Low-latitude Locations Research Articles

Interhemispheric anti-phasing of rainfall during the last glacial period

We have obtained a high-resolution oxygen isotopic record of cave calcite from Caverna Botuverá (27°13′S, 49°09′W), southern Brazil, which covers most of the last 36 thousand years (ka), with an average resolution of a few to several decades. The chronology was determined with 46 U/Th ages from two stalagmites. Tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change. We interpret our record in terms of meteoric precipitation changes, hence the variability of South American Monsoon (SAM) intensity. The oxygen isotopic profile broadly follows local insolation changes and shows clear millennial-scale variations during the last glacial period with amplitudes as large as 3‰ but with smaller centennial-scale shifts ( < 1 ‰ ) during the Holocene. The overall record is strikingly similar to, but strongly anti-correlated with, a number of records from the Northern Hemisphere. We compared our record to other precisely dated contemporaneous records from Hulu Cave eastern China. Minima in δ 18O (wet periods, intense SAM) at our site are synchronous with maxima in δ 18O (dry periods, weak East Asian Monsoon, EAM) in eastern China (within precise dating errors) and vice versa. This anti-phased precipitation relationship between two low-latitude locations may be interhemispheric in extent, based on comparison with records from other sites. Precipitation anti-phasing may be related to north–south shifts in the mean position of the intertropical convergence zone (ITCZ) and asymmetry in Hadley circulation in two hemispheres, associated not with seasonal changes as observed today, but with millennial-scale climate shifts. The millennial-scale atmospheric see-saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor's greenhouse properties.

Geomagnetic signatures during the intense geomagnetic storms of 29 October and 20 November 2003

Solar cycle 23 in its declining phase witnessed the most pronounced space weather events during October–November 2003. A series of powerful solar flares and associated geoeffective Coronal Mass Ejections (CMEs) travelling at ∼2000 km/s drove shock fronts that impacted the Earth's magnetic field consecutively on 29 and 30 October, resulting in intense geomagnetic disturbances during 29–31 October. Another intense geomagnetic storm activity occurred during 20–21 November, resulting from a solar flare that had an associated geoeffective CME travelling at a speed of ∼1100 km/s. Digital ground magnetic field measurements from the equatorial and low-latitude locations in the Indian longitude zone, in conjuction with the interplanetary solar wind and magnetic filed parameters, are used to study the characteristics of these storms. Maximum magnitude of the total magnetospheric energy injected into the magnetosphere during the mainphase of the three major storm amounts to approximately 4.5×10 13, 3.6×10 13, 2.8×10 13 W. Another salient feature brought out is the close correspondence between the magnitude of the peak of the southward component of the interplanetary magnetic field B z and the strength of the storm intensity as inferred from the D st and low-latitude digital magnetic records.

Coral climate proxy data from a marginal reef area, Kuwait, northern Arabian–Persian Gulf

We present the first coral climate proxy data (Sr / Ca, δ 18O, δ 13C) from the Persian–Arabian Gulf using a Porites lutea coral collected offshore Kuwait. Both oxygen isotope- and Sr / Ca-ratios of the coral core investigated are statistically correlated with instrumental sea surface temperatures (SST) for the area from 1980–2002. However, the temperature calibration curve for Sr / Ca differs from other published curves in that its slope is much lower. The correlation coefficient between δ 18O coral and instrumental SST is at the lower limit as compared to other sites. These differences most likely relate to insufficient data representation of cold winters and hot summers, and indicates that much higher sampling resolution is needed in marginal reef areas such as Kuwait with extreme environmental fluctuations, i.e., intra-annual SST-variation of 16–34 °C, as compared to low latitude locations with relatively low SST variation. Variations of carbon isotopes of the coral skeleton do not exhibit climate control, however, a negative excursion in 1991 might be attributed to the massive oil field fires of the II. Gulf War.

Changes in Northeast Atlantic temperature and carbon flux during the Cenomanian/Turonian paleoceanographic event: the Goban Spur stable isotope record

Stable isotopes of bulk sediment and well preserved tests of planktonic and benthic foraminifera from mid-latitude NE Atlantic DSDP Site 551 (Goban Spur) provide the first estimates of carbon isotope gradients within the water column at a lower bathyal site during the Cenomanian/Turonian boundary interval (CTBI). The CTBI carbon isotope excursion is prominent (up to 2‰ shift in δ 13C) in the bulk (coccolith) signal, but less pronounced (approximately 0.5‰ shift in δ 13C) in planktonic and benthic foraminifera. This difference indicates a very steep 13C gradient in the upper water column and a very efficient biological pump during the CTBI carbon isotope excursion. We suggest significantly increased seasonal primary production in the uppermost water column with an enhanced shallow water chlorophyll maximum as a cause for this steep carbon isotope gradient. Deep-water and surface-water temperature changes during the CTBI are estimated using benthic and planktonic foraminiferal oxygen isotopes. Warm deep-water masses (13–16°C) and a low temperature gradient within the water column prevailed in the late Cenomanian. Additional warming (approximately 2°C for both surface and deep water) occurred in the latest Cenomanian prior to CTBI black shale deposition. This pattern of CTBI black shale deposition during a temperature maximum is also evident at two low latitude locations (ODP Site 1050, Blake Nose and Tarfaya, southern Morocco).

IRI related studies in Argentina: Topside profile and electron content

This paper reviews and extends studies of the ability of the International Reference Ionosphere (IRI) to make predictions of the topside electron density (N) and the total electron content for low latitude locations. Members of the Argentine scientific community have performed the reported studies. Topside Profile : Measurements of N obtained with the Hinotori and Taiyo satellites were used to check the ability of the IRI to predict monthly average value of N at 600 km of altitude (N 600) in the low latitude region and, also, that corresponding to a particular day and time so-called “instanteneous N value”. The comparison showed that the best agreement between IRI derived average N 600 values and measurements was obtained for a station near the magnetic equator and, in general, for the period of minimum ionisation for stations not near the magnetic equator. When ground ionosonde data obtained at Tucumán (26.9° S, 294.6°E; mag. latitude:−15.5) and Huancayo (12.0° S, 284.7° E; mag. latitude:−0.7) were used as input to the model the predicted instantaneous N 600 values were better than those obtained using either the CCIR or URSI options. Additional studies, covering different heights in the F region at low latitudes for low solar activity, showed that the IRI gives better N predictions than those obtained for high solar activity. Electron Content : Measurements of vertical total electron content (VTEC) over Tucumán, Havana (23.1° N, 277.5° E, mag. latitude: 34.2) and Arequipa (16.5° S, 289.0° E; geoma. Lat.:−5.1) obtained from geosynchronous and GPS satellites signals during high solar activity periods, have been used to study the IRI performance as predictor of TEC. Both versions, IRI-90 and IRI-95, overestimate VTEC at the southern crest of the equatorial anomaly for hours around the daily minimum, and underestimate it the rest of the day. This suggests that the main reason for the disagreement is the shape of the N profile assumed by the model. Good predictions were obtained for hours of maximum ionisation over Arequipa in some cases. For Havana, which is not under the northern peak of the equatorial anomaly, the IRI generally gives good predictions for the period around the daily minimum, and for all hours in some cases. Moreover, IRI has been used to calculate the ionospheric slant electron content (SEC) for the ATS 6 geosynchronous satellite — Boulder (40.13° N, 254.76° E) ratio signal path. The comparison with measurements obtained during low solar activity showed cases with good agreement among predictions and measurements. But, in general, the model underestimates SEC. The need for additional studies covering more stations and conditions to complete the validation of IRI model is stressed.

Ionospheric F3 layer: Implications for the IRI model

Recent studies have revealed the existence of an additional layer in the topside equatorial ionosphere, which was named the F 3 layer. Statistical studies using Fortaleza ionograms show that the F 3 layer is present on 75% of the days during December solstices (local summer) of low solar activity period, becoming a very common feature on that location. During those events, the F peak density becomes smaller than its usual value and the peak height can be much higher than the usual h max value. Vertical profiles obtained from ionograms, using true height inversion methods, show the existence of a very broad F layer peak, with a very slow height gradient. In this work we compare the observed F layer critical frequency and peak height with their IRI predictions (calculated using the URSI and CCIR coefficients). It was found that the IRI model overestimates the observed frequency by up to 3 MHz and underestimates the peak height by up to 100 km, when the F 3 layer phenomena is present. The vertical distribution of the F region electron density calculated by IRI also differs significantly from the observations during periods of F 3 layer occurrence. It is suggested that this phenomenon could account for some of the discrepancies between IRI predictions and observations, at least for daytime low-latitude locations.

Mesospheric gravity wave imaging at a subauroral site: First results from Millstone Hill

Optical detections of short‐period quasi‐monochromatic gravity waves in the mesospheric nightglow emissions of OH (695–950 nm), O (557.7 nm), and Na (589.3 nm) have been obtained for the first time from Millstone Hill, Massachusetts (42.6°N, 71.5°W). Using a new all‐sky bare charge‐coupled device (CCD) imaging system at this subauroral site, we obtained 12 nights of observations during the period February 1 to July 21, 1998. Wave structure was observed during ∼55% of the total observing time, and 35 events were identified and analyzed. Two types of wave structure were observed: extensive, long‐lasting (∼1–5 hours) waves known as bands and small‐scale, short‐lived (∼5–40 min) features called ripples. The band events are believed to be due to freely propagating or ducted internal gravity waves. The mean measured parameter values of the band events were horizontal wavelength (λh) = 21±7 km, phase speed (cob) = 47±20 m s−1, and observed period (τob) = 8.5±4.6 min. The intrinsic wave parameters (cin and τin) and the vertical wavelength (λz) of the band events were calculated using available meteor radar winds for nine of the events. Of the deduced λz values, five were evanescent, and the remaining four ranged from 12 to 21 km, with a mean of 17±4 km. Calculated energy and momentum fluxes for two case study events yielded 4–14 × 10−3 W m−2 and 5–30 m2 s−2, respectively. Possible sources of the waves were investigated using additional data obtained during 1999. The majority of the events exhibited poleward propagation, even during periods of visible auroral activity. This indicates that although the thermosphere above Millstone Hill exhibits subauroral behavior during enhanced levels of geomagnetic activity (Kp&gt;3), the propagation characteristics and frequency of occurrence of mesospheric gravity waves at this location differ very little from other middle‐ and low‐latitude locations.

The Abnormal Quiet Days in SQ(H) in Mid and Low Latitudes Regions

From the analysis and study of abnormal quiet days (AQDs) at mid and low latitudes locations, it was found that there is difference between the characteristics of phase variability Sq (H) of the low latitude locations. This suggests that the origin and cause of AQDs are of different sources in the two latitude regions. The AQDs events are independent of local time and could be of extra-terrestrial origin. The lunar contribution as seen from the analysis carried out has insignificant effects on the AQDs in both latitudes. Nigeria Journal of Pure and Applied Physics VOLUME 1, AUGUST 2000, pp. 1-2

Gravity wave spectra, directions and wave interactions: Global MLT-MFR network

Observations of winds and gravity waves (GW) by MF radars from the Arctic to the Equator are used to provide frequency spectra and spectral variances of horizontal motions, and information on the predominant azimuthal directions of propagation for the waves. The years used are mainly 1993/4; the height layer 76–88 km; and the GW bands 10 100 min. and 1–6 hrs. The high/mid-latitude locations of Tromso, Saskatoon, London/Urbana, Yamagawa, generally demonstrate similar behaviour: the monthly spectra have slopes near −5/3 in winter months, but smaller (absolute) slopes at higher frequencies (<2 hrs.) in summer. Corresponding to this, the spectral densities (10–100 min.) are larger for conditions of higher mean background windspeed—this is related by means of a new correlation-vector technique to GW propagating anti-parallel to the mean zonal winds, and the closure of the solstitial mesospheric jets. Also consistent with this, the sizes and orientations of perturbation ovals (fitted to the wind variations), demonstrate strong semi-annual-oscillations (SAO), and generally similar monthly and latitudinal directions. This suggests strong control, especially of the high-frequency GW band, by the dominant zonal wind-structures of the mesosphere. In contrast the low-latitude locations of Hawaii and Christmas Island demonstrate uniquely different behaviours, with indications of significant inter-annual variability. The frequency spectra for all months tend to have smaller slopes at higher frequencies. Also the dependence of spectral density in both GW bands, upon background wind speed, is negative rather than positive, and is shown to be generally consistent with GW propagating parallel to the mean-global winds. This is consistent with weaker vertical shears in the zonal winds (76–88 km), and lower GW momentum depositions. The perturbation ovals reveal much weaker SAO, and more variable orientations, consistent with more dependency upon GW sources, and less control by the mean winds of the mesosphere.

Temporal dynamics of fish communities on an exposed shoreline in Hawaii

Environmental conditions on higher latitude coral reefs can be extremely variable, and may structure fish communities in ways not previously observed in the more stable, low latitude locations where communities have usually been studied. Temporal changes in fish community structure were examined in an intensive two-year study of the reef fishes of Hanalei Bay, Kauai, Hawaii. Hanalei Bay is directly exposed to winter swells with high surf, as well as frequent heavy winter rainfall and high river discharge. Twenty-two transects (25 × 5 m) were established in a wide variety of habitats and censused monthly (N = 1052 censuses). Over 121 000 sightings of individuals from 150 species were made during the study. Seasonal patterns in number of species, number of individuals, species diversity, and evenness were observed, with winter values usually lowest. Values of these ensemble variables tended to be higher at deeper sites and at sites with greater habitat complexity. Surf height and degree of wave exposure were negatively correlated with several measures of community organization. Groups of fishes with different levels of spatial mobility tended to occupy depths consistent with their various abilities to respond to events of heavy weather. The rank abundance of fish taxa tended to be more stable seasonally at sites with less exposure to high wave energy. These seasonal effects may suggest some type of short-range movement from more exposed and monotypic habitats to locations that are deeper or otherwise provide refuge from seasonally heavy seas.

Plasma drifts inferred from thermospheric neutral winds and temperature gradients observed at low latitudes

Low-latitude plasma drifts (zonal and meridional) in the F-region are inferred from the observed night-time thermospheric neutral wind velocities and temperature gradients, together with models for the neutral density (MSIS-86 model) and the electron density (IRI model). The thermospheric neutral winds and temperatures are derived from measurements of Doppler shifts and widths of the Oi 630.0 nm airglow emission line, respectively, using a Fabry-Perot interferometer at Cachoeira Paulista (23°S, 45°W), Brazil. The equations considered are the ideal gas law and the momentum equation for the thermosphere, which includes the time variation of the neutral wind, the pressure gradient which is related to the temperature and density gradients and the ion drag force. The present method to infer the night-time plasma drift using observed neutral parameters (time variation of neutral wind velocities and temperature gradients) showed results that are in reasonable agreement with our calculated plasma drifts and those observed in other low-latitude locations. On the other hand, it is surprising that sometimes the winds flow from the observed coldest sector to the hottest part of the thermosphere during many hours, suggesting that plasma drift can drive the neutral winds at low latitudes for a period of time.

The role of the Hermanus Magnetic Observatory in geomagnetic field research

Geomagnetic field research carried out at the Hermanus Magnetic Observatory over the past decade is reviewed. An important aspect of this research has been the study of geomagnetic field variations, with particular emphasis on ULF geomagnetic pulsations. Features of geomagnetic pulsations which are unique to low latitude locations have been investigated, such as the cavity mode nature of low latitude Pi 2 pulsations and the role played by ionosphericO + ions in the field line resonances responsible for Pc 3 pulsations. A theoretical model has been developed which is able to account for the observed relationships between geomagnetic pulsations and oscillations in the frequency of HF radio waves traversing ionospheric paths. Other facets of the research have been geomagnetic field modelling, aimed at improving the accuracy and resolution of regional geomagnetic field models, and the development of improved geomagnetic activity indices.

Upper atmosphere research at INPE

INPE — the Brazilian National Space Research Institute - carries out research in the upper atmosphere and ionosphere at a number of low latitude locations including São José dos Campos (23°S, 46°W), Cachoeira Paulista (23°S, 45°W), Fortaleza (4°S, 38°W) and Alcântara (2°S, 44°W). This brief review, which is mainly concerned with work related to the chemistry and dynamics of the neutral mesosphere and lower thermosphere, describes INPE's facilities for research in these areas, and presents some recent results.

Variations of the geomagnetic field and the geomagnetic effects of three earthquakes (M s&gt;5.0) in Jiangsu Province, China

Since 1979 the repeated observations and experiments of geomagnetic total intensity and vertical component have been carried out for ten years in the geomagetic network which is located in Jiangsu Province, China. Three earthquakes aboveM s 5.0 occurred during the decade, and some seismomagnetic effects were observed. The observation results show that the anomalies of the vertical geomagnetic component can’t be observed untill some months before the earthquake (M s>5.0) in this area. In this paper it is suggested that a densely distributed network for continuous observation of geomagnetic vertical component may catch seismomagnetic anomalies and thus improve earthquake prediction in the light of the geomagnetic measurements of the mid — or — low latitude locations.

Spectra of the Far–Ultraviolet Background Shortward of 1200 Å from Voyager 2

Deep spectra of the far-ultraviolet sky background have been obtained by the Voyager 2 Ultraviolet Spectrometer (UVS) in the 500 to 1200 Å wavelength range. New diffuse galactic background results—particularly in the 900 to 1100 Å region—are presented, which extend the North Galactic Pole results of Holberg (1986) over a range of lower galactic latitudes. Twenty-four lines of sight are analyzed for the presence of residual diffuse continua. Typical upper limits of several hundred photons cm−2 s−1 Å−1 sr−1 are obtained for most lines of sight; however, a faint residual continuum is detected at several low-latitude locations. In Ophiuchus, a diffuse stellarlike continuum is attributed to scattering from interstellar dust.

Oceans on Titan?

If global oceans of methane exist on Titan, the atmosphere above them must be within 2 percent of saturation. The two Voyager radio occultation soundings, made at low latitudes, probably occurred over land, since they imply a relative humidity less, similar 70 percent near the surface. Oceans might exist at other low-latitude locations if the zonal wind velocities in the lowest 3 kilometers are </= 4 centimeters per second.

Preliminary Synthesis of Eocene-Oligocene Stable Isotope Data from Atlantic, Indian, and Pacific Ocean Sites: ABSTRACT

Generally consistent patterns of Eocene-Oligocene oxygen and carbon stable isotopic change are emerging from all ocean basins where well-preserved pelagic carbonates have been studied. The major features of benthic foraminiferal oxygen isotopic change are enrichment of close to 1^pmil in 18O associated with the middle-late Eocene boundary and the Eocene-Oligocene boundary at locations which differ by more than 3 km (2 mi) paleodepth. The Eocene-Oligocene enrichment occurs mostly in the earliest Oligocene and is clearly isochronous according to planktonic foraminiferal biostratigraphy. Completion of the Eocene-Oligocene event took less than one million years, with maximum earliest Oligocene ^dgr18O values giving rise to lower values later in the Oligo ene. Surface-dwelling planktonic foraminifera from low latitude locations become enriched in 18O by only a few tenths ^pmil from the Eocene to the Oligocene, whereas at high latitude locations they show the same enrichment as benthic foraminifera (about 1^pmil). Down-core oxygen isotopic trends are best interpreted as reflecting changes in the thermal or density structure of the ocean from Eocene to Oligocene time. This reasoning follows from the general lack of covariance of benthic and planktonic ^dgr18O from tropical locations, which in the Quaternary is the strongest evidence of large glacial-interglacial changes in seawater isotopic composition. Thus, high latitude locations where the planktonic ^dgr18O increased the most were probably the source of dense water to the deep ocean everywhere. A major increase in the density structure of the ocean should be evident in plots of planktonic foraminiferal ^dgr18O versus paleolatitude and benthic foraminiferal ^dgr18O versus paleodepth. Our preliminary data indicate the ^dgr18O versus latitude gradient increased from Eocene to Oligocene times, although more data from non-upwelling areas is required to establish the significance of this change. Reconstructions of vertical ^dgr18O structure in the late Eocene suffer from too little data from individual basins, although our synthesis of the early Oligocene North Atlantic reveals about a 1^pmil ^dgr18O increase between 1 and 2 km (0.6 to 1.2 m) paleodepth. Carbon isotopic results apparently do not vary systematically in time series, but display differences which probably reflect the hydrographic conditions overlying each site. For example, there are large variations in planktonic foraminiferal ^dgr13C, which vary symmetrically about the equator, as does the ^dgr13C of total CO2 today. Some of the lowest ^dgr13C values come from sites which backtrack to equatorial latitudes, perhaps reflecting upwelling. Benthic foraminiferal ^dgr13C varies by as much as 1^pmil within the North Atlantic during the Oligocene, unlike today when similar variability is only seen between deep basins. This probably reflects dramatically different Paleogene circulation patterns due at least, in part, to the absence of the Panama Isthmus as a barrier between the Atlantic and Pacific. End_of_Article - Last_Page 493------------

A reevaluation of the spatial and temporal distribution of sedimentary phosphate deposits in the light of plate tectonics

A new compilation of paleolatitudes of sedimentary phosphate deposits using worldwide paleomagnetic data confirms the hypothesis that the majority formed at low-latitude locations. A number of Jurassic and possibly some Cambrian deposits formed at intermediate latitudes (30 degrees -50 degrees ). Phosphorites range in age from lower Proterozoic to Recent. There has been sporadic deposition of phosphate during this time, as continents drifted into low-latitude locations. It is proposed that there is no direct genetic link between periods of volcanism, orogenesis, formation of evaporites, and episodes of phosphogenesis. All these features are indirectly genetically related through plate tectonics to particular phases in the rifting and separation of continental and oceanic components of plates. In the case of diverging plates, volcanism and tectonism are an early-stage feature, evaporites intermediate-, and phosphorites late-stage features. Models are developed to explain the different sedimentary sequences that develop in north-south and east-west seaways. No model is presently available for epeiric seas.A temporal model is also proposed to explain the distribution of phosphorus in sediments in the last 3,400 m.y. The association of phosphorus with iron and organic matter in the hydrosphere is particularly critical to the model. Even the temporal model is influenced to some extent by plate tectonics.