Space-time structure of the morning aurora inferred from coincident DMSP-F6, -F8, and Søndrestrøm incoherent scatter radar observations

On rare occasions, observations from the DMSP-F6 and -F8 spacecraft and the Søndrestrøm incoherent scatter radar coincide in space. Such coincidence offers a unique opportunity to study temporal vs spatial variations on a small scale. We discuss data from one of those occasions, with observations made in the dawn sector in the presence of moderate auroral precipitation during a magnetically quiet period. The DMSP satellites measured vertical electron and ion flux and cross-track plasma drift while the radar measured the ionospheric electron density distribution and line-of-sight plasma velocities. We combine these data sets to construct a two-dimensional map of a possible auroral pattern above Søndrestrøm. It is characterized by the following properties. No difference is seen between the gross precipitation patterns measured along the DMSP-F6 and -F8 trajectories (separated by 32 km in magnetic east-west direction and some 4 s in travel time in magnetic north-south direction), except that they are not exactly aligned with the L shells. However, F6 and F8 observed minor differences in the small-scale structures. More significant differences are found between small-scale features in the DMSP precipitation measurements and in radar observations of the E-region plasma density distribution. These measurements are separated by 74 km, equivalent to 2.4°, in magnetic longitude, and 0–40 s in time along the spacecraft trajectories (varying with magnetic latitude). Large-scale magnetospheric-ionospheric surfaces such as plasma flow reversal, poleward boundary of the keV ion and electron precipitation, and poleward boundary of E-region ionization, coincide. The combined data suggest that the plasma flow reversal delineates the polar cap boundary, that is, the boundary between precipitation characteristic for the plasma mantle and for the plasma sheet boundary layer.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—I. A method of estimating electric fields

Using the measured Doppler spectra of the VHF backscatter radar signals from type II ionization irregularities in the equatorial electrojet (EEJ) at Thumba (dip. 56′S), the height profiles of the phase velocity V_p of the plasma waves in the EEJ are determined. It is shown that the east-west electrostatic field E_y in the EEJ can be deduced from the experimental height profiles of V_p using an appropriate model of ion and electron collision frequencies. The theoretical basis and the practical application of the method for deducing E_y are described. The usefulness of the method even when type I irregularities are present at the higher altitudes of the EEJ is demonstrated.It is shown that the collision frequencies of ions and electrons are likely to have a significant diurnal variation, which may be caused by diurnal variations of the neutral densities and temperatures in the E-region.     

Three-component a.c. E-field observations during the rocket and scatter experiment in 1988–1989 under radar auroral conditions in northern Scandinavia

During the ROSE (ROcket and Scatter Experiments) campaigns of 1988–1989 in northern Scandinavia a newly designed spherical probe (short name: KUSO, KUgelSOnde) was used to determine, in the frequency range 120–3500 Hz, the three components of the a.c. electric field being present under radar auroral conditions in the auroral E-region. Different from the normally utilized large d.c. double probes observing potential differences, the small (d = 23 cm) metallized KUSO probe measured the a.c. currents flowing through the sphere. These are mainly determined by the a.c. electric field existing in the surrounding plasma, and by the resitivity of the (small) Debye layer around its surface. Thus, if the relevant parameters are measured at the same time on one rocket the required a.c. E-field vectors can be deduced. With the aid of a simultaneously recording star sensor the rocket attitude could further be traced back to a non-rotating ground-based coordinate system. Afterwards the directions of the oscillating E-fields were deduced, both in the plane perpendicular to the Earth's magnetic (H-)field and relative to the H-field vector at the same time. The frequency distributions of these directions for about every second of flight time, for example, of more than 8000 E-field vectors contained in one such distribution, are discussed. Moreover, using FFT analyses, the spectral energy distributions of the total a.c. E-fields are obtained for the different heights. Different smaller spectral ranges of the E-field could be synthesized within the total bandwidth. The results of all the analyses are compared with the STARE, the EISCAT and the d.c. E-field observations. The electron densities measured by the Digital Impedance Probe (DIP) are also presented.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—II. Characteristics of electric fields on quiet and disturbed days

Using the Doppler spectra of VHF radar signals, the height profiles of the phase velocity (V_p) of 2.7 m irregularities in the equatorial electrojet (EEJ) over Thumba (dip: 56′S) are obtained. The day-time east-west electric fields (E_y) are deduced by matching experimentally observed V_p profiles with theoretically deduced ones for a number of quiet and disturbed days. The experimental E_y values show: (i) a large day-to-day variability; (ii) a large decrease in the afternoon hours on some days (quiet and disturbed); (iii) the frequent presence of short period fluctuations with amplitudes of 30–50% of the background value and with typical time scales of 30–60 min, on moderately disturbed days (9 ⩽ A_p ⩽ 30); (iv) a significant decrease of the average E_y on disturbed days compared to that on quiet days during 0900–1200 h L.T.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—III. Comparison of observed ΔH values with those estimated from measured electric fields

From VHF backscatter radar measurements at Thumba (dip: 56′S) of the phase velocities of type II irregularities in the equatorial electrojet (EEJ), electric field (E_y) values are estimated for different times of the day. Using the electric field values thus deduced and the Pedersen and Hall conductivities calculated using model values of electron densities and the collision frequencies of ions and electrons, the height integrated current intensity in the EEJ is estimated. The surface level geomagnetic field perturbation ΔH produced by this ionospheric current is then calculated. The calculated values of ΔH are compared with observed values of ΔH (after subtracting the magnetospheric contribution of D_st) for a number of days. The comparisons show good agreement between observed ΔH values and those calculated from measured electric fields. The agreement is found to be good even when type I irregularities are present at higher altitudes in the EEJ. This comparative study demonstrates the validity of estimating electric field values from VHF radar measurements and it indicates the possibility of deducing electric field values from ground level ΔH values, at least for statistical studies.     

Tidal winds from the MLT global radar network during the first LTCS campaign—September 1987

Winds and tides were measured by a number of MLT (Mesosphere, Lower Thermosphere) radars with locations varying from 43–70°N, 35–68°S, during the first LTCS (Lower Thermosphere Coupling Study) Campaign, 21–25 September 1987. The mean winds were globally westerly, consistent with early winter-like (NH) and late winter (SH) circulations.The semi-diurnal tide had vertical wavelengths near or less than 100 km at most locations, with some latitudinal variation (longer/shorter at lower latitudes in the NH/SH)—amplitudes decreased at high latitudes. The global tide was closer to anti-symmetric, with northward components being in phase at 90 km. Numerical model calculations [Forbes and Vial (1989), J. atmos. lerr. Phys. 51, 649] for September have rather similar wavelengths and amplitudes; however, the global tide was closer to symmetric, and detailed latitudinal trends differed from observed.The diurnal tide had similar wavelengths in each hemisphere, with short values (~30 km) at 35°, long (evanescence) at 68–70°, and irregular phase structures at mid-latitudes. The tide was neither symmetric nor anti-symmetric. Model calculations for the equinox [Forbes. S and Hagan (1988), Planet. Space Sci. 36, 579] were by nature symmetric, and showed the short wavelengths extending to mid-latitudes (43–52°). Southern hemisphere phases were significantly (6–8 h) different from observations. Amplitudes decreased at high latitudes in model and observation profiles.     

Results inferred from electron density measurements at Saskatoon,Canada (L = 4.4) by a partial reflection technique—I. Variations of nitric oxide in the D-region during quiet periods

Measurements of electron densities at Saskatoon (52°N, 106° W, L = 4.4) from 1976 to 1979 reveal seasonal variations which cannot be explained solely by solar zenith angle variations. These profiles have been used to infer the variations of nitric oxide density with season and solar activity for quiet-time conditions. It is found that while nitric oxide varies with season, it remains unchanged with the change in solar activity. The summer and spring profiles are much lower than the measured values of Baker et al. (1977) for heights below 85 km, while the winter estimated values show differences from the measured values in the height range 77.5–85 km. Above 85 km the values for all the seasons are close to the measured values. A dip in the nitric oxide distribution is obtained in all the cases around 80–82 km and the values of nitric oxide at the minimum are less than those measured by Baker et al. (1977) or Meira (1971).     

Dynamic regime of the mesopause—lower thermosphere at mid-latitudes of the northern hemisphere by radio meteor observations

The coherent pulse Meteor Automatic Radar System (MARS) based at Kharkov (49°30′N, 36°51′E) was used to measure zonal winds in the altitude range 80–105 km in the period from November 1986 to December 1990. It was found that, for the greater part of the year, the zonal prevailing wind component was in the eastward direction. The change from eastward to westward direction begins in the lower thermosphere in February–March, propagating downwards to the mesosphere, and it remains there until June–July. The structure of semidiurnal tides has general regularities at different sites. Annual variations in the monthly mean values of semidiurnal vertical wavelengths are practically the same, both in the northern and southern hemispheres. Wavelengths are more than 100 km in summer months, whereas they are less than 60 km in winter months.Studies of internal gravity wave (IGW) parameters in the height range of 80–105 km have shown that the internal gravity wave amplitude does not exceed 30 m/s, the vertical wavelength is in the range of 10–30 km, the horizontal wavelengths are 100–800 km and the horizontal phase velocities are in the range 20–160 m/s. The propagation and breaking of upward and downward IGW at heights of 80–100 km have been recorded.     

The investigation of electromagnetic interactions between regions surrounding the Earth—2. Results of the numerical simulation

A numerical simulation of the penetration of the global atmospheric electric field from the troposphere into the ionosphere has been performed. The numerical model, which we use here, has been described in paper 1 (Kuznetsov et al., 1995). Evaluations of the amplitude of the electric mode penetrating into the upper regions, which depends on the frequency, the characteristic spatial scale of the field and the wave electric field orientation relative to the geomagnetic field have been made using realistic models of the anisotropic conductivity. We show that in an inhomogeneous medium the high-altitude behaviour of the electric mode is characterized by the appearance of regions with either vertical or horizontal currents being dominant. The possibility of using an analogy with the laws for calculating the currents in different regions, namely Kirchhoff's laws for circuits with lumped parameters, is demonstrated. Characteristics of the magnetic modes excited in the ionosphere due to the action of the penetrating atmospheric electric fields are also described.     

Meteorological effects in the variations of ionospheric parameters—I. The lower thermosphere

The influence of the troposphere and stratosphere on variations of ionospheric D-region parameters are discussed. It is found that variations of the minimum observable ionosonde frequency (f_min) and horizontal winds are closely related to temperature and pressure fields in the lower atmosphere. Experimental data for the Central Asian and East Siberian U.S.S.R. regions have been used.     

A numerical model of the ionospheric dynamo—I. Formulation and numerical technique

A new method of numerically solving a suitably formulated ionospheric wind dynamo equation for electrostatic potential and field is developed. Unlike in many other dynamo models, the upper boundary does not exist and the formulation asymptotically approaches the equatorial boundary condition. Therefore, it naturally incorporates the symmetric, asymmetric E- and F-region dynamo actions in any given ionosphere and any given global or local wind field. It also enables the equation to be posed as an initial value problem and solved numerically using an efficient, accurate, stable and fast integration method of ordinary differential equations. The numerical technique can be extended to compute three dimensional dynamo-generated electric currents in the ionosphere.     

What’s the point? Retouched bladelet variability in the Protoaurignacian. Results from Fumane,Isturitz, and Les Cottés

The Protoaurignacian is considered a cultural proxy for one of the first expansions of anatomically modern humans across Europe. The stabilization of bladelet industries that characterizes this techno-complex is therefore often used as supporting evidence for the break from previous stone knapping traditions and also for the increase of human mobility through wider territories. Despite the cultural importance that bladelets have gained, a careful inter-regional comparison, stressing similarities and differences, has not yet been attempted. Moreover, the use of traditional typologies has blurred the morpho-metrical variability that characterizes lamellar tools. Here, a study has been carried out on retouched bladelets from three pivotal sites: Fumane (northeast Italy), Isturitz (southwest France), and Les Cottés (northern France). By using morphological, dimensional, and retouching attributes, and by evaluating the statistical significance of the main differences, the first detailed analysis of the variability of retouched bladelets within the Protoaurignacian has been documented. The results indicate that the features that best discriminate the bladelet assemblages are the presence and the relative variability of bladelets with convergent retouch, although a reassessment of existing studies and new methodological approaches are required to test the latter hypothesis. Throughout this paper, we demonstrate the merits of using a unified classification of retouched bladelets for comparing behavior in between groups distant in space. We hope that this paper will be a new incentive to develop unified taxonomies for the study of Early Upper Paleolithic lithics in Western Eurasia.     

Results inferred from electron density measurements at Saskatoon,Canada (L=4.4) by a partial reflection technique—II. Ion production rates and nitric oxide in the D-region during post-storm periods

Electron density profiles measured, at Saskatoon (52°N, L=4.4) during and after magnetic disturbances have been examined. It is seen that there is an enhanced electron density during magnetic disturbances, the enhancement being different at different heights. This enhancement is maintained for a number of days after the end of the disturbance. An increase of ion production rate and an increase of nitric oxide density are required for the maintenance of increased electron density in the post-storm period.     

A numerical study of the lunar tide in the mid-latitude F2-region of the ionosphere—I. Oscillations of the electron density

The continuity equations for four ion species and the momentum equations for the ion, electron and neutral gases are solved numerically. The momentum equations for ions and electrons include the effects of the electric field, and that for the neutral gas includes the effects of the collision between ions and neutrals, Coriolis force, the viscosity and the horizontal pressure force. It is found that the calculated semi-monthly tidal oscillations in NmF2 during the day and night-time are almost out of phase to each other, while the phase of the oscillations in hmF2 is weakly dependent on the solar time. The amplitude and phase of the calculated semi-diurnal lunar tide averaged over the solar times are 1.7% and 9.1 h (in NmF2) and 4.6 km and 8.1'h (in hmF2) respectively, at 45.0° dip. The height distribution of the semi-monthly lunar tide in the electron density is also obtained. The results show that the phase is 2 h for the lower region (below 200 km) and shifts to 9 h above about 250 km in the daytime, whereas it is nearly constant with height in the night-time. The calculated results are in good agreement with observations except for the phase at night. Thus it may be concluded that the major features of the observed lunar tidal variations in the mid-latitude F2-region can be explained by the electromagnetic drifts due to the solar and lunar electric fields.     

Small preys and plant exploitation by late pleistocene hunter–gatherers. A case study from the Northeast of the Iberian Peninsula

The aim of this paper is to explain the subsistence strategies of late hunter–gatherers from the Northeast of the Iberian Peninsula on the basis of zooarchaeological and archaeobotanical records. The study is based on the Molí del Salt archaeological site which has yielded an Upper Palaeolithic and Mesolithic sequence. Each of the disciplines shows us a various type of assemblage to approach the same problem. Archaeobotanical materials, including charcoal and seeds provide data on the strategies related to firewood gathering and vegetal food supply, which was a basic subsistence resource for these groups. The archaeobotanical assemblage is mainly formed by conifers, but other species related to edible plants are also important. Bone assemblage, mainly formed by Oryctolagus cuniculus, show how hunting and meat processing is highly related. The environmental constraints as well as mobility are discussed in order to achieve an ampler knowledge on human activities.     

The amplitude of auroral backscatter—I. Model estimates of the dependence on electron density

A model of the auroral backscatter amplitude, in the form discussed by Uspensky and Oksman et al., has been derived for the radar geometry appropriate to joint observations by the PGI auroral radars at Karmaselga and Essoyla and the EISCAT incoherent scatter radar. The model shows how refraction effects cause a strongly non-linear dependence of backscatter amplitude on electron density in the E-region. It also explains why the macro aspect sensitivity for auroral radar operating at a frequency of about 45 MHz is only 1–2 dB per degree for aspect angles greater than 5°.