PUBLICATIONS. Radio Science. Analysis of traveling ionospheric disturbances (TIDs) in GPS TEC launched by the 2011 Tohoku earthquake

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1 PUBLICATIONS RESEARCH ARTICLE Special Section: Ionospheric Effects Symposium 2015 Key Points: TIDs associated with the Tohoku tsunami traveled over long distances to the west coast of the U.S. The propagation of the TIDs over the U.S. matches the tsunami speed generated by the earthquake After the forcing stopped at the shore, TIDs continued to propagate inland and gradually decayed Correspondence to: I. Azeem, Citation: Crowley, G., I. Azeem, A. Reynolds, T. M. Duly, P. McBride, C. Winkler, and D. Hunton (2016), Analysis of traveling ionospheric disturbances (TIDs) in GPS TEC launched by the 2011 Tohoku earthquake, Radio Sci., 51, , doi:. Received 1 DEC 2015 Accepted 30 APR 2016 Accepted article online 4 MAY 2016 Published online 24 MAY American Geophysical Union. All Rights Reserved. Analysis of traveling ionospheric disturbances (TIDs) in GPS TEC launched by the 2011 Tohoku earthquake Geoff Crowley 1, Irfan Azeem 1, Adam Reynolds 1, Timothy M. Duly 1, Patrick McBride 1, Clive Winkler 1, and Don Hunton 1 1 ASTRA, Boulder, Colorado, USA Abstract Traveling ionospheric disturbances (TIDs) have been detected using various measurement techniques, including HF sounders, incoherent scatter radars, in situ measurements, and optical techniques. However, observations of TIDs have tended to be sparse and there is a need for additional observations to provide new scientific insight into the geophysical source phenomenology and wave propagation physics. The dense network of GPS receivers around the globe offers a relatively new data source to observe and monitor TIDs. In this paper, we use total electron content (TEC) measurements from ~4000 GPS receivers throughout the continental United States to observe TIDs associated with the 11 March 2011 Tohoku tsunami. The tsunami propagated across the Pacific to the U.S. west coast over several hours, and we show that corresponding TIDs were observed in the US. Using this network of GPS receivers we present a 2D imaging of TEC perturbations and calculate various TID parameters, including horizontal wavelength, speed, and period. Well-formed, planar TIDs were detected over the west coast of the U.S. ~10 h after the earthquake. Fast Fourier transform analysis of the observed waveforms revealed that the period of the wave was 15.1 min with a horizontal wavelength of km, phase velocity of m/s, and an azimuth of (propagating nearly due east in the direction ofthetsunamiwave).theseresultsareconsistent with the TID observations in airglow measurements from Hawaii earlier in the day and with other GPS TEC observations. 1. Introduction The major source of error in geolocation is traveling ionospheric disturbances (TIDs), both for long ranges (over-the-horizon radar) and short ranges (high frequency direction finding). TIDs are perturbations in ionospheric electron density caused by atmospheric gravity waves (AGWs) via ion-neutral collisions as they travel through the thermosphere/ionosphere from their source region. At ionospheric heights the motion of the neutral gas in the AGW sets the ionosphere into motion. Thus, the signature of the AGW is manifested as variations of electron density in the ionosphere, resulting in a TID. As a result of these ionospheric perturbations, TIDs have been detected by various radio techniques for many years, including ionosondes, incoherent scatter radars, and HF Doppler sounders. HF sounder measurements of TIDs have been presented by a number of authors [e.g., Crowley, 1985; Waldock and Jones, 1987; Crowley et al., 1987; Crowley and McCrea, 1988; Crowley and Rodrigues, 2012; and included references]. A newer technique for measuring TIDs is the use of GPS total electron content (TEC), which is the focus of this paper. The technique of producing 2D images of the TIDs using GPS TEC data has been demonstrated for locations, such as Japan [Tsugawa et al., 2007, 2011; Liu et al., 2011] and the United States [Nishioka et al., 2013]. AGWs are buoyancy waves generated as a consequence of gravity attempting to restore the equilibrium of an atmospheric perturbation [Hines, 1960; Yeh and Liu, 1974; Francis, 1975]. There are many different generation mechanisms for AGWs. In the auroral regions they can be caused by Joule heating associated with geomagnetic storms [e.g., Richmond, 1978; Hunsucker, 1982]. Severe meteorological events, such as thunderstorms and tornadoes, and tropospheric deep convection cells [Fovell et al., 1992; Alexander et al., 1995; Lane et al., 2001] have been shown to generate AGWs, and their signatures have been recorded in the thermosphere [Taylor and Hapgood, 1988; Nishioka et al., 2013]. Several studies have suggested that convective storms are one of the primary drivers for AGWs that propagate upward into the mesopause region [Alexander, 1996; Holton and Alexander, 1999; Walterscheid et al., 2001]. AGWs can also be man-made, as nuclear detonations have been shown to launch the waves into Earth s atmosphere [Row, 1967]. AGWs CROWLEY ET AL. TIDS IN GPS TEC 507

2 transport energy and momentum upward and are a prime example of coupling lower atmospheric processes (i.e., in the troposphere) into the upper atmosphere (i.e., in the thermosphere/ionosphere). Natural disasters, including earthquakes [Hasbi et al., 2011, and references therein], and tsunamis [Makela et al., 2011] have also generated AGWs and associated ionospheric signatures such as TIDs. The 11 March 2011 Tohoku earthquake and subsequent tsunami off the east coast of Japan created AGWs, which propagated on global spatial scales. The signature of the AGWs was observed with airglow imagers located in Maui, Hawaii [Makela et al., 2011]. In their study, Makela et al. [2011] observed AGWs that were consistent with numerical modeling of AGWs generated by an earthquake. Occhipinti et al. [2013] reviewed far-field measurements of earthquake-related ionospheric signatures and also presented near-field GPS-based total electron content (TEC) measurements associated with the Tohoku event. Komjathy et al. [2012] also reported on the Tohoku earthquake and the ionospheric signatures that resulted from it. They showed that a global network of GPS receivers was able to detect the earthquake, but they did not include measurements at specific positions across the globe to give indications of the motion or velocity of the TEC disturbances. For the same catastrophic event, spatially resolved measurements of TEC variations, measured with a network of GPS receivers in Japan and Taiwan, revealed an initial ionospheric TEC enhancement about 7 min after the earthquake located at an ionospheric epicenter about 200 km distant from the geologic epicenter [Tsugawa et al., 2011; Liu et al., 2011]. Concentric rings of TEC enhancements and depletions then propagated radially outward from this ionospheric epicenter. Tsugawa et al. [2011] reported velocities in the range m/s, while Liu et al. [2011] observed initial velocities of m/s and later detected concentric rings with a lower velocity close to the tsunami wave velocity of m/s. The tsunami propagated across the Pacific to the U.S. west coast over several hours, and corresponding TIDs were observed over Japan [Galvan et al., 2012; Saito et al., 2011; Maruyama et al., 2011]. In this paper, we extend the spatially resolved measurements near Japan to the continental Unites States. We analyze TEC measurements from the dense network of GPS receivers throughout the U.S. to observe TIDs associated with the 11 March 2011 Tohoku earthquake and tsunami. The network of GPS receivers provides a 2-D spatial map of TEC perturbations, which we have used to calculate TID parameters, including horizontal wavelength, speed, and period. The results presented here are consistent with the previous studies of TIDs associated with the Tohoku event. Although coupling between the ocean and ionosphere associated with tsunamis was predicted as early as the 1970s, much work is still needed in order to fully understand the geophysical source phenomenology, the ocean-atmosphere coupling mechanisms, and the wave propagation physics. 2. GPS TEC Data We used GPS TEC data from over 4000 sites in the United States to detect and image TIDs. Our approach to processing the GPS data to derive vertical TEC maps was similar to that presented by Tsugawa et al. [2007] and Nishioka et al. [2013]. In brief, we used the pseudorange and phase measurements of GPS signals at L1 ( MHz) and L2 ( MHz) frequencies to derive slant TEC values, which were then converted to vertical TEC (VTEC) using the obliquity factor model described by Kaplan and Hagerty [2006]. We used the International Reference Ionosphere model [Bilitza et al., 2012] to calculate the height of maximum F region electron density (h m F 2 ) and treated it as the altitude of the ionospheric pierce point. For the solar and geomagnetic conditions on 11 March 2011 the model h m F 2 was 250 km. We then computed perturbations in TEC by detrending VTEC using a 20 min running mean for each PRN followed by binning in latitude and longitude and horizontal smoothing of the resulting TEC map using a 2-D Gaussian filter with a full width at half maximum of 0.75 in both dimensions. 3. Results A magnitude 9.0 earthquake occurred on 11 March 2011 at 05:46:23 universal time (UT) near the northeast coast of Honshu, Japan. The earthquake spawned a tsunami that resulted in widespread destruction along the coast of the northern part of Japan and threatened coastal areas throughout the Pacific. Figure 1 shows the NOAA tsunami travel time (TTT) map for the 2011 Tohoku tsunami. The Tohoku tsunami propagated throughout the Pacific Ocean region, thereby affecting the entire Pacific coast of North and South America from Alaska to Chile [Tang et al., 2012]. Warnings were issued and evacuations carried out in many countries CROWLEY ET AL. TIDS IN GPS TEC 508

3 Radio Science Figure 1. NOAA National Geophysical Data Center s map of tsunami arrival time (in hours) during the 2011 Japan tsunami ( bordering the Pacific [Fraser et al., 2012; Suppasri et al., 2013]. While the tsunami had significant impact on the eastern coast of Japan, the extent of damage on the eastern side of the Pacific was minor. Chile s Pacific coast, one of the furthest from Japan at about 17,000 km (11,000 mi) distant, was struck by waves 2 m (6.6 ft) high, compared with an estimated wave height of 38.9 m (128 ft) at Omoe peninsula, Miyako city, Japan. The TTT map in Figure 1 shows that the tsunami wave reached the west coast of the United States about 10 to 12 h after the earthquake struck Japan. The tsunami arrival time on the west coast of the United States is Figure 2. (a) Time series of TEC perturbations measured by a Continuously Operating Reference Station GPS receiver located in Shasta Lake, CA (40.79 N latitude, W longitude) on 11 March 2011 between 15:00 UT and 20:00 UT. (b) The corresponding elevation angle of the GPS satellite SVN 15 used for computing TEC in Figure 2a. CROWLEY ET AL. TIDS IN GPS TEC 509

4 Figure 3. 2-D maps of TEC perturbations at (a) 15:30 UT, (b) 16:40 UT, (c) 17:10 UT, and (d) 19:20 UT. These maps show planar TID wavefronts over the west coast of the United States. confirmed by the Bottom Pressure Recorded data (not shown here) on the Deep-ocean Assessment and Reporting of Tsunami (DART) buoys in the Pacific region. A detailed discussion of the DART data from the Pacific region is beyond the scope of this paper but we have utilized multiple DART buoys off the coast of California to validate tsunami arrival times shown in Figure 1. Figure 2 shows TEC perturbations measured by a GPS receiver located in Shasta Lake, CA (40.79 N, W) before and the after the tsunami event. The corresponding elevation angles of the GPS satellite being tracked (PRN 21) are shown in Figure 2b. The dashed curve in the figure shows the band-pass-filtered VTEC from a receiver-satellite pair on 10 March The magnitude of the VTEC perturbations on the quiet day was approximately 0.05 total electron content unit (TECU; 1 TECU = el m 2 ), close to the noise floor of most dual frequency GPS receivers. The solid line in Figure 2a shows the TEC response on 11 March 2011 measured about 11 h after the Tohoku earthquake in Japan. The amplitude of the TEC perturbation was about 1.1 TECU (1 TECU = el/m 2 ). During the main phase of the TEC perturbations in Figure 2a, the elevation of the GPS satellite PRN 21 was well above 40 (see Figure 2b), which suggests that the observed VTEC variations were not caused by low-elevation effects and multipath but rather by geophysical variations in the ionosphere. Similar perturbations were also seen on many other receiver-satellite links for receivers distributed across the Contiguous United States (CONUS), confirming that the perturbations seen in this figure were widespread. Figure 3 shows some results of our GPS TEC analysis to produce TID maps over CONUS. We show GPS TEC maps over the continental U.S. for four different UT times on 11 March 2011 (15:30, 16:40, 17:10, and 19:20 UT) to illustrate the extent and propagation of the TIDs. We caution the reader that the images in Figure 3 have been purposely saturated by limiting the grey scale to ±0.03 TECU in order to draw attention to the TID wave train. Thus, at first glance, the scales of the TEC perturbations in the maps could mistakenly be assumed to be on the order of 0.03 TECU. However, the TID amplitudes are much larger than the scale in Figure 3 suggests. The amplitude of the TID in raw TEC data was shown (in Figure 2) to be ±1.1 TECU. The filtering and binning reduce that to about 0.4 TECU. At 15:30 UT (Figure 3a), there was no indication of TIDs in the northwest region. TIDs were first seen in the GPS TEC measurements at 15:46 UT (not shown), and by 16:40 UT (Figure 3b), planar wavefronts had clearly formed above the U.S. West Coast with local orientation in a north-east/south-west direction. These TID wave trains are also present in the TEC map at 17:10 UT CROWLEY ET AL. TIDS IN GPS TEC 510

5 Radio Science Figure 4. 3-D FFT analysis for the TID passing over the Western US. (a) Selected raw data. The max amplitude of the FFT is calculated, and (b and c) the spatial slice and temporal frequencies, respectively, are shown. (d) The reconstructed wave from the 3-D FFT analysis is also depicted. CROWLEY ET AL. TIDS IN GPS TEC 511

6 (Figure 3c). At 18:00 UT (not shown), the wave trains were breaking up, and by 19:20 (Figure 3d) they had largely dissipated, and there was again no clear evidence of TIDs over the West Coast. A full TEC map sequence (not shown here) at a cadence of 30 s shows that these TID wavefronts propagated from the direction of the earthquake at an azimuth of (measured clockwise from North). In total, the TIDs persisted for about 4 h after they were first seen. The TID azimuth is in good agreement with the propagation direction of the tsunami near the Pacific coast of the U.S. We also performed a 3-D fast Fourier transform (FFT) analysis on the GPS TEC data set to calculate various TID wave parameters. Based on the FFT analysis we estimate the dominant TID period to be 15.1 min, the horizontal wavelength to be km, and the phase velocity to be m/s. We note that this measured TID phase speed is in good agreement with the phase speed of the tsunami, which was about 221 m/s [Galvan et al., 2012]. The total horizontal distance from the earthquake epicenter to central Washington is approximately 7500 km. The delay between the earthquake (5:46 UT) and the first detection of TIDs in the northwest (15:46 UT) was 10 h, which corresponds to a propagation speed of about 208 m/s and approximately to the propagation velocity across the Pacific as shown in Galvan et al. [2012]. The tsunami travel time is consistent with the NOAA TTT map (Figure 1) and the data from DART buoys in the Pacific region. This agreement in timing suggests that the source of the observed TIDs is most likely to be the tsunami as it approached the west coast of the U.S. In Figure 4a, a subset of data was selected covering the Western United States of ~4 4 in latitude and longitude, which is approximately km at the ionospheric pierce point. Within this region, a 2 h time window was selected from 17:03:30 to 19:03:30 UT, representative of the TID passing through this region. This interval was selected because it represented the largest amplitude of TIDs propagating from the west coast of the US. A 3-D FFT was calculated for this 3-D block, and the data are zero padded to provide interpolation in the frequency domain. From this calculation, the maximum amplitude was found, and Figure 4b shows the k x versus k y slice of the maximum value of the FFT, which is equivalent to λ = km and θ = (azimuth angle measured clockwise from North). Next, Figure 4 c shows the FFT of the third dimension (i.e., time) of the 3-D FFT block and shows the maximum value at Hz (T = 15.1 min). Finally, Figure 4d shows a snapshot of the wave recovered from the analysis, representative of the wave shown in Figure 4a. Next we use the Boussinesq dispersion relation to estimate the vertical wavelengths, λ z of the TID. We use the dispersion relation as described by Vadas and Crowley [2010]: ω 2 Ir ¼ m 2 þ k 2 (1) H where ω 2 Ir is the intrinsic frequency of the wave, k H is the horizontal wave number, N is Brunt-Väisälä frequency, and m is the vertical wave number (m =2π/λ z ). The relationship between the observed (ω r ) and intrinsic frequencies is given by ω r ¼ ω Ir þ k x u þ k y v (2) where k x is the zonal wave number, k y is the meridional wave number, u is the zonal neutral wind, and v is the meridional neutral wind. We use the Thermosphere-Ionosphere-Mesosphere Electrodynamics General Circulation Model [Roble and Ridley, 1994] to estimate neutral winds and Brunt-Väisälä frequency at the time and location of the observed TIDs. The model zonal and meridional winds at 18:00 UT and at 40 N and 120 W were 143 m/s and 15 m/s, respectively. Using these values, we find that λ z = 43.5 km, however, in this case we have no way to independently confirm this estimated value because TEC is a height integrated measurement. 4. Discussions and Conclusions Whenever TIDs are observed, the question of their source arises. In the case of the current event, one wonders whether the source was the earthquake, initial surge, or the tsunami that traveled across the ocean. The scenario observed in the current study is somewhat unique, because the tsunami was abruptly stopped by the west coast of the US, and the source of AGWs was removed. This makes it possible for us to investigate the source of the TIDs observed across the U.S. For the TIDs observed in the GPS TEC data, we compute the propagation angle (ξ) predicted by equation (3) for zero-wind conditions [Vadas and Crowley, 2010]. k2 H N2 sinðþ ξ e τ B =τ (3) CROWLEY ET AL. TIDS IN GPS TEC 512

7 where ξ is the angle between the horizontal and the direction of propagation of the AGW and τ B is the buoyancy period [e.g., Vadas et al., 2009]. Using an average buoyancy period (between z ~ 0 to 250 km) of τ B ~8 min, from equation (1) we calculate ξ = ~32 for the AGW with τ = 15.1 min. Then assuming an observation height of 250 km, and knowing the longitude of the observation, we can predict the source location. We have applied this analysis to a broad range of TEC measurements across the West Coast for this tsunamirelated event. For each location, we find that the source appears to lie just off the western coast of the U.S. This strongly suggests that the tsunami was the source of the observed TIDs/AGWs, thereby supporting our hypothesis that the observed TIDs were forced by the tsunami as it approached the western coastline of the United States. In this paper, we used TEC measurements from 4000 GPS receivers throughout the continental U.S. to image, as far as we know for the first time, TIDs over the U.S. associated with the 11 March 2011 Tohoku tsunami. The tsunami propagated across the Pacific to the Western Coast of the U.S. during a 10 h period, at approximately the same time as it arrived at the U.S. West Coast corresponding TIDs were observed in ionospheric TEC measurements. We used a 3-D FFT analysis on a sequence GPS TEC images over the Western U.S. to derive TID wave parameters. The period of the wave was 15.1 min with a horizontal wavelength of km, phase velocity of m/s, and an azimuth of (propagating in the direction of the tsunami wave). These results are consistent with TID observations in airglow measurements from Hawaii earlier in the day, and other GPS TEC observations. The vertical wavelength of the TID was estimated to be 43.5 km. This paper demonstrates that the 2011 tsunami generated TIDs which propagated inland about 1500 km from their source at the West Coast, and the TIDs persisted for about 4 h over North America. These results have significant implications for advancing our understanding of the full chain of coupled plasma processes in the atmosphere-ionosphere system. For example, assuming the tsunami continuously launches AGWs, when the source stops at the west coast, the propagation and decay of the TIDs inland provides a rare controlled experiment with which to test theories and models of AGW propagation and dissipation. Any model that claims to accurately simulate this event must include the lack of TIDs more than about 1500 km inland and must also explain why the TIDs dissipate after about 4 h. The event described here represents a unique controlled experiment with which to test models of how GWs are launched, propagated, and dissipated in relation to tsunamis. The observations also provide information regarding the transportation of wave energy and momentum over large distances, and the morphological characteristics of TIDs. Using the same techniques as discussed here, we have now examined maps of the TEC perturbations over the U.S. for many days covering several years and all seasons, and we can confidently assert that this event was unique in its clarity and direction of propagation, and we have never seen an event comparable to the TIDs shown here. Acknowledgments The authors acknowledge contract N from the Office of Naval Research, NASA LWS Contract NNH13CJ33C, NSF grant AGS , and subcontracts NWRA-12-S-151 (under NASA contract NNH12CE58C) and NWRA-14-S-176 (under AFRL contract FA C-0306). We acknowledge the use of publicly available ground-based GPS TEC data from the Southern California Integrated GPS Network, International GPS Service for Geodynamics, UNAVCO, Hartebeesthoek Radio Astronomy Observatory, Natural Resources Canada, Geoscience Australia, the Brazilian Institute of Geography and Statistics, University of New Brunswick, National Oceanic and Atmospheric Administration, and National Aeronautics and Space Administration. TID data presented in this paper are available from the corresponding author upon request. References Alexander, M. J. (1996), A simulated spectrum of convectively generated gravity waves: Propagation from the tropopause to the mesopause and effects on the middle atmosphere, J. Geophys. Res., 101, , doi: /95jd Alexander, M. J., J. R. Holton, and D. R. Durran (1995), The gravity wave response above deep tropical convection in a squall line simulation, J. Atmos. Sci., 52(12), Bilitza, D., D. Altadill, Y. Zhang, C. Mertens, V. Truhlik, P. 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