Estimates of electromagnetic and turbulent energy dissipation rates under the existence of strong wind shears in the polar lower thermosphere from the European Incoherent Scatter (EISCAT) Svalbard radar observations

Abstract

[1] Turbulent and electromagnetic energy dissipation rates in the altitude range of ∼98–116 km have been estimated using data obtained from the European Incoherent Scatter Svalbard radar (ESR) observations on 22 September 1998 and 12 March 1999. Solar and geomagnetic activities were moderate and quiet during the observational periods, respectively. The horizontal neutral wind fields derived from the ESR observations show strong vertical shears and temporal variations. The Richardson numbers for the wind fields sometimes show values less than 0.25. The turbulent energy dissipation rates estimated from the wind shears have wavy structures depending on the shear distributions and decrease with height rapidly from ∼110 km. The turbulent energy dissipation rates also have maxima, which sometimes exceed 1 W/kg, in the region ∼102–108 km. The electromagnetic energy dissipation rates are strongly controlled by the neutral wind in the region below ∼110 km, while the Pedersen conductivity affects the energy dissipation above ∼110 km. On average, the two energy dissipation rates change their relative importance at ∼109 km on 22 September 1998 and at ∼112 km on 12 March 1999, depending on the wind shear, the electric field, and the Pedersen conductivity. When the electromagnetic energy fluxes are small, the contribution of the neutral wind to the electromagnetic and turbulent energy dissipation becomes large and the region, where turbulent energy dissipation is dominant, also expands upward. The magnitude and the dissipation height of the energy poured from the upper and lower regions seem to be strongly dependent on the neutral wind particularly below ∼110 km. The variations of the two energy dissipation rates may have significant effects on energetics in the polar lower thermosphere.