Deg Latitude Research Articles

Dispersed ion structures at the poleward edge of the auroral oval: Low‐altitude observations and numerical modeling

We have compared the AUREOL 3 (A3) observations of auroral ion precipitation, particularly ion beams, with the results from the global kinetic model of magnetotail plasma of Ashour‐Abdalla et al. (1993). We have identified 101 energetic 2‐20 keV H+ velocity dispersed precipitating ion structures (VDIS) with fluxes above 10−3 ergs. cm−2. s−1 in the A3 record between the end of 1981 and mid‐1984. These beams display a systematic increase in energy with increasing latitude and were observed in a narrow region within less than 1 deg in latitude of the polar cap boundary. The VDIS are the most distinctive feature in the auroral zone of the plasma sheet boundary layer. We report first on a statistical analysis of the possible relationships between magnetic activity or substorm phase and the VDIS properties. The VDIS are found on 15‐18% of the A3 orbits. In general their frequency of occurrence is not correlated with activity. However in the 2200‐0200 MLT sector, the probability of observing more energetic VDIS increases for larger values of the AE index. Our particle simulations of the precipitating ions have been extended by using a series of modified versions of the Tsyganenko (1989) magnetic field model and by varying the cross‐magnetosphere electric field. In the simulations, plasma from a mantle source is subject to strong nonlinear acceleration, forming beams which flow along the PSBL. Only 3 to 4% of these beams precipitate into the ionosphere to form the VDIS while the majority return to the equatorial plane after mirroring and form the thermalized central plasma sheet. The final energy and the dispersion of the beams in the model depend on the amplitude of the cross‐tail electric field. Two unusual observations of low‐energy (< 5 keV) O+ VDIS, shifted by 4°‐5° in invariant latitude equatorward of H+ VDIS are analyzed in detail. The sparsity of such O+ events and the absence of the changes in the flux and frequency of occurrence indicate a solar wind origin for the plasma. Finally, large‐scale kinetic modeling, even with its simplifications and assumptions (e.g., static magnetic field, solar wind source), reproduces low‐altitude auroral ion features fairly well; it may therefore be presented as an appropriate framework into which data on energization and transport of the hot plasma, obtained in the equatorial plane, could be inserted in the near future.

Spots on RS CVn from spectroscopy and photometry

We have used contemporaneous spectra and V light curves to form spot models for RS CVn in 1991 and 1992. More than two spots are needed to fit all the properties of the observations. In fact, moderately small spots (22 x 28 deg in latitude and longitude) having only a slight effect on the rotational light curve were eclipsed in both years, and we find that a collection of 6-8 such moderate spots is required to fit the line profiles in each year. These groups of spots also account naturally for a difference in level of light between the two years. There is no evidence for polar spots larger than 18 degrees in radius. We have also derived new orbits from radial velocities of the stars, which give the mass ratio Mc/Mh = 1.04 +/-0.02.

A Passive Microwave Algorithm for Tropical Oceanic Rainfall

This study discusses a rainfall algorithm utilizing six channels of microwave radiance data from the Nimbus-7 Scanning Multifrequency Microwave Radiometer. The algorithm is intended for short-term climate studies over the ocean at low latitudes. To find a set of functional relationships, rain rates are regressed on brightness temperatures for each channel. Next, these functions are integrated over a class of rain-rate distributions to find relations between mean brightness temperatures and mean rain rates. This step accounts for beam filling. Finally, weights are obtained for combining the rain rates from the individual channels. The weights vary with the rain rates, so that the optimum combination of channels is always used. Results are stored in a database grid 1 deg latitude x 1 deg longitude by one month. To test the algorithm, three years (1979-81) of data from the Indian Ocean are processed.

A Diagnosis of the Explosive Development of Two Extratropical Cyclones

This paper examines the 24-h explosive development periods of two extratropical cyclones, the first occurring over the Gulf Stream off the coast of New England from 18 to 19 January 1979 and the second occurring over the southeastern United States from 20 to 21 January 1979. The data used in this study are the First GARP (Global Atmospheric Research Program) Global Experiment (FGGE) level IIIb (SOP I) global analyses on a 4 deg latitude x 5 deg longitude grid. The parameter used to diagnose development is the geostrophic relative vorticity tendency calculated using an extended form of the Zwack-Okossi development equation. This development equation is similar to the Petterssen-Sutcliffe development equation, but is shown to be more complete by explicitly coupling surface development with forcing at all levels above the surface. Cyclonic-vorticity advection, warm-air advection, and latent heat release act to develop the two cyclones, while adiabatic cooling in the ascending air opposes development. Further, vertical profiles of the development quantities for these two cases reveal that vorticity and temperature advection maximize in the 200-300-mb layer, while the latent heat release maximum is typically below 500 mb.

The Structure and Annual Variation of Antisymmetric Fluctuations of Tropical Convection and Their Association with Rossby–Gravity Waves

The present study explores the signature of 4-5-day-period Rossby-gravity waves in the tropical convection field across the Indian and Pacific oceans. The convergence/divergence field of these waves in the lower troposphere is anticipated to produce an antisymmetric fluctuation in tropical convection. Antisymmetric fluctuations of tropical convection are shown to exhibit a pronounced spectral peak at a 4-5-day period only during boreal fall and only within about 30 deg longitude of the date line. The peak amplitude occurs around 7.5 deg latitude. These fluctuations propagate westward at 15-20 m/s with zonal wavelength of about 7000-9000 km. The fluctuations of convections are coherent and out of phase with the equatorial meridional wind, which also exhibits a pronounced spectral peak at a 4-5-day period in the lower troposphere near the date line. The horizontal distributions of the 4-5-day power and coherence of the winds and convection are consistent with that produced by a convectively coupled Rossby-gravity wave that is confined near the date line.

Phase Speed Spectra of Transient Eddy Fluxes and Critical Layer Absorption

Tropospheric zonal mean eddy fluxes of heat and momentum, and the divergence of the Eliassen-Palm flux, are decomposed into contributions from different zonal phase speeds. Data analyzed are the European Center for Medium Range Weather Forecasts operational global analyses covering 1980-1987. Eastward moving medium-scale waves (zonal waves 4-7) dominate the spectra of lower tropospheric heat fluxes in both hemispheres and all seasons. Upper tropospheric wave flux spectra are similar to the low level spectra in midlatitudes, but shift to slower zonal phase speeds as low latitudes are approached. The cause of this shift is the selective absorption of faster moving components in midlatitudes as the waves propagate meridionally. Latitude-phase speed distributions of eddy fluxes are constructed and compared to the zonal mean wind structure. These results demonstrate that upper tropospheric eddies break and decelerate the zonal mean flow approximately 10-20 deg in latitude away from their critical line (where phase speed equals zonal wind speed). Comparisons are also made with results from the middle stratosphere.

Global, Seasonal Cloud Variations from Satellite Radiance Measurements. Part II. Cloud Properties and Radiative Effects

Cloud and surface radiative properties and their effects on the earth and surface radiation budgets are obtained based on global daily visible and IR radiance measurements. The magnitude of cloud property variations and their effects on radiation increase strongly with decreasing space/time scales. Cloud properties are systematically different between land and ocean, with ocean having larger cloud cover with somewhat larger optical thicknesses and lower cloud top altitudes. Although cloud variations appear to be the primary cause of regional radiation budget variability at 5-30 daytime scales, the effects of their seasonal variations at larger spatial scales are less important than the changes associated with changes in solar declination and atmospheric/surface temperatures. The largest seasonal variations in radiation occur in the 30-60 deg latitude band in each hemisphere. Although clouds have a net cooling effect on the global, annual mean radiation balance at both the top of the atmosphere and the surface, their net effect on regional, seasonal balances is much more varied.

Oceanic tides from Geosat altimetry

Direct tidal analysis of the altimetry from the Geosat Exact Repeat Mission's first year is used to derive estimates of the diurnal and semidiurnal oceanic tides. The geoid is removed by collinear differences at 34.1‐day separation, and the orbit error is reduced by subtracting a slowly modulated 1 cycle/revolution term. A sequence of independent analyses at grid areas of 1° latitude×1.5° longitude using “orthotide” functions ensures complete definition of the diurnal and semidiurnal species. Global admittance maps for M2 and S2 within the latitudes 58°N and 59°S compare well with ground truth at 66 open‐ocean sites. Maps of differences between Geosat and the Naval Surface Weapons Center model show important areas of the order of 10–15 cm. The integral for the global power input from the moon gives 2.51 TW (M2), 0.44 TW (S2), lower than many previous estimates, but in agreement with figures deduced from a recent analysis of satellite orbits.

Rainfall variation, snowline depression and vegetational shifts in Chile during the Pleistocene

Palynological, geomorphological, and relict vegetation evidence point to the existence of cooler and more humid conditions along semiarid and temperate Chile during the Pleistocene. Departing from an actualistic model, and utilizing a regression technique that includes significant independent variables on the basis of R 2 and F statistics, the best fit multivariable model was produced for annual rainfall and snowline elevation. Predicted values for rainfall are obtained by controlling sea surface temperatures and air temperatures (the most significant variables in the model) at different latitudes. A variation of only 1 °C of the winter sea and air temperatures induces more than a doubling of the annual precipitation in north-central Chile, and increases by nearly fifty percent in southern Chile. Entering the predicted values of precipitation and lowering the winter temperatures by 1 or 2 °C produces a slight depression of the snowline in semiarid north-central Chile and a significant descent in southern Chile. The predicted depression of the snowline coincided well with geomorphological evidence of glacial advances and fossil periglacial phenomena in the Andes. Cooling and increased precipitation during the Pleistocene pluvial elicited northward shifts of the temperate rainforest of southern Chile in the order of 7 deg latitude.

Impacts of carbon dioxide warming on climate and man in the semi-arid tropics

Tropical semi-arid climates occur between 10 and 35 deg latitude and are characterised by highly variable summer rainfall of between 300 and 750 mm in a rainy season of at least 4 months, generally adequate for rainfed cropping but with considerable drought risk. They support a mesic savanna vegetation. They have a land extent of 4.5 million km2, mainly occupied by Third World nations with rapidly increasing populations which in the main are predominantly rural and largely agricultural with low per capita incomes, consequently vulnerable to climate change. A doubling of atmospheric CO2 by the year 2030 is predicted to cause a rise in equilibrium mean temperature of 1–3 °C; however there is continuing uncertainty regarding the consequences for rainfall amount, variability and intensity, length of rainy season or the frequency of extreme rainfall events. Two scenarios are considered, with reduction and increase in rainfall respectively, involving corresponding latitudinal shifts in present climatic boundaries of about 200 km. Because of their variability, a clear signal of the greenhouse effect on these climates may be delayed, whilst regional responses may differ. Vegetational and hydrological responses under the alternative scenarios are considered. The possible consequences for rainfed and irrigated agriculture, water and energy supplies and disease and pest ecology are discussed. Lands of the semi-arid tropics are already extensively desertified, with consequent lowered productivity and heightened vulnerability to drought, and the possible impacts of greenhouse warming on desertification processes and on measures for land rehabilition to the year 2030 are reviewed. Measures to conserve the biological diversity of savanna lands in face of greenhouse warming are discussed.

Low-Frequency Oscillations of the Large-Scale Stratospheric Temperature Field

Low-frequency analyses are reported for four years of three-day-mean satellite microwave and infrared data representative of temperatures in the stratosphere. In data representative of 30-150 mb temperatures, oscillations with 39-51 day periods are observed as a tropical dipole pattern in the Indonesia/central Pacific. In addition, the first evidence is presented for such oscillations in the southeast Pacific. Furthermore, significant 39-51 day oscillations are observed in the mid- and upper stratosphere, centered near 60 deg latitude.

Beaming of Uranian kilometric radiation and implications for the planetary magnetic field

view Abstract Citations (4) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Beaming of Uranian kilometric radiation and implications for the planetary magnetic field Barbosa, D. D. Abstract Radio waves from Uranus at 481.2 kHz detected by Voyager 2 show a strong beaming effect. When the data are analyzed according to the magnetic latitude of the observer in an offset tilted dipole field model, a sharply defined emission beam extending from 25 to 55 deg latitude is evident consistent with a source over the south magnetic pole. The lack of prominent emission from the north magnetic pole can be attributed to the large spatial offset of the magnetic dipole resulting in much lower source frequencies. For proper alignment of the beam, an advance of the dipole axis longitude by 23 deg from the standard model is required. This phase shift is interpreted as the result of higher order magnetic moments producing a skew of the field lines at low altitudes in the polar source region. This conclusion further substantiates models of the interior of Uranus which predict a complex intrinsic field with significant nondipolar contributions to the surface magnetic field. Publication: The Astrophysical Journal Pub Date: April 1987 DOI: 10.1086/184878 Bibcode: 1987ApJ...315L.151B Keywords: Auroral Irradiation; Kilometric Waves; Magnetic Properties; Planetary Magnetic Fields; Radio Waves; Uranus (Planet); Magnetic Dipoles; Planetary Rotation; Radio Bursts; Voyager 2 Spacecraft; URANUS; KILOMETER WAVES; RADIATION; BEAMING; MAGNETIC FIELDS; LATITUDE; RADIO WAVES; EMISSIONS; VOYAGER 2; SPACECRAFT OBSERVATIONS; ANALYSIS; MODELS; SOURCE; MAGNETIC POLES; DIPOLE; MAGNETIC MOMENTS; INTERIORS; MAGNETOSPHERE; PLANETARY RADIO ASTRONOMY INSTRUMENT; FREQUENCIES; PRA; Lunar and Planetary Exploration; Uranus full text sources ADS |

Heliospheric shocks (excluding planetary bow shocks)

Even though it took place less than halfway through the last four year period, the AGU Chapman Conference in Napa Valley, California, during February, 1984, highlighted in many ways current U. S. contributions to the study of heliospheric shocks. Considerable recent progress in the theoretical understanding of these discontinuities has been summarized in the collections of the review papers from the Conference, so that those excellent papers have been an important source for the preparation of this Report.In addition, detailed analyses of ISEE‐3 interplanetary shock observations, and reports on additional data from the outer heliosphere, particularly major disturbances during 1982, have been published since the last Quadrennial Report, and are summarized hereafter. The Pioneer and Voyager spacecraft in the outer heliosphere also have traveled closer to the heliospheric boundary, and further inferences of the boundary location have been made, although it has not yet been directly detected. In addition, details are not yet available concerning the type of shock transition with which it may be associated. More general characteristics of shocks in the outer heliosphere, and out of the solar equatorial plane to ∼25 deg latitude or more, are available from the Pioneer and Voyager data. Also, collection and analyses of data from the inner heliosphere have been extended. Finally, new analyses and observations of coronal shocks have been done, and theoretical studies of this phenomenon have been advanced.

The abundance and distribution of water vapor in Jupiter's atmosphere

view Abstract Citations (82) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Abundance and Distribution of Water Vapor in Jupiter's Atmosphere Bjoraker, Gordon L. ; Larson, Harold P. ; Kunde, Virgil G. Abstract The atmospheric transmission window between 1800 and 2250/cm in Jupiter's atmosphere was observed from the Kuiper Airborne Observatory and by the IR spectrometer (IRIS) on Voyager. The vertical distribution of H2O was derived for the 1-6 bar portion of Jupiter's troposphere. The spatial variation of H2O was measured using IRIS spectra of the Hot Spots in the North and South Equatorial Belts (NEB, SEB) and the Equatorial Zone and for an average of the North and South Tropical Zones. The H2O column abundance above the 4 bar level is the same in the zones as in the SEB Hot Spots, about 20 cm amagats. The NEB Hot Spots are desiccated by a factor of 3 with respect to the rest of Jupiter. For an average between -40 and +40 deg latitude, the H2O mole fraction, qH2O, is saturated for P less than 2 bars, qH2O = 4 millionths in the 2-4 bar range, and it increases to 3/100,000 at 6 bars. A similar vertical profile applies to the spatially resolved zone and belt spectra, except that H2O falls off more rapidly at P less than 4 bars in the NEB Hot Spots. A massive H2O cloud at 5 bars, T = 273 K is inconsistent with the observations. Instead, a thin H2O ice cloud would form at 2 bars, T = 200 K. The O/H ratio in Jupiter, inferred from H2O measurements in both belts and zones at 6 bars, is depleted by a factor of 50 with respect to the sun. Publication: The Astrophysical Journal Pub Date: December 1986 DOI: 10.1086/164842 Bibcode: 1986ApJ...311.1058B Keywords: Abundance; Infrared Spectroscopy; Jupiter Atmosphere; Vertical Distribution; Water Vapor; Atmospheric Temperature; Ice Clouds; Spatial Distribution; Voyager Project; Lunar and Planetary Exploration; INFRARED: SPECTRA; PLANETS: ABUNDANCES; PLANETS: ATMOSPHERES; PLANETS: JUPITER; PLANETS: SPECTRA full text sources ADS |

Photovoltaic-Electrolyzer System Transient Simulation Results

Brookhaven National Laboratory has developed a Hydrogen Technology Evaluation Center to illustrate advanced hydrogen technology. The first phase of this effort investigated the use of solar energy to produce hydrogen from water via photovoltaic-powered electrolysis. A coordinated program of system testing, computer simulation, and economic analysis has been adopted to characterize and optimize the photovoltaic-electrolyzer system. This paper presents the initial transient simulation results. Innovative features of the modeling include the use of real weather data, detailed hourly modeling of the thermal characteristics of the PV array and of system control strategies, and examination of systems over a wide range of power and voltage ratings. The transient simulation system TRNSYS was used, incorporating existing, modified or new component subroutines as required. For directly coupled systems, we found the PV array voltage which maximizes hydrogen production to be quite near the nominal electrolyzer voltage for a wide range of PV array powers. The array voltage which maximizes excess electricity production is slightly higher. The use of an ideal (100 percent efficient) maximum power tracking system provides only a six percent increase in annual hydrogen production. An examination of the effect of PV array tilt indicates, as expected, that annual hydrogen production is insensitive to tilt angle within ± 20 deg of latitude. Summer production greatly exceeds winter generation. Tilting the array, even to 90 deg, produces no significant increase in winter hydrogen production.

A Simple Model for 100K-Year Oscillations in Glaciation

A simple climatic model which produces glaciation cycles of 100 K periods in response to forcing by 20 K, 40 K, and 100 K periods is described. The model is based on Milankovitch's (1930) hypothesis that glaciation fluctuations are forced by orbital variation and the associated change in insolation. The sea ice/snow cover line for the model, and the relation between heat variations and the ice/snow line are analyzed. The sea ice/snow cover line for the model is between the pole and 53 deg latitude and the line's position is the forcing for the glaciation cycle. Examples of the model's response to forcing are presented and evaluated. The negative glaciation permitted by the model is studied. The role of CO2 feedback in the glaciation cycle is investigated.

The Effect of Increased Horizontal Resolution on GLA Fourth Order Model Forecasts

A benchmark series of ten-day weather forecasts has been run with the GLA Fourth Order GCM with both a 4 deg latitude by 5 deg longitude resolution and a 2 deg latitude by 2.5 deg longitude resolution. Ensemble statistics of forecast skill and maps of systematic error fields have been generated for both resolutions. The enhanced resolution added 24 hours of useful predictive skill to the sea level pressure forecasts and 6 hours to the 500 mb height forecasts, but 5 to 6 days into the forecasts the advantage of the finer resolution was lost. The systematic error fields showed that by 8 days the 'climate drift' of the 2 x 2.5 deg forecasts had become pronounced and had caused the loss of predictive skill relative to the 4 x 5 deg forecasts. Additional results indicate that a gravity wave drag parameterization scheme might alleviate the climate drift problem.

Zonal Winds and the Angular Momentum Balance of Venus’ Atmosphere within and above the Clouds

Temperatures and pressures inferred from radio occultation data acquired by the Pioneer Venus orbiter between September 1982 and November 1983 are used to derive cyclostrophic zonal winds in the middle atmosphere of Venus (1350 to 2.1 mb, 10 deg to 70 deg latitude). The main feature of the wind field is a jet positioned just above the cloud tops at 70 km and approximately 48 deg latitude. The maximum speed of the jet is about 130 m/s. A comparison with results of similar analyses on Pioneer Venus radio occultation data obtained between December 1978 and October 1981 suggests an equatorward shift of the jet and a decrease in jet speed during this five-year time interval. It is proposed that the poleward transport of westward zonal momentum by the upper branch of the cloud level Hadley cell supplies the excess momentum of the jet and maintains it against dissipation. The location of the jet thereby provides a minimum estimate of the latitudinal extent of the Hadley cell. Cyclostrophic zonal wind velocities decrease with height above about 70-75 km. It is suggested that this deceleration of the superrotation in equatorial latitudes is due to the dissipation of vertically propagating thermal tides forced primarily at altitudes around 65 km.

Wave Instability in the Polar Region of Venus

Infrared and radio observations of the upper cloud region of Venus indicate that the north polar region contains features of large thermal contrast. A cold collar, encompassing a region of temperature inversions, lies between latitudes of about 65 and 75 deg, and a pair of warm features, separated by about 180 deg of longitude and centered near 80 deg latitude, rotate about the pole with a period of about 2.9 days. It is shown that the cold temperatures associated with the inversions lead to an enhancement in the mean zonal wind in a localized area near the pole, and that this enhancement makes the mean flow barotropically unstable. Since data for this region are limited, a model for the thermal structure has been used for calculating growth times and phase periods of the unstable modes. Choosing model parameters to agree as closely as possible with available data, it has been determined that the rotating warm features are likely to be manifestations of barotropically unstable waves.

The Latitude Dependence of the Variance of Zonally Averaged Quantities

Geometric characteristics of the spherical earth are shown to be responsible for the increase of variance with latitude of zonally averaged meteorological statistics. An analytic model is constructed to display the effect of a spherical geometry on zonal averages, employing a sphere labeled with radial unit vectors in a real, stochastic field expanded in complex spherical harmonics. The variance of a zonally averaged field is found to be expressible in terms of the spectrum of the vector field of the spherical harmonics. A maximum variance is then located at the poles, and the ratio of the variance to the zonally averaged grid-point variance, weighted by the cosine of the latitude, yields the zonal correlation typical of the latitude. An example is provided for the 500 mb level in the Northern Hemisphere compared to 15 years of data. Variance is determined to increase north of 60 deg latitude.