]. However, TEC is the integration of Ne from the ground to the GPS satellite altitude (~20,200 km), which cannot provide vertical structures of STIDs. On the other hand, ionosondes have been utilized to observe earthquake-perturbed vertical Ne structures in the lower part of the ionosphere (below the F 2 peak) [Liu and Sun, 2011; Maruyama et al., 2011; Maruyama et al., 2012; Maruyama and Shinagawa, 2014]. Liu and Sun [2011] reported ionogram signatures of STIDs triggered by the 11 March 2011 M w 9.0 Tohoku-Oki earthquake and showed that the speeds of STIDs induced by Rayleigh waves, acoustic gravity waves mainly traveling in the ionosphere, and tsunami waves are m/s, 900 m/s, and 200 m/s, respectively. Maruyama et al. [2011, 2012] and Maruyama and Shinagawa [2014] examined ionograms near the epicenters of various earthquakes and found that the vertical structures of the earthquake-induced Ne disturbances have scales of few tens of kilometers in the lower part of the ionosphere. Several studies have reported the earthquake-induced atmospheric and ionospheric perturbations detected by satellites. Garcia et al. [2013] presented the first in situ sounding of a postseismic infrasound wavefront induced by the great Tohoku-Oki earthquake using data from the Gravity field and steady state Ocean Circulation Explorer mission at 270 km altitude. Yang et al. [2014] showed the atmospheric density perturbations SUN ET AL. IONOSPHERE PERTURBED BY NEPAL EARTHQUAKE 5778

2 Figure 1. Ground-based GPS TEC perturbed by the 25 April 2015 Nepal earthquake. (a) Locations of ground-based GPS receivers (triangles) and ionospheric F3/C RO profiles (curves with asterisk). Asterisk denotes the location of the h m F 2. Circles with s1 4 show the seismic traveling ionospheric disturbances (STIDs) recorded by the ground-based GPS receivers. Pentacle indicates the epicenter. (b i) The absolute vertical TEC and the corresponding filtered TEC. A high-pass filter was applied to remove signals with periods longer than 10 min. The vertical black line indicates the initial earthquake rupture. The vertical gray lines with circle denote the beginnings of the STIDs. recorded by Gravity Recovery and Climate Experiment spacecraft near 450 km altitude. Coïsson et al. [2015] analyzed the FORMOSAT-3/COSMIC (F3/C) Ne profiles and for the first time observed the vertical structure of the gravity wave excited by the tsunami generated by the Tohoku-Oki earthquake. The ionospheric Ne profiles measured by the F3/C radio occultation (RO) sounding technique have led to significant improvement of our understanding of global and three-dimensional (3-D) electron density structures of the ionosphere [e.g., Lin et al., 2007, 2009]. The COSMIC Data Analysis Archive Center (CDAAC) processes the F3/C altitudinal Ne profiles from the calibrated TEC through the Abel inversion technique under the assumption of local spherical symmetry around the ray tangent point [cf. Hajj and Romans, 1998; Lei et al., 2007]. The calibrated TEC stands for the path TEC derived from the occulting GPS phase observations below the orbits of the low Earth orbit (LEO) satellite. The Ne profiles retrieved from the Abel inversion generally could yield large errors below about 200 km, and however, those near and above the altitude of peak Ne (h m F 2 ) are reliable [Yue et al., 2010; Liu et al., 2010]. A magnitude M w 7.8 earthquake struck Nepal on 25 April 2015 at 06:11:26 UT. Reddy and Seemala [2015] analyzed ground-based GPS TEC observations in Asia and found the ionospheric response due to the shock acoustic (average velocity ~ 1180 m/s) and Rayleigh waves (average velocity ~ 2400 m/s) induced by the Nepal earthquake. Note that the ground-based GPS TEC observations simply detect waves propagating in the horizontal direction. In this paper, we combine the Ne profiles of F3/C RO sounding and the colocated TEC of ground-based GPS receivers to three-dimensionally study STIDs of the electron density induced by the Nepal earthquake. 2. Results Figure 1a shows four GPS stations at: Lhasa, China (LHAZ); Hyderabad, India (HYDE); Bangalore, India (IISC); and Port Blair, India (PBRI), recorded seismic traveling ionospheric disturbances (STIDs) in the vicinity and the south of the epicenter of the Nepal earthquake (28.15 N, E). The amplitudes of these STIDs are less than 1 TECU (Figures 1f 1i) nearly 1% of the background TEC (Figures 1b 1e). The height assumed for the ionospheric sounding point of GPS TEC is 350 km. Figure 2 illustrates that the STIDs take 9 min to reach SUN ET AL. IONOSPHERE PERTURBED BY NEPAL EARTHQUAKE 5779

3 Figure 2. Time travel diagram of the STIDs. The color of the dots represents the intensity of the STIDs. The gray and red lines denote the linear regression of the beginnings and maxima, respectively, of the STIDs. The linear regression lines show that the mean velocity of the earthquake-triggered Ne disturbances propagating in the horizontal direction is near 2.4 km/s. The regression line of the beginning of the STIDs intersects distance = 0 km at 06:20:30 UT. It reveals that the STIDs take 9 min to reach ionosphere altitudes. The vertical line indicates the initial earthquake rupture. Circles and asterisks indicate the locations of the STIDs (s1 4) and the peak density of three F3/C profiles (p1 3), respectively. The curves overlapping with the asterisks are the tangent points of the three profiles from 350 to 500 km altitude. ionosphere altitudes and propagate away from the epicenter for 2000 km with a speed of 2.4 km/s. The results agree with previous studies using ground-based GPS receiving networks [Liu et al., 2011, 2012; Tsai et al., 2011; Chen et al., 2011; Rolland et al., 2011; Saito et al., 2011; Astafyeva et al., 2011, 2013; Komjathy et al., 2012; Reddy and Seemala, 2015]. The vertical variations associated with such disturbances cannot be resolved from these observations, since TEC is an integral value. RO is one of the techniques that can resolve the vertical ionospheric Ne structures. On the day of the 2015 Nepal earthquake, three F3/C RO events nearly simultaneously (1 min difference) scanned the vertical structures of the northern equatorial ionization anomaly (EIA) crest, where TEC values exceed 80 TECU (Figure 3a), approximately 16 min after the initial earthquake rupture. The location of the tangent points of profile 1 (p1) from 350 to 500 km altitude intersects with the regression line of the STID maxima. Figure 3. Vertical Ne structure of the ionosphere perturbed by the Nepal earthquake. (a) Ray paths (straight lines) between the F3/C LEO satellite (orbit altitude: ~800 km) and the GPS satellites G01, G11, and G04 (orbit altitude: ~20,200 km) slice through the northern equatorial ionization anomaly (EIA) crest. The bright white area corresponds to the EIA, which is obtained from Center for Orbit Determination in Europe s (CODE) global ionospheric map on 25 April 2015 at 0600 UT. The three curves indicate the location (tangent point) where the Ne profiles are sampled for comparison. (b) The ionospheric Ne profiles observed by F3/C. Here the F3/C RO technique scans the ionospheric Ne structures from the top to the bottom. The red, black, and gray curves are profiles 1, 2, and 3, respectively. Horizontal lines indicate the h m F 2 s. (c) These F3/C Ne profiles are collected within the region of N, E (gray rectangle shown in Figure 1a) during the period of UT from 26 March to 25 May The blue error bar indicates the standard deviation of the h m F 2 s. SUN ET AL. IONOSPHERE PERTURBED BY NEPAL EARTHQUAKE 5780

4 Figure 4. Sound speed profile calculated from the NRLMSISE-00 simulations at the location of p1 on the earthquake day. The black circle and dot indicate the mean sound speeds averaged from km and km, respectively. The red circle and dot denote the mean velocities of the seismotraveling atmospheric disturbances those range from 648 m/s to 926 m/s. By contrast, there is no intersection between the regression line and the tangent point locations of profiles 2 (p2) and 3 (p3) from 350 to 500 km altitude (Figure 2). The intersection indicates that the Ne structure of the F 2 region recorded by p1 is most likely perturbed by the Nepal earthquake. Figure 3b illustrates that p2 and p3 are nearly identical; however, p1 shows an Ne perturbation with the vertical scale near 150 km between 350 and 500 km altitude. To show that the anomalous Ne perturbation on p1 is not due to the presence of the EIA, the background variation in the F3/C Ne profiles is examined. Figure 3c displays the Ne profiles collected within the region of N, E (see Figure 1a), between 0530 and 0730 UT from 26 March to 25 May 2015 (1 month before and after the earthquake). p1 is the only profile that shows the significant vertical Ne structure, and its h m F 2 yields the highest value among all the h m F 2 s. The mean value of the h m F 2 s observed during that period is km, and the corresponding standard deviation is 19.3 km. The h m F 2 of the disturbed Ne profile (p1) (~440 km) exceeds one standard deviation away from the mean value ( = km). These results suggest that the perturbation of p1 is unlikely to result from the regional and usual dynamics of the ionosphere in this active equatorial ionosphere region. Accordingly, the mean velocity of the earthquake-induced STIDs propagating in the vertical direction ranges from 648 m/s (350 km/9 min) to 926 m/s (500 km/9 min), which is faster than the mean sound speed (~600 m/s), i.e., supersonic, in the Earth s atmosphere below 500 km altitude (Figure 4). The sound speed in p the ideal gas (C s ) shown in Figure 4 is defined as C s ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffi γrt=m, in which γ (7/5) is the adiabatic index; R ( J mol 1 K 1 ) is the molar gas constant; the molecular weight (M), and the absolute temperature (T) are obtained from the Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar model (NRLMSISE-00) [Picone et al., 2002]. The integral values of p1, p2, and p3 from 200 to 700 km are 51.7, 51.6, and 51.9 TECU, respectively. The amplitude of the vertical Ne perturbation is cm 3 nearly 10% of the two nondisturbed profiles (Figure 3b), which is much greater than the perturbation in TEC from both F3/C and ground-based GPS receivers. Moreover, h m F 2 was uplifted by 34.4 km (difference between p1 and p2, Figure 3b) within 1 min (time difference between p1 and s2, Figure 2). The uplifting velocity of the peak Ne layer ranges from 573 (34.4 km/1 min) to 1147 (34.4 km/0.5 min) m/s, which may exceed the sound speed at h m F 2 (~800 m/s) (Figure 4). Note that the RO sounding event p1 takes near 1 min to scan the Ne perturbation between 350 and 500 km. Due to the rapid change of the profile, the observed h m F 2 and peak Ne (N m F 2 ) might not be the maximum ones in the real condition. It is possible that the disturbed h m F 2 and N m F 2 can be even larger. Maruyama et al. [2012] showed that the vertical scale of the Tohoku-Oki earthquake-induced Ne disturbances is km in the lower part of the ionosphere. It is surprising that the F3/C Ne profile shows a large vertical scale (~150 km) near the peak Ne altitude (Figure 3b). Due to the fact that the atmospheric density decreases SUN ET AL. IONOSPHERE PERTURBED BY NEPAL EARTHQUAKE 5781

5 Figure 5. Simulation of the amplification of air perturbation. (a) NRLMSIS-00 simulation of total mass density of neutral air, (b) wave amplification factor, and (c) wave amplitude along altitude over the epicenter. Note that no attenuation effects are taken into account in this amplification model. almost exponentially with altitude (Figure 5a), the amplification factor (f) of the oscillation amplitude (A) is given as f =(ρ G /ρ I ) 1/2 [Liu et al., 2012] (Figure 5b), where ρ stands for the mass density of neutral air [Picone et al., 2002]. The sudden vertical displacement on the ground (A G ) triggered by the initial earthquake rupture was about 0.6 m at Kathmandu, Nepal (obtained from Center for Engineering Strong Motion Data, The wave amplitude in ionosphere A I (=fa G ) is about 40 km at 200 km altitude and is longer than 150 km above 300 km altitude (Figure 5c). The simulated wave amplitudes and the observed vertical scales have the same order of magnitude. 3. Discussion and Conclusion With regard to the earthquake-induced ionospheric Ne disturbances in the vertical direction, the F3/C observed 34 km uplifting of h m F 2 is significant. Several studies have employed theoretical models to simulate the perturbation in thermospheric neutral density after the initial rupture of the Tohoku-Oki earthquake [Yang et al., 2014; Coïsson et al., 2015]. Yang et al. [2014] showed that the vertical scale of the neutral density perturbation is on the order of tens of kilometers at ionospheric altitudes. Coïsson et al. [2015] showed the maximum displacement of the air parcels is 25 km near 200 km altitude. Development of a model for the coupling between the atmospheric neutral density and ionospheric Ne perturbations through the neutral-ion collision process will substantially benefit the research on explaining the significant uplifting of h m F 2. Astafyeva et al. [2011, 2013] analyzed 1 Hz GPS TEC data during the Tohoku-Oki earthquake and suggested that the early arrivals of TEC perturbations (420 s after the earthquake) possibly due to the shock-acoustic waves propagating at supersonic speed. The altitudinal Ne profiles shown in Figure 3 confirm that the velocity of the vertically propagating acoustic waves induced by a strong earthquake can be supersonic. Under geomagnetic quiet conditions, the uplift velocity of h m F 2 is only tens of meters per second near the dip equator [e.g., Oyekola et al., 2008; Yue et al., 2008]. In contrast, a geomagnetic storm is capable of significantly moving h m F 2 up and down. One of the most extreme records was observed during the 30 October 2003 super geomagnetic storm. The storm induced an abnormally large prompt penetrating eastward electric field that uplifted the ionosphere with a velocity near 1000 m/s at the dip equator [Abdu et al., 2008]. The uplifting velocities of the ionosphere due to the super storm and the Nepal earthquake (range from 573 to 1147 m/s) are comparable. The magnetic activity was low, and no penetration of electric fields occurred during the Nepal earthquake. It reveals that a strong earthquake-triggered ground perturbation should be considered as one of the powerful natural forces that can physically change the vertical structure of ionosphere. Note that an earthquake-induced effect should be very local as compared to a storm time effect. The electrodynamics process, which is controlled by the changes of thermospheric neutral winds and tidal waves, affects the behavior of ionosphere mainly above 200 km altitude under quiet geomagnetic conditions. Possible changes in the ionospheric dynamics [Sun et al., 2011] and the chemical process [Kakinami et al., 2012] during earthquake periods need further investigation. SUN ET AL. IONOSPHERE PERTURBED BY NEPAL EARTHQUAKE 5782

6 The F3/C RO profile p1 shows the Ne perturbation mainly in the F 2 region (Figure 3b). The earthquake may also perturb the lower parts of these profiles since they were sounded later than their peak densities. The obscuration of the possible perturbation at the lower part is due to the fact that the density there is near 10 times weaker than the peak density (see Figure 3b). In fact, the amplification factor at the lower part about 200 km altitude is also 10 times smaller than at the h m F 2 near 400 km altitude (see Figure 5b) since the neutral density decreasing inversely to the altitude. Therefore, based on the above facts, it will be a challenge detecting the coseismic disturbances at the lower part especially about 200 km altitude. On the other hand, some small vertical disturbances in ionospheric electron density have been recorded by F3/C after the occurrence of the 2011 M w 9.0 Tohoku earthquake [Liu et al., 2013]. The magnitude of the Nepal earthquake (M w 7.8) is much smaller than that of the Tohoku earthquake (M w 9.0). It is the possible reason that the F3/C did not observe the disturbance after the arrival of the vertical earthquake disturbance of the Nepal earthquake. Various types of numerical approaches, such as data assimilation and tomography, have been used to reconstruct 3-D ionospheric Ne structures from ground-based GPS TEC and space-based RO observations [Hirooka et al., 2012; Lin et al., 2015]. Here the observations from F3/C (Figure 3) and ground-based GPS receivers (Figure 1) show that the earthquake-generated Ne perturbation in the vertical direction (~10%) is much more significant than the TEC perturbation in the horizontal direction (~1%). The large difference between the Ne and TEC perturbations is due to the fact that the coseismic wave propagates mainly upward near the location of those observations. With the help of vertical information from F3/C, the earthquake-perturbed 3-D Ne structures can be better reconstructed by using those approaches. Knowing the 3-D Ne structures may improve the early warning systems of earthquakes and tsunamis [Occhipinti et al., 2008]. In conclusion, the F3/C Ne profiles illustrate that the Nepal earthquake-triggered STIDs propagate supersonically in the vertical directions (~800 m/s). On the other hand, in the horizontal direction, the ground-based GPS TEC observations show the STIDs southwardly propagate away from the epicenter with a velocity of 2.4 km/s. The earthquake perturbed the vertical Ne structure by 10% near h m F 2. The air perturbation triggered by the initial earthquake rupture near the ground can be magnified times (~150 km) near h m F 2 (~400 km) due to the decrease of atmospheric density with altitude. The peak Ne layer is uplifted by about 34 km within 1 min, and the corresponding uplifting velocity is near supersonic. The analysis process (Figure 2) shown in this study can be applied to examine other earthquake events, e.g. the M w 9.0 Tohoku-Oki earthquake which is near 15 times more powerful than the M w 7.8 Nepal earthquake. A stronger earthquake may cause more significant changes in the vertical Ne structure of ionosphere. Acknowledgments This study is supported by the grant MOST M Loren C. Chang is supported by grant MOST M MY3. The authors acknowledge the Scripps Orbit and Permanent Array Center ( ucsd.edu) for providing ground-based GPS data; COSMIC Data Analysis and Archival Center (CDAAC, and Taiwan Analysis Center for COSMIC (TACC, for providing FORMOSAT-3/COSMIC radio occultation data; U.S. Geological Survey (USGS) and Center for Engineering Strong Motion Data (CESMD, for providing the earthquake information; and Center for Orbit Determination in Europe (CODE) for providing global ionospheric map ftp://cddis.gsfc.nasa. gov/gps/products/ionex). References Abdu, M. A., et al. (2008), Abnormal evening vertical plasma drift and effects on ESF and EIA over Brazil-South Atlantic sector during the 30 October 2003 superstorm, J. Geophys. Res., 113, A07313, doi: /2007ja Artru, J., T. Farges, and P. Lognonné (2004), Acoustic waves generated from seismic surface waves: Propagation properties determined from Doppler sounding observations and normal-mode modelling, Geophys. J. Int., 158, , doi: /j x x. Astafyeva, E., P. Lognonné, and L. Rolland (2011), First ionospheric images of the seismic fault slip on the example of the Tohoku-Oki earthquake, Geophys. Res. Lett., 38, L22104, doi: /2011gl Astafyeva, E., L. Rolland, P. Lognonné, K. Khelfi, and T. Yahagi (2013), Parameters of seismic source as deduced from 1 Hz ionospheric GPS data: Case study of the 2011 Tohoku-Oki event, J. Geophys. Res. Space Physics, 118, , doi: /jgra Blanc, E. (1985), Observations in the upper atmosphere of infrasonic waves from natural or artificial sources: A summary, Ann. Geophys., 3, Calais, E., and J. B. Minster (1995), GPS detection of ionospheric perturbations following the January 17, 1994, Northridge Earthquake, Geophys. Res. Lett., 22, , doi: /95gl Chen, C. H., A. Saito, C. H. Lin, J. Y. Liu, H. F. Tsai, T. Tsugawa, Y. Otsuka, M. Nishioka, and M. Matsumura (2011), Long-distance propagation of ionospheric disturbance generated by the 2011 off the Pacific coast of Tohoku Earthquake, Earth Planets Space, 63, , doi: / eps Coïsson, P., P. Lognonné, D. Walwer, and L. M. Rolland (2015), First tsunami gravity wave detection in ionospheric radio occultation data, Earth Space Sci., 2, , doi: /2014ea Ducic, V., J. Artru, and P. Lognonné (2003), Ionospheric remote sensing of the Denali earthquake Rayleigh surface waves, Geophys. Res. Lett., 30(18), 1951, doi: /2003gl017812,18. Garcia, R. F., S. Bruinsma, P. Lognonné, E. Doornbos, and F. Cachoux (2013), GOCE: The first seismometer in orbit around the Earth, Geophys. Res. Lett., 40, , doi: /grl Hajj, G. A., and L. J. Romans (1998), Ionospheric electron density profiles obtained with the Global Positioning System: Results from the GPS/MET experiment, Radio Sci., 33(1), , doi: /97rs Hirooka, S., K. Hattori, M. Nishihashi, S. Kon, and T. Takeda (2012), Development of ionospheric tomography using Neural Network and its application to the 2007 Southern Sumatra earthquake, Electr. Eng. Jpn., 181(4), 9 18, doi: /eej Kakinami, Y., M. Kamogawa, Y. Tanioka, S. Watanabe, A. R. Gusman, J.-Y. Liu, Y. Watanabe, and T. Mogi (2012), Tsunamigenic ionospheric hole, Geophys. Res. Lett., 39, L00G27, doi: /2011gl SUN ET AL. IONOSPHERE PERTURBED BY NEPAL EARTHQUAKE 5783

7 Komjathy, A., D. A. Galvan, P. Stephens, M. D. Butala, V. Akopian, B. Wilson, O. Verkhoglyadova, A. J. Mannucci, and M. Hickey (2012), Detecting ionospheric TEC perturbations caused by natural hazards using a global network of GPS receivers: The Tohoku case study, Earth Planets Space, 64, , doi: /eps Lei, J., et al. (2007), Comparison of COSMIC ionospheric measurements with ground-based observations and model predictions: Preliminary results, J. Geophys. Res., 112, A07308, doi: /2006ja Lin, C. H., W. Wang, M. E. Hagan, C. C. Hsiao, T. J. Immel, M. L. Hsu, J. Y. Liu, L. J. Paxton, T. W. Fang, and C. H. Liu (2007), Plausible effect of atmospheric tides on the equatorial ionosphere observed by the FORMOSAT-3/COSMIC: Three-dimensional electron density structures, Geophys. Res. Lett., 34, L11112, doi: /2007gl Lin, C. H., J. Y. Liu, C. Z. Cheng, C. H. Chen, C. H. Liu, W. Wang, A. G. Burns, and J. Lei (2009), Three-dimensional ionospheric electron density structure of the Weddell Sea Anomaly, J. Geophys. Res., 114, A02312, doi: /2008ja Lin, C. Y., T. Matsuo, J. Y. Liu, C. H. Lin, H. F. Tsai, and E. A. Araujo-Pradere (2015), Ionospheric assimilation of radio occultation and ground-based GPS data using non-stationary background model error covariance, Atmos. Meas. Tech., 8, , doi: / amt Liu, J. Y., and Y. Y. Sun (2011), Seismo-traveling ionospheric disturbances of ionograms observed during the 2011 M w 9.0 Tohoku Earthquake, Earth Planets Space, 63, , doi: /eps Liu, J. Y., H. F. Tsai, and T. K. Jung (1996), Total electron content obtained by using the Global Positioning System, Terr. Atmos. Ocean. Sci., 7, Liu, J. Y., Y. B. Tsai, S. W. Chen, C. P. Lee, Y. C. Chen, H. Y. Yen, W. Y. Chang, and C. Liu (2006), Giant ionospheric disturbances excited by the M9.3 Sumatra earthquake of 26 December 2004, Geophys. Res. Lett., 33, L02103, doi: /2005gl Liu, J. Y., C. Y. Lin, C. H. Lin, H. F. Tsai, S. C. Solomon, Y. Y. Sun, I. T. Lee, W. S. Schreiner, and Y. H. Kuo (2010), Artificial plasma cave in the low-latitude ionosphere results from the radio occultation inversion of the FORMOSAT-3/COSMIC, J. Geophys. Res., 115, A07319, doi: /2009ja Liu, J. Y., C. H. Chen, C. H. Lin, H. F. Tsai, C. H. Chen, and M. Kamogawa (2011), Ionospheric disturbances triggered by the 11 March 2011 M9.0 Tohoku earthquake, J. Geophys. Res., 116, A06319, doi: /2011ja Liu, J. Y., Y. Y. Sun, H. F. Tsai, and C. H. Lin (2012), Seismo-traveling ionospheric disturbances triggered by the 12 May 2008 M 8.0 Wenchuan Earthquake, Terr. Atmos. Ocean. Sci., 23, 9 15, doi: /tao (t). Liu, J.-Y., C.-Y. Chen, and Y.-Y. Sun (2013), Wavy structures in vertical electron density profile of the ionosphere triggered by the 2011 M9.0 Tohoku earthquake, EGU General Assembly, Vienna, Austria, Apr. Maruyama, T., and H. Shinagawa (2014), Infrasonic sounds excited by seismic waves of the 2011 Tohoku-Oki earthquake as visualized in ionograms, J. Geophys. Res. Space Physics, 119, , doi: /2013ja Maruyama, T., T. Tsugawa, H. Kato, A. Saito, Y. Otsuka, and M. Nishioka (2011), Ionospheric multiple stratifications and irregularities induced by the 2011 off the Pacific coast of Tohoku Earthquake, Earth Planets Space, 63, , doi: /eps Maruyama, T., T. Tsugawa, H. Kato, M. Ishii, and M. Nishioka (2012), Rayleigh wave signature in ionograms induced by strong earthquakes, J. Geophys. Res., 117, A08306, doi: /2012ja Mutschlecner, J. P., and R. W. Whitaker (2005), Infrasound from earthquakes, J. Geophys. Res., 110, D01108, doi: /2004jd Occhipinti, G., A. Komjathy, and P. Lognonné (2008), Tsunami detection by GPS: how ionospheric observation might improve the Global Warning System, GPS World, 19(2), Oyekola, O. S., A. Ojo, and J. Akinrimisi (2008), A comparison of ground and satellite observations of F region vertical velocity near the dip equator, Radio Sci., 43, RS1005, doi: /2007rs Picone, J. M., A. E. Hedin, D. P. Drob, and A. C. Aikin (2002), NRLMSISE-00 empirical model of the atmosphere: Statistical comparisons and scientific issues, J. Geophys. Res., 107(A12), 1468, doi: /2002ja Reddy, C. D., and G. K. Seemala (2015), Two-mode ionospheric response and Rayleigh wave group velocity distribution reckoned from GPS measurement following M w 7.8 Nepal earthquake on 25 April 2015, J. Geophys. Res. Space Physics, 120, , doi: / 2015JA Rolland, L. M., P. Lognonné, E. Astafyeva, E. A. Kherani, N. Kobayashi, M. Mann, and H. Munekane (2011), The resonant response of the ionosphere imaged after the 2011 off the Pacific coast of Tohoku earthquake, Earth Planets Space, 63, , doi: / eps Saito, A., T. Tsugawa, Y. Otsuka, M. Nishioka, T. Iyemori, M. Matsumura, S. Saito, C. H. Chen, Y. Goi, and N. Choosakul (2011), Acoustic resonance and plasma depletion detected by GPS total electron content observation after the 2011 off the Pacific coast of Tohoku Earthquake, Earth Planets Space, 63, , doi: /eps Sun, Y. Y., K.-I. Oyama, J. Y. Liu, H. K. Jhuang, and C. Z. Cheng (2011), The neutral temperature in the ionospheric dynamo region and the ionospheric F region density during Wenchuan and Pingtung Doublet earthquakes, Nat. Hazard. Earth Syst. Sci., 11, , doi: /nhess Tsai, H. F., J.-Y. Liu, C.-H. Lin, and C.-H. Chen (2011), Tracking the epicenter and the tsunami origin with GPS ionosphere observation, Earth Planets Space, 63, , doi: /eps Yang, Y.-M., X. Meng, A. Komjathy, O. Verkholyadova, R. B. Langley, B. T. Tsurutani, and A. J. Mannucci (2014), Tohoku-Oki earthquake caused major ionospheric disturbances at 450 km altitude over Alaska, Radio Sci., 49, , doi: /2014rs Yue, X., W. Wan, J. Lei, and L. Liu (2008), Modeling the relationship between E B vertical drift and the time rate of change of h m F 2 (Δh m F 2 /Δt) over the magnetic equator, Geophys. Res. Lett., 35, L05104, doi: /2007gl Yue, X., W. S. Schreiner, J. Lei, S. V. Sokolovskiy, C. Rocken, D. C. Hunt, and Y.-H. Kuo (2010), Error analysis of Abel retrieved electron density profiles from radio-occultation measurements, Ann. Geophys., 28, , doi: /angeo SUN ET AL. IONOSPHERE PERTURBED BY NEPAL EARTHQUAKE 5784