Showing papers on "Longitude published in 1998"

Global Distribution of Density Irregularities in the Equatorial Ionosphere

Abstract: We analyzed measurements of ion number density made by the retarding potential analyzer aboard the Atmosphere Explorer-E (AE-E) satellite, which was in an approximately circular orbit at an altitude near 300 km in 1977 and later at an altitude near 400 km. Large-scale (>60 km) density measurements in the high-altitude regions show large depletions of bubble-like structures which are confined to narrow local time, longitude, and magnetic latitude ranges, while those in the low-altitude regions show relatively small depletions which are broadly distributed in space. For this reason we considered the altitude regions below 300 km and above 350 km and investigated the global distribution of irregularities using the rms deviation ΔN/N over a path length of 18 km as an indicator of overall irregularity intensity. Seasonal variations of irregularity occurrence probability are significant in the Pacific regions, while the occurrence probability is always high in the Atlantic-African regions and is always low in the Indian regions. We find that the high occurrence probability in the Pacific regions is associated with isolated bubble structures, while that near 0° longitude is produced by large depletions with bubble structures which are superimposed on a large-scale wave-like background. Considerations of longitude variations due to seeding mechanisms and due to F region winds and drifts are necessary to adequately explain the observations at low and high altitudes. Seeding effects are most obvious near 0° longitude, while the most easily observed effect of the F region is the suppression of irregularity growth by interhemispheric neutral winds.

Equatorial electrojet at African longitudes: first results from magnetic measurements

Abstract: . In the framework of the French participation in the International Equatorial Electrojet Year (IEEY), ten magnetotelluric stations were installed between November 1992 and November 1994 along a 1200-km-long meridian profile, between Lamto (latitude 6.2°N, Cote d'Ivoire) to the south and Tombouctou (latitude 16.7°N, Mali) to the north. These stations measured digitally the three components of the magnetic field and the two components of the telluric electric field, and operated over a period of 20 months. The magnetic data is used to study the features of the equatorial electrojet (EEJ) in West African longitude. The measurement of the telluric electric field variations will be presented elsewhere. Hourly mean values are used to study the morphological structure of the regular diurnal variation SR of the three components (H, D, and Z) of the earth magnetic field and to characterize the EEJ during magnetically quiet days. The occurrences of the counter-electrojet (CEJ) are set forth, emphasizing its seasonal variability. Assumed to be a current ribbon, the EEJ main parameters (the position of its center, its width, and the amplitude of its current density at the center) are determined. A preliminary analysis of the time variations of these parameters is presented over period of 20 months (from January 1993 to August 1994). Results are compared with those obtained during previous experiments of the same kind.

Pi1 magnetic pulsations in space and at high latitudes on the ground

Abstract: Nightside, high-latitude (68°–80° magnetic) Pi1 waves, measured with a ground array of induction magnetometers, are studied and compared with magnetic field measurements made at synchronous orbit near the meridian of the ground measurements. The objectives of the study are to relate the ground signatures of Pi1B and PiC to the auroral substorm and its manifestation at synchronous orbit in an attempt to understand the origin of the Pi1 waves. Pi1 waves are measured in the equatorial plane by the GOES spacecraft and appear to be initiated by the dipolarization process of the nightside tail magnetic field at the onset of substorms. Across two meridional arrays of ground stations the earliest onset of Pi1B generally occurred at the lowest-latitude station and, in many instances, this burst was superimposed on a ground signature of a sudden onset of the westward electrojet. In one instance, where good coverage of ground optical data was available, this sudden onset Pi1B was time related to the overhead passage of a westward traveling auroral surge. The timing of maximum Pi1B across the array in both longitude and latitude agrees with the westward motion of the local auroral surge and the poleward motion of the aurora after the surge has arrived at a given site, suggesting a local ionospheric source for some of the Pi1 waves. However, across the entire array, extending about 12° in latitude and 20° in longitude, there often was nearly simultaneous Pi1B wave power at all sites which occurred before the maximum signal at a given site (and presumably local aurora), suggesting horizontal ducting of wave power from the onset of PiIB seen earlier at the auroral zone latitude. Prompt turn-on of PiC waves across the array also indicates ducting of these waves. The narrow bandwidth of the PiC waves themselves suggests a resonant cavity source for them which would indicate that some wave power enters the ionosphere from space (consistent with the GOES in situ data) as the trigger wave for this resonant source mechanism. In conclusion, this study finds evidence for local ionospheric currents, magnetospheric waves, and resonant cavity modes as sources for Pi1 ground waves.

Equatorial 150‐km irregularities observed at Pohnpei

Abstract: First observations of meter-scale equatorial 150-km plasma irregularities outside the American longitude sector are reported. The observations conducted at Pohnpei (western Pacific) with a wind profiling ST radar demonstrate the feasibility of making ionospheric electric field measurements with low cost instruments and also reveal a potentially important anisotropy in the angular distribution of 150-km VHF radar echoes.

Longitudinal and interhemispheric variations of auroral ionospheric electrodynamics in a realistic geomagnetic field

Abstract: We investigate the influences of nondipolar features of the geomagnetic field (field strength, orientation, and magnetic coordinate distortion) on auroral ionospheric electrodynamics. We present a conceptual model in which both the electric potential and the distribution functions of precipitating auroral particles are invariant in the magnetic latitude/magnetic local time reference frame, and we explore the predictions of this model concerning longitudinal (or universal time) and interhemispheric (north/south) variations of auroral electrodynamic parameters at ±68° magnetic latitude for a fixed magnetic local time and solar zenith angle. The conductances, electric fields and currents, ion drift velocity components, and Joule heating tend to have two minima and maxima with respect to longitude in the northern hemisphere but have a single minimum and maximum in the southern hemisphere. Particularly large variations are found for the field-aligned current density (maximum/minimum = 1.76) and the regional Joule heating (maximum/minimum = 1.88), with maxima near northwest Iceland and northwest Alaska, and minima over north-central Siberia and Hudson Bay. The variations of field-aligned current intensity may imply a tendency for more frequent field-aligned electron acceleration and thus brighter aurora near northwest Iceland and northwest Alaska than elsewhere. The longitudinal variations of Joule heating may contribute significantly to universal time variations of total hemispheric Joule heating. Whether or not the conceptual model is correct, at least some of the various electrodynamic parameters must have longitudinal and hemispheric variations of the general magnitudes the model predicts.

Unusual longitude modification of the night‐time midlatitude F2 region ionosphere in July 1980 over the array of tectonic faults in the Andes area: Observations and interpretation

Abstract: Ionosphere topside sounding data from the INTERCOSMOS-19 satellite have been utilized to study longitude distribution of the night-time midlatitude F2 region peak electron density NmF2 in the Southern hemisphere over the array of tectonic faults in the Andes area for low geomagnetic activity periods in July 1979 and in July 1980 when solar activity was high and nearly the same, however, in July 1980 there was intensified tectonic activity in the Andes area, and in July 1979 the tectonic activity was lower. The results show that in July 1980 the observed value of NmF2 as a function of longitude decreased by a factor of ∼3 with the change of longitude from 280° to 295°E over the latitude range 30°–45°S, whereas the appropriate longitude profiles of NmF2 observed in July 1979 and reproduced by the ionospheric model IRI-90 do not reveal any appreciable longitude variations above the Andes area. Using model simulation we interpret the observed longitude anomaly of NmF2 in July 1980 as a result of the F2 region modification caused by a hypothetical large-scale long-living perturbation of the vertical electrostatic field on the Earth's surface associated with intensified tectonic activity.

Fabry-Perot interferometer measurements of neutral winds and F2 layer variations at the magnetic equator

Abstract: . This letter presents some night-time observations of neutral wind variations at F2 layer levels near the dip equator, measured by the Fabry-Perot interferometer set up in 1994 at Korhogo (Ivory Coast, geographic latitude 9.25°N, longitude 355°E, dip latitude –2.5°). Our instrument uses the 630 nm (O1D) line to determine radial Doppler velocities of the oxygen atoms between 200 and 400 km altitude. First results for November 1994 to March 1995 reveal persistent eastward flows, and frequent intervals of southward winds of larger than 50 ms–1 velocity. Compared with the simultaneous ionospheric patterns deduced from the three West African equatorial ionosondes at Korhogo, Ouagadougou (Burkina-Faso, dip latitude +1.5°) and Dakar (Senegal, dip latitude +5°), they illustrate various impacts of the thermospheric winds on F2 layer density: (1) on the mesoscale evolution (a few 103 km and a few 100 minutes scales) and (2) on local fluctuations (hundreds of km and tens of minutes characteristic times). We report on these fluctuations and discuss the opportunity to improve the time-resolution of the Fabry-Perot interferometer at Korhogo. Key words. Ionosphere (Equatorial ionosphere; Ionosphere-atmosphere interaction) · Meteorology and Atmospheric Dynamics (General circulation)

The Effects of Climatological and Persistence Variables on the Intensities of Tropical Cyclones over the Eastern North Pacific Ocean

Abstract: The effects of 10 climatological and persistence variables (latitude, maximum wind speed, 12-h change of maximum wind speed, longitude, distance to land, Julian date, sea surface temperature, speed of movement, zonal component of motion, and meridional component of motion) on changes of intensities of tropical cyclones over the eastern North Pacific Ocean were examined for the periods 1982–87 and 1988–93. Backward multiple regressions were performed to relate these 10 variables to changes in maximum intensity (as determined by wind speed) over periods ranging from 12 to 72 h. Latitude, maximum wind speed, and the 12-h change of maximum wind speed were the most significant variables. Each of the 10 variables was statistically significant at the 95% level at one or more of the time periods. Speed of movement, the component of motion, and the meridional component of motion were the least significant factors. The statistical relationships were tested using independent data from 1994. The mean absolut...

Total electron content measurements in the southern hemisphere using GPS satellites, 1991 to 1995

Abstract: The seasonal, diurnal, and latitudinal variations of total electron content (TEC) were determined using Global Positioning System (GPS) satellite signals over approximately 5 days per month during almost half a sunspot cycle (July 1991 to June 1995) at Salisbury (latitude 34.77°S, longitude 138.63°E), South Australia. These are the only such extensive southern hemisphere data till 1995 that have been recorded and analyzed. A selection of the data is presented, discussed, and compared with other workers' observations. Examples of Australian Surveying and Land Information Group (AUSLlG) data in the Australian region are presented. Some model predictions for northern hemisphere ionospheric TEC are compared with the GPS southern hemisphere observations of the combined TEC of the ionosphere and protonosphere. (The two models employed are the international reference ionosphere (IRI-90) and the paramaterized ionospheric model (PIM)(version 1.4, February 1996)). They are considered to be global models, even though the IRJ model is based primarily, but not exclusively, on northern hemisphere TEC data, and PIM is based on a theoretical model and is thus not directly based on TEC data.

Photospheric magnetic activity in a proxy for the young sun: hd 134319

Abstract: We present the results of a long-term photometric monitoring of the young single main-sequence star HD134319. It shows short-term variability of the optical-band continuum flux with a period of 4.448 days. The variability is attributed to dark spots or spots groups unevenly distributed in longitude on the star's photosphere, whose visibility is modulated by the star's rotation. Maps of the photospheric spot pattern have been obtained with light curve inversion techniques based on the Maximum Entropy and the Tikhonov regularization criteria. The overall spot pattern shows evidence for two long-lasting active longitudes located about 180° apart, with a total area of at least 16% of the stellar surface (assuming an inclination of the stellar rotation axis of 90° on the line of sight). The longitude distribution of the spot pattern and its total area do not show any clear evidence for a long-term variation along the five years of observations. A comparison with recent mean field dynamo models is also addressed, suggesting a possible interpretation of such a behaviour. Singularity, spectral type, youth and a high level of photospheric and chromospheric activity make HD134319 a suitable proxy for studying the magnetic activity of the young Sun not far after its arrival on the zero age main sequence.

Trajectory optimization using eccentric longitude formulation

Abstract: Theproblemofoptimaltransfer usinga set ofnonsingularorbit elementswhere theeccentric longituderepresents the sixth state variable is extended and completed by fully considering the six-state dynamics.Because the eccentric longitude appears in the right-hand sides of the dynamic equations, the use of this particular formulation removes the need for solving the transcendental Kepler equationat each integration step, thereby easing, to some extent, the numerical computations. Furthermore, because the eccentric longitude is being integrated directly, it effectively becomes an independent orbital element such that the adjoint differential equations are derived by assuming that this longitude is independent of the other elements. The variational Hamiltonian is constant throughout the transfer with the boundary conditions given simply in terms of the elements. An example of a general minimumtime transfer using continuous low-thrust is generated duplicating a previous result using the mean longitude formulation to validate the mathematical derivations.

Topography effects on small-scale precipitation variability in the Swiss pre-Alps

Abstract: Long-term precipitation and snow-cover measurements were carried out to estimate the areal pattern of precipitation in a small, rugged Swiss pre-Alpine basin. The dense networks used consisted of 34 storage gauges and 32 pit gauges with orifice parallel with slope, the latter being corrected for wetting losses. Consequently, the corrected data was very near to the true precipitation at a particular point. In the winter season also snow-cover characteristics were measured at 11 locations. Altitude turned out to be the best overall predictor for corrected data (R 2 = 0.67) and longitude for uncorrected data (R 2 = 0.67 vs only 0.58 for altitude!). The combination of altitude, longitude and latitude explains almost 75% of the spatial variance of summer season precipitation and snow-cover characteristics. However, the three variables used in the analysis were found to be interrelated, particularly in networks of storage gauges and snow-cover measurements (R 2 = 0.40-0.70).

ULF waves and the ground magnetic field

Abstract: A theory is put forward to account for the horizontal structure and polarization of the magnetic field of low-frequency ULF waves on the ground at low latitudes. The theory is based on the condition that the component of the rotation of magnetic field normal to the Earth's surface vanishes in the neutral atmosphere and also on an assumption that the phase in the H component of the ground magnetic field is constant latitudinally at low latitudes. When the H component phase is independent of the latitude, the D component amplitude increases monotonically from the magnetic equator to higher latitudes. The D component is 180° out of phase in the northern and southern hemispheres as well as east and west of longitude of the wave source. The sense of rotation of the magnetic perturbation vector in the H–D plane is clockwise and counterclockwise for the eastward and westward propagations, respectively, in the northern hemisphere and vice versa in the southern hemisphere. The major axis of the H–D plane ellipse orients in the northeast and northwest directions west and east of the source longitude, respectively, in the northern hemisphere and vice versa in the southern hemisphere. For compressional MHD disturbances emitted from a distant source region, their wave fronts surround the Earth in the inner plasmasphere owing to the slowdown of the Alfven speed just inside the plasmapause and wind around it near the Earth. This suggests that the latitudinally constant phase of the H component is a common feature in low-latitude ULF waves.

Variation of suicide and homicide rates by longitude and latitude

Abstract: State suicide rates in 1980 varied gradually, rather than abruptly, with longitude, as did homicide rates with latitude.

A reexamination of the local time asymmetry of lobe encounters at geosynchronous orbit: CRRES, ATS 5, and LANL observations

Abstract: The local time of occurrence of a geosynchronous plasma flux dropout or lobe encounter is dependent on the longitude of the observing spacecraft. Satellites located on or west of the International Geomagnetic Reference Field (IGRF) model magnetic dip prime meridian (magnetic dip meridian) observe almost exclusively (77{percent}) dawnside lobe encounters, while satellites located east of the magnetic dip meridian observe mostly duskside lobe encounters (67{percent}). However, the transition from observing dawnside or duskside lobe encounters is not symmetric about the magnetic dip meridian. Three satellites (ATS 5, 1991-080, and 1990-095) located essentially at the magnetic dip meridian observed a majority (73{percent}) of dawnside encounters. This study therefore concludes that there is a dawn-dusk asymmetry of the synchronous orbit altitude magnetic equatorial plane during active geomagnetic periods with the dawnside magnetosphere experiencing more radical distortions than the duskside. A case study of near-simultaneous lobe observations from CRRES and 1989-046 supports this conclusion. Note that it is found that the IGRF model dip prime meridian better organizes the local time occurrence frequency than does the magnetic dipole magnetic meridian, which has been shown to organize the seasonal dependence of geosynchronous lobe encounters in previous studies (i.e., Thomsen {ital et al.}, 1994). Hencemore » this study emphasizes the importance of considering the relative position of satellites with respect to both the magnetic dipole equator and the geomagnetic equator when utilizing geosynchronous plasma, magnetic field, and energetic particle data as diagnostics of the energy state of the magnetosphere. {copyright} 1998 American Geophysical Union« less

A fast moving easterly wave of the west Africa troposphere

Abstract: This paper documents a tropospheric synoptic-scale wavelike disturbance over Northern Africa and the tropical Atlantic during summer, in the 3.–5.day band period, different from the African wave. It has a velocity of 10–12 degree longitude per day instead of 6–8, a slighter shorter period, i.e. 3.5 instead of 4.5 days. It is best seen between 5°–7° N whilst the African wave has its largest amplitude about 12° N. The data used are NCEP/NCAR reanalyses and radiosonde data.

Cumulative ecological significance of oil and gas structures in the Gulf of Mexico: A Gulf of Mexico fisheries habitat suitability model. Phase 2 model description

Abstract: Habitat suitability indices (HSI) were developed for 560 model cells located west of 87 degrees west longitude in the nearshore Gulf of Mexico for seven species/ages subject to catch in trawls, and six adult reef fish species. Each model cell is then minutes of latitude by ten minutes of longitude (approximately 129 square miles).

Ground‐based spectroscopic observations of atmospheric ozone from 142 to 359 GHz in southern Europe

Abstract: During different periods of time between 1991 and 1996 we have achieved ground-based observations of rotational lines of atmospheric ozone using two different ground-based radio telescopes located in southern Europe: the 30 meters IRAM radio telescope at Pico Veleta (latitude 37:04:05.60°N, longitude 3:23:58.10°W, altitude 2850 m) and POM-2 at Plateau de Bure, (Hautes Alpes, France, latitude 44:38:02.00°N, longitude 5:54:28.50°E, altitude 2550 m). A total of eight different ozone lines have been observed with frequencies in the range 142-359 GHz. Several back-end spectrometers were able to analyze the incoming signal with very different frequency resolutions and total spectral coverage. This has allowed us to retrieve information about the vertical profile of ozone over a relatively wide range of altitudes. The retrieval itself has been achieved by means of a nonlinear least squares fit of the observed line profile, taking into account the spectroscopic, observational, and instrumental parameters of the different observations. A comparison between the ozone amounts retrieved from observations of different ozone lines in winter and summertime is given separately. Ozone amounts obtained from the different instruments and ozone lines are consistent.

Tropospheric atmospheric gravity waves and their relationship with geomagnetic activity

Abstract: Ground-level atmospheric gravity waves (GL-AGWs), recorded at Brisbane, Australia, from 1963 to 1966 by microbarographs, have been investigated for possible associations with geomagnetic activity. Two sets of well-defined GL-AGWs have been used. There were 54 events associated with the passage of cold-fronts at Brisbane, (frontal events), while another 67 well-defined GL-AGWs were called non-frontal events. Superposed-epoch analyses have been done to investigate (i) AE indices, (ii) three-hourly K indices and (iii) geomagnetic ΔH components at stations close to Brisbane's longitude, using these events as controls. Cross-correlation analyses have also been performed. For all these investigations, except for the cross-correlation analyses, it was found at a reasonably-high level of significance that the non-frontal events were delayed after geomagnetic activity by 3 days and the frontal events by 4 days. It is suggested that these GL-AGWs may be associated with sub-auroral and high-midlatitude ionospheric absorption produced by charge particles which enter the radiation belt at times of geomagnetic activity, and defuse to lower L shells. The experimental evidence for the bifurcation of the associated enhanced geomagnetic events at specific local times, as well as the evidence for other delays around 1 or 2 days has also been reported.

Method for short-term forecast of high-power solar flares

Abstract: FIELD: prediction of high-power solar flares. SUBSTANCE: method involves recording full and circularly polarized microwave radiation of active region with resolution not worse than 30 and determining probability of high-power solar flare with 24-hour term of forecast. Method is characterized in that solar latitude &; and solar longitude lambda of active region, its length l, angle of inclination of axis of sun-spot group relative to sun equator Δ, magnitude and polarity of magnetic field in sun-spots are measured, and magnetic class of active region is determined. Using these data, visible surface of Sun is divided into longitude zones; to this end, boundary positions of zones with known normal, flare-safe distribution of polarization in each separate zone are calculated. Flare danger of active region is recognized by deviation of visible distribution of polarization from normal for zone where analyzed active region is located. Boundaries of longitude zones are calculated every day for better estimation of the effect of evolution of active region. EFFECT: improved confidence of forecast results. 1 tbli

The active RS Canum Venaticorum binary II Pegasi

Abstract: Using new high-resolution and high signal-to-noise ratio spectroscopic observations carried out in 1992-1996, nine surface images of II Peg are obtained. The inversion technique applied is the recently developed Occamian approach, which does not put any artificial constraints on the solution and pro- vides an error analysis of the solution. The surface imaging is applied to Cai ,F ei, and Nii lines simultaneously, a number of blending atomic and molecular lines being included into the line list. Two high-latitude active regions are found to dominate in all seasons, which determine the spectroscopic and photometric variability. No cool polar cap is seen. The positions of the spots are constantly migrating to earlier orbital phases with approx- imately the same rate. This motion of the spot configuration means a shorter rotational period, which is just about the mean photometric period. The mean longitude separation between the active regions is about 180. This is considered as two active longitudes. The largest of the two spots seems to be close to the central meridian, i.e. tends to be faced toward the secondary. In 1994 it changed its position from one active longitude to an- other, showing the effect of switching the activity between the longitudes.

Mad display method and map display device

Abstract: PROBLEM TO BE SOLVED: To perform an endless map data display as if the earth is rotated at least in one direction of a longitude direction or a latitude direction by performing a coordinate transformation such as map data are not interrupted in terminal ends of a longitude and a latitude even though the terminal ends of the longitude and the latitude are present in map data. SOLUTION: When the world map of a planar rectangular coordinate system is displayed in accorance with a reduction ratio through the screen of a display, even though terminal ends of the longitude and the latitude (180 degrees of east longitude, 180 degrees of west longitude, 90 degrees of north latitude, 90 degrees of south latitude) are present in map data A, B stored in a fixed storage device, at the time of performing a screen display processing, a coordinate transformation processing such as the map data are not interrupted is performed. As a result, it is possible to display the world map data endlessly in an interval in which the earth rotates in the longitude direction or the latitude direction through the screen. Thus, it is possible to easily perform retrievals even to map data A'B' equivalent to longitude areas of the opposite side or the rear side of the earth.

Observations of the longitude effects in the mid-latitude ionospiieric f-region and comrisons with models

Abstract: Longitude effect in the mid-latitude F region is studied in this paper, using monthly median value f0F2 and f0F1 observed at Changchun and Urumqi,China from 1956 to 1994. Results are compared with the IRI-90 model. Physical mechanism leading to the longitude effect is discussed with the help of the IRI-90 and MSISE90 models.