Showing papers in "Earth, Planets and Space in 1999"
TL;DR: In this article, a new technique for continuation of the ground magnetic field caused by ionospheric currents to the ionosphere in spherical geometry is presented that makes use of elementary ionosphere current systems, which were introduced by Amm (1997) in extension of an earlier work by Fukushima (1976).
Abstract: A new technique for continuation of the ground magnetic field caused by ionospheric currents to the ionosphere in spherical geometry is presented that makes use of elementary ionospheric current systems, which were introduced by Amm (1997) in extension of an earlier work by Fukushima (1976). The measured ground magnetic disturbance is expanded in terms of the ground magnetic effect of a spatial distribution of such elementary current systems. Using a matrix inversion technique, the scaling factors for each elementary current system, and therefrom the ionospheric equivalent currents are calculated. The technique can be applied to both global and local scales. Its advantages compared to the common field continuation techniques with Fourier (local scale), spherical cap (local to medium scale), or spherical (global scale) harmonic expansions are: 1) No fixed limitation of the spectral content has to be given for the whole analysis area, as it has to be done for the other techniques by truncation of a series expansion. 2) The locations of the elementary current systems can be chosen freely, such that they are most suitable with respect to the available measurement sites or the type of current system to be analysed. Results of the new technique are discussed in comparison to results of the spherical cap harmonic expansion method for a model of a Cowling channel.
296 citations
TL;DR: The Optical Mesosphere Thermosphere Imagers (OMTI) as discussed by the authors consist of an imaging Fabry-Perot interferometer, three all-sky cooled-CCD cameras, three tilting photometers and a Spectral Airglow Temperature Imager (SATI) with two container houses to install them in.
Abstract: The Optical Mesosphere Thermosphere Imagers (OMTI) have been developed to investigate the dynamics of the upper atmosphere through nocturnal airglow emissions. The OMTI consist of an imaging Fabry-Perot interferometer, three all-sky cooled-CCD cameras, three tilting photometers, and a Spectral Airglow Temperature Imager (SATI) with two container houses to install them in. These instruments measure wind, temperature and 2-dimensional airglow patterns in the upper atmosphere at multi-wavelengths of OI (557.7 nm and 630.0 nm), OH (6–2) bands, O2 (0, 1) bands, and Na (589.3 nm), simultaneously. Examples of the data are shown for the cameras, the photometers, and the SATI based on the airglow observation at a mid-latitude station in Japan. Good correlation of the photometer and SATI observations is obtained. A comparison is shown for small- and large-scale wave structures in airglow images at four wavelengths around the mesopause region using four cooled-CCD cameras. We found an event during which large-scale bands, small-scale row-like structures, and large-scale front passage occur simultaneously.
181 citations
TL;DR: In this paper, experiments of pulse laser irradiation, proton implantation, and laser irradiated to proton implanted samples were performed and reflectance spectra of altered materials were measured.
Abstract: For the purpose of simulating the surface alteration process called “space weathering”, experiments of pulse laser irradiation, proton implantation, and laser irradiation to proton implanted samples were performed and reflectance spectra of altered materials were measured. To simulate the impact heating by micrometeorite bombardments, we made a new apparatus using a pulse laser whose pulse duration is 6‐8 nanoseconds, comparable with a timescale of micrometeorite impacts. We find that the degree of space weathering, i.e., change of reflectance spectrum should depend on mineral composition. Laser irradiation onto olivine produces the largest reduction of albedo and the highest reddening of reflectance spectrum. In general, variation of olivine spectra is much larger than that of pyroxenes. Depths of absorption bands do not change in the scaled spectra. The olivine spectrum after the laser irradiation can match spectra of some olivine asteroids within a subtype of S-type asteroids. Comparison of Vesta spectrum with altered pyroxene spectra suggests that Vesta surface would be relatively older than olivine asteroids. We also investigate the influence of solar wind proton and pyroxene FeO content. The proton implantation causes small changes in olivine and enstatite spectra. Implanted protons do not influence spectral change by the laser irradiation: the laser irradiation and the proton implantation do not produce multiplicative but additive changes on the reflectance spectra. FeO content of pyroxenes does not relate to the degree of reflectance change.
178 citations
TL;DR: In this article, a wide-view CCD imager for airglow observations was operated at the MU radar site in Shigaraki, Japan (35°N, 136°E). From the 18 months' observation, dominant gravity wave components were extracted, and seasonal variation of the characteristics of the waves was investigated.
Abstract: A wideview CCD imager for OH airglow observations was operated at the MU radar site in Shigaraki, Japan (35°N, 136°E). From the 18 months’ observation, dominant gravity wave components in the OH images are extracted, and seasonal variation of the characteristics of the waves is investigated. These waves typically have short horizontal wavelengths (5 km–60 km) and short periods (5 min–30 min), with horizontal phase speeds of 0–100 m/s. All the wave events are separated into two groups by a boundary of a horizontal wavelength of 17.5 km, which is close to the boundary between ripples and bands. For the waves with larger horizontal wavelengths, the horizontal propagation direction showed clear seasonal variation with summer eastward and winter westward preferences, with a change of direction in mid-March and at the end of September. This suggests that these waves are propagated from the lower atmosphere and filtered in the middle atmosphere by the mean winds. However, the small scale waves propagate in almost all azimuths with a slight seasonal variation. Therefore, in-situ generation would be the major source of such waves although the wavelength as a physical boundary between the two groups could be smaller than 17.5 km. The seasonal variation of the wave parameters especially between summer/winter and equinoctial months is also discussed. The waves with small horizontal wavelengths ( 10 min), and slow horizontal phase speeds (<30 m/s) are mainly seen in summer/winter.
152 citations
TL;DR: A set of numerical experiments have been conducted using the National Center for Atmospheric Research Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (NCAR Time-GCM) to understand the effects of the quasi-two-day wave (QTDW) on the middle atmosphere horizontal wind and temperature fields as mentioned in this paper.
Abstract: A set of numerical experiments have been conducted using the National Center for Atmospheric Research Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (NCAR TIME-GCM) to understand the effects of the quasi-two-day wave (QTDW) on the middle atmosphere horizontal wind and temperature fields. A zonal wavenumber three perturbation with a period of 48 hours and a latitudinal structure identical to the (3, 0) Rossby-gravity mode has been included at the lower-boundary of the model. A response in the middle atmosphere horizontal wind fields is observed with a structure qualitatively similar to observations and other model results. There is also some evidence to suggest an increase in the lower-thermosphere QTDW response due to the interaction with gravity waves. Changes are observed in the zonal mean wind and temperature fields that are clearly related to the QTDW, however it is unclear if these changes are the direct result of wave driving due to the QTDW or are from another source. Evidence for nonlinear interactions between the QTDW and the migrating tides is presented. This includes significant (40–50%) decreases in the amplitude of the migrating tides when the QTDW is present and the generation of wave components which can be tracked back to an interaction between the QTDW and the migrating tides. Clear evidence for the existence of a westward propagating zonal wavenumber six nonmigrating diurnal tidal component which results from the nonlinear interaction between the QTDW and the migrating tides is also presented.
112 citations
TL;DR: In this article, the formation of spatial structure in dense, self-gravitating particle systems such as Saturn's B-ring through local N-body simulations to clarify the intrinsic physics based on individual particle motion was investigated.
Abstract: We investigate the formation of spatial structure in dense, self-gravitating particle systems such as Saturn’s B-ring through local N-body simulations to clarify the intrinsic physics based on individual particle motion. In such a system, Salo (1995) showed that the formation of spatial structure such as wake-like structure and particle grouping (clump) arises spontaneously due to gravitational instability and the radial velocity dispersion increases as the formation of the wake structure. However, intrinsic physics of the phenomena has not been clarified. We performed local N-body simulations including mutual gravitational forces between ring particles as well as direct (inelastic) collisions with identical (up to N ∼ 40000) particles. In the wake structure particles no longer move randomly but coherently. We found that particle motion was similar to Keplerian motion even in the wake structure and that the coherent motion was produced since the particles in a clump had similar eccentricity and longitude of perihelion. This coherent motion causes the increase and oscillation in the radial velocity dispersion. The mean velocity dispersion is rather larger in a more dissipative case with a smaller restitution coefficient and/or a larger surface density since the coherence is stronger in the more dissipative case. Our simulations showed that the wavelength of the wake structure was approximately given by the longest wavelength λcr = 4π2GΣ/κ2in the linear theory of axisymmetric gravitational instability in a thin disk, where G, Σ, and κ are the gravitational constant, surface density, and a epicyclic frequency.
88 citations
TL;DR: In this article, the behavior of the 5-day and 16-day waves in the mesopause region is examined by using a general circulation model, and the results are as follows.
Abstract: The behavior of the 5-day and 16-day waves in the mesopause region is examined by using a general circulation model. The results are as follows. The 5-day wave is largely unaffected by the zonal mean zonal wind distribution, and the symmetric structure about the equator is clearly seen in the mesopause region. The amplitude of the 16-day wave in the summer hemisphere of the stratosphere is small. However, above the upper mesosphere, the 16-day wave appears not only in the winter hemisphere but also in the summer hemisphere. The penetration of the 16-day wave from the winter hemisphere to the summer hemisphere occurs near the mesopause region. The 16-day wave is mainly excited by heating due to the moist convection in the troposphere, and the vertical penetration into the middle atmosphere occurs. Furthermore, a correlation between the geomagnetic variation and the wind variation associated with the 5-day and 16-day waves is discussed.
82 citations
TL;DR: In this article, a simple, generalized parameter constraint using a priori information to obtain a stable inverse of geophysical data is presented. But the constraint is not restricted to positive values.
Abstract: This paper presents a simple, generalized parameter constraint using a priori information to obtain a stable inverse of geophysical data. In the constraint the a priori information can be expressed by two limits: lower and upper bounds. This is a kind of inequality constraint, which is usually employed in linear programming. In this paper, we have derived this parameter constraint as a generalized version of positiveness constraint of parameter, which is routinely used in the inversion of electrical and EM data. However, the two bounds are not restricted to positive values. The width of two bounds reflects the reliability of ground information, which is obtained through well logging and surface geology survey. The effectiveness and convenience of this inequality constraint is demonstrated through the smoothness-constrained inversion of synthetic magnetotelluric data.
78 citations
TL;DR: In this paper, a computationally inexpensive spectral gravity wave parameterization scheme was proposed, whose predictions approximate those of a full three-dimensional (in spectral space) spectral model of atmospheric gravity waves.
Abstract: This paper reports first steps toward a computationally inexpensive spectral gravity wave parameterization scheme whose predictions approximate those of a full three-dimensional (in spectral space) spectral model of atmospheric gravity waves. A reduction to two dimensions, as proposed by Hines, requiring the neglect of Coriolis and non-hydrostatic effects, is explored on the basis of comparisons with a full three-dimensional power-spectral model that includes Coriolis and non-hydrostatic effects. The reduction tries to be more realistic in terms of spectral shapes, though simpler in terms of wave-breaking criteria. It works remarkably well in the absence of, but less well in the presence of, background shear. The reasons for the discrepancies are under investigation, as are the implications for two-dimensional schemes, including Hines’ as well as ours.
76 citations
Journal Article•
TL;DR: In this paper, the authors presented high-resolution Nd isotope records for crusts previously dated using 10 Be=9 Be chronology, consistent with a likely relationship with the closure of the Central American Isthmus and intensification of Northern Hemisphere Glaciation, and in accordance with changes seen in other physical and chemical records.
Abstract: Hydrogenetic ferromanganese (Fe‐Mn) crusts from the western North Atlantic record variations in the Nd and Pb isotopic composition of Cenozoic deep water preserved during their growth. The timing and cause of the most striking change have been the subject of debate. Some have proposed that the shift took place after 4 Ma in response to the closure of the Panama gateway. Others have argued that the major change in isotope composition occurred as early as 8 Ma. This study presents high-resolution Nd isotope records for crusts previously dated using 10 Be= 9 Be chronology. These data confirm that the shifts in Nd occurred after 4 Ma, consistent with a likely relationship with the closure of the Central American Isthmus and intensification of Northern Hemisphere Glaciation, and in accordance with changes seen in other physical and chemical records. These results illustrate the need for both a robust chronological framework and high-resolution records before a reliable paleoceanographic interpretation can be made of the variations recorded by Fe‐Mn crusts. © 1999 Elsevier Science B.V. All rights reserved.
74 citations
TL;DR: In this paper, a short review of the relevant turbulent spectra and equations are considered, and their applications in middle atmosphere studies are outlined, and new developments with regard to some of this theory, and especially new understandings about the dynamical parameters used in some of these applications (often referred to as the constants of the equations) are described.
Abstract: The study of turbulent heating and diffusion in the middle atmosphere is complicated by some subtle points relating to the application of existing theory. Incorrect interpretation of turbulent spectra can result, leading to errors in estimates of the strengths of turbulence by factors of 5 and more. In this short review, the relevant turbulent spectra and equations are considered, and their applications in middle atmosphere studies are outlined. New developments with regard to some of this theory, and especially new understandings about the dynamical parameters used in some of these applications (often referred to as the “constants” of the equations) are described. Current areas of uncertainty are also considered, both in relation to turbulent energy dissipation as well as diffusion over various scales.
TL;DR: In this paper, a series of numerical experiments conducted with the global-scale wave model (GSWM) and designed to investigate the effects of the quasi-biennial oscillation (QBO) on the migrating diurnal tide were conducted.
Abstract: We report on a series of numerical experiments conducted with the global-scale wave model (GSWM) and designed to investigate the effects of the quasi-biennial oscillation (QBO) on the migrating diurnal tide. Our results indicate that the diurnal tidal response in the upper mesosphere and lower thermosphere (MLT) is significantly affected by the QBO in zonal mean zonal winds, but largely insensitive to the QBO in stratospheric ozone. We discuss the variable mean wind results in light of previous analytic attempts to describe the diurnal tide in the presence of mean winds and dissipation. Our calculations do not explain the interannual tidal variations observed by the High Resolution Doppler Interferometer (HRDI) on the Upper Atmosphere Research Satellite (UARS).
TL;DR: In this paper, two independent codes were developed to integrate numerically the orbits of test particles around irregularly shaped primary bodies, based on a representation of the central body in terms of discrete spherical masses and a polyhedron with a variable number of triangular faces.
Abstract: The new profiles of the space missions aimed at asteroids and comets, moving from fly-bys to rendezvous and orbiting, call for new spaceflight dynamics tools capable of propagating orbits in an accurate way around these small irregular objects. Moreover, interesting celestial mechanics and planetary science problems, requiring the same sophisticated tools, have been raised by the first images of asteroids (Ida/Dactyl, Gaspra and Mathilde) taken by the Galileo and NEAR probes, and by the discovery that several near-Earth asteroids are probably binary. We have now developed two independent codes which can integrate numerically the orbits of test particles around irregularly shaped primary bodies. One is based on a representation of the central body in terms of “mascons” (discrete spherical masses), while the other one models the central body as a polyhedron with a variable number of triangular faces. To check the reliability and performances of these two codes we have performed a series of tests and compared their results. First we have used the two algorithms to calculate the gravitational potential around non-spherical bodies, and have checked that the results are similar to each other and to those of other, more common, approaches; the polyhedron model appears to be somewhat more accurate in representing the potential very close to the body’s surface. Then we have run a series of orbit propagation tests, integrating several different trajectories of a test particle around a sample ellipsoid. Again the two codes give results in fair agreement with each other. By comparing these numerical results to those predicted by classical perturbation formulae, we have noted that when the orbit of the test particle gets close to the surface of the primary, the analytical approximations break down and the corresponding predictions do not match the results of the numerical integrations. This is confirmed by the fact that the agreement gets better and better for orbits farther away from the primary. Finally, we have found that in terms of CPU time requirements, the performances of the two codes are quite similar, and that the optimal choice probably depends on the specific problem under study.
TL;DR: In this paper, the authors used the hydrodynamic equations to model entrainment of the mixing ratios of ionized constituents hypothesized to be present in the upper polar mesosphere to calculate the spectra and cospectra of ions and electrons.
Abstract: The temporal evolution of a turbulent layer is calculated in detail by solving the hydrodynamic equations. The turbulence is initiated by a Kelvin-Helmholtz instability. The field of potential-temperature fluctuations serves as a tracer for modeling entrainment of the mixing ratios of ionized constituents hypothesized to be present in the upper polar mesosphere. This entrainment modeling provides the input to a turbulence advection model capable of calculating the spectra and cospectra of ions and electrons. The turbulence advection model is used as a subgrid-scale model and is required because, given present or foreseeable computer capabilities, numerical solutions cannot span the enormous range of spatial scales from the depth of the shear layer to the smallest scales on which the most massive ions diffuse. The power spectrum of electron number-density fluctuations obtained from the turbulence advection model is compared with that measured by a rocket during the STATE (Structure and Atmospheric Turbulence Environment) experiment; agreement is found for a case of massive ions. The radar cross section for Bragg scattering is calculated from the electron number-density power spectrum and is used to calculate the signal-to-noise ratio (S/N) for the Poker Flat 50 MHz radar. The resultant S/N is then compared with the radar measurements obtained during the STATE experiment. These comparisons support the hypothesis that massive ions can cause polar mesosphere summer echoes from turbulent layers. Large-scale morphology of the turbulent layer obtained from rocket and radar measurements is reproduced by the hydrodynamic solution.
TL;DR: In this article, the amplitude and frequency variations of long-period oscillations as a function of height and time were studied using power spectral techniques, and a mean climatology of these variations taken from years 1990-1997 was presented.
Abstract: Zonal and meridional winds in the equatorial mesosphere and lower thermosphere (65–98 km) measured at two sites separated by 94° in longitude are used to study the zonal structure of planetary-scale waves. The data were obtained with MF radars located at Pontianak (0°N, 109°E)and Christmas Island (2°N, 157°W). The data at Christmas Island were collected from January 1990 to December 1997 and the observations at Pontianak were made from November 1995 to July 1997. Power spectral techniques are used to study the amplitude and frequency variations of long-period oscillations as a function of height and time. A mean climatology of these variations taken from years 1990–1997 is presented. Strong peaks in zonal and meridional winds are found at tidal periods and for the quasi 2-day wave. Zonal spectra exhibit considerable power at periods of 3–10 days, with transient oscillations with periods near 3.5 day and 6.5 days being especially prominent. The 6.5-day wave is particularly strong during April and September. Examination of the phase differences obtained from cross-spectra between the two stations show that the 6.5-day wave is westward propagating with zonal wavenumber 1, while the 3.5 day wave is eastward propagating with wavenumber 1. The 6.5-day wave is identified as a manifestation of an unstable mode, while the 3.5-day wave is identified as an ultrafast Kelvin wave. There are significant longitudinal variations in the amplitudes and inferred momentum fluxes of the 3.5-day wave, amplitudes being larger in the Asian region than in the central Pacific.
TL;DR: In this paper, the long-term behavior of Kelvin waves with the wave period ranging from 3 to 4 days, which are generally called an ultra-fast Kelvin (UFK) wave was studied.
Abstract: This paper is concerned with observations of the long-term behavior of Kelvin waves with the wave period ranging from 3 to 4 days, which are generally called an ultra-fast Kelvin (UFK) wave. Horizontal wind velocity at 74–110 km altitudes observed with a meteor wind radar (MWR) near Jakarta (6.4°S, 106.7°E) for five years during November 1992 and December 1997 and daily radiosonde profiles in Bandung (6.9°S, 107.6°E) collected between October 1993 and March 1996 and have been analyzed. In the mesosphere and lower thermosphere (MLT) region, the UFK wave activity, defined by the spectral density of zonal wind perturbations at the 3.0–3.8 day period, is strongly enhanced twice a year. An interaction between UFK waves and a semiannual oscillation in the mesosphere (MSAO) can be suggested, although an exact mechanism is uncertain. We also have investigated seasonal variation of 3.0–3.8 day oscillations of zonal winds in the stratosphere, excluding gravity wave components, but, we have not detected an evidence of semiannual periodicity. The UFK wave activity in the MLT region exhibited intraseasonal variations, which showed some correlation with the amplitudes of zonal wind in the troposphere.
TL;DR: In this paper, the authors use an equatorial beta-plane model to demonstrate how such a modulation could come about through selective damping of the equatorial wave spectrum excited by deep convection.
Abstract: Recent satellite and radar observations suggest that the semiannual oscillation (SAO) in the mesosphere is modulated by the stratospheric quasibiennial oscillation (QBO). The modulation is only apparent during the SAO easterly phase, which is considerably stronger when QBO winds are westerly than when they are easterly. We use an equatorial beta-plane model to demonstrate how such a modulation could come about through selective damping of the equatorial wave spectrum excited by deep convection. The waves affected most strongly are easterly inertia-gravity waves of phase speeds slower than ∼ −40 m s−1. This is close to the zonal wind speed during the easterly phase of the QBO (−30 to −35 m s−1), so the waves suffer strong thermal damping or even absorption as they propagate through the stratosphere. Because these waves are important for driving the easterly phase of the mesopause SAO in the model, that phase is weaker when the stratospheric QBO winds are easterly. A similar modulation of the westerly phase of the SAO does not occur for two reasons: (1) QBO westerlies are only half as strong as QBO easterlies, and (2) much of the driving of the westerly phase of the SAO is accomplished by Kelvin waves of phase speed ∼40–60 m s−1. As a consequence, the QBO winds have negligible influence on the vertical propagation of waves with westerly phase velocity and hence on the westerly phase of the modeled SAO.
TL;DR: In this article, a model based analysis of rocket borne common volume measurements of electron number densities and aerosol charge densities during the ECHO campaign in 1994 is presented, and the existence of positively charged aerosols can be explained if they mainly consist of a substance with a sufficiently low work function.
Abstract: We present a model based analysis of rocket borne common volume measurements of electron number densities and aerosol charge densities during the ECHO campaign in 1994. During that campaign a sounding rocket was launched into a noctilucent cloud (NLC) as detected by a ground based lidar. At NLC altitudes a particle impact detector gave strong evidence for positively charged aerosols, and an electron probe measured a significant electron enhancement. We have applied a model of aerosol charging to these measurements and find that the existence of positively charged aerosols can be explained if they mainly consist of a substance with a sufficiently low work function. The electron enhancement as well as the aerosol size and number density deduced from our model are consistent with the electron probe and lidar measurements, respectively. Considering the photoelectrical properties of various metals we conclude that only sodium and potassium have a sufficiently low work function to allow for significant photoemission. Even under very favourable conditions the maximum positive charge accumulated on the aerosols is only approximately 4 elementary charges which is much less than discussed in some of the current theories for the creation of polar mesosphere summer echoes. We note that the amount of sodium or potassium required to form these particles is far above the natural abundances at NLC altitudes. The exact abundance and composition of the aerosols need to be known at the time of the in situ measurements in order to make more sophisticated comparisons between measurements and models.
TL;DR: In this paper, a linked stress release model is proposed for the analysis of spatial interaction of earthquake occurrences through stress transfer within a large area of the Earth's crust, which has implications for the possible triggering of earthquakes at long distances from the origin event.
Abstract: A linked stress release model is proposed for the analysis of spatial interaction of earthquake occurrences through stress transfer within a large area of the Earth’s crust. As an example, the model is used for statistical analysis for the Japanese historical earthquakes in central Japan and offshore in the Nankai and Sagami troughs with magnitude M ≥ 6.5 during the period from 1400 to 1997. This area is divided into four smaller regions of roughly comparable size and activity. Based on the Akaike information criterion (AIC), the results demonstrate the existence of coupling between certain of the regions. With the evidence that the crust may lie in a near-critical state, this has implications for the possible triggering of earthquakes at long distances from the origin event. In particular, we find evidence for the dependence of Nankai trough events on the Chubu/Kinki triangle region, whose events are themselves dependent on the the Fossa Magna/Sagami trough. Evidence for the validity of the model includes simulation results indicating that the model had a higher forecast hazard post-1991 for an event in the Chubu/Kinki triangle region than did models not incorporating regional coupling.
TL;DR: In this paper, the authors studied the effect of solar flares on the geomagnetic field at the Indo-USSR chain of magnetic observatories extending from the magnetic latitudes 0° to 45°N, a network not available any where else in the world.
Abstract: The paper describes the study of solar flare effects (sfe) on horizontal (H), eastward (Y) and vertical (Z) components of the geomagnetic field at the Indo-USSR chain of magnetic observatories extending from the magnetic latitudes 0° to 45°N, a network not available any where else in the world. Events are selected when strong, normal and reversed (counter) equatorial electrojet are in existence as well as when only a partial counter electrojet is present. During a strong and normal electrojet event time, sfe consists of a positive impulse in H at all stations, the amplitude of H following the latitudinal variation similar to that of the quiet day monthly mean, Sq (H). The sfe in Y is negative at all the stations. Sfe in Z shows positive impulse at the four-electrojet stations and negative at other stations. During a counter electrojet period the effect of solar flare on H field is negative impulse at electrojet stations, positive at low latitude and again negative at stations north of Sq focus. Sfe in Y is small at all the equatorial nd negative at higher latitude stations. However, sfe (Z) is negative at the equatorial latitudes. During a partial counter electrojet period the observed effect is the combination of an increase of the planetary current component and the decrease of the electrojet component, giving rise to a negative impulse in H at equatorial stations and apositive impulse in H, increasing with increasing distance from the equator. These results are presented and discussed.
TL;DR: In this article, the occurrence and properties of the mid-latitude terdiurnal (8-hr) tide at near mesopause altitudes (80-105 km) have been investigated using two different instrumental techniques, an Na Wind/Temperature Lidar and an OH Mesospheric Temperature Mapper.
Abstract: Results from two different instrumental techniques, an Na Wind/Temperature Lidar and an OH Mesospheric Temperature Mapper, have been combined to investigate the occurrence and properties of the mid-latitude terdiurnal (8-hr) tide at near mesopause altitudes (80–105 km). High-resolution Na lidar measurements were taken throughout the diurnal and annual cycle (1996–98) at Urbana, Illinois (40°N, 88°W) to characterize the seasonal behavior of the 24, 12, 8 and 6-hr tides. Complementary measurements using a recently developed CCD imager capable of mapping OH temperature (at ∼87 km altitude) were made from Bear Lake Observatory, Utah (41.9°N, 111.6°W) and Ft. Collins, Colorado (40.6°N, 105°W) within the same time period. The “mean day” lidar data for the spring and fall periods investigated here each indicate an average amplitude variation of ∼2–5 K over the depth of the OH layer but distinct phases of <1-hr LST and ∼7-hr LST, respectively, for the 8-hr component. The Temperature Mapper data are in excellent agreement with these findings but in addition have shown that the amplitude of this tidal component can vary by as much as an order of magnitude (1.5–15 K) on a night-by-night basis resulting in an apparent 8-hr dominance of the nocturnal variation during investigated portions of the spring and fall seasons with little or no diurnal and semi-diurnal variability evident. Reports of terdiurnal tidal measurements in the mid-latitude nightglow emissions are exceptionally rare and have yet to be modeled. These innovative joint measurements pave the way for new research in this important area.
TL;DR: A meteor radar was operated at Amundsen-Scott Station, South Pole, from January 19, 1995 through January 26, 1996 and from November 21, 1996 through January 27, 1997.
Abstract: A meteor radar was operated at Amundsen-Scott Station, South Pole, from January 19, 1995 through January 26, 1996 and from November 21, 1996 through January 27, 1997. Hourly wind measurements were obtained nearly continuously over these time periods, at an approximate altitude of 95 km and at about 2° latitude from South Pole along the longitude meridians 0°, 90°E, 90°W, and 180°. The scientific advances achieved to date through analyses of these data are presented, including updates to several of our previously published works. The findings addressed herein include the following: (1) Strong divergences of zonal-mean meridional winds occasionally occur over South Pole, implying extreme vertical winds; (2) The monthly mean zonally asymmetric (zonal wavenumber s = 1) wind component varies during the year in a manner consistent with migration of the center of the polar vortex with respect to the geographic (rotational) pole; (3) Strong (>15 m/s) westward-propagating migrating diurnal (s = 1) and non-migrating semidiurnal (s = 1) oscillations exist except during winter months; (4) Long-period (∼2–10 days) waves exist during winter months which are primarily eastward-propagating; (5) Intradiurnal (periods ∼6–11.5 hours) westward-propagating oscillations exist, which are thought to be gravitational normal modes, or “Lamb” waves.
TL;DR: In this paper, the authors present radar measurements of the 12-hour wave, a zonal wavenumber 1 westward propagating wave that exists in the southern polar mesopause region winds.
Abstract: We present radar measurements of the 12-hour wave, a zonal wavenumber 1 westward propagating wave that exists in the southern polar mesopause region winds (Hernandez et al., 1993; Forbes et al., 1995). MF radar measurements of the horizontal winds at McMurdo (77.8°S, 166.67°E) show that the 12-hour wave is highly seasonal, occurring during the austral summer solstice. During these seasonal occurrences, the wave is highly intermittent with amplitude peaks of ≳30 m s−1. The burst-like occurrences of large 12-hour wave amplitudes are highly correlated between the zonal and meridional direction. The diurnal tide over McMurdo has a more constant amplitude, but it is also an almost exclusively summertime phenomenon. Inertia-gravity wave activity is evident at periods less than 12 hr during the austral winter months. The weakening of gravity wave activity during the summer is probably due to critical layer filtering by the zonal mean wind, 12-hour wave and diurnal tide which are all strong during this season. The 12-hour wave is confined in height to the vicinity of the zero crossing in the zonal winds above the westward jet. Extreme distortion is observed in the vertical phase fronts of the 12-hour wave which could signify either refraction or in situ forcing. The distortion in the phase fronts and localization of the 12-hour wave in time and height is apparently responsible for departures in period from the nominal 12 hours. We do not find the wave period to be systematically different from 12 hours. The association of the 12-hour wave events with shear in the mean wind suggests that refractive effects could conceivably cause a dilation in wave amplitude. However, the shear is of the opposite sign to cause this dilation unless the wave originates at higher altitudes and propagates downward into the mesosphere. Investigations are made of the zonal structure of the 12-hour wave by comparing phases of the 12-hour wind component between McMurdo and the dynasonde at Halley (75.8°S, 26.4°W). The phase is found to be stable and consistent with a westward propagating zonal wavenumber 2 structure during seasons when the 12-hour wave is weak. The migrating semidiurnal tide evidently dominates during these times of the year. During seasons when the 12-hour wave amplitude is large, the zonal structure is highly unstable and there is not an obvious dominant zonal wavenumber.
TL;DR: The neural network model suggests that the minimum Dst of a storm is significant in the storm recovery process, and more than 90% of the observed Dst variance is predictable in the model.
Abstract: In order to enhance the reproduction of the recovery phase Dst index of a geomagnetic storm which has been shown by previous studies to be poorly reproduced when compared with the initial and main phases, an artificial neural network with one hidden layer and error back-propagation learning has been developed. Three hourly Dst values before the minimum Dst in the main phase in addition to solar wind data of IMF southward-component Bs, the total strength Bt and the square root of the dynamic pressure, $sqrt {n{V^2}}$
, for the minimum Dst, i.e., information on the main phase was used to train the network. Twenty carefully selected storms from 1972–1982 were used for the training, and the performance of the trained network was then tested with three storms of different Dst strengths outside the training data set. Extremely good agreement between the measured Dst and the modeled Dst has been obtained for the recovery phase. The correlation coefficient between the predicted and observed Dst is more than 0.95. The average relative variance is 0.1 or less, which means that more than 90% of the observed Dst variance is predictable in our model. Our neural network model suggests that the minimum Dst of a storm is significant in the storm recovery process.
TL;DR: In this paper, the anisotropy of local turbulence in the Earth's core is examined and the effect of the local turbulence on the diffusion of large-scale fields is found.
Abstract: The anisotropy of local turbulence in the Earth’s core is examined. It is recognized that small-scale motions in the core are strongly influenced by the Earth’s rotation and its magnetic field. A small region of the core is simulated (the computational box), across which the prevailing large-scale (toroidal) magnetic field is supposed to be uniform and in which the temperature or compositional gradient providing the buoyancy that powers the turbulence is parallel to the (uniform) gravitational field. The simulations are used to estimate the turbulent fluxes of mean fields and their dependence on the latitude at which the computational box is situated. It is found that the effect of local turbulence on the diffusion of large-scale fields is significant, and that turbulent transport is anisotropic. It is believed that the results of the present study will prove useful in determining geophysically realistic diffusivities for use in future global geodynamo simulations.
TL;DR: In this paper, the authors investigated the source rupture process using teleseismic data at IRIS network as well as local data at Jayapura, Irian Jaya, station.
Abstract: A large earthquake (Ms 7.1) occurred off northwest coast of Papua New Guinea (PNG), and a massive tsunami attacked villages to cause a devastating damage. In an attempt to ascertain the tsunami source, we investigate the source rupture process using teleseismic data at IRIS network as well as local data at Jayapura, Irian Jaya, station. The source parameters obtained are: (strike, dip, slip) = (301°, 86°, 91°); the seismic moment = 4.3 × 1019 Nm (Mw = 7.0); the duration of main rupture = 19 s; the centroid depth = 20 ± 5 km; the extent of rupture along the fault strike = 40 km; the average dislocation = 1.8 m; the stress drop = 7.3 MPa. The tsunami magnitude Mt determined from tide-gage data at long distance is 7.5, significantly larger than Ms, so that the PNG earthquake is characterized as a tsunami earthquake. Tsunami earthquakes might have been caused by slow rupture, submarine landslide, and high-angle dip-slip. Our teleseisimic analysis precludes the first two candidates and favors the third one as a source of the present earthquake, although it does not necessarily exclude the possibility of an aseismic landslide induced by the main shock or its aftershocks.
TL;DR: In this article, the complex image method (CIM) is used to calculate the electromagnetic field at the earth's surface produced by 3D ionospheric current systems when the earth has a layered conductivity structure.
Abstract: The complex image method (CIM) is an efficient tool to calculate the electromagnetic field at the earth’s surface produced by 3D ionospheric current systems when the earth has a layered conductivity structure. The calculations are applicable to the estimation of source effects on magnetotelluric data. In this paper CIM is used in connection with some typical high-latitude ionospheric events: a westward travelling surge, a Harang discontinuity, an omega band, and a giant pulsation. The complicated ionospheric current systems are constructed of short horizontal current filaments with vertical currents at both ends. The currents are given numerically on a 50 km × 50 km grid covering a region of even 1000 km × 2000 km. The investigations indicate that the source distortion very much depends on the event, and may be significant in a wide period range, especially for a resistive earth structure. The source effect seems quite unpredictable. Sometimes the apparent resistivity is larger and sometimes smaller than the plane wave value. At times the source effect is very small even if the ionospheric current is strongly inhomogeneous.
TL;DR: The long-term behavior of atmospheric tides in the mesosphere and lower thermosphere has been observed with the meteor wind radar (MWR) in Jakarta, Indonesia (6°S, 107°E) from November 1992 to August 1997 as discussed by the authors.
Abstract: The long-term behavior of atmospheric tides in the mesosphere and lower thermosphere has been observed with the meteor wind radar (MWR) in Jakarta, Indonesia (6°S, 107°E) from November 1992 to August 1997. The amplitudes and phases of the diurnal tides show systematic seasonal variations, particularly distinct in the meridional component. In addition, substantial interannual variability is evident, characterized by a biennial periodicity of tidal parameters, and considerably small tidal amplitudes exclusively seen in 1996. The MWR results are compared with the Global Scale Wave Model (GSWM) as well as MF radar data collected in two equatorial sites in Pontianak (0.03°N, 109°E) and Christmas Island (2°N, 158°W) for November 1995–July 1997 and January 1996–October 1997, respectively. Comparison studies of these radar data have revealed the detailed latitudinal structure of the diurnal tide near the equator. The GSWM has successfully described the general characteristics of the radar results, although some discrepancies are recognized. In 1996 when radar data are available at all the three sites, the monthly mean values of tidal amplitudes at 90 km agreed very well between Jakarta and Pontianak, while significant discrepancy was found for Christmas Island, suggesting the existence of geographical effects such as non-migrating tides.
TL;DR: In this paper, a two-dimensional MHD model has been developed with an emphasis placed on the structure of the ionosphere of Mars, and it is found that the modeled electron density profile in the upper ionosphere strongly depends on the solar wind dynamic pressure as well as the solar zenith angle.
Abstract: The ionosphere of Mars is expected to be significantly affected by the solar wind because Mars does not possess a significant intrinsic magnetic field which deflects the solar wind. Despite a number of plasma measurements made near Mars, the nature of the solar wind-Mars interaction has not yet been fully understood. In order to self-consistently study the solar wind interaction with the ionosphere of Mars, a two-dimensional MHD model has been developed with an emphasis placed on the structure of the ionosphere of Mars. It is found that the modeled electron density profile in the upper ionosphere strongly depends on the solar wind dynamic pressure as well as the solar zenith angle. The ionosphere in the model tends to have an ionopause-like sharp drop of the electron density at some altitude for realistic solar wind dynamic pressures. Such behavior is not consistent with most of the observed electron density profiles, which exhibit relatively large and constant scale height in most of the dayside region. While the observed electron density profiles of the Venus ionosphere have been reproduced reasonably well by ionospheric models as well as recent three-dimensional MHD models, the electron density profiles of the Martian ionosphere have not been successfully reproduced by theoretical models including this study. This fact implies that processes not present in the Venus ionosphere, such as crustal magnetic fields and the rotation of the planet, may have significant effects on the structure and the dynamics of the ionosphere of Mars.
TL;DR: In this paper, the frequency spectra and spectral variances of horizontal motions of gravity waves are used to provide information on the predominant azimuthal directions of propagation for the waves.
Abstract: 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.