Stable and unstable response of the plasma in the ionospheric F region to the spatial packet of atmospheric gravity waves (AGW) of the lithospheric origin

Rapoport, Yu. G.
Kiev National Taras Shevchenko University, Kiev, Ukraine
Boundary conditions for excitation of AGW by the lithospheric source are obtained. It is
proposed to take into account spectral peculiarities of the excited AGW including the presence of
reactive (non-propagating vertically) modes. Spatial asymmetry of the ionospheric response (IR)
in the oblique geomagnetic field and presence of the maximum of dependence of the IR on the
AGW period are shown using 2D model. Connection between spatial shapes of the IR and
lithospheric source is found using 2D and 3D models.
The new analytical and numerical model for the response of unstable near-equatorial
ionosphere is proposed. We propose that the observed seismogenic excitations of plasma
concentration in the equatorial F-region after strong earthquakes could be connected with the
excitation of the spatial packet of AGW by the lithospheric source and subsequent development
of the Rayleigh-Taylor instability (RTI). In distinction to the known theory of RTI, we took into
account the driving force for the RTI connected not only with vertical, but also with horizontal
component of the AGW velocity; moreover, the last exceeds the other components of the driving
force. Development of the plasma instability in the presence of not a single harmonic AGW, but
of a wave packet excited by the lithospheric source is searched. Results of calculations
correspond qualitatively to the main features of observed structures in near-equatorial plasma
after strong earthquakes [1]: (1) regions of plasma perturbations are shifted for few thousand
kilometres to the west relatively to the maximum of AGW field in the presence of the east wind
(Figs. 1, 2); (2) spatial periodicity with typical periods ~ 600-800 km is revealed both in neutral
and plasma oscillations (Figs. 1b, 2b); (3) regions of plasma perturbations are localized and have
typical dimensions of order of 1000 km (Figs. 1b, 2b); (4) maximal value of amplitude of order
of 60-80% could be reached, which proves a presence of instability. We propose that observed
structure of the IR could be explained by the dynamics of two groups of (possibly) unstable
plasma modes (in the presence of wind and ion motion): modes with horizontal wavelength
(~800 km), very closed to the boundary between vertically propagating and reactive modes, and
with much wider spectrum, shorter wavelengths, lower amplitudes and (possibly) larger
increments of instability. The effect of localised structure formation could be “linear analogy in
the unstable media” (with subsequent nonlinear stabilization) of nonlinear development of
narrow wave fronts.

Fig. 1b. Spatial distribution of normalized relative change of electron concentration in the
absence of wind. Horizontal and vertical velocities of ions are -50 m/s and -10 m/s, respectively.
Time since the moment of “switching on” the influence of AGW on the development of plasma
instability is 74 min.

Fig. 2b. Spatial distribution of normalized relative change of electron concentration in the
presence of wind with horizontal velocity 60 m/s. Horizontal and vertical velocities of ions are
20 m/s and -5 m/s, respectively. Time since the moment of “switching on” the influence of
AGW on the development of plasma instability is 3h20 min.
[1] Fedorenko А. К., Lizunov G. V., Rotkel Х. Satellite measurements of quasiwave
perturbations of the atmosphere on the altitudes of F region caused by powerful earthquakes //
Geomagnetism and aeronomy.- 2005.- V. 45, № 3.-P. 403-410.

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