# Magnetospheric–Ionospheric–Lithospheric Coupling Model. 1: Observations during the 5 August 2018 Bayan Earthquake

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## Abstract

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## 1. Introduction

## 2. Data and Methods

#### 2.1. Atmospheric Temperature and Acoustic Gravity Waves Evaluation

^{th}generation atmospheric data set produced by the European Centre for Medium-Range Weather Forecasts [25]. The model produces global and hourly temperature profiles with high resolution (137 different pressure levels) from near surface up to 0.01 hPa (∼80 km altitude) using observations from satellites, radiosondes, dropsondes, aircraft, and radars [25]. The horizontal resolution is ∼0.28${}^{\circ}$ in both longitude and latitude.

#### 2.2. The Vertical Total Electron Content (vTEC)

#### 2.3. China Seismo-Electromagnetic Satellite (CSES) Data

#### 2.4. Ground Magnetometer and Magnetospheric Field Line Resonance Frequency Estimation

#### 2.5. Non-Stationary Signal Decomposition and Their Multiscale Statistical Analysis: The Fast Iterative Filtering Algorithm

## 3. The Bayan 5 August 2018 Earthquake, Co-seismic Observations

#### 3.1. Acoustic Gravity Waves Observations

#### 3.2. Vertical Total Electron Content Observations

- We identified 10 days of August 2018 characterized by both low solar activity (i.e., −10 nT < Sym-H < 5 nT and $AE<100$ nT, Sym-H and AE being the geomagnetic disturbed time index [70] and Auroral Electrojet index [71], respectively) and low seismic activity (i.e., M < 2, M being the EQ magnitude) in an area of 3${}^{\circ}$× 3${}^{\circ}$ lat × lon around the EE;
- We decomposed the diurnal vTEC observations using the FIF method, which we briefly recalled in Section 2.5. The interested reader can find more details on this algorithm and its pseudo-code in [59,61] FIF code for Matlab is freely available at www.cicone.com);
- We evaluated the 10-day average relative energy spectrogram (${\overline{\u03f5}}_{rel}$) after removing the long term trend;
- ${\overline{\u03f5}}_{rel}$ is the vTEC background.

#### 3.3. Magnetospheric Field Line Resonance (FLR) Frequency Observations

## 4. The Bayan 5 August 2018 Earthquake, Pre-Seismic Observations

#### 4.1. Atmospheric Temperature Observations

#### 4.2. CSES Satellite Ionospheric Observations

- The entire electric and magnetic field dataset is divided into two subsets depending on different seismic conditions: ${M}_{L}$ defined for low seismic activity (i.e., $M\le 2$); ${M}_{H}$ defined for high seismic activity (i.e., $M>2$);
- ${M}_{L}$ and ${M}_{H}$ is divided into three groups according to the geomagnetic activity. This procedure made use of Sym-H and AE geomagnetic indices. The three subgroups correspond to low, moderate and high geomagnetic activity, namely: ${I}_{k,1}$ - Sym-H = [10 nT, −10 nT] and AE < 100 nT; ${I}_{k,2}$ - Sym-H = [−10 nT, 80 nT] and AE < 200 nT; ${I}_{k,3}$ - Sym-H $\le -80$ nT and AE ≥ 200 nT;
- A cell ${3}^{\circ}\times {3}^{\circ}$ in latitude–longitude centered over the EE, in which we evaluated the time-frequency average ${\overline{\u03f5}}_{rel}$, is selected. The mean operation is applied only if the ratio ${R}_{\u03f5}\left(f\right)=\frac{{\u03f5}_{rel}}{{\overline{\u03f5}}_{rel}}=1\pm 4\sigma \left(f\right)$, $\sigma \left(f\right)$ being the standard deviation of ${\overline{\u03f5}}_{rel}$ evaluated for each frequency scale.
- Each frequency scale showing a ${K}_{ex}$ almost null and correspondingly a relative maximum in the Shannon entropy (I) was not considered in the evaluation of the relative energy, since it can be represented as a Gaussian fluctuation characterized by high “degree of randomness”—i.e., instrumental noise [58].

## 5. Discussion

- An AGW is generated around the EE, propagating through the atmosphere;
- The AGW interacts mechanically with the ionosphere, creating a local instability in the plasma distribution through a pressure gradient. Such plasma variation put the ionosphere into a “meta-stable” state, giving rise, in the E-layer, to a local non-stationary electric current. This, in turn, generates an electromagnetic (EM) wave.
- The interaction of such EM waves with the magnetospheric field causes a change in the eigenfrequency of the field line, whose ionospheric footprint is located over the radial projection of the EE.

## 6. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Abbreviations

AGW | Acoustic Gravity Wave |

AE | Auroral electroject |

CSES | China-Seismo-Electromagnetic satellite |

EE | Earthquake epicenter |

EFD | Electric field Detector |

EQ | Earthquake |

FIF | Fast iterative Filtering |

FLR | Field Line Resonance |

GNSS | Global Navigation Satellite System |

IMF | Intrinsic Mode function |

LAP | Langmuir Probe |

M.I.L.C. | Magnetospheric–Ionospheric–Lithospheric Coupling |

SCM | Search-Coil Magnetometer |

TEC | Total Electron Content |

vTEC | Vertical TEC |

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**Figure 1.**Co-seismic vertical profiles of: (

**a**) temperature; (

**b**) background temperature; (

**c**) temperature deviation; (

**d**) square term of $Brunt-V\ddot{a}is\ddot{a}l\ddot{a}$ frequency; (

**e**) potential energy at 12:00 UT on 5 August 2018. Red horizontal dashed lines represent the AGW peaks identified; the green horizontal dashed line identify the tropopause peak.

**Figure 2.**Co-seismic ${E}_{P}$ maps from 3 to 5 August 2018. The date and altitude are indicated in each panel (

**a**–

**c**). The earthquake epicenter is marked by a black dot.

**Figure 3.**vTEC relative energy spectrogram evaluated over the earthquake epicenter. Panel (

**a**) vTEC relative energy background for solar quiet conditions for August 2018; panel (

**b**) vTEC relative energy evaluated on 5 August 2018. Colors are representative of the ${\u03f5}_{rel}$ values. The yellow dashed line represents the time of the earthquake occurrence.

**Figure 4.**The cross-phase dynamical spectrogram between two low-latitude ground stations near the earthquake epicenter (GNG-LRM; GNG: $\lambda =31.{36}^{\circ}$ S and $\varphi =115.{71}^{\circ}$E; LRM: $\lambda =22.{22}^{\circ}$S and $\varphi =114.{1}^{\circ}$E). Each spectrum has been evaluated over a 1 h interval. Spectra have been smoothed both in time and frequency domains (7 frequency bands and 15 temporal bands). The green vertical line represents the earthquake occurrence time. The red vertical line represents the occurrence time of the first FLR frequency decrease. $\Delta \psi $ in the color-bar represents the phase difference in degrees between the equatorward (LRM) and poleward (GNG) ground magnetometer.

**Figure 5.**Pre-seismic vertical profiles of: (

**a**) temperature; (

**b**) background temperature; (

**c**) temperature deviation; (

**d**) square term of $Brunt-V\ddot{a}is\ddot{a}l\ddot{a}$ frequency; (

**e**) potential energy 6:00 UT on 5 August 2018. Red horizontal dashed lines represent the AGW peaks identified; the green horizontal dashed line identify the tropopause peak.

**Figure 6.**Pre-seismic ${E}_{P}$ maps from 3 to 5 August 2018. The date and altitude are indicated in each panel (

**a**–

**c**). The earthquake epicenter is marked by a black dot.

**Figure 7.**CSES satellite Electric Field detector observations. Box (

**A**) Environmental background evaluated over the earthquake epicenter in terms of ${\overline{\u03f5}}_{rel}$ vs. time and frequency for the reference quiet condition ($M\le 2$; Sym-H = [10 nT, −10 nT]; AE < 100 nT) for the three components of the electric field; Box (

**B**) ${\u03f5}_{rel}$ evaluated over the earthquake epicenter on 5 August 2018 for the three components of the electric field. Magenta and red vertical dashed lines represent anomalous peaks at ∼180 Hz and at ∼630, respectively. Colors are representative of the ${\u03f5}_{rel}$ values.

**Figure 8.**CSES satellite Search Coil Magnetometer observations. Box (

**A**) Environmental background evaluated over the earthquake epicenter in terms of ${\overline{\u03f5}}_{rel}$ vs. time and frequency for the reference quiet condition ($M\le 2$; Sym-H = [10 nT, −10 nT]; AE < 100 nT) for the three components of the magnetic field; Box (

**B**) ${\u03f5}_{rel}$ evaluated over the earthquake epicenter on 5 August 2018 for the three components of the magnetic field. Magenta vertical dashed lines represent anomalous peaks at ∼180 Hz. Colors are representative of the ${\u03f5}_{rel}$ values.

**Figure 9.**CSES satellite LAP observations. Panel (

**a**) Ionospheric plasma density: blue line is the observations for the entire 5 August 2018 orbit; red dashed line is baseline; black line represents the plasma density fluctuations; Panel (

**b**) environmental background evaluated over the earthquake epicenter in terms of ${\overline{\u03f5}}_{rel}$ vs. time and frequency for the reference quiet condition ($M\le 2$; Sym-H = [10 nT, −10 nT]; AE < 100 nT); panel (

**c**) ${\u03f5}_{rel}$ evaluated over the earthquake epicenter on 5 August 2018; panel (

**d**) plasma density anomaly ($\rho *$) obtained by the superposition of the two frequency components, ${T}_{1}=67\pm 1$ s and ${T}_{2}=111\pm 1$ s. Orange horizontal dashed line represents the time at which the satellite flew over the EE position. Colors in panels (

**b**,

**c**) are representative of the ${\u03f5}_{rel}$ values.

**Figure 11.**Behavior of the FLR eigen-frequency as modeled by an atmospheric pressure gradient caused by a seismic event.

**Table 1.**Geographic location of the International GNSS Service stations used for the vTEC evaluation.

Site | City | Country | Latitude | Longitude |
---|---|---|---|---|

ALIC00AUS | Alice Springs | Australia | −23.67${}^{\circ}$ | 133.89${}^{\circ}$ |

BAKO00IDN | Cibinong | Indonesia | −6.49${}^{\circ}$ | 106.85${}^{\circ}$ |

BNOA00IDN | Benoa | Indonesia | −8.7465${}^{\circ}$ | 115.21${}^{\circ}$ |

BTNG00IDN | Bitung | Indonesia | 1.4389${}^{\circ}$ | 125.19${}^{\circ}$ |

CIBG00IDN | Cibinong | Indonesia | −6.4904${}^{\circ}$ | 106.85${}^{\circ}$ |

DARW00AUS | Darwin | Australia | −12.8437${}^{\circ}$ | 131.13${}^{\circ}$ |

JOG200IDN | Yogyakarta | Indonesia | −7.7638${}^{\circ}$ | 110.37${}^{\circ}$ |

KAT100AUS | Katherine | Australia | −14.3760${}^{\circ}$ | 132.15${}^{\circ}$ |

MRO100AUS | Boolardy Station | Australia | −26.6966${}^{\circ}$ | 116.64${}^{\circ}$ |

NTUS00SGP | Singapore | Singapore | 1.3458${}^{\circ}$ | 103.68${}^{\circ}$ |

PGEN00PHL | General Santos City | Philippines | 6.0649${}^{\circ}$ | 125.13${}^{\circ}$ |

PPPC00PHL | Puerto Princesa City | Philippines | 9.7729${}^{\circ}$ | 118.74${}^{\circ}$ |

XMIS00AUS | Christmas Island | Australia | −10.4499${}^{\circ}$ | 105.69${}^{\circ}$ |

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Piersanti, M.; Materassi, M.; Battiston, R.; Carbone, V.; Cicone, A.; D’Angelo, G.; Diego, P.; Ubertini, P.
Magnetospheric–Ionospheric–Lithospheric Coupling Model. 1: Observations during the 5 August 2018 Bayan Earthquake. *Remote Sens.* **2020**, *12*, 3299.
https://doi.org/10.3390/rs12203299

**AMA Style**

Piersanti M, Materassi M, Battiston R, Carbone V, Cicone A, D’Angelo G, Diego P, Ubertini P.
Magnetospheric–Ionospheric–Lithospheric Coupling Model. 1: Observations during the 5 August 2018 Bayan Earthquake. *Remote Sensing*. 2020; 12(20):3299.
https://doi.org/10.3390/rs12203299

**Chicago/Turabian Style**

Piersanti, Mirko, Massimo Materassi, Roberto Battiston, Vincenzo Carbone, Antonio Cicone, Giulia D’Angelo, Piero Diego, and Pietro Ubertini.
2020. "Magnetospheric–Ionospheric–Lithospheric Coupling Model. 1: Observations during the 5 August 2018 Bayan Earthquake" *Remote Sensing* 12, no. 20: 3299.
https://doi.org/10.3390/rs12203299