1. Alexander S. Medvedev1, Erdal Yiğit2 and Paul Hartogh1
Ion friction and geomagnetic influence on gravity wave propagation in the thermosphere/ionosphere
SCOSTEP, Toronto, 9-13 July, 2018
Alexander S. Medvedev1, Erdal Yiğit2 and Paul Hartogh1
Erdal Yiğit

2. Atmosphere-Ionosphere system
Atmosphere and ionosphere coexist.
If there was no geomagnetic field, ions would behave as other trace gases.
B imposes electromagnetic forces, and ions move differently than neutrals.
Ions affect neutrals via ion drag

3. Gravity waves and the thermosphere
Thermosphere is regularly disturbed by gravity waves.
In many cases, sources can be traced back to the troposphere.
Numerical experiments demonstrated that certain waves can penetrate high into the upper atmosphere despite the growing molecular diffusion and heat conduction.
However the dynamical importance of GWs in the thermosphere was not fully appreciated.

4. Gravity wave field But they are difficult/senseless to model4
Gravity waves occur every time and everywhere.
A divergent component of the wind field at p=10 mb, NCEP reanalysis data
But they are difficult/senseless to model

5. Parameterization N – number of harmonics in the spectrum5
Source:
N – number of harmonics in the spectrum
The goal is to quantify bion

6. GW and ion drag (acting on the mean flow)
June solstice
September equinox
GW drag (upper panels) and ion drag (lower panels).
GWs substantially modulate the ion drag via changes in the neutral circulation.
How ion drag influences GW effects?
Yiğit et al. [2009]
Yiğit et al. [2012]
Erdal Yiğit

7. Electromagnetic influence on GWs
0 for a quasi-neutral plasma
The generalized Ohm’s law

8. The physics Gyrofrequency >> nni >> wGW v
b is the normalized B/|B|, i.e., damping depends on the shape of the magnetic field, but not on its strength.

9. Undisturbed magnetic field
The latitudinal structure due to the dipole
shape

10. GW vertical damping
ky=0 (east-west propagating GW): latitudinally independent.
kx=0 (north-south propagation): damping is stronger in high latitudes
Latitudinal dependence of the form-factor for three characteristic propagation azimuths

11. Drag and heating/cooling rates

12. Some inferences
Magnetic field influences gravity waves via ion friction.
The GW damping is a function of the magnetic filed geometry but not strength.
Ion friction effects on GWs are anisotropic.
GW damping by ions depends on the latitude and direction of propagating harmonics.
Influence on the circulation will be studied with a thermospheric GCM.
Is there a link between GW-induced cooling rates and the magnetic field?
Medvedev, A. S., E. Yiğit, E., P. Hartogh (2017). Ion friction and quantification of the geomagnetic influence on gravity wave propagation and dissipation in the thermosphere-ionosphere. J. Geophys. Res. Space Physics, 122, 12,464–12,475.