Zonal neutral winds at equatorial and low latitudes

Abstract

Fabry–Perot interferometric (FPI) measurements of thermospheric zonal neutral winds at Arequipa, Peru ( 16.7° S, 71.5° W, −2.7° dip ), and Carmen Alto, Chile ( 23.1° S, 69.4° W, −10.2° dip ), were collected during the solar minimum periods of September–October 1996 and 1997. The data set included 39 nights from Arequipa and 14 nights for Carmen Alto, with 8 nights of simultaneous observations. Analysis of averaged results found the peak evening zonal neutral wind speed of ∼127±15 m/ s eastward for the Arequipa observatory, which is located near the magnetic equator, to occur between 21:30 and 22:30 LT. In contrast, the peak evening zonal winds of ∼100±10 m/ s eastward observed from Carmen Alto, which is located near the crest of the equatorial ionization anomaly (EIA), occurred ∼0.5– 1 h later. These measurements represent the first case of groundbased FPI observations of the so called equatorial temperature and wind anomaly (ETWA) over such a small latitude range in the same longitude sector. This reduction in speed of ∼20–25% at Carmen Alto relative to Arequipa is attributed to increased ion drag at Carmen Alto caused by the higher electron density within the EIA region at altitudes of 220– 300 km . Model studies were conducted using electron density and neutral atmosphere parameters form the parameterized ionospheric model (PIM) and the mass spectrometer incoherent scatter (MSIS) models, respectively, to calculate the ratio of ion–neutral collision frequencies at the two sites. We found that the increase in electron density within the EIA was sufficient to account for the observed reduction in the zonal wind. Thus, this analysis confirms the dominant role of ion drag in modulating thermospheric dynamics at equatorial latitudes. A comparison of the FPI results with the predictions by two current neutral wind models, the Horizontal Wind Model-90 and the NCAR Thermospheric Ionosphere Electrodynamics General Circulation Model (TIEGCM), reveals that neither is able to reproduce accurately the latitude dependence reported here. Model refinements for electrodynamics and improved resolution are suggested.