Latitudinal variations of neutral wind structures in the lower thermosphere for the March equinox period

Abstract

Neutral winds in the lower thermosphere (95– 130 km ) measured during the March equinox period (1991–1992) by ground-based incoherent scatter radars at Arecibo (18°N), Millstone Hill (42.5°N), and Sondrestrom (67°N) and by the space-based wind imaging interferometer (WINDII) are compared and show overall good agreement but some differences. At 18°N, the wind field in the altitude region of 95– 110 km displays prevailing upward propagating diurnal tides with wavelengths of about 22 km . The diurnal structure is affected by the semidiurnal tide resulting in regular minima separated by 11– 12 h . At altitudes above 110 km , the diurnal tide dominant wind structure changes to the semidiurnal tide dominant structure as illustrated clearly by WINDII data with 24 h coverage. Winds at 42.5°N and 67°N show similar structures in which winds at 105– 115 km are generally anti-sunward. Daytime ISR winds show prevailing upward propagating semidiurnal tides with wavelengths of 35– 70 km . Winds from WINDII reveal the existence of the in situ thermospheric diurnal tide with amplitudes comparable to those of the semidiurnal tide. The superimposition of the two tides result in a wind field stronger during daytime than during nighttime at mid- and high-latitudes. Geomagnetic influence on neutral winds is negligible at low- and mid-latitudes under solar quiet conditions, but is observed at high-latitudes, where wind vectors follow a clockwise one-cell pattern at altitudes above about 118 km in geomagnetic coordinates. Most recent simulations for the three latitudes provided by the NCAR thermosphere/ionosphere/mesosphere electrodynamics general circulation model are compared to the observations. The results at low- and mid-latitudes agree well with the observed winds in both wind structures and magnitudes, and reveal details of wave transition. Simulations for high-latitudes are less satisfactory, and require further improvements.