High‐latitude lower thermospheric neutral winds at EISCAT and Sondrestrom during LTCS 1

Abstract

The incoherent scatter radar located at S0ndre Str0mfjord, Greenland (67øN, 51øW, 74.5øA) and the EISCAT incoherent scatter facility located in northern Scandinavia (69.5øN, 19øE, 66.3øA) both obtained E and F region measurements during the first campaign of the Lower Thermosphere Coupling Study (LTCS 1, September 21-25, 1987). Neutral winds deduced from these measurements have been analyzed for their mean flow and tidal components. A number of the altitude profiles for the mean winds and the diurnal and semidiurnal wave components at the two radar locations show similar variations with height, indicating that latitudinal rather than longitudinal effects are dominant in determining the observed wind field. Diurnal tidal amplitudes and phases are reasonably well represented by theoretical model results (Forbes, 1982). The semidiurnal amplitudes and phases, although somewhat consistent between the two radars, are not well represented in equinox tidal model results (Forbes and Vial, this issue). Results from both radars indicate a vertical wavelength for the zonal semidiurnal oscillation of approximately 60 km. During a period of impulsive magnetospheric forcing (September 22-23), winds deduced from measurements at both radars show enhanced eastward flows near midnight accompanied by equatorward winds at Sondrestrom. Comparison with the results of a National Center for Atmospheric Research thermosphere-ionosphere general circulation model (TIGCM) simulation of the LTCS I interval shows generally better agreement with the observations at EISCAT than at Sondrestrom. During the period of activity on September 22-23 the TIGCM is reasonably successful at simulating the eastward surge near midnight in the EISCAT (but not in the Sondrestrom) observations and the equatorward flow after midnight in the Sondrestrom results. The observed winds in magnetic latitude and magnetic local time coordinates indicate the presence of anticyclonic divergent flow near dusk and cyclonic converging flow near dawn. and therefore geographically controlled, can be expected to be similar, assuming that longitudinal effects are negligible. This paper presents a comparison of the neutral winds derived from the measurements obtained at the EISCAT and Sondrestrom radars during the period of simultaneous observations. A companion paper presents discussion of the measurements obtained at Sondrestrom alone [Johnson, this issue]. The following section briefly outlines the data acquisition modes utilized at the two facilities (see Johnson [this issue] for further details). An analysis of the deduced neutral winds follows, focusing on both the average flow and tidal structures observed during the interval as well as on the response of the neutral winds to an interval of impulsive forcing. Comparisons with appropriate theoretical models are introduced in the discussion of the radar results.

Studies of the E-Region Neutral Wind in the Auroral Ionosphere Using Two Long-Run Data.

“…The period of the 21-25 September 1987 was the core period of the first Lower Thermosphere Coupling Study (LTCS 1) campaign and the dataset obtained by EISCAT has been analyzed in terms of mean flow and tidal components of the neutral wind by Johnson and Virdi (1991). They also compared neutral wind data obtained both by EISCAT and Sondre Stromfjord (67°N, 51°W, 74.5 °A) radars.…”

“…In our work we use MSIS-86 model (Hedin, 1987) for neutral densities and the formula by Schunk and Walker (1973) for a derivation of ion-neutral collision frequency. Both are also used in the studies by Johnson (1991) and Johnson and Virdi (1991) for the Sondre Stromfjord data, and Brekke et al (1994) and Nozawa and Brekke (1995) of averaged neutral winds in quiet-and disturbed-times, respectively. Comparison of mean winds as well as tidal components with the Sondre Stromfjord results by Johnson (1991) will be given in Subsection 4.2.…”

“…For the EISCAT measurements, they used CIRA72 model for neutral densities and the expression for the ion-neutral collision frequency given by Banks and Kockarts (1973), and then derived three tidal components such as 24h, 12h and 8h using a LSF technique. As mentioned in Johnson and Virdi (1991), a choice of a neutral model atmosphere is important as the derived neutral wind velocities depend very much on the model chosen. Differences in collision frequencies from the two models are expected to amount to 35 % (Johnson and Virdi, 1991).…”

“…As mentioned in Johnson and Virdi (1991), a choice of a neutral model atmosphere is important as the derived neutral wind velocities depend very much on the model chosen. Differences in collision frequencies from the two models are expected to amount to 35 % (Johnson and Virdi, 1991). At present it is rather difficult to determine what model can give the best neutral densities and ion-neutral collision frequencies.…”

“…Usually the tidal components at a particular height are derived by the method of a least squares fit (LSF). The fit is often made by determining the mean wind and amplitude and phase of one or more tidal components such as the diurnal, semidiurnal and shorter period tides; this has been done in the works of Johnson et al (1987), Williams and Virdi (1989), Manson et al (1990Manson et al ( , 1992, Johnson (1991), Johnson and Virdi (1991), Kunitake and Schlegel (1991), Virdi and Williams (1993), and Kofman et al (1996). Brekke et al (1994) introduced the method of Fast Fourier Transform (FFT; discrete Fourier transform) for spectral analysis of averaged 24-hour data sets of the neutral wind velocities at different E-region heights.…”