Large field-aligned velocities observed by EISCAT

Large upward ion fluxes are observed near the polar cap/auroral zone boundary in the midnightdawn sector. Results from a recent model show that large upward velocities will occur in response to strong frictional heating and these results are consistent with the observations presented in this paper. There is particularly good agreement between the spatial and temporal morphology of the heating and the upward flux of ions which favours this mechanism to explain the observations. However, the time and location are also those associated with other acceleration mechanisms which cause upward motion of energised ions and these may also contribute to the large velocities observed.     

Real and spurious anti-correlations between short-period variations of field-parallel and field-perpendicular plasma velocities in EISCAT data

Measurement of the observed anti-correlation between the field-perpendicular component of F-region plasma velocity in the north-south plane and the downward, field-parallel component has been proposed as a way to determine the value of the O-O⁺ collision frequency. However, random noise errors in measurements of plasma velocity made at EISCAT may combine in analysis to induce a spurious anti-correlation between the derived values of these components which is hard to distinguish from any genuine anti-correlation.     

A comparison of ionization densities determined from spacecraft and incoherent scatter radar data

By comparing direct measurements taken from onboard Atmosphere Explorer spacecraft (AE), in eccentric orbit, with incoherent scatter radar (ISR) measurements taken from the ground, we illustrate both the merits and the difficulties involved in such comparisons. Five altitude profiles of ionization determined from AE, in near coincidence with ground stations making ISR measurements, compared favorably with the ISR data so long as the AE measurements were properly analyzed for the effects of variations in latitude and solar zenith angle along the spacecraft orbit.     

Model predictions of the occurrence of non-Maxwellian plasmas,and analysis of their effects on EISCAT data

The recent identification of non-thermal plasmas using EISCAT data has been made possible by their occurrence during large, short-lived flow bursts. For steady, yet rapid, ion convection the only available signature is the shape of the spectrum, which is unreliable because it is open to distortion by noise and sampling uncertainty and can be mimicked by other phenomena. Nevertheless, spectral shape does give an indication of the presence of non-thermal plasma, and the characteristic shape has been observed for long periods (of the order of an hour or more) in some experiments. To evaluate this type of event properly one needs to compare it to what would be expected theoretically. Predictions have been made using the coupled thermosphere-ionosphere model developed at University College London and the University of Sheffield to show where and when non-Maxwellian plasmas would be expected in the auroral zone. Geometrical and other factors then govern whether these are detectable by radar. The results are applicable to any incoherent scatter radar in this area, but the work presented here concentrates on predictions with regard to experiments on the EISCAT facility.     

EISCAT observations of tidal modes in the lower thermosphere

EISCAT has made regular measurements of plasma velocity at heights between 101 and 133 km in the E-region and at 279 km in the F-region as part of the Common Programme CP1. Correcting for the effect of the electric field as determined in the E-region, it is possible to estimate the neutral wind velocity in the E-region for a number of days in the period 1985–1987 when magnetic conditions were relatively quiet. These velocities display diurnal and semi-diurnal tidal oscillations. The diurnal tide varies considerably from day to day in both amplitude and phase. The semi-diurnal tide also varies in amplitude but displays a fairly consistent phase at each height and the variation of phase with height below 110 km indicates a dominant (2,4) mode. Above 120 km the variation of phase with height is slower which suggests that at these heights the (2, 4) mode is attenuated and the (2, 2) mode is more important. The results agree well with previous measurements at high latitude.     

Electron temperature enhancements in the polar E-region measured with EISCAT

We have found electron temperature enhancements up to 1000K at 110 km altitude using the EISCAT multipulse method which allows high spatial resolution measurements within E-region. This electron temperature enhancement which is closely related to the d.c. electric field strength, is in good agreement with theoretical estimates based on wave heating. The results of the measurements are presented together with a discussion of the electron gas heating, its height variations and its difference in the eastward and westward electrojet.     

Temperatures in the plasmasphere determined from VLF observations

Satellite and ground-based VLF recordings were made at SANAE, Antarctica from 1976 to 1979. In this paper we combine ground and satellite observations to determine temperatures in the plasmasphere. Scale heights in the plasmasphere are determined at high altitudes using a diffusive equilibrium model and measurements of equatorial electron densities and densities at about 3000 km. The temperatures corresponding to these scale heights show a gradual increase with increasing L-value and sharp increases of about 2000 K just inside the plasmapause.     

EISCAT observations of ion composition and temperature anisotropy in the high-latitude F-region

The papers by Winseret al. [(1990) J. atmos. terr. Phys.52, 501] and Häggström and Collis [(1990) J. atmos. terr. Phys.52, 519] used plasma flows and ion temperatures, as measured by the EISCAT tristatic incoherent scatter radar, to investigate changes in the ion composition of the ionospheric F-layer at high latitudes, in response to increases in the speed of plasma convection. These studies reported that the ion composition rapidly changed from mainly O⁺ to almost completely (>90%) molecular ions, following rapid increases in ion drift speed by >1 km s⁻¹. These changes appeared inconsisent with theoretical considerations of the ion chemistry, which could not account for the large fractions of molecular ions inferred from the obsevations. In this paper, we discuss two causes of this discrepancy. First, we reevaluate the theoretical calculations for chemical equilibrium and show that, if we correct the derived temperatures for the effect of the molecular ions, and if we employ more realistic dependences of the reaction rates on the ion temperature, the composition changes derived for the faster convection speeds can be explained. For the Winser et al. observations with the radar beam at an aspect angle of ϕ = 54.7° to the geomagnetic field, we now compute a change to 89% molecular ions in < 2 min, in response to the 3 km s⁻¹ drift. This is broadly consistent with the observations. But for the two cases considered by Häggström and Collis, looking along the field line (ϕ = 0°), we compute the proportion of molecular ions to be only 4 and 16% for the observed plasma drifts of 1.2 and 1.6 km s⁻¹, respectively. These computed proportions are much smaller than those derived experimentally (70 and 90%). We attribute the differences to the effects of non-Maxwellian, anisotropic ion velocity distribution functions. We also discuss the effect of ion composition changes on the various radar observations that report anisotropies of ion temperature.     

Preliminary results from a study of the F-region heating during an intense aurora observed by EISCAT

Observations of large time variations in the ionospheric F-region temperature derived from EISCAT are compared with simultaneous observations of the E- and F-region plasma densities. The observations suggest that the F-region may be heated by current driven instabilities generated during intense precipitation of auroral electrons.     

A comparison of plasma densities by EISCAT and the Dynasonde from auroral altitudes: evidence of intense structure

Under conditions of moderately-energetic particle precipitation typical of the equatorward side of the auroral oval, plasma densities obtained from routine analysis of EISCAT Common Program data are often a factor 2 to 5 smaller than those suggested by co-located digital ionograms. We consider the reasons for this disagreement, and in particular we reject the implications of diffractive and multiplyrefractive scatter as alternatives to the usual plasma-frequency interpretation of ionogram echoes. We examine the effects of the (5 min and shorter) temporal averaging applied to the EISCAT data and conclude that together with the evidently small size (perhaps as little as 20 km) and high velocity of these structures, this accounts for much, if not all, of the disagreement. We point out the significance of the higher plasma densities in the 100–150 km height range for estimates of Joule and particle heating.     

The effects of differential ion flows on EISCAT observations in the auroral ionosphere

During geomagnetic storms different partial pressure gradients in the auroral ionosphere may result in H⁺, He⁺, O⁺ and molecular ions drifting with different velocities along the Earth's magnetic field line. For relative drift velocities ⪡ 400 m s⁻¹ it is shown that differential ion flows may be identified by two signatures in the autocorrelation function (ACF) measured by EISCAT. For larger relative drifts numerical simulations show that these signatures still exist and may result in an asymmetry in the incoherent scatter spectrum for O⁺ and molecular ions. It is demonstrated that UHF data can be reliably analysed for k²λ_D² ≲ 1, but at high altitudes, where O⁺–H⁺ flows are expected, UHF observations will be restricted by large Debye lengths (k²λ_D² > 1). Examples of ACFs based on polar wind theory are presented and discussed for the VHF system and finally it is shown that large ion temperature ratios (T_i(H⁺) >T_i(O⁺)) can significantly affect the velocity determination.     

Temperature anisotropy of drifting ions in the auroral F-region,observed by EISCAT

During relative drifts between the ions and the neutrals perpendicular to the geomagnetic field, the ion temperature in the auroral F-region becomes anisotropic with a higher temperature perpendicular than parallel to the magnetic field (T_⊥ >T_∥). It has been shown that for a gyrotropic ion velocity distribution the ion temperatures T_⊥ and T_∥ can be expressed as a function of the neutral temperature and of the squared normalized relative ion-neutral drift, with parameters β_⊥ and β_∥ describing the anisotropy and the collision process.In this paper, five increases of the F-layer ion temperature and ion drift velocity, found in EISCAT-CP1F data, were analyzed to obtain information about the anisotropy and the collision process. In the CP1F experiment, the angles between the magnetic field line ending in Tromsø and the antenna directions remain small, and the ion drift velocities of the investigated events in general were below 1500 m/s. Thus the ion velocity distributions were approximated by a bi-Maxwellian, and NO⁺ was assumed to remain a minor constituent at the F-layer maximum. For a quantitative analysis, generalized theoretical β-values for a bi-Maxwellian ion velocity distribution drifting through a mixture of different neutral components and for arbitrary observation directions were calculated. With these expressions it was possible to compare the drift dependence of the measured ion temperature for every antenna position directly with the theory. A statistical analysis of the heating events showed a good correlation between the ion temperatures of Tromsø, Kiruna and Sodankylä and the squared normalized ion drift, and values β_T, β_K, β_S could be calculated by linear regression. The fitted curves corresponded well with theoretical curves for a bi-Maxwellian velocity distribution of O⁺ ions drifting through a neutral atmosphere consisting of O and N₂.     

点校本《明史》校读札记

中华书局点校本《明史》有不少史实记载错误或字句失校之处,本文从三个方面对部分进士传记进行考证:一是进士所赐赠官和谥号的漏误;二是进士登科时间的错误;三是进士任职的职官漏误。     

Seasonal and tidal variations of neutral temperatures and densities in the high latitude lower thermosphere measured by EISCAT

Measurements of ion temperature, ion-neutral collision frequency and ion drift in the E-region from the period December 1984 to November 1985 are used to derive neutral temperatures, densities and meridional winds in the altitude intervals 92–120 km, 92–105 km and 92–120 km, respectively. Altitude profiles of temperature and density and their seasonal variations are compared with the CIRA 1972 and MSIS 1983 models and the effects of geomagnetic activity are demonstrated. Semi-diurnal tidal variations in all three parameters are derived and the comparison with lower latitude measurements is discussed.     

F-region drift velocities in the dusk sector mid-latitude trough

Analysis of 6 months of ground-based ionosonde data from mid/high-latitude Digisonde stations at Millstone Hill, Argentina and Goose Bay, shows the relation between the formation of the mid-latitude trough in the dusk sector and the measured F-region drift velocities. The observed westward drift velocities in the trough are comparable in magnitude with the velocity of the Earth's rotation as required by the stagnation theory of trough formation. Using the Digisonde database of 15 min samples of electron density profiles and F-region drifts, a new trough detection algorithm automatically identifies the occurrence of the trough at any of the three stations. Correlating trough occurrence with the measured drift velocities indicates that troughs develop due to an increase in the horizontal westward velocity component. The extent of the trough formation relates to the magnitude of the horizontal velocity.     

A comparison of three methods of measuring tidal oscillations in the lower thermosphere using EISCAT common programmes

The EISCAT Common Programme can be used in three ways to monitor tidal oscillations in the lower thermosphere. In Common Programme One (CPI) tristatic observations provide measurements of the ion-velocity vector at several heights in the E-region and one height in the F-region. In Common Programme Two (CP2) monostatic measurements give profiles of ion velocity in the E-region while tristatic measurements give continuous measurements of ion velocity in the F-region. From the ion velocities and the ion-neutral collision frequency, the vector of the E-region neutral wind can be determined and both east-west and north-south components of the diurnal, semi-diurnal and ter-diurnal oscillations can be identified. CP1 and CP2 also provide profiles of the field-aligned ion velocity, and these can be used to calculate the north-south component of the neutral wind without knowing the ion-neutral collision frequency, but the result is affected by any vertical component of neutral velocity. The three methods are compared and the advantages of CP2 demonstrated.     

Field-aligned and field-perpendicular velocities in the ionospheric F2-layer

Incoherent scatter observations have shown that there is sometimes a detailed anticorrelation or ‘mirroring’ between V_∥ and V_⊥, the components of F2-layer plasma velocity parallel to and perpendicular to the geomagnetic field. In this paper we develop a simple theoretical model of the F2-layer and compute its response to applied perturbations of V_⊥, both steplike and oscillatory; in particular we investigate the phase and amplitude relationships between V_∥ and V_⊥ resulting from ion-drag and plasma diffusion. For periods of a few hours, the oscillations of V_∥ lag behind exact anticorrelation with V_⊥ by 0.1–0.2 cycle, but the time lags corresponding to these phase differences are only a fraction of 1 h and seem broadly compatible with observations previously reported from Arecibo and Malvern. We do not study the question of what causes the velocities to fluctuate in the first place.     

《宋史》讹误考校五则

将中华书局本《宋史》．与其他宋代史料对校,就会发现其中有三类讹误：一是地名讹,二是人名讹,三是文字讹。在此基础上,本文进行了考校,共订正其中讹误五则。