Search

A set of tabulated functions called ‘Hough Mode Extensions’ (HMEs), which represent numerical extensions of classical Hough modes into the viscous regime of the thermosphere, are used to least-squares fit a climatological data base of tidal measurements. The data base consists of monthly average vertical profiles of semidiurnal amplitudes and phases at 17 radar sites accessing some part of the 80–150 km height region. The radars are distributed between 78 S and 70 N latitude, and each one provides measurements of one or more of the following: eastward wind, southward wind, perturbation temperature. As a result of the fitting process, a single complex normalizing coefficient is derived for each month and for each of the four HMEs, designated (2,2), (2,3), (2,4) and (2,5) after their classical Hough function designations. Once the complex coefficients are derived, reconstruction by weighted superposition of the HMEs results in globally continuous specifications of semidiurnal horizontal and vertical wind, temperature, pressure, and density throughout the 80–150 km height region. The tidal variations in density, in particular, provide greater accuracy for several aerospace applications. The methodology developed here can also be utilized to derive tidal lower boundary conditions for Thermospheric General Circulation Models (TGCMs), or as a basis for future empirical model development. Comparisons are also made with HME coefficients and global tidal fields from the Forbes and Vial [(1989) J. atmos. terr. Phys. 51 , 649] numerical tidal model.

On decrit les observations de la region F a moyenne latitude, aux environs de L=4, le jour. On a utilise une methode de Doppler differentielle de mesures au sol. Cette methode a permis de separer les effets en temps local et temps universel

The eastward component of the electric field was measured simultaneously at ionospheric heights and in the equatorial plane of the magnetosphere by using the incoherent scatter radar at Millstone Hill and whistler data from Siple, Antarctica, respectively. The locations of the two data sets correspond to approximately magnetically conjugate areas within the outer plasmaphere, and thus a unique opportunity arises for testing the electric field mapping hypothesis and for intercomparing the measurement techniques. The whistler results are averaged for L = 3.5-4.7, while the radar data correspond to L = 4.4. The measurements represent time averages over about 20 minutes. There is excellent correlation during most of the 12 hour period of simultaneous data on 10 July 1978. The period was one of moderate disturbance (Kp = 2-3). A 500 gamma substorm developed around 0830 UT (0330 LT). During this substorm, both of the electric field measurements showed a large westward perturbation, in agreement with previous observations. The meridional component of the electric field was also measured at Millstone Hill. Taken together, these data indicate an enhanced flow with components inward and towards dawn during the substorm. Except for a time near magnetic midnight the magnitude of the zonal field scales from the ionosphere to the equator in direct ratio with the spreading of magnetic field lines in the IGRF 1975 model. The whistler data clearly show that the measurements were obtained inside the plasmasphere. The middle latitude substorm perturbations appeared to be related to a reconfiguration of the magnetospheric current system.

The Atmospheric Explorer-C (AE-C) satellite passed almost directly over the Millstone Hill incoherent scatter radar station on 14 February 1974 and passed within the near vicinity of the station on 15 February 1974. Measurements of ionospheric and atmospheric properties were made simultaneously by the incoherent scatter radar and the AE-C satellite instruments. The incoherent scatter radar measured vertical profiles of the electron and ion temperatures and electron density and these data were used to derive a neutral gas temperature profile. The AE-C satellite measured the electron and ion densities and electron and ion temperatures, neutral gas composition, solar EUV flux, photoelectron spectra, the 6300 A volume emission rate profile and the distribution of NO along the satellite path. These simultaneous measurements provide a consistent set of data to examine current F-region theory in the daytime ionosphere. We used a time-dependent coupled model of the ionospheric E- and F-region to calculate the ionospheric properties over Millstone Hill at the times of the AE-C crossings and then compared the calculated structure to the observed structure. The results show good agreement between the incoherent scatter radar measurements and the model calculations. There is also good agreement among satellite and incoherent scatter radar measurements and model calculations for the altitude of the satellite crossing, 161 km. The satellite measurements along the orbital path, however, reveal considerable horizontal gradients in the measured ionospheric properties.

Model predictions of the diurnal variations of plasma convection velocities and electron densities in the high-latitude ionosphere were compared with Millstone Hill incoherent scatter radar observations for an equinox day on which there was moderate magnetic activity. On the observation day, three major morphological features were evident at 500 km, including a dayside high density region, a nightside mid-latitude trough, and a region of slightly enhanced densities in the auroral zone. Although the dayside high density region was due to solar EUV radiation, it was not symmetrical about local noon (1000-1900 LT sector) owing to the effect of horizontal transport. The nightside mid-latitude trough was the deepest, the widest, and reached its most equatorward latitude in the morning sector. The model was able to reproduce these two features quite accurately. In the dusk sector, the trough was filled in, and its latitudinal extent was restricted by a discrete auroral arc, a feature not included in the ionospheric model. Except for this arc region, the enhanced electron densities in the auroral zone were adequately described by the average precipitation fluxes used in the model. The observed plasma drift velocities were consistent with a two-cell, asymmetric convection pattern with enhanced flow in the dusk sector. Outside the polar cap, the fall-off of the magnetospheric potential with latitude was proportional to the inverse of the sine of colatitude to the fourth power. The convection pattern employed in the model included these features and had a 60 kV cross-polar-cap potential. Efforts to reproduce the observed behavior by using a larger cross-polar-cap potential (90 kV) or a symmetric pattern are also presented. These were generally less successful and demonstrate the sensitivity of the morphology of the F region at high latitudes to the convection process.

The Atmosphere Explorer-C satellite (AE-C) is uniquely suited for correlative studies with ground-based stations because its on-board propulsion system enables a desired ground station overflight condition to be maintained for a period of several weeks. It also provides the first low-altitude (below 260 km) comparison of satellite and incoherent scatter electron and ion temperatures. More than 40 comparisons of remote and in situ measurements were made by using data from AE-C and four incoherent scatter stations (Arecibo, Chatanika, Millstone Hill, and St. Santin). The results indicate very good agreement between satellite and ground measurements of the ion temperature, the average satellite retarding potential analyzer temperatures differing from the average incoherent scatter temperatures by −2% at St. Santin, +3% at Millstone Hill, and +2% at Arecibo. The electron temperatures also agree well, the average satellite temperatures exceeding the average incoherent scatter temperatures by 3% at St. Santin, 2% at Arecibo, and 11% at Millstone Hill. Several temperature comparisons were made between AE-C and Chatanika. In spite of the highly variable ionosphere often encountered at this high-latitude location, good agreement was obtained between the in situ and remote measurements of electron and ion temperatures. Longitudinal variations are found to be very important in the comparisons of electron temperature in some locations. The agreement between the electron temperatures is considerably better than that found in some earlier comparisons involving satellites at higher altitudes.