Simulations of Neutral Structure in the E region at High Latitudes

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0310 Airglow And Aurora, 3369 Thermospheric Dynamics (0358), 3384 Acoustic-Gravity Waves, 3389 Tides And Planetary Waves

Scientific paper

Measurements made during the JOULE and ARIA (Atmospheric Response in Aurora) rocket campaigns reveal large winds and strong shears in the high latitude E region. Large vertical shears are seen in neutral winds in the auroral region below 120 km altitude, with large maxima between around 110 and 120 km. Observations suggest the regions below these shears may be unstable, with Richardson number less than 0.25. If instabilities are present they may have important effects, including changes in turbulent diffusion and composition, and the generation of waves. The effects of the dissipation of these strong shears on the general circulation and energy budget are unknown, but may be important. The neutral wind vector in this region is found to rotate with height, suggesting the presence of propagating tides, and the magnitude of the wind maximum around 110 to 120 km altitude also appears to have a dependence on geomagnetic activity. In order to understand the observations, simulations have been performed using a high resolution limited area model, using values of electric fields and ion densities measured during the rocket campaigns to provide input parameters, and with global background winds and tides provided by the CTIP (Coupled Thermosphere Ionosphere Plasmasphere) model. The simulations reproduce many features of the observations, which appear to be related to a combination of auroral and tidal forcing, but the strengths of the shears around 110 to 120 km altitude are not reproduced for any values of tidal and auroral forcing parameters. It is therefore possible that other processes may be needed to generate the strong shears observed. One mechanism might be gravity wave forcing, since gravity waves are frequently observed in this altitude range, and may be dissipated at these altitudes, and also tend to have short vertical wavelengths, consistent with the vertical scale lengths of the observed structure. Model runs have been performed using the high resolution model, to simulate the effects of gravity wave forcing in the high latitude E region, during periods of enhanced aurora forcing. Considerable vertical structure is produced in initial simulations, and the results are compared with observations made during the rocket campaigns.

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