# Alternative Approach for Tsunami Early Warning Indicated by Gravity Wave Effects on Ionosphere

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## Abstract

**:**

_{2}) data obtained from the ionosonde data also showed clear disturbances that were consistent with the GPS observations. IGWs and tsunami waves have similar propagation properties, and IGWs were detected about 25 min faster than tsunami waves in GPS ground stations at the United States west coast, located about 7900 km away from the tsunami’s epicenter. As IGWs have a high vertical propagation velocity, and propagate obliquely into the atmosphere, IGWs can also be used for tsunami early warning. To further investigate the spatial variation in ionospheric electron density (IED), ionospheric profiles from FORMOSAT-3/COSMIC (F3/C) satellites were investigated for both reference and observation periods. During the tsunami, the reduction in IED started from 200 km and continued up to 272 km altitude. The minimum observed reduction was 2.68 × 10

^{5}el/cm

^{3}, which has happened at 222 km altitude. The IED increased up to 767 km altitude continuously, such that the maximum increase was 3.77 × 10

^{5}el/cm

^{3}at 355 km altitude.

## 1. Introduction

## 2. Materials

## 3. Methods

#### 3.1. TEC Measurements

#### 3.2. TEC Disturbances

#### 3.3. Ionospheric Irregularities

## 4. Results

#### 4.1. IGWs

#### 4.1.1. Horizontal Speed Phase of IGWs

#### 4.1.2. Ionosphere Response to IGWs

#### 4.2. Tsunami Waves Activity

^{2}, and H is the ocean depths in meters. The ocean depth value was obtained from the DART data. The tsunami propagation velocity varies from 163.8 to 204.3 m/s. Table 2 shows the variation range of the tsunami propagation velocity at each DART station. Finally, the average velocity of the tsunami waves is 182.25 ± 18 m/s.

#### 4.3. Validation of Results

#### 4.3.1. Using Ionosonde Data

_{2}data observed by the Point Arguella (PA836) ionosonde station were used for validation of the ionospheric disturbances. The data for this station are available at (ftp://ftp.ngdc.noaa.gov/ionosonde, accessed on 20 June 2020). The foF

_{2}can be converted to electron density using Equation (12).

_{2}parameter from the 10 to 12 March is shown in Figure 15 with a red color. The range of variation in foF

_{2}under the quiet period (10 and 12 March) was calculated using Equation (13).

_{2}values increased suddenly after 15:00 UT and then decreased back to the normal value at 20:00 UT on 11 March, compared to the quiet days (10 and 12 March). The foF2 values were not between the lower and upper limit from 15:00 UT to 20:00 UT on 11 March (Figure 15b), while on the 10th and 12th of March (Figure 15a,c) the values were completely between the lower and upper limit. Figure 16 shows the difference between the upper value and the diurnal variation in the foF2 parameter in each epoch from 14:00 UT to 22:00 UT on 11 March 2011. The ionosonde station did not have data from 17:00 to 18:15 UT on 11 March (Figure 15b). Only the PA836 station had data in the Western United States on 11 March 2011. The abnormal onset and end times of the GPS observation were 15:10 UT and 20:00 UT. Therefore, the results of ionosonde are consistent with the GPS analysis. It is also worth noting that there were no obvious disturbances on the day before (10 March) and the day after (12 March), using the ionosonde data.

#### 4.3.2. Using Electron Density Disturbances

^{5}el/cm

^{3}(~27%), which happened at the 222 km altitude. The IED increased up to 767 km altitude continuously, such that the maximum increase was 3.77 × 10

^{5}el/cm

^{3}(~64%) at 355 km altitude. The electron density peak was 12.22 × 10

^{5}el/cm

^{3}at 291 km altitude on 11 March, and the value was 12.18 × 10

^{5}el/cm

^{3}at 258 km altitude. Generally, it can be inferred that the tsunami induced an increase in the IED peak value and altitude.

## 5. Discussion

^{5}el/cm

^{3}(~27%), which happened at 222 km altitude. The IED increased up to 767 km altitude continuously, such that the maximum increase was 3.77 × 10

^{5}el/cm

^{3}(~64%) at 355 km altitude. Generally, the tsunami induced an increase in the IED peak of value and altitude. Occhipinti et al. showed that about one hour after the occurrence of the tsunami, the main part of the IGWs energy reaches the altitude of 300 km. At this height, the electron density value becomes significant. This effect in the upward velocity adjusts the tsunami waveform as it propagates from the surface of the ocean surface to high altitudes. The ionosphere responds instantaneously to the IGW forcing, and produces a passing wave that disappears with the diffusion and chemical loss after some time. On the contrary, ion production and loss plays a crucial role; the signatures of the IGWs in the plasma are maximized in the direction of the magnetic field [31]. In addition, Occhipinti et al. demonstrated that when a tsunami occurs in the Northern Hemisphere, the perturbed electron density would not exceed 10% in both the E and F regions; however, when it travels south, the electron density could reach to up 80% [31].

## 6. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Travel time map of the 2011 Tohoku tsunami. The spatial distribution of tide gauge (orange point), DART (red point), and location of the epicenter (yellow star) are shown in the figure [17].

**Figure 2.**Case study and the spatial distribution of ground-based GPS receivers (red triangle), DART stations (yellow diamond), tide gauge stations (dark-blue circle), and ionosonde station (green asterisk) considered in this study.

**Figure 5.**Location of sample GPS stations and IPP paths between 15:00 UT and 20:00 UT over west American coast on 11 March. The spatial distribution of GPS stations are shown with different colors. For each station, IPPs of only one satellite path is depicted, which has the same color.

**Figure 6.**${\mathsf{\Delta}}^{2}TEC$ time series of PRN 18, PRN22, and PRN29 observed from different GPS stations between 14:30 UT and 21:00 UT on 11 March (the day of the tsunami). Results from each station are vertically shifted from the previous station by two TECU in (

**a**) and (

**b**), and results from each station are vertically shifted from the previous station by one TECU in (

**c**).

**Figure 7.**Location of IPPs for PRN 18 (purple line), PRN 22 (blue line), and PRN 29 (green line) observed from all the GPS stations from 15:10 UT to 20:00 UT on 11 March.

**Figure 8.**Time series for PRN18 (

**a**), PRN22 (

**c**), and (

**e**) PRN29 observed from GPS station PABH. (

**b**), (

**d**), and (

**f**) the time-frequency analysis for the ${\mathsf{\Delta}}^{2}TEC$ time series.

**Figure 9.**${\mathsf{\Delta}}^{2}TEC$ time series of PRN 18, PRN22, and PRN29 observed from different GPS stations between 14:30 UT and 21:00 UT on 10 March (a day before the tsunami). Results from each station are vertically shifted from the previous station by two TECU in (

**a**) and (

**b**), and results from each station are vertically shifted from the previous station by one TECU in (

**c**).

**Figure 10.**${\mathsf{\Delta}}^{2}TEC$ time series of PRN 18, PRN22, and PRN29 observed from different GPS stations between 14:30 UT and 21:00 UT on 12 March (a day after the tsunami). Results from each station are vertically shifted from the previous station by two TECU in (

**a**) and (

**b**), and results from each station are vertically shifted from the previous station by one TECU in (

**c**).

**Figure 11.**Spatial distribution of DART (yellow diamond), tide gauge stations (dark-blue circle), GPS station CABL (red triangle), and location of IPPs for PRN15 and PRN16 observed from GPS station CABL on 11 March 2011.

**Figure 12.**Time series for PRN16 (

**a**) and PRN15 (

**c**) observed from GPS station CABL. (

**b**) and (

**d**) the time-frequency analysis for the ${\mathsf{\Delta}}^{2}TEC$ time series. ROTI time series for PRN16 (

**e**) and PRN15 (

**f**) observed from GPS station CABL on 11 March 2011.

**Figure 13.**Average ROTI values for all satellites observed from all GPS stations on March 10 (green curve), 11 March (red curve), and 12 March (blue curve).

**Figure 14.**Sea level time series for tide gauges at (

**a**) Arena, and (

**c**) West Port stations on 11 March 2011. The red points indicate the time of arrival of the tsunami waves at the stations, and the time-frequency analysis for tide gauges at (

**b**) Arena, and (

**d**) West Port stations.

**Figure 15.**Variations in the ionospheric parameter foF

_{2}obtained at PA836 from 10 to 12 March 2011 (red line), for (

**a**) 10th, (

**b**) 11th, and (

**c**) 12th March 2011. The solid blue line is lower values of foF

_{2}parameter on quiet days (10 and 12 March 2011). The solid green line is upper values of foF

_{2}parameter on quiet days (10 and 12 March 2011). Blue dot line is the average of quiet day values (10 and 12 March 2011).

**Figure 16.**The difference between the upper value and the diurnal variation of the foF2 parameter in each epoch from 14:00 UT to 22:00 UT on 11 March 2011.

**Figure 17.**IPP tracks of GPS PRN 18 from the four southward stations and tangent point projections of F3/C RO observations.

**Figure 18.**(

**a**) The red, green, and blue curves show the IED profiles that occurred during the tsunami (reference period), one day after the tsunami (observation period), and 14 March 2011, respectively. (

**b**) The difference in electron density between the reference and observation period.

**Figure 19.**${\mathsf{\Delta}}^{2}TEC$ time series of PRN18 observed from four GPS stations on 10, 11, and 12 March 2011. Results from each station are vertically shifted from the previous station by 1.5 TECU.

Station | Tsunami Time Arrival (UT) | Latitude (° N) | Longitude (° W) |
---|---|---|---|

Alameda | 16:36 | 37.77 | −122.3 |

Arena | 15:34 | 38.91 | −123.72 |

Astoria | 16:24 | 46.21 | −123.77 |

Cherry Point | 17:08 | 48.86 | −122.76 |

Jolla | 16:22 | 32.87 | −117.26 |

San Fran | 16:15 | 37.81 | −122.47 |

South Beach | 15:42 | 44.63 | −124.04 |

West Port | 15:39 | 46.91 | −124.11 |

46404 | 14:33 | 45.85 | −128.78 |

46407 | 14:39 | 42.71 | −128.83 |

46411 | 14:59 | 39.34 | −127.07 |

Station | Tsunami Propagation Velocity (m/s) |
---|---|

46404 | 163.8 |

46407 | 178.9 |

46411 | 204.3 |

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**MDPI and ACS Style**

Foroodi, Z.; Alizadeh, M.; Schuh, H.; Tsai, L.-C.
Alternative Approach for Tsunami Early Warning Indicated by Gravity Wave Effects on Ionosphere. *Remote Sens.* **2021**, *13*, 2150.
https://doi.org/10.3390/rs13112150

**AMA Style**

Foroodi Z, Alizadeh M, Schuh H, Tsai L-C.
Alternative Approach for Tsunami Early Warning Indicated by Gravity Wave Effects on Ionosphere. *Remote Sensing*. 2021; 13(11):2150.
https://doi.org/10.3390/rs13112150

**Chicago/Turabian Style**

Foroodi, Zahra, Mahdi Alizadeh, Harald Schuh, and Lung-Chih Tsai.
2021. "Alternative Approach for Tsunami Early Warning Indicated by Gravity Wave Effects on Ionosphere" *Remote Sensing* 13, no. 11: 2150.
https://doi.org/10.3390/rs13112150