The Experts below are selected from a list of 1950 Experts worldwide ranked by ideXlab platform
Diego Janches - One of the best experts on this subject based on the ideXlab platform.
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Aspect sensitivity of VHF echoes from field aligned irregularities in Meteor Trails and thin ionization layers
Atmospheric Chemistry and Physics Discussions, 2015Co-Authors: Q. H. Zhou, Y. T. Morton, J D Mathews, Diego JanchesAbstract:The aspect sensitivity of VHF echoes from field aligned irregularities (FAI) within Meteor Trails and thin ionization layers is studied using numerical models. Although the maximum power is obtained when a radar is pointed perpendicular to the field line (perpendicular to B), substantial power can be obtained off the perpendicular to B direction if the ionization trail/layer is thin. When the FAI length is 20 m, the power observed 6° off perpendicular to B is about 10 db below that perpendicular to the B direction. Meteoric FAI echoes can potentially be used to determine the diffusion rate in the mesopause region. Based on the aspect sensitivity analysis, we conclude that the range spread trail echoes far off perpendicular to B observed by powerful VHF radars are likely due to overdense Meteors. Our simulation also shows that ionospheric FAI echoes can have an altitude smearing effect of about 4 km if the vertical extension of a FAI layer is around 100 m, which has often been observed at Arecibo. The altitude smearing effect can account for the fact that the Es layers observed by the Arecibo incoherent scatter radar are typically much narrower than FAI layers and the occurrence of double spectral peaks around the Es layer altitude in FAI echoes.
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Aspect sensitivity of VHF echoes from field aligned irregularities in Meteor Trails and thin ionization layers
Atmospheric Chemistry and Physics, 2004Co-Authors: Q. H. Zhou, Y. T. Morton, J D Mathews, Diego JanchesAbstract:The aspect sensitivity of VHF echoes from field aligned irregularities (FAI) within Meteor Trails and thin ionization layers is studied using numerical models. Although the maximum power is obtained when a radar is pointed perpendicular to the field line (B), substantial power can be obtained off the B direction if the ionization trail/layer is thin. When the FAI length along B is 20 m, the power observed 6° off B is about 10 db below that perpendicular to the B direction. Meteoric FAI echoes can potentially be used to determine the diffusion rate in the mesopause region. Based on the aspect sensitivity analysis, we conclude that the range spread trail echoes far off B observed by powerful VHF radars are likely due to overdense Meteors. Our simulation also shows that ionospheric FAI echoes can have an altitude smearing effect of about 4 km if the vertical extension of a FAI-layer is around 100 m, which has often been observed at Arecibo. The altitude smearing effect can account for the fact that the Es-layers observed by the Arecibo incoherent scatter radar are typically much narrower than FAI-layers and the occurrence of double spectral peaks around the Es-layer altitude in FAI echoes.
Lars P. Dyrud - One of the best experts on this subject based on the ideXlab platform.
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Multilayer detection and classification of specular and nonspecular Meteor Trails
Radio Science, 2011Co-Authors: Siming Zhao, Lars P. Dyrud, Julio Urbina, R. SealAbstract:[1] Meteor radar data are continuously collected by different radar systems that operate throughout the year. Analyzing this fast growing, large data set requires efficient and reliable detection routines. Currently most Meteor echo routines search for underdense Meteor Trails, often discarding overdense and nonspecular Meteor Trails. This is because their main purpose is the study of mesospheric winds. But the study of Meteor flux requires the unique identification of each type of Meteor reflections. In this paper, a multilayer radar detection and classification algorithm is proposed to correctly identify multiple types of Meteor trail reflections. The process consists of two steps. The first step is based on the time-frequency waveform detector. In this step, we start by selecting low signal-to-noise ratio (SNR) values in order to detect all types of radar echoes; however, a high probability offalse alarm is often produced. In the second step, several features from the detected echoes in step one are extracted and a support vector machine (SVM) classifier is constructed to further classify these echoes. The algorithm was tested using data collected from a 50-MHz radar stationed near Salinas, Puerto Rico, on April 5, 1998. A total of 270 detected echoes were labeled as underdense, overdense, nonspecular, other ionospheric echoes, and noise. We used 50% of the labeled echoes as training samples and divided the rest 50% testing samples as 10 subsets for testing. This technique successfully classified about 85% of the testing samples. Details concerning implementation, feature extraction, and data visualization are presented and discussed.
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Diurnal variation of non-specular Meteor Trails
Annales Geophysicae, 2009Co-Authors: J. Hinrichs, Lars P. Dyrud, Julio UrbinaAbstract:We present results of simulated radar observations of Meteor Trails in an effort to show how non-specular Meteor Trails are expected to vary as a function of a number of key atmospheric, ionospheric and Meteoroid parameters. This paper identifies which geophysical sources effect the variability in non-specular trail radar observations, and provides an approach that uses some of these parameter dependencies to determine Meteoroid and atmospheric properties based upon the radar Meteor observations. The numerical model used follows Meteor evolution from ablation and ionization to head echo plasma generation and through formation of field aligned irregularities (FAI). Our main finding is that non-specular Meteor trail duration is highly sensitive to the presence of lower thermospheric winds or electric fields and the background ionospheric electron density. In an effort to make key predictions we present the first results of how the same Meteoroid is expected to produce dramatically different Meteor Trails as a function of location and local time. For example, we show that mid-latitude trail durations are often shorter lasting than equatorial trail observations because of the difference in mid-latitude wind speed and equatorial drift speed. The simulated Trails also account for observations showing that equatorial nighttime non-specular Meteor Trails last significantly longer and are observed more often than daytime Trails.
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Modeling long duration Meteor Trails
Journal of Geophysical Research, 2007Co-Authors: Lars P. Dyrud, Erhan Kudeki, Meers OppenheimAbstract:[1] We constructed a model of the plasma evolution of Meteor Trails in order to simulate high-power-large-aperture radar observations. This model follows Meteor evolution from ablation and ionization to head echo formation and through non-specular trail reflections. Meteor plasma field aligned irregularities (FAI) result in radar reflections called non-specular Meteor Trails. We have updated our model to include the polarizing effects of winds and background electric fields on Meteor trail instability. This model incorporates existing Meteor physics together with knowledge we gained from the simulations, such as instability physics and anomalous diffusion. Comparing results from this model with large radar observations of head echoes and non-specular Trails shows that we can reproduce many of the observed features, such as the detailed altitude profile and duration of head echoes and non-specular Trails. Specifically when external electric fields are included in our model, the same model predicts both short duration (order of 1 s) and long duration Meteor Trails (several minutes) depending upon the characteristics of the Meteoroid and atmosphere. The addition of a background E-field or wind also reproduces a commonly observed non-specular trail feature we call an “extended tail” where the delay time between head echo and non-specular reflection at the lower altitude portion of the trail increases as the Meteor gets lower in the atmosphere. We also demonstrate a dependence on trail duration with the electron density of the background ionosphere.
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Modelling high-power large-aperture radar Meteor Trails
Journal of Atmospheric and Solar-Terrestrial Physics, 2005Co-Authors: Lars P. Dyrud, Meers Oppenheim, Sigrid Close, Licia C Ray, Kelly D. DenneyAbstract:Despite decades of research, many questions remain about the global flux of Meteoroids at Earth, their influence on the atmosphere, and their use as upper atmospheric diagnostics. We see high-power large-aperture (HPLA) radar observations of Meteor phenomena called head echoes and non-specular Trails as a valuable tool for answering these questions. In the past we conducted plasma simulations demonstrating that Meteor Trails are unstable to growth of Farley-Buneman gradient-drift (FBGD) waves that become turbulent and generate large B-field aligned irregularities (FAI). These FAI result in reflections called non-specular Meteor Trails. Using these and other results, we have developed a model that follows Meteor evolution from ablation and ionization through the creation of radar head echoes and non-specular trail reflections. This paper presents results from this model, showing that we can reproduce many aspects of these large radar observations, such as the general altitude profile of head echoes and non-specular Trails. Additionally we show that trail polarization due to E-fields or neutral winds causes a noticeable trail feature as well as may be responsible for Trails lasting longer than about 1 s. We also demonstrate how such a model is a valuable tool for deriving Meteoroid properties such as flux, mass, and velocity. Finally, such a model could also provide some composition information, and diagnose the atmosphere and ionosphere where Meteors produce their Trails.
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Plasma instabilities in Meteor Trails: 2-D simulation studies
Journal of Geophysical Research: Space Physics, 2003Co-Authors: Meers Oppenheim, Lars P. Dyrud, Axel F. Vom EndtAbstract:[1] Field-aligned plasma density irregularities detected as nonspecular echoes by radars with large aperture-power products indicate the presence of plasma turbulence within Meteor Trails. This paper presents two-dimensional simulations of Meteor trail instabilities and compares these results with theory and observations. In particular, this paper describes techniques for simulating trail turbulence and then discusses two sample cases using realistic plasma density gradients, masses, and atmospheric conditions appropriate for a 102-km altitude. In the first case, the trail lies along the geomagnetic field, B. In the second, it lies perpendicular to B and is subject to a small external electric field pointing parallel to it. These cases show the spontaneous development of instabilities leading to turbulence and field-aligned irregularities. These irregularities can create nonspecular echoes with broad spectral lines and small Doppler shifts similar to those observed by radars with large aperture-power products. The simulations also show turbulence-enhanced cross-field diffusion rates. Finally, the paper describes simulations of Trails containing multiple ion species and shows how turbulent mixing greatly reduces species fractionation.
Q. H. Zhou - One of the best experts on this subject based on the ideXlab platform.
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Aspect sensitivity of VHF echoes from field aligned irregularities in Meteor Trails and thin ionization layers
Atmospheric Chemistry and Physics Discussions, 2015Co-Authors: Q. H. Zhou, Y. T. Morton, J D Mathews, Diego JanchesAbstract:The aspect sensitivity of VHF echoes from field aligned irregularities (FAI) within Meteor Trails and thin ionization layers is studied using numerical models. Although the maximum power is obtained when a radar is pointed perpendicular to the field line (perpendicular to B), substantial power can be obtained off the perpendicular to B direction if the ionization trail/layer is thin. When the FAI length is 20 m, the power observed 6° off perpendicular to B is about 10 db below that perpendicular to the B direction. Meteoric FAI echoes can potentially be used to determine the diffusion rate in the mesopause region. Based on the aspect sensitivity analysis, we conclude that the range spread trail echoes far off perpendicular to B observed by powerful VHF radars are likely due to overdense Meteors. Our simulation also shows that ionospheric FAI echoes can have an altitude smearing effect of about 4 km if the vertical extension of a FAI layer is around 100 m, which has often been observed at Arecibo. The altitude smearing effect can account for the fact that the Es layers observed by the Arecibo incoherent scatter radar are typically much narrower than FAI layers and the occurrence of double spectral peaks around the Es layer altitude in FAI echoes.
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Aspect sensitivity of VHF echoes from field aligned irregularities in Meteor Trails and thin ionization layers
Atmospheric Chemistry and Physics, 2004Co-Authors: Q. H. Zhou, Y. T. Morton, J D Mathews, Diego JanchesAbstract:The aspect sensitivity of VHF echoes from field aligned irregularities (FAI) within Meteor Trails and thin ionization layers is studied using numerical models. Although the maximum power is obtained when a radar is pointed perpendicular to the field line (B), substantial power can be obtained off the B direction if the ionization trail/layer is thin. When the FAI length along B is 20 m, the power observed 6° off B is about 10 db below that perpendicular to the B direction. Meteoric FAI echoes can potentially be used to determine the diffusion rate in the mesopause region. Based on the aspect sensitivity analysis, we conclude that the range spread trail echoes far off B observed by powerful VHF radars are likely due to overdense Meteors. Our simulation also shows that ionospheric FAI echoes can have an altitude smearing effect of about 4 km if the vertical extension of a FAI-layer is around 100 m, which has often been observed at Arecibo. The altitude smearing effect can account for the fact that the Es-layers observed by the Arecibo incoherent scatter radar are typically much narrower than FAI-layers and the occurrence of double spectral peaks around the Es-layer altitude in FAI echoes.
Meers Oppenheim - One of the best experts on this subject based on the ideXlab platform.
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Modeling long duration Meteor Trails
Journal of Geophysical Research, 2007Co-Authors: Lars P. Dyrud, Erhan Kudeki, Meers OppenheimAbstract:[1] We constructed a model of the plasma evolution of Meteor Trails in order to simulate high-power-large-aperture radar observations. This model follows Meteor evolution from ablation and ionization to head echo formation and through non-specular trail reflections. Meteor plasma field aligned irregularities (FAI) result in radar reflections called non-specular Meteor Trails. We have updated our model to include the polarizing effects of winds and background electric fields on Meteor trail instability. This model incorporates existing Meteor physics together with knowledge we gained from the simulations, such as instability physics and anomalous diffusion. Comparing results from this model with large radar observations of head echoes and non-specular Trails shows that we can reproduce many of the observed features, such as the detailed altitude profile and duration of head echoes and non-specular Trails. Specifically when external electric fields are included in our model, the same model predicts both short duration (order of 1 s) and long duration Meteor Trails (several minutes) depending upon the characteristics of the Meteoroid and atmosphere. The addition of a background E-field or wind also reproduces a commonly observed non-specular trail feature we call an “extended tail” where the delay time between head echo and non-specular reflection at the lower altitude portion of the trail increases as the Meteor gets lower in the atmosphere. We also demonstrate a dependence on trail duration with the electron density of the background ionosphere.
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Modelling high-power large-aperture radar Meteor Trails
Journal of Atmospheric and Solar-Terrestrial Physics, 2005Co-Authors: Lars P. Dyrud, Meers Oppenheim, Sigrid Close, Licia C Ray, Kelly D. DenneyAbstract:Despite decades of research, many questions remain about the global flux of Meteoroids at Earth, their influence on the atmosphere, and their use as upper atmospheric diagnostics. We see high-power large-aperture (HPLA) radar observations of Meteor phenomena called head echoes and non-specular Trails as a valuable tool for answering these questions. In the past we conducted plasma simulations demonstrating that Meteor Trails are unstable to growth of Farley-Buneman gradient-drift (FBGD) waves that become turbulent and generate large B-field aligned irregularities (FAI). These FAI result in reflections called non-specular Meteor Trails. Using these and other results, we have developed a model that follows Meteor evolution from ablation and ionization through the creation of radar head echoes and non-specular trail reflections. This paper presents results from this model, showing that we can reproduce many aspects of these large radar observations, such as the general altitude profile of head echoes and non-specular Trails. Additionally we show that trail polarization due to E-fields or neutral winds causes a noticeable trail feature as well as may be responsible for Trails lasting longer than about 1 s. We also demonstrate how such a model is a valuable tool for deriving Meteoroid properties such as flux, mass, and velocity. Finally, such a model could also provide some composition information, and diagnose the atmosphere and ionosphere where Meteors produce their Trails.
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Plasma instabilities in Meteor Trails: 2-D simulation studies
Journal of Geophysical Research: Space Physics, 2003Co-Authors: Meers Oppenheim, Lars P. Dyrud, Axel F. Vom EndtAbstract:[1] Field-aligned plasma density irregularities detected as nonspecular echoes by radars with large aperture-power products indicate the presence of plasma turbulence within Meteor Trails. This paper presents two-dimensional simulations of Meteor trail instabilities and compares these results with theory and observations. In particular, this paper describes techniques for simulating trail turbulence and then discusses two sample cases using realistic plasma density gradients, masses, and atmospheric conditions appropriate for a 102-km altitude. In the first case, the trail lies along the geomagnetic field, B. In the second, it lies perpendicular to B and is subject to a small external electric field pointing parallel to it. These cases show the spontaneous development of instabilities leading to turbulence and field-aligned irregularities. These irregularities can create nonspecular echoes with broad spectral lines and small Doppler shifts similar to those observed by radars with large aperture-power products. The simulations also show turbulence-enhanced cross-field diffusion rates. Finally, the paper describes simulations of Trails containing multiple ion species and shows how turbulent mixing greatly reduces species fractionation.
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Plasma Instabilities in Meteor Trails: Linear Theory
Journal of Geophysical Research: Space Physics, 2003Co-Authors: Meers Oppenheim, Lars P. Dyrud, Licia C RayAbstract:Ablation of microMeteoroids between 70 and 130 km altitude in the atmosphere creates plasma columns with densities exceeding the ambient ionospheric electron density by many orders of magnitude. Density gradients at the edges of these Trails can create ambipo- lar electric fields with amplitudes in excess of 100 mV/m. These fields combine with diamag- netic drifts to drive electrons at speeds exceeding 2 km/s. The fields and gradients also ini- tiate Farley-Buneman and gradient-drift (FBGD) instabilities. These create field-aligned plasma density irregularities which evolve into turbulent structures detectable by radars with a large power-aperture product, such as those found at Jicamarca, Arecibo, and Kwajalein. This pa- per presents a theory of Meteor trail instabilities using both fluid and kinetic methods. In par- ticular, it discusses the origin of the driving electric field, the resulting electron drifts, and the linear plasma instabilities of Meteor Trails. It shows that, though the ambipolar electric field changes amplitude and even direction as a function of altitude, the electrons always drift in the positive direction where is the density and the geomagnetic field. The lin- ear stability analysis predicts that instabilities develop within a limited range of altitudes with the following observational consequences: (1) non-specular Meteor trail echoes will be field aligned; (2) non-specular echoes will return from a limited range of altitudes compared with the range over which the head echo reflection indicates the presence of plasma columns; and (3) anomalous cross-field diffusion will occur only within this limited altitude range with con- sequences for calculating diffusion rates and temperatures with both specular and non-specular radars.
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Interpretation of non‐specular radar Meteor Trails
Geophysical Research Letters, 2002Co-Authors: Lars P. Dyrud, Meers Oppenheim, Sigrid Close, Stephen HuntAbstract:[1] Radar data of non-specular Meteor Trails shows two clear and consistent features: (1) non–specular Meteor Trails are observed from a narrower altitude range than are head echoes and (2) an approximately 20 ms delay between Meteor head echoes and trail radar scatter. This paper shows that both features can result from Meteor trail plasma instability. Simulations have demonstrated that Trails often develop Farley-Buneman/gradient-drift (FBGD) waves which become turbulent and generate field aligned irregularities (FAI). Plasma stability analysis shows that Trails are only unstable within a limited altitude range, matching the observed altitudes of non–specular Trails to within 1–2 km. The simulations show that instability develops into turbulence in ∼20 ms and appears to be the only Meteor trail process that can explain both the observed delay between head and trail echoes and generate coherent scatter at both UHF and VHF wavelengths.
Y. T. Morton - One of the best experts on this subject based on the ideXlab platform.
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Aspect sensitivity of VHF echoes from field aligned irregularities in Meteor Trails and thin ionization layers
Atmospheric Chemistry and Physics Discussions, 2015Co-Authors: Q. H. Zhou, Y. T. Morton, J D Mathews, Diego JanchesAbstract:The aspect sensitivity of VHF echoes from field aligned irregularities (FAI) within Meteor Trails and thin ionization layers is studied using numerical models. Although the maximum power is obtained when a radar is pointed perpendicular to the field line (perpendicular to B), substantial power can be obtained off the perpendicular to B direction if the ionization trail/layer is thin. When the FAI length is 20 m, the power observed 6° off perpendicular to B is about 10 db below that perpendicular to the B direction. Meteoric FAI echoes can potentially be used to determine the diffusion rate in the mesopause region. Based on the aspect sensitivity analysis, we conclude that the range spread trail echoes far off perpendicular to B observed by powerful VHF radars are likely due to overdense Meteors. Our simulation also shows that ionospheric FAI echoes can have an altitude smearing effect of about 4 km if the vertical extension of a FAI layer is around 100 m, which has often been observed at Arecibo. The altitude smearing effect can account for the fact that the Es layers observed by the Arecibo incoherent scatter radar are typically much narrower than FAI layers and the occurrence of double spectral peaks around the Es layer altitude in FAI echoes.
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Aspect sensitivity of VHF echoes from field aligned irregularities in Meteor Trails and thin ionization layers
Atmospheric Chemistry and Physics, 2004Co-Authors: Q. H. Zhou, Y. T. Morton, J D Mathews, Diego JanchesAbstract:The aspect sensitivity of VHF echoes from field aligned irregularities (FAI) within Meteor Trails and thin ionization layers is studied using numerical models. Although the maximum power is obtained when a radar is pointed perpendicular to the field line (B), substantial power can be obtained off the B direction if the ionization trail/layer is thin. When the FAI length along B is 20 m, the power observed 6° off B is about 10 db below that perpendicular to the B direction. Meteoric FAI echoes can potentially be used to determine the diffusion rate in the mesopause region. Based on the aspect sensitivity analysis, we conclude that the range spread trail echoes far off B observed by powerful VHF radars are likely due to overdense Meteors. Our simulation also shows that ionospheric FAI echoes can have an altitude smearing effect of about 4 km if the vertical extension of a FAI-layer is around 100 m, which has often been observed at Arecibo. The altitude smearing effect can account for the fact that the Es-layers observed by the Arecibo incoherent scatter radar are typically much narrower than FAI-layers and the occurrence of double spectral peaks around the Es-layer altitude in FAI echoes.