Thermospheric wind measurements with EISCAT

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Thermospheric wind measurements with the EISCAT UHF radar around the evening Harang discontinuity are presented both in the E- and F-layers. Within the E-layer auroral oval the Lorentz and Coriolis force are shown to be more or less in balance. The neutral velocity is a factor of the order of two smaller than the ion velocity and is on average advanced 90° in a clockwise direction compared to the ion velocity. In the low electron density region just before the Harang discontinuity and outside the auroral oval a large (~250 m s −1), thermally dominated neutral wind is closely followed by the ion wind in the antisolar direction. There is also a large downward flow present just before the Harang discontinuity. In the F-layer the neutral wind approximately follows the ion convection pattern, except for a couple of hours after the sudden change in the ion convection just after the passage of the evening Harang discontinuity. The close resemblance between the equilibrium ion and neutral flow when the neutral-ion collision frequency is close to twice the Earth's angular velocity may be connected to back pressures created by Joule heating in the case of an appreciable ion-neutral velocity difference.