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.     

Field-perpendicular and field-aligned plasma flows observed by EISCAT during a prolonged period of northward IMF

The effect of a prolonged period of strongly northward Interplanetary Magnetic Field (IMF) on the high-latitude F-region is studied using data from the EISCAT Common Programme Zero mode of operation on 11–12 August 1982. The analysis of the raw autocorrelation functions is kept to the directly derived parameters N_e, T_e, T_i and velocity, and limits are defined for the errors introduced by assumptions about ion composition and by changes in the transmitted power and system constant. Simple data-cleaning criteria are employed to eliminate problems due to coherent signals and large background noise levels. The observed variations in plasma densities, temperatures and velocities are interpreted in terms of supporting data from ISEE-3 and local riometers and magnetometers. Both field-aligned and field-perpendicular plasma flows at Tromsø showed effects of the northward IMF: convection was slow and irregular and field-aligned flow profiles were characteristic of steady-state polar wind outflow with flux of order 10¹² m⁻² s⁻¹. This period followed a strongly southward IMF which had triggered a substorm. The substorm gave enhanced convection, with a swing to equatorward flow and large (5 × 10¹² m⁻² s⁻¹), steady-state field-aligned fluxes, leading to the possibility of O⁺ escape into the magnetosphere. The apparent influence of the IMF over both field-perpendicular and field-aligned flows is explained in terms of the cross-cap potential difference and the location of the auroral oval.     

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.     

EISCAT results during the ROSE campaign and comparison with STARE measurements

EISCAT measurements were performed during the four ROSE rocket launches. The results are presented. It is shown that the upper altitude limit of instabilities observed by in-situ measurements agrees with calculations using EISCAT results of drift and ion sound speed and assuming the two-stream-instability mechanism. The EISCAT results together with the STARE observations were used to calculate the ion velocity and the ψ-values from the dispersion relation of two-stream-instabilities. A comparison of EISCAT, STARE and in-situ measurements is discussed.     

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.     

D-region signatures of substorm growth phase and onset observed by EISCAT

High resolution electron density measurements by EISCAT during the pre-onset phase and onset of an auroral absorption substorm are used to investigate the characteristics of electron precipitation during these substorm phases. The development of the pre-onset phase is the result of a uniform increase of electron fluxes with energies of a few tens of keV, with no particularly hard component. The absorption spike observed at substorm onset contains fine structure when investigated at 10 s resolution, indicating a rapid hardening of the precipitating spectrum at the onset.     

Auroral riometer absorptions and the F-region disturbances observed over a wide range of latitudes

Standard riometer data from a southern auroral station were compared with ionograms obtained at five stations positioned from sub-auroral to equatorial latitudes. The rapid onset in riometer absorption, during intense substorm activities in an equinoctial period, was associated with a sequential propagation of ionospheric disturbances deduced from the F-region parameters h′F and range spread-F. The time shift between absorption maxima and extrapolated commencement times of the disturbances was consistent with the presence of large-scale travelling ionospheric disturbances (TIDs), propagating equatorwards with velocities lying typically in the range 600–900 m s⁻¹, and with a median velocity of 720 m s⁻¹. It is suggested that the onset of TIDs is associated with high-energy particle precipitation, manifested by the occurrence of auroral absorption events. Similarity of absorption increases at the southern and northern conjugate points, found from a previous riometer study, would indicate that large-scale TIDs are simultaneously generated in both hemispheres.     

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 10⁴ cm⁻³. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.     

Mesospheric studies using the EISCAT UHF and VHF radars: a review of principles and experimental results

Studies of the mesosphere and ionospheric D-region are carried out with the incoherent scatter as well as with the MST radar technique. We briefly describe the principal differences between these techniques and the corresponding scattering mechanisms. Particular emphasis is placed on the observation of the coherent echoes from the summer mesosphere, which the EISCAT VHF and UHF radars have recently also detected. We present an overview of the historical development of EISCAT observations and results in these areas, and suggest possible directions of progress which would make the EISCAT radar systems even more suitable for mesosphere measurements.     

Spread-F-like irregularities observed by the Jicamarca radar during the day-time

Until now the presence of F-region irregularities responsible for spread-F (sp-F) traces in ionograms has been considered as a purely night-time phenomenon extending sporadically to the early morning hours. We herein report that, on two occasions (26 March 1974 and 1 February 1984) similar irregularities were observed between 1400 and 1600 hours local time with the Jicamarca radar. These irregularities caused enhancements in the power of the radar echo of as much as two orders of magnitude, were found over a region of a few hundred kilometers on the topside of the F-region extending from around 600 to 1000 km altitude, and persisted for 1–2 h. The irregularities were aspect sensitive (aligned with the magnetic field) and produced echoes with a fading rate of the order of one to a few seconds. The background zonal electric field, inferred from the vertical drift velocity, was fairly constant in altitude, with values smaller than 0.1 mV m⁻¹. During the duration of the events, zonal components of both signs occurred, with the component passing through zero several times. We have no information on the vertical component of E. These irregularities could not be observed with ground-based ionosondes, since they are on the topside of the F-region. They may be related to fossil bubbles that are responsible for HF ducting observed by satellites.     

Characteristics of polar mesosphere summer echoes (PMSE) observed with the EISCAT 224 MHz radar and possible explanations of their origin

We describe experiments carried out with the EISCAT VHF radar during the MAC/SINE campaign. These experiments included observations of the polar mesosphere summer echoes (PMSE), which were studied with a high spectral resolution program. The fine structure of the spectra imply that very thin and non-random transient structures of reflectivity occur frequently in the mesopause region. We find no clear relation between the echo power and the coherence time which could support the hypothesis of scatter from turbulence or partial reflection. In addition, the estimates of radar reflectivity let us discard incoherent scatter and pure turbulence scatter as the cause of the PMSE. We also discuss the relation of the PMSE and cluster ions, electric fields, charge accumulation and atmospheric gravity waves.     

Ion composition changes during F-region density depletions in the presence of electric fields at auroral latitudes

F-region density depletions in the afternoon/evening sector of the auroral zone are studied with the EISCAT UHF radar. Four case studies are presented, in which data from three experiment modes are used. In each case the density depletion can be identified with the main ionospheric trough. For the two cases occurring in sunlit conditions the electron densities recovered significantly after the trough minimum. Tristatic ion velocity measurements show the development of poleward electric fields of typically 50–100 m Vm⁻¹, which maximize exactly in the trough minimum. A special analysis technique for incoherent scatter measurements is introduced, based on the ion energy equation. By assuming that the ion temperature should obey this equation it is possible to fix this parameter in a second analysis and to allow the ion composition to be a free parameter. The results from two experiments with accurate velocity measurements indicate that the proportion of O⁺ near the F-region peak decreased from 100% in the undisturbed ionosphere to only 10% and 30%, respectively, in the density minimum of the trough. The loss of O⁺ is explained by the temperature dependence of recombination with nitrogen molecules. Temperatures derived from radar measurements are very sensitive to the assumed ion composition. For the above case of 10% O⁺ the deduced electron temperature in the trough was transformed from a local minimum of < 2000 K to a local maximum of 4000 K.     

Steepening of reflectivity structures detected in high-resolution Doppler spectra of polar mesosphere summer echoes (PMSE) observed with the EISCAT 224 MHz radar

Polar mesosphere summer echoes observed with the EISCAT 224 MHz radar frequently exhibit significant discontinuous offsets or jumps in the Doppler frequency. We can explain these frequency jumps as a result of a lifting of partially reflecting or scattering layers, which are distorted by bumps. These bumps can be caused by steepened refractivity variations, i.e. reflectivity structures. These suggestions are supported by model computations. We also notice that a relation exists between these structure shapes and gravity waves, which are steepened, but which do not necessarily break into enhanced turbulent velocity fluctuations.     

Damage caused by insects during the mummification process: an experimental study

The process of mummification had been known since the Fourth Dynasty of the Old Kingdom (ca. 2600 bc) and continued to develop throughout ancient Egyptian history. Although mummification protected the body from decay, especially by microbes, some mummification techniques left the body susceptible to insect attack. Certain types of insects have been detected in the mummies. In scholarly publications, most authors have dealt with microorganisms, while few have concerned themselves in depth with the effect of insects on the mummies. This study aims to discuss the significance of insects and the changes they affected to the mummies during embalming. To achieve this goal, experiments were carried out replicating various mummification techniques using albino Wistar rats. Analysis and investigative techniques used included visual observation, Fourier transform infrared spectroscopy, investigation of the surface morphology by a scanning electron microscope, and color change by a spectrophotometer. The following insects could be identified as being present during the second and third processes of mummification: Dermestes maculatus, Necrobia rufipes, Saprinus gilvicornis, Chrysomya albiceps, Wohlfahrtia magnifica, and Attagenus fasciatus. In addition, the majority of our findings confirmed that the degradation by insects increased with the second and third methods of mummification. Finally, the experimental study conducted using the mummification techniques of the New Kingdom (ca. 1570–1070 bc) indicated that they were more resistant to insect attack than the other types.     

A scintillation theory of the fading of HF waves returned from the F-region: receiver near transmitter

Scintillation theory is used to study the fading of HF radio waves returned from the ionospheric F-region to a receiver close to the transmitter. Estimates are made of

• 1.(i) the fluctuations of phase both for long term (∼ an hour) and for short term (∼ a fading correlation time),
• 2.(ii) the correlation distance,
• 3.(iii) the quasi-period of fading,
• 4.(iv) the angular divergence in the direction of arrival around the zenith and
• 5.(v) the correlation bandwidth.

The calculations are made as a function of wave frequency for two ionospheric penetration frequencies representative of high day-time values and low pre-sunrise values. Results are compared with observations of fading made with ionosondes over the past 40 years. Precise comparison is rendered impossible by omissions in the experimental data caused by lack of guidance from scintillation theory. Nevertheless, agreement is promising. When fading is deep but spread-F-region is not well-developed there is a slow modulation of the fading. This is what, for optical propagation in the troposphere, is called twinkling. The slow fluctuations observed by Bramley and Ross in the HF band constitute ionospheric twinkling. Calculated quasi-periods of twinkling range from about an hour down to about a couple of minutes, while calculated quasi-periods of fading range from about a couple of minutes down to about a tenth of a second.     

Determination of velocity vectors of thermospheric wind from dispersion relations of TID's observed by an HF Doppler array

A method of determining horizontal velocity vectors and temperature of thermospheric winds from azimuthally different ω-κ diagrams derived from the data of traveling ionospheric disturbances (TID's) observed by an HF Doppler array has been developed. In this method, an analogous property between the Brunt-Vaisala frequency in the atmospheric oscillations and the plasma frequency in the plasma oscillations is applied to the analysis of the observed data. A declination of the resonance branches of the Brunt-Vaisala frequency in the ω-κ diagrams due to a Lorentz transformation is used for the estimation of the velocity vector of the thermospheric wind. This method makes it possible to separate the velocity vector of the thermospheric wind and the propagating directions of the TID's from the azimuthally different ω-κ diagrams. Several results of the velocity vectors during the daytime data are consistent with the results obtained by incoherent scatter radars and theoretical results. Furthermore, a typical dispersion curve which agrees well with those of the acoustic gravity waves derived from the linear perturbation theory under the isothermal atmospheric condition has been also obtained.     

Preliminary results from the URSI/IAGA cooperative tidal observations program (CTOP)

This paper presents an overview of the results of the first three intervals of the Cooperative Tidal Observation Program (CTOP). The program has been undertaken as a collaborative effort by the incoherent scatter radar network, and the Global Radio Meteor Wind Studies Project (GRMWSP) under joint URSI/IAGA sponsorship to provide input to global models of the atmospheric circulation in the upper mesosphere and lower thermosphere. The data contributing to this overview are published in detail in nine companion papers in this issue.     

Identifying the sources of Inka period ceramics from northern Chile: results of a neutron activation study

To examine regional patterns of ceramic production and distribution during the era of Inka domination in northern Chile, we determined the elemental compositions of 157 samples of archaeological ceramics and geological clays from the sites of Catarpe and Turi using instrumental neutron activation analysis. We identified two major and three minor composition groups in the ceramics. The major groups, High Cr and Low Cr, are linked to clays from two broad geological contexts within the region, while the minor Low Na group is made up of ceramics imported from northwestern Argentina. The distribution of the composition groups indicates that, in the Catarpe–Turi region, patterns of ceramic production differed for different vessel types: jars were made from clay and temper acquired near the sites where the jars were used, while bowls were made of material coming from more distant sources. The geographical distribution of the analyzed ceramics indicates that bowls were exchanged between Catarpe and Turi in a pattern more similar to tribute/extraction than to market exchange, with Catarpe being the dominant site. The compositional analysis also demonstrates that Inka-style ceramics were being locally produced at sites in this region during the era of Inka domination.     

Applicability of the two-layer model for simulation of non-linear evolution of a large scale plasma cloud in the ionospheric F-region

The adequacy of the two-layer model of Lloyd and Haerendel for describing the behaviour of an ionospheric irregularity is verified by numerical simulation of large plasma cloud dynamics. The background ionosphere is approximated by a set of conductive layers with ion mobilities and concentrations corresponding to the real ionospheric conditions. Polarization electric field produces positive and negative image clouds, i.e. plasma density enhancements and depletions in each layer. Their intensity, form and orientation turn out to change with height depending on the local conditions. However, the drift and deformation of the released cloud slightly differ from the case when the ionosphere is characterized by constant, height averaged parameters, at least if altitude dependent neutral wind and photochemical processes are ignored.