Longitude

About: Longitude is a research topic. Over the lifetime, 2260 publications have been published within this topic receiving 54988 citations. The topic is also known as: angle of longitude.

Rates of scattering occurrence in routine HF radar observations during solar cycle maximum

Abstract: The HF radars of the Super Dual Auroral Radar Network (SuperDARN) provide continuous monitoring of the high-latitude ionosphere over large areas in both the northern and southern hemispheres. The HF technique generates estimates of the E×B convective drift of ionospheric plasma when suitable small-scale (∼10 m) irregularities are present and viewed under favorable magnetic aspect conditions (radar k vector ⊥ to B). In this paper we report on the statistics of HF scattering occurrence. The study period encompasses 5.5 years (1988 to mid-1993) of routine observations with the HF radar located at Goose Bay, Labrador, Canada, and corresponds to the most recent period of solar cycle maximum. The data were carefully filtered for those instances when high-confidence F region velocity measurements were obtained within 1°×3° magnetic latitude/longitude regions and 12-min UT intervals. For average conditions the rates at which scatter occurred at particular values of invariant latitude ranged from 40% with MLT. The probability of making velocity measurements somewhere in the radar field of view varied from ∼80% on the nightside to ∼45% on the noon meridian. When scatter was detected, the average latitudinal coverage varied from a minimum of 4° Λ on the dayside to over 6° Λ on the nightside. There were significant dependencies on Kp and season. The highest occurrence rates (>60%) were obtained on the nightside for quiet conditions and in the afternoon for disturbed conditions. Winter was the most active season. HF scattering was limited by the equatorward boundary of the Feldstein oval but extended poleward of the oval into the polar cap. Scattering in the noon sector was associated with the aggregate cusp/mantle/low-latitude boundary layer region. The scattering activity was notably suppressed in the morning sector under disturbed conditions and in the noon and afternoon sectors in summer. We discuss the geophysical factors that influence the likelihood of generating HF scatter. These results have value for the design of experiments involving the SuperDARN HF radars.

MAVEN/IUVS Stellar Occultation Measurements of Mars Atmospheric Structure and Composition

Abstract: The Imaging UltraViolet Spectrograph (IUVS) instrument of the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has acquired data on Mars for more than one Martian year. During this time, beginning with March 2015, hundreds of stellar occultations have been observed, in 12 dedicated occultation campaigns, executed on average every two to three months. The occultations cover the latitudes from 80° S to 75° N and the full range longitude, and local times with relatively sparse sampling. From these measurements we retrieve CO 2 , O 2, and O 3 number densities as well as temperature profiles in the altitude range from 20 to 160 km, covering eight order of magnitudes in pressure from ∼2 × 10 1 to ∼4 × 10 −7 Pa. These data constrain the composition and thermal structure of the atmosphere. The O2 mixing ratios retrieved during this study show a high variability from 1.5 × 10 −3 to 6 × 10 −3 ; however, the mean value seems to be constant with solar longitude. We detect ozone between 20 and 60 km. In many profiles there is a well defined peak between 30 and 40 km with a maximum density of 1 – 2 × 10 9 cm −3 . Examination of the vertical temperature profiles reveals substantial disagreement with models, with observed temperatures both warmer and colder than predicted. Examination of the altitude profiles of density perturbations and their variation with longitude shows structured atmospheric perturbations at altitudes above 100 km that are likely non‐migrating tides. These perturbations are dominated by zonal wavenumber 2 and 3 with amplitudes greater than 45 %.

Statistical analysis of injection/dispersion events in Saturn's inner magnetosphere

Abstract: [1] In the inner magnetosphere of a rapidly rotating planet such as Jupiter and Saturn, radial transport of plasma mainly comprises hot, tenuous plasma moving inward and cold, denser plasma moving outward. A distinctive phenomenon resulting from the drift dispersion of injecting hot plasma provides direct evidence for this convective motion. Particle instruments aboard the Cassini spacecraft, including the Magnetospheric Imaging Instrument (MIMI) and the Cassini Plasma Spectrometer (CAPS), have made numerous observations of such signatures. The statistics of the properties of such events are studied in this paper by analyzing CAPS data from 26 Cassini orbits. A statistical picture of their major characteristics is developed, including the distributions of ages, longitudinal widths, radial distances, and longitudes and local times of injection. An unexpected longitude modulation of these events appears in the old SLS longitude system, which is based on the Saturn kilometric radiation (SKR) observations by Voyager around 1980, while no such modulation seems to exist in the new SKR longitude system of the Cassini era. A Lomb periodogram analysis, however, reveals no significant periodic modulation of these events. The injection structures are found to occupy a small fraction (∼5–10) of the available longitude space.

An Estimation of Sea-Level and Surface-Current Anomalies during the 1972 El Niño and Consequent Thermal Effects

Abstract: This paper has two main parts. First, it is shown that if the dynamics is simplified by assuming a one-mode, low-frequency (or long-wave) model, the whole field of anomalous motion in the tropical region can he deduced from the time series of sea-level anomaly at the eastern boundary. This is because the amplitudes of all the equatorial waves are proportional to sea level at this longitude, and the amplitudes at other longitudes can he deduced by integrating along characteristics using the method of Gill and Clarke (1974). The integration proceeds forward in time for the planetary waves and backward in time for the Kelvin wave. The calculation requires a knowledge of wind stress anomalies, although the wind effect only becomes significant after integration to large distances from the eastern boundary. The western boundary is irrelevant to the calculations. The technique is applied to estimate zonal surface current anomalies in the equatorial Pacific for the period 1971-73 spanning a major El Nino...

Spatio‐temporal variability of ocean temperature in the Portugal Current System

Abstract: A dynamic process convolution model (DPCM) is used to investigate the evolution and spatial distribution of monthly ocean temperature anomalies in the Portugal Current System. The analysis is performed with 20th century standard depth measurements from the National Oceanographic Data Center, ranging from the surface to 500 m depth. The proposed DPCM decomposes the temporal variability into short-term non-linear components and long-term linear trends, with both components varying smoothly across latitude, longitude and depth. An important feature of the DPCM is that it allows the assessment of trend significance without ad hoc corrections, since the residuals are spatially and temporally uncorrelated. In the analyzed period, an overall warming of coastal surface waters off the west Iberian Peninsula is found, together with fading cross-shelf temperature gradients and increased coastal stratification. Since previous studies also found that upwelling-favorable winds have weakened from the 1940s onward, these results most likely reflect a long-term weakening of the coastal upwelling regime. Transient periods of temperature change are also described and associated with known variability in the North Atlantic, and a final discussion on the link between the observed trends and anthropogenic forcing on climate is presented.