Modeling of Sporadic-E Structures from Wind-Driven Kelvin-Helmholtz Turbulence

Atmospheric wind shears control the irregularity structure in the E-region. Near 100 km altitude, where the ion collision frequency is much larger than the ion cyclotron frequency, the degree of shear turning and the direction of maximum shear gradient determine the location of the E-layer peak. The effects of speed shears on the three-dimensional structure of the E-layer are modeled using a system of coupled equations for continuity and momentum that describes both the neutrals and plasma. In the coupled model, the large amplitude components of the neutral wind shear drive the neutral atmosphere unstable and produce KelvinHelmholtz (K-H) billows. At the same time, the three-dimensional structure of the same wind shear compresses the ions vertical profile that is not necessarily centered on the node of the wind shear. The shear component is the source of Kelvin-Helmholtz turbulence and it produces quasi-periodic (Q-P) irregularities in the layer along the horizontal direction of the shear. The location of the turbulence in the ions is dependent on the offset lifting or lowering by the turning component of the neutral wind shear. The numerical results of the model study are consistent with the radar and optical observations. 1.0 INTRODUCTION In the E-region between 90 and 120 km altitude, because the plasma density is one-million times less than the neutral density, neutral wind dynamics plays a controlling role for determining the plasma irregularity structures. Several sources of neutral drivers of E-region irregularities have been proposed including (1) gravity waves [Woodman et al., 1991], (2) plasma instabilities driven directly by neutral wind shears [Cosgrove and Tsunoda, 2003; 2004; Tsunoda and Cosgrove, 2004], and (3) the Kelvin-Helmholtz (KH) instability generated by wind shears that produces turbulence that couples to the plasma [Larsen, 2000; Bernhardt, 2002]. This latter process is described in the next sections. First, the strong evidence of neutral Bernhardt, P.A.; Werne, J.; Larsen, M.F. (2006) Modeling of Sporadic-E Structures from Wind-Driven Kelvin-Helmholtz Turbulence. In Characterising the Ionosphere (pp. 34-1 – 34-14). Meeting Proceedings RTO-MP-IST-056, Paper 34. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 01 JUN 2006 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Modeling of Sporadic-E Structures from Wind-Driven Kelvin-Helmholtz Turbulence 5a. CONTRACT NUMBER