Work Package 6 - Theoretical investigation on the coupling between Earth’s lithosphere, atmosphere, and ionosphere


Objectives

The objectives of WP6 were:

  1. The investigation of the coupling of the long scale atmospheric structures with internal gravity waves.
  2. Numerical models of TGW passage from the earthquake zones up to the ionosphere and subsequent perturbation of the ionosphere.
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Description

A number of different theoretical models describing the influence of seismic activity upon the atmosphere and ionosphere have been proposed to explain the observed seismo-electromagnetic phenomena. Currently, there are two main concepts used to explain energy penetration from the earthquake origin through the atmosphere and into the ionosphere, namely the modification of electric fields or the excitation of internal gravity waves. The first mechanism was detailed by Pulinets and Boyarchuk (2005). This mechanism seems to be improbable, because it assumes the presence of an unrealistically large ground level ground electric field. The existence of such a field covering an area of over 100 km2 for a long time interval is not supported by observations.

Low-frequency vibrations of the Earth, atmospheric heating, and gas emanation can lead to the generation and upward propagation of gravity waves and subsequently a perturbation of the ionosphere (Mareev et al., 2002). The existence of such sources has been confirmed by the observation of temperature variations of a few degrees in the near Earth’s surface layer that accompany seismic activity (Tronin, 1999).

A new paradigm in upper atmospheric and ionospheric physics has begun to emerge, starting with discoveries (Immel et al. 2009) from the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) mission. These observations show that the ionosphere regularly responds to large-scale tropospheric structures such as planetary waves, zonal winds, and tidal waves. In this project we will investigate the relative importance of the coupling due to the variability of the zonal winds and ionospheric conductivity, changes in the neutral and ion composition, large scale atmospheric waves, and zonal flows that actually propagate to higher altitudes directly forcing the ionosphere. We has used results obtained for the interpretation of satellite observations of large scale ion and electron density perturbations found during earthquakes. Furthermore, we also investigated the role of gravity waves in producing large scale (tidal) variability in the ionosphere.

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Highlights

Task 6.1 - Increment of the parametric instability of drift type waves characterising generation of zonal winds

A theoretical investigation of the coupling between the lithosphere, atmosphere, and ionosphere by internal gravity waves (IGW) was carried out. Previous studies have shown that IGW may be generated by earthquakes, volcanic eruptions, large explosions, and the launch of powerful rockets. Task T6.1 investigated how these waves propagate into the atmosphere and ionosphere, dissipating their energy and creating observable perturbations.

T6.1 demonstrated how nonlinear zonal structures (convection cells) can be excited by finite amplitude IGW based on the parametric decay instability. The spatial scales of the structures created are consistent with observations.

Task 6.2 - Analysis of the ionospheric response to the atmosphere tidal motion

Task 6.2 investigated a theoretical justification of the internal gravity waves (IGWs) propagation in the model of the atmosphere taking into account finite scale of the vertical gradient of the zonal winds. The results described here build on those already obtained in WP6.

Ionospheric disturbances above seismically active regions appear as specific inhomogeneities typically a few days before strong earthquakes. These inhomogeneities, with characteristic scales of the order of several hundreds of kilometers, are thought to be caused by internal gravity waves propagating from the neutral atmosphere to the ionosphere. The meteorological conditions associated with such seismic processes indicate a clear change in atmospheric parameters before strong earthquakes (EQs).

The IGW disturbances grow in amplitude by several orders of magnitude as they attain ionospheric altitudes. The nonlinear interaction of IGWs with zonal winds has been investigated as part of Task 6.1. The generation of zonal structures limits the growth of IGW perturbations. This mechanism provides an effective channel for the transfer of energy from small-scale turbulence of the waves to global convective motions of the atmosphere. This process is an example of an inverse cascade paradigm that exists in the theory of two-dimensional anisotropic turbulence. Nonlinear IGWs can exist in the form of vortex structures. In these studies it was shown that the horizontal velocity of the vortices should be of the order of or greater than the speed of sound. Such supersonic motion of the structures should initiate shock waves and, therefore, is inconsistent to the initial assumptions. These models also neglect the effects of the vertical zonal wind gradients on the propagation of IGWs. It is known that in a real atmosphere the zonal winds are inhomogeneously distributed with height.

Nonlinear IGWs in a turbulent atmosphere with the zonal winds can generate the vortex structures. However, the effects due to the vertical zonal wind shear were neglected in earlier studies that showed that the solitary vortex propagation velocity should equal or even exceed the speed of sound. In the Earth’s atmosphere with zonal wind shear vortex structures (convective cells) can exist when where U(z) is the wind velocity, v is the horizontal speed of vortices in a coordinate system connected with the Earth, U’=dU/dz and U’’=d2U/dz2. When the effects related to the zonal wind shear are neglected and γ=1.4 the vortex velocity in the coordinate system associated with the zonal wind should be of the order of or greater than the sound velocity cs, |U-v|>0.9cs. From observations the speed of the zonal wind in the atmosphere at altitudes 35-100km is of the order of 10ms-1 with vertical shears (shift) of the zonal wind U’ at an altitude of 80-100km of the order 2x10-2s-1 and at the height of 100 km can attain the value 4x10-2s-1 . From these observations it follows that may be of the order or even higher than the Brunt-Vaisala frequency, ωg≈2x10-2s-1. Assuming, |U’’| << |U’|/H, |U’|=(2-4)x10-2s-1 and H=7km one can obtain from (1) that |U-v| > (60-110)ms-1. It follows that in an atmosphere that has a shear in the zonal wind velocity the vortex velocity can be substantially smaller than the velocity of sound in the atmosphere. This condition for the existence of the IGWs vortices is more realistic. Owing to collective vortex motions in the atmosphere and related motion in the ionosphere, the plasma and atomic oxygen densities increase. This agrees with DEMETER observations.

Task 6.3 - Numerical calculations of the ray tracing trajectories of gravity waves

The objective of task 6.3 was to investigate the profiles of the zonal winds, temperature, and density of the upper atmosphere in the form of analytic models. These models can subsequently be used as the basis for atmospheric models for investigations of the propagation of Internal Gravity Waves in the upper atmosphere and ionosphere.

Internal Gravity Waves may be created as a result of seismic activity. There is experimental evidence that IGW may be excited by low frequency vibrations of the Earth’s surface, atmospheric heating, or the emission of gasses from the crust. The IGW carry energy into the upper atmosphere where they eventually dissipate, perturbing the ionospheric/atmospheric environment in the process. The propagation of these waves depends very much on the properties of the atmosphere. These properties vary depending on location of observation, time of the year, etc.. The goal of this task was to determine analytic functions for the zonal winds and the Brunt-Vaisala frequency from experimental data.

Changes in wave power spectra

Figure 6.1: Zonal winds measurements (blue) and the analytic approximation (red).

These variations need to be modelled analytically in order for them to be used within numerical computations for the propagation of IGW using ray tracing techniques. Figure 6.1 shows measurements of the zonal winds measured over India at 12UT (blue) and their analytic approximation (red). The resulting analytic function can then be used in the numerical ray tracing calculations to investigate how IGW are modified as they propagate through the upper atmosphere and ionosphere.

The results of the ray tracing studies indicate that the propagation of IGW from their source region into the ionosphere depends critically of the choice of parameters used to define the properties of the local atmosphere.

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Reports

D6.1 Increment of the parametric instability of drift type waves characterising generation of zonal winds.
D6.2 A model for the generation of tidal ionosphere structures above the earthquake epicentres: Theory and comparison with satellite observations.
D6.3 Results of the numerical calculations of the ray trajectories of the gravity waves.

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