Diurnal variations in the flux of ionisation above the F2 peak in the northern and southern hemispheres

The flux of ionisation at 850 km height is calculated using the MSIS atmospheric model, a simplified form for the continuity equation at the peak of the F2-layer, and observed values of NmF2. Results are given for stations at latitudes of 32°N, 21°N, 21°S and 37°S during 1971 and for Tahiti (18°S) in 1980. Changes in the neutral atmosphere and in the hmF2 model have minor effects at low latitudes, where the fluxes are larger, but can appreciably alter the results at mid latitudes. Increased recombination due to N₂ vibrational excitation produces a large afternoon decrease in NmF2 in summer, near solar maximum, and an increased downward flux. At all stations the day-time flux has a much larger downward component in winter than in summer. Because of the eastward magnetic declination, zonal winds produce opposite effects on the diurnal variations of hmF2, NmF2 and flux in the northern and southern hemispheres. Downward fluxes are largest in the morning in the southern hemisphere and in the late afternoon and evening in the north. At ± 21° latitude, neutral winds have a major effect on the distribution of ionisation from the equatorial fountain. Thus, at the solstices the day-time flow is about 4 times larger in winter than in summer. Averaged over both hemispheres, the total flow at 21° latitude is approximately the same for solstice and equinox conditions. At mid latitudes there is a downwards flux of about 1–2 × 10¹² m² s⁻¹ into the night ionosphere.     

On the IRI model predictions for the low-latitude ionosphere

Measurements of ionospheric electron density vertical profiles, carried out at a magnetic equatorial station located at Fortaleza (4°S, 38°W; dip latitude 2°S) in Brazil, are analyzed and compared with low-latitude electron density profiles predicted by the International Reference Ionosphere (IRI) model. The analysis performed here covers periods of high (1979/1980) and low (1986) solar activities, considering data obtained under magnetically quiet conditions representative of the summer, winter and equinox seasons. Some discrepancies are found to exist between the observed and the IRI model-predicted ionospheric electron density profiles. For high solar activity conditions the most remarkable one is the observed fast upward motion of the F-layer just after sunset, not considered in the IRI model and which precedes the occurrence of nighttime ionospheric plasma irregularities. These discrepancies are attributed mainly to dynamical effects associated with the low latitude E × B electromagnetic plasma drifts and the thermospheric neutral winds, which are not satisfactorily reproduced either in the CCIR numerical maps or in the IRI profile shapes. In particular, the pre-reversal enhancement in the vertical E × B plasma drifts around sunset hours has a great influence on the nighttime spatial distribution of the low-latitude ionospheric plasma. Also, the dynamical control exerted by the electromagnetic plasma drifts and by the thermospheric neutral winds on the low-latitude ionospheric plasma is strongly dependent on the magnetic declination angle at a given longitude. These important longitudinal and latitudinal dependences must be considered for improvement of IRI model predictions at low latitudes.     

Variations of the gravity wave characteristics with height,season and latitude revealed by comparative observations

This paper reviews some recent observations of gravity wave characteristics in the middle atmosphere, revealed by co-ordinated observations with the MU radar in Shigaraki (35°N, 136°E) and nearby rocketsonde experiments at Uchinoura (31°N, 131°E). We further summarize the results of comparative studies on the latitudinal variations of the gravity wave activity, which were detected by additionally employing data obtained with MF radars at Adelaide (35°S, 139°E) and Saskatoon (52, 107W) and lidar observations at Haute Provence (44, 6E).The seasonal variation of gravity wave activity detected with the MU radar in the lower stratosphere showed a clear annual variation with a maximum in winter, and coincided with that for the jet-stream intensity, indicating a close relation between the excitation of gravity waves and jet-stream activity at middle latitudes. The long-period (2–21 h) gravity waves seemed to be excited near the ground, presumably due to the interaction of flow with topography, and the short-period (5 min 2 h) components had the largest kinetic energy around the peak of jet-stream.We found an increase with height in the vertical scales of dominant gravity waves, which can be explained in terms of a saturation of upward propagating gravity waves. The values of the horizontal wind velocity variance generally increased in the stratosphere and lower mesosphere, but they became fairly constant above about 65 km due to the wave saturation, resulting in the active production of turbulent layers.Although the gravity wave energy showed an annual variation in the lower atmosphere, it exhibited a semiannual variation in the mesosphere, with a large peak in summer and a minor enhancement in winter. Lidar observations reasonably interpolated the seasonal variations in the intermediate height regions.The gravity wave energy in the mesosphere, with periods less than about 2 h, was consistently larger in summer than in winter at all the stations, i.e. at 35N, 44N,52 N and 35 S. However, the values were generally larger at 35 N than at 52 N. which was found from a comparison of l-yr observations at Shigaraki and Saskatoon. Furthermore, a comparison between Shigaraki and Adelaide, located at the conjugate points relative to the equator, revealed that the gravity-wave energy in the mesosphere was found to be fairly similar, when we compared the values in summer/winter in each hemisphere.     

Shellfish Exploitation in the Western Canary Islands Over the Last Two Millennia

ABSTRACT

The residents of the Canary Archipelago consumed limpets since the arrival of humans ～2500 yrs. ago, and these harvested gastropods were deposited in large coastal shell middens. This work preliminarily explores shell margin oxygen isotope composition (δ¹⁸O) and body size of the black limpet (Patella candei d’Orbigny, 1840) from archaeological sites in the Canary Islands to assess possible seasonal variability and intensity of shellfish collection throughout the late Holocene. The shell margin δ¹⁸O values of 100 shells (radiocarbon dated between ～500 and ～1800 cal. yr BP) were analysed to estimate sea surface temperature (SST) at time of death. Paleotemperature estimates suggest shellfish harvesting was not year-round, and was avoided in the cooler months (when SST?<?20°C). This pattern differs from most higher latitude Mesolithic and Neolithic human groups, which gathered shellfish year-round, targeting winter more heavily. Preliminary body-size measurements suggest shell sizes have experienced a decline from aboriginal times to the present, which possibly resulted from increasing anthropogenic pressures. During aboriginal inhabitation, maximum adult shell size remained stable, suggesting that present-day harvesting practices are more intense than harvesting from aboriginal human groups. This intensive collection has likely diminished the average adult size of limpet populations in the islands by ～27%.     

An approximate form for the Chapman grazing incidence function

Numerical modelling of the ionosphere requires repeated evaluation of the Chapman function Ch(x, χ). This normally involves a numerical integration of the Chapman expression. Approximate values can be obtained using accurate tables of the error function integral, but large numerical errors make this approach unusable for values of χ less than about 80°. Algebraic expressions are obtained for the rapid, direct calculation of the Chapman function, at any zenith angle and for scale heights from 6 to 120 km. The maximum errors in this calculation correspond to an error of about 0.03° in the zenith angle, while the rms error is half this amount.     

Effect of the electric field on the equatorial ionospheric plasma distribution

It is known that on a counter electrojet day the noontime electron density at the equator shows enhanced values with no bite-out. The consequences of the absence of the normal equatorial electrojet on the electron density distribution at the equatorial station Kodaikanal (dip latitude 1.4°N, long. 77.5°E) and at an anomaly crest location Ahmedabad (dip latitude 18°N, long. 73°E) are discussed for a strong electrojet (SEJ) day and a counter electrojet (CEJ) day. The electron density distribution with height for a pair of SEJ and CEJ days at the two equatorial stations Kodaikanal and Huancayo (dip latitude 1°N, long. 75°W) are studied. The F-region peak height, hm and the semi-thickness parameter ym on the SEJ day followed a similar variation pattern. On the CEJ days ym exhibited a substantially low and mostly flattened daytime variation compared to the peaked values on the SEJ day. An attempt is made to interpret these differences in terms of the changes in the vertical drift pattern resulting from the E × B drift of plasma at the equator and the varying recombination rate β, which is also a height dependent and a local time dependent parameter.     

A Late Pleistocene coral fauna from Evans Head,N.S.W.

An assemblage of hermatypic scleractinian corals occurring landwards of the Pleistocene sandy Inner Barrier system has been referred to the last interglacial period. It comprises at least 20 species, many of which are in growth position, and is accompanied by a substantial association of molluscans. The richness of the assemblage is indicative of good access of oceanic waters at the time of its formation, so the deposit predates barrier emplacement. The coral occurrences are compared with present-day southern ranges of the scleractinian species (all extant), and the implications for climatic and sea-level conditions in the last interglacial are discussed. A sea-level stand of 4–6 m above that at present obtaining (in accord with Marshall & Thom, 1976) and a climatic shift towards a cooler regime equivalent to a minimum of 2° of latitude are concluded.     

EVIDENCE FOR CHURCH ORIENTATION BY MAGNETIC COMPASS IN TWELFTH-CENTURY DENMARK

The orientations of some 570 Romanesque churches (one-third of the total) from four regions in Denmark show significant differences in the claimed east-west orientation, being typically rotated 5-15° clockwise from true east-west. Magnetic declination was predominantly easterly between AD 1000 and 1600; the church deviations probably originate from sometimes using a magnetic compass, other simple and plausible causes for this ‘skewness’, for example orientation by the sun at specific periods of the year or astronomical alignment, being hard to design. Hence, some 25% of the churches were probably oriented by means of a magnetic compass. As a tool for dating, the orientations have not yet been successful due to statistical scatter. Church building in Denmark boomed in the twelfth century. The early history of the compass is somewhat uncertain. The first European reference known is from about 1190-1200, while the oldest Norse source is from about 1225 (Landnámabók), saying that the lodestone was not known before. Danish churches thus bring new information about the early use of the magnetic compass in Europe.     

Longitudinal differences and inter-annual variations of zonal wind in the tropical stratosphere and troposphere

A quantitative assessment has been made of the longitude-dependent differences and the interannual variations of the zonal wind components in the equatorial stratosphere and troposphere, from the analysis of rocket and balloon data for 1979 and 1980 for three stations near ±8.5° latitude (Ascension Island at 14.4°W, Thumba at 76.9°E and Kwajalein at 67.7°E) and two stations near 21.5° latitude (Barking Sands at 159.6°W and Balasore at 86.9°E). The longitude-dependent differences are found to be about 10–20 m s⁻¹ (amounting to 50–200% in some cases) for the semi-annual oscillation (SAO) and the annual oscillation (AO) amplitudes, depending upon the altitude and latitude. Inter-annual variations of about 10 m s⁻¹ also exist in both oscillations. The phase of the SAO exhibits an almost 180° shift at Kwajalein compared to that at the other two stations near 8.5°, while the phase of the AO is independent of longitude, in the stratosphere.The amplitude and phase of the quasi-biennial oscillation (QBO) are found to be almost independent of longitude in the 18–38 km range, but above 40 km height the QBO amplitude and phase have different values in different longitude sectors for the three stations near ±8.5° latitude. The mean zonal wind shows no change from 1979 to 1980, but in the troposphere at 8.5° latitude strong easterlies prevail in the Indian zone, in contrast to the westerlies at the Atlantic and Pacific stations.     

Optical interferometer measurements of thermospheric temperature at Kavalur (12.5°N, 78.5°E), India

First results on the behaviour of thermospheric temperature over Kavalur (12.5°N, 78.5°E geographic; 2.8°N geomagnetic latitude) located close to the geomagnetic equator in the Indian zone are presented. The results are based on measurements of the Doppler width of O(¹D) night airglow emission at 630 nm made with a pressure-scanned Fabry-Perot interferometer (FPI) on 16 nights during March April 1992. The average nighttime (2130-0430 IST) thermospheric temperature is found to be consistently higher than the MSIS-86 predictions on all but one of the nights. The mean difference between the observed nightly temperatures and model values is 269 K with a standard error of 91 K. On one of the nights (9/10 April 1992, Ap = 6) the temperature is found to increase by ~250 K around 2330 IST and is accompanied by a ‘midnight collapse’ of the F-region over Ahmedabad (23°N, 72°E, dip 26.3°N). This relationship between the temperature increase at Kavalur and F-region height decrease at Ahmedabad is also seen in the average behaviour of the two parameters. The temperature enhancement at Kavalur is interpreted as the signature of the equatorial midnight temperature maximum (MTM) and the descent of the F-region over Ahmedabad as the effect of the poleward neutral winds associated with the MTM.     

ARCHAEOMAGNETIC TILT CORRECTION ON BRICKS

Fired bricks are used in archaeomagnetic work to determine the Inclination I of the earth's magnetic field at the time of firing. However, in addition to the usual mean error of between ± 1/2° and ± 2° due mainly to inexact levelling of the bricks during the firing, there is a systematic error in the measured mean value of the Inclination that may amount to 2° or 3° at high magnetic latitudes (I? 80°), while at lower magnetic latitudes (I< 60°) it is usually less than 1/2°. A method of correction based on the measured statistical spread of the bricks is suggested.     

Diurnal propagating tides in the low-latitude middle atmosphere

Using an equivalent gravity wave f-plane model it is shown that longitude variations in diurnal insolation absorption by tropospheric H₂O can account for longitudinal variations of at least ± 12–15% about zonal mean values in the diurnal wind amplitude at low latitudes (0–20°) between 80 and 100 km, by virtue of the non-migrating propagating tidal modes which are excited. Phase variations of about ± 0.75 h also occur. These percentage variations are conservative estimates, since the background migrating (1,1,1) mode appears to be slightly (20–25%) overestimated in amplitude. In addition, the assumed eddy dissipation values, which appear necessary to model the breaking (1,1,1) mode, are larger than generally considered ‘reasonable’ by photochemical modellers. For a photochemically more reasonable eddy diffusion profile, estimates of longitude differences in diurnal wind amplitude are quite similar to the above values below 87 km, but increase to ± 17–25% near 100 km, with accompanying phase variations of ± 1–2 h about zonal mean values. In addition, it is shown that radiative damping by CO₂ parameterized by a scale-dependent Newtonian cooling coefficient accounts for no more than a 20% reduction in the amplitudes of diurnal propagating tides above 80 km.     

A model for the electron temperature variation along geomagnetic field lines and its effect on electron density profiles and VLF paths

A realistic model for the temperature variation along geomagnetic field lines is described. For high altitudes (>1500 km) the temperature is taken to increase as the nth power of radial distance (n−2), giving temperatures consistent with those measured in situ by high altitude satellites. For realistic temperatures at low altitude an extra term is included. The temperature gradient along the field line is then 0.9–1.6° km⁻¹ during the day and 0.5–0.7° km⁻¹ during the night at 1000 km, reducing to about half these values at 2000 km, for the latitude range 35–50°. This is consistent with calculations made from nearly simultaneous satellite measurements at 1000 and 2500 km. It is shown that assuming diffusive equilibrium, including the new temperature model, more realistic equatorial electron density profiles result than for isothermal field lines.The temperature gradient model is also purposely formulated to be of a form that enables the temperature modified geopotential height to be obtained without numerical integration. This renders the model particularly suitable for ray-tracing calculations. A ray-tracing model is developed and it is shown that unducted ray paths are significantly altered from the corresponding paths in an equivalent isothermal model; there is greater refraction and magnetospheric reflection takes place at lower altitudes. For summer day conditions, an inter-hemispheric unducted ray path becomes possible from 26° latitude that can reach the ground at the conjugate.     

Experimental estimates of electron density variations at the reflection height of VLF signals

To study the behaviour of the electron concentration at the reflection level of very low frequency (VLF) waves, two years of phase and amplitude records of the 12.9 kHz signals emitted from Omega-Argentina (43.20°S; 294.60°E) and received at Tucumán (26.90°S; 294.70°E) have been used. The experimental results are compared with values derived from the International Reference Ionosphere model (IRI-79). The experimental data show a seasonal variation not predicted by the model. Differences are explained in terms of changes of night-time atomic oxygen concentration, which control the electron density profile at the base of the night-time D-region, not taken into account in the IRI model. Values of atomic oxygen necessary to explain VLF data are comparable with published data.     

Solar cycle variation of the total electron content around equatorial anomaly crest region in east asia

The monthly mean hourly values of total electron content data obtained at Lunping Observatory (geographic coordinates 25.00°N, 121.17°E; geomagnetic coordinates 14.3°N, 191.3°E) by using the ETS2 satellite beacon signal during the period from March 1977 to December 1990 have been used to analyze the solar cycle variations of total electron content (TEC) around equatorial anomaly crest region in East Asia. Positive, correlations were found between the 12 month running average of monthly mean TECs and sunspot numbers. By using the linear regression analysis method, the contour charts for real diurnal and seasonal variations of TEC at certain sunspot numbers were constructed and described. The diurnal variation of TEC was represented by the sum of its diurnal mean and first three harmonic components. The solar cycle variations of these components have also been discussed.     

Comparing different post-IR IRSL approaches for the dating of Holocene coastal foredunes from Ruhnu Island,Estonia

Different post-IR Infrared Stimulated Luminescence (IRSL) approaches are applied to sediments from a Holocene coastal foredune sequence on Ruhnu Island in the eastern Baltic Sea. The comparison of D _e-values and ages determined by the different approaches is complimented by fading and bleaching experiments. The fading experiments imply strong fading of IRSL (50°C) signals and no fading of any of the post-IR IRSL signals, but this is not confirmed by the determined D _e-values. In fact, post-IR IRSL (150°C) D _e-values agree within errors with those calculated for IRSL (50°C). From the bleaching experiments it is inferred that the higher values observed for post-IR IRSL at more elevated stimulation temperatures (225°C/290°C) are likely related to either thermal transfer and/or slow-to-bleach components within the signal. For the dating of the Holocene foredune sequence of Ruhnu Island, the post-IR IRSL (150°C) approach is preferred and these agree with the limited independent age control available from radiocarbon dating. Accordingly, the sequence formed between ca. 7.0 ka and 2.5 ka ago.     

Origin of palaeo‐waters in the Ordovician carbonates in Tahe oilfield,Tarim Basin: constraints from fluid inclusions and Sr,C and O isotopes

Petrographic features, isotopes, and trace elements were determined, and fluid inclusions were analyzed on fracture‐filling, karst‐filling and interparticle calcite cement from the Ordovician carbonates in Tahe oilfield, Tarim basin, NW China. The aim was to assess the origin and evolution of palaeo‐waters in the carbonates. The initial water was seawater diluted by meteoric water, as indicated by bright cathodoluminescence (CL) in low‐temperature calcite. The palaeoseawater was further buried to temperatures from 57 to 110°C, nonluminescent calcite precipitated during the Silurian to middle Devonian. Infiltration of meteoric water of late Devonian age into the carbonate rocks was recorded in the first generation of fracture‐ and karst‐filling dull red CL calcite with temperatures from <50°C to 83°C, low salinities (<9.0 wt%), high Mn contents and high ⁸⁶Sr/⁸⁷Sr ratios from 0.7090 to 0.7099. During the early Permian, ⁸⁷Sr‐rich hydrothermal water may have entered the carbonate rocks, from which precipitated a second generation of fracture‐filling and interparticle calcite and barite cements with salinities greater than 22.4 wt%, and temperatures from 120°C to 180°C. The hydrothermal water may have collected isotopically light CO₂ (possibly of TSR‐origin) during upward migration, resulting in hydrothermal calcite and the present‐day oilfield water having δ¹³C values from ?4.3 to ?13.8‰ and showing negative relationships of ⁸⁷Sr/⁸⁶Sr ratios to δ¹³C and δ¹⁸O values. However, higher temperatures (up to 187°C) and much lower salinities (down to 0.5 wt%) measured from some karst‐filling, giant, nonluminescent calcite crystals may suggest that hydrothermal water was deeply recycled, reduced (Fe‐bearing) meteoric water heated in deeper strata, or water generated from TSR during hydrothermal water activity. Mixing of hydrothermal and local basinal water (or diagenetically altered connate water) with meteoric waters of late Permian age and/or later may have resulted in large variations in salinity of the present oilfield waters with the lowest salinity formation waters in the palaeohighs.     

Angular characteristics of radio pulses reflected from the subpolar ionosphere

This paper presents the results derived by measuring angular spectra of HF-radio pulses reflected from the subpolar ionospheric F2-region (62°N) using vertical-incidence soundings and a phase direction finder with Doppler filtering. The results correspond to three main types. One is the classical mirror reflection from the undisturbed ionospheric F2-region, typical of mid-latitudes (deviations from zenith do not exceed 3°; the angular spectrum width is less than 1°). The second type includes oblique diffuse reflections with a deviation from zenith of from 10 to 45°. The azimuth of arrival of these reflections is distributed in the range from 0 to 360°, the angular spectrum width is from 5 to 10°, and the range varies from 400 to 600 km. The third type includes anomalous mirror reflections with small deviations from zenith (not greater than 3°) but with substantially larger detection ranges (for example, 500km) as compared with the main reflections (250–300 km).     

Fracture mineralization and fluid flow evolution: an example from Ordovician–Devonian carbonates,southwestern Ontario,Canada

Petrography, geochemistry (stable and radiogenic isotopes), and fluid inclusion microthermometry of matrix dolomite, fracture‐filling calcite, and saddle dolomite in Ordovician to Devonian carbonates from southwestern Ontario, Canada, provide useful insights into fluid flow evolution during diagenesis. The calculated δ¹⁸O_fluid, ΣREE, and REE_SN patterns of matrix and saddle dolomite suggest diverse fluids were involved in dolomitization and/or recrystallization of dolomite. The ⁸⁷Sr/⁸⁶Sr ratios of dolomite of each succession vary from values in the range of coeval seawater to values more radiogenic than corresponding seawater, which indicate diagenetic fluids were influenced by significant water/rock interaction. High salinities (22.4–26.3 wt. % NaCl + CaCl₂) of Silurian and Ordovician dolomite–hosted fluid inclusions indicate involvement of saline waters from dissolution of Silurian evaporites. High fluid inclusion homogenization temperatures (>100°C) in all samples from Devonian to Ordovician show temperatures higher than maximum burial (60–90°C) of their host strata and suggest involvement of hydrothermal fluids in precipitation and/or recrystallization of dolomite. A thermal anomaly over the mid‐continent rift during Devonian to Mississippian time likely was the source of excess heat in the basin. Thermal buoyancy resulting from this anomaly was the driving force for migration of hydrothermal fluids through regional aquifers from the center of the Michigan Basin toward its margin. The decreasing trend of homogenization temperatures from the basin center toward its margin further supports the interpreted migration of hydrothermal fluids from the basin center toward its margin. Hydrocarbon‐bearing fluid inclusions in late‐stage Devonian to Ordovician calcite cements with high homogenization temperatures (>80°C) and their ¹³C‐depleted values (approaching ?32‰ PDB) indicate the close relationship between hydrothermal fluids and hydrocarbon migration.     

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.