Head Echo Research Articles

Preliminary experimental results by the prototype of Sanya Incoherent Scatter Radar

In the past decades, the Incoherent Scatter Radar (ISR) has been demonstrated to be one of the most powerful instruments for ionosphere monitoring. The Institute of Geology and Geophysics at the Chinese Academy of Sciences was founded to build a state-of-the-art phased-array ISR at Sanya (18.3°N, 109.6°E), a low-latitude station on Hainan Island, named the Sanya ISR (SYISR). As a first step, a prototype radar system consisting of eight subarrays (SYISR-8) was built to reduce the technical risk of producing the entire large array. In this work, we have summarized the preliminary experimental results based on the SYISR-8. The amplitude and phase among 256 channels were first calibrated through an embedded internal monitoring network. The mean oscillation of the amplitude and phase after calibration were about 1 dB and 5°, respectively, which met the basic requirements. The beam directivity was confirmed by crossing screen of the International Space Station. The SYISR-8 was further used to detect the tropospheric wind profile and meteors. The derived winds were evaluated by comparison with independent radiosonde and balloon-based GPS measurements. The SYISR-8 was able to observe several typical meteor echoes, such as the meteor head echo, range-spread trail echo, and specular trail echo. These results confirmed the validity and reliability of the SYISR-8 system, thereby reducing the technical risk of producing the entire large array of the SYISR to some extent.

Characteristics of very faint (+16) meteors detected with the Middle Atmosphere ALOMAR Radar System (MAARSY)

In September/October 2016 a dedicated specular meteor trail experiment with the high-power, large-aperture MAARSY radar was conducted, combining the high-power density of the radar with the large scattering target of trail echoes with the goal of probing the population of very small, slow-moving meteoroids. In contrast to meteor head echo observations, we find our raw velocity distribution peaks around 19 km/s for meteoroids with a limiting radar magnitude of +16. The correction for different observing biases to an equivalent limiting mass leads to a peak in the meteoroid influx at 13 km/s. Our calculated meteoroid masses range from 10−11 to 10−7 kg. Using an ablation model we show that our detections are near the micrometeorite ablation limit at low speeds. A tentative comparison with a Jupiter Family Comets (JFC) dynamical meteoroid model shows an apparent excess of large, slow meteoroids not predicted by the model.

A technique for inferring lower thermospheric neutral density from meteoroid ablation

In this paper, we present a novel technique for calculating the lower thermosphere neutral density using meteoroid ablation estimated via radar head echos observations. These observations were conducted by making use of ALTAIR, a high power larger aperture radar. We employ a semi-analytical scattering model to correlate a meteor’s head echo return to its plasma density, its mass, and couple this with a continuous single particle ablation model and an optimization technique in order to infer estimates of the surrounding neutral density. We compare our results with the popular MSIS-90 and obtain averaged values 20–40% lower than those computed by the semi-empirical global model. The discrepancies found between our methodology and MSIS-90 are consistent with the differences obtained between in-situ measurements and the semi-empirical global model. In addition, we are capable of observing short term changes in density of a magnitude consistent with experimental results. This suggests that the presented methodology is capable to estimate the thermospheric neutral density. Nonetheless the high error bar associated with our computation, as well as the diversity of the meteoroid flux, and the choice of our ablation model present some limitation to the application of the proposed technique.

Formation of Plasma Around a Small Meteoroid: Electrostatic Simulations

AbstractObtaining meteoroid mass from head echo radar cross section depends on the assumed plasma density distribution around the meteoroid. An analytical model presented in Dimant and Oppenheim (2017a, https://doi.org/10.1002/2017JA023960; 2017b, https://doi.org/10.1002/2017JA023963) and simulation results presented in Sugar et al. (2018, https://doi.org/10.1002/2018JA025265) suggest the plasma density distribution is significantly different than the spherically symmetric Gaussian distribution used to calculate meteoroid masses in many previous studies. However, these analytical and simulation results ignored the effects of electric and magnetic fields and assumed quasi‐neutrality. This paper presents results from the first particle‐in‐cell simulations of head echo plasma that include electric and magnetic fields. The simulations show that the fields change the ion density distribution by less than ∼2% in the meteor head echo region, but the electron density distribution changes by up to tens of percent depending on the location, electron energies, and magnetic field orientation with respect to the meteoroid path.

Modeling the Altitude Distribution of Meteor Head Echoes Observed with HPLA Radars: Implications for the Radar Detectability of Meteoroid Populations

Abstract The altitude distribution of meteors detected by a radar is sensitive to the instrument’s response function and can thus provide insight into the physical processes involved in radar measurements. This, in turn, can be used to determine the rate of ablation and ionization of the meteoroids and ultimately the input flux on Earth. In this work, we model the radar meteor head echo altitude distribution for three High Power and Large Aperture radar systems, by considering meteoroid populations from the main cometary family sources. In this simulation, we first use the results of a dynamical model of small meteoroids impacting Earth’s upper atmosphere to model the incoming mass, velocity, and entry angular distributions. We then combine these with the Chemical Ablation Model and establish the meteoroid ionization rates as a function of mass, velocity, and entry angle in order to determine the altitude at which these radars should detect the produced meteors and the portion of produced meteors from each population that are detected by these radars. We explore different sizes of head plasma as well as the possible effects on radar scattering of the head echo aspect sensitivity. We find that the modeled altitude distributions are generally in good agreement with measurements, particularly for ultra-high-frequency radars. In addition, our results indicate that the number of particles from Jupiter Family Comets (JFCs) required to fit the observations is lower than predicted by astronomical models. It is not clear yet if this discrepancy is due to the overprediction of JFC meteoroids by dynamical models or due to unaccounted physical processes in the treatment of ablation, ionization, and detections of meteoroids as they pass through Earth’s atmosphere.

Meteor head and terminal flare echoes observed with the Gadanki MST radar

Meteor observations with Gadanki MST radar usually detect variety of meteor echoes that includes head echoes, specular and non-specular trail echoes. Sometimes, but not always head echoes are followed by a sudden increase in signal strength amounting to many decibels at terminal end point of the trail, known as terminal flare echoes - a feature mostly observed with optical and high power large aperture (HPLA) radar systems. In this study, we report some examples of terminal flare echoes observed with Gadanki MST radar. Because these echoes provide valuable insight into the role of diffusion and plasma instabilities in the formation and evolution of meteor trail. From the observations, it has been noticed that the head echoes at higher altitudes are generating non-specular trail echoes, whereas they disintegrate as terminal flares associated with meteoroid fragmentation in lower altitudes. Although meteoroid fragmentation is a common phenomenon, but terminal flaring is a rare feature observed with Gadanki radar. A small, but non-negligible fraction of meteor events (∼2.5% of all head echo events) showed flaring apparently produced by terminal destruction of a meteoroid fragmentation along with the insights into the fragmentation and terminal flaring process.

Formation of Plasma Around a Small Meteoroid: Simulation and Theory

AbstractHigh‐power large‐aperture radars detect meteors by reflecting radio waves off dense plasma that surrounds a hypersonic meteoroid as it ablates in the Earth's atmosphere. If the plasma density profile around the meteoroid is known, the plasma's radar cross section can be used to estimate meteoroid properties such as mass, density, and composition. This paper presents head echo plasma density distributions obtained via two numerical simulations of a small ablating meteoroid and compares the results to an analytical solution found in Dimant and Oppenheim (2017a, https://doi.org/10.1002/2017JA023960, 2017b, https://doi.org/10.1002/2017JA023963). The first simulation allows ablated meteoroid particles to experience only a single collision to match an assumption in the analytical solution, while the second is a more realistic simulation by allowing multiple collisions. The simulation and analytical results exhibit similar plasma density distributions. At distances much less than λT, the average distance an ablated particle travels from the meteoroid before a collision with an atmospheric particle, the plasma density falls off as 1/R, where R is the distance from the meteoroid center. At distances substantially greater than λT, the plasma density profile has an angular dependence, falling off as 1/R2 directly behind the meteoroid, 1/R3 in a plane perpendicular to the meteoroid's path that contains the meteoroid center, and in front of the meteoroid. When used for calculating meteoroid masses, this new plasma density model can give masses that are orders of magnitude different than masses calculated from a spherically symmetric Gaussian distribution, which has been used to calculate masses in the past.

A meteoroid stream survey using meteor head echo observations from the Middle Atmosphere ALOMAR Radar System (MAARSY)

Results from a meteor head echo shower survey using the quasi continuous meteor observations of the high power large aperture radar MAARSY, located in northern Norway (69.30°N, 16.04°E) are presented. The data set comprises 760 000 head echoes detected during two and half years sensitive to an effective limiting masses below 10−8 kg. Using a wavelet shower search algorithm, we identified 33 meteor showers in the data set all of which are found in the IAU meteor shower catalog. We find ∼ 1% of all measured head echoes at these masses are associated with meteor showers. Comparison of shower radiants from this survey with the observation of the Canadian Meteor Orbit radar (CMOR) transverse scattering radar system shows generally good agreement, although there are large differences in the measured durations of some meteor showers. Differential mass indices (s) of ∼ 1.5–1.6 are measured for the Perseids (PER), Geminids (GEM) and Quadrantids (QUA) showers. The Orionids (ORI) show a much steeper mass index of 2.0, in agreement with other observations at small particle sizes, suggesting the Halleyid showers, in particular, are rich in very small meteoroids.

Radar Detectability Studies of Slow and Small Zodiacal Dust Cloud Particles. III. The Role of Sodium and the Head Echo Size on the Probability of Detection

Abstract We present a path forward on a long-standing issue concerning the flux of small and slow meteoroids, which are believed to be the dominant portion of the incoming meteoric mass flux into the Earth’s atmosphere. Such a flux, which is predicted by dynamical dust models of the Zodiacal Cloud, is not evident in ground-based radar observations. For decades this was attributed to the fact that the radars used for meteor observations lack the sensitivity to detect this population, due to the small amount of ionization produced by slow-velocity meteors. Such a hypothesis has been challenged by the introduction of meteor head echo (HE) observations with High Power and Large Aperture radars, in particular the Arecibo 430 MHz radar. Janches et al. developed a probabilistic approach to estimate the detectability of meteors by these radars and initially showed that, with the current knowledge of ablation and ionization, such particles should dominate the detected rates by one to two orders of magnitude compared to the actual observations. In this paper, we include results in our model from recently published laboratory measurements, which showed that (1) the ablation of Na is less intense covering a wider altitude range; and (2) the ionization probability, β ip , for Na atoms in the air is up to two orders of magnitude smaller for low speeds than originally believed. By applying these results and using a somewhat smaller size of the HE radar target we offer a solution that reconciles these observations with model predictions.

Results of the first continuous meteor head echo survey at polar latitudes

We present the first quasi continuous meteor head echo measurements obtained during a period of over two years using the Middle Atmosphere ALOMAR Radar System (MAARSY). The measurements yield information on the altitude, trajectory, vector velocity, radar cross section, deceleration and dynamical mass of every single event. The large statistical amount of nearly one million meteor head detections provide an excellent overview of the elevation, altitude, velocity and daily count rate distributions during different times of the year at polar latitudes. Only 40% of the meteors were detected within the full width half maximum of the specific sporadic meteor sources. Our observation of the sporadic meteors are compared to the observations with other radar systems and a meteor input function (MIF). The best way to compare different radar systems is by comparing the radar cross section (RCS), which is the main detection criterion for each system. In this study we aim to compare our observations with a MIF, which provides information only about the meteoroid mass. Thus, we are using a statistical approach for the elevation and velocity dependent visibility and a specific mass selection. The predicted absolute count rates from the MIF are in a good agreement with the observation when it is assumed that the radar system is only sensitive to meteoroids with masses higher than one microgram. The analysis of the dynamic masses seems to be consistent with this assumption since the count rate of events with smaller masses are low and decrease even more by using events with relatively small errors.

Simultaneous optical and meteor head echo measurements using the Middle Atmosphere Alomar Radar System (MAARSY): Data collection and preliminary analysis

The initial results of a two year simultaneous optical-radar meteor campaign are described. Analysis of 105 double-station optical meteors having plane of sky intersection angles greater than 5° and trail lengths in excess of 2km also detected by the Middle Atmosphere Alomar Radar System (MAARSY) as head echoes was performed. These events show a median deviation in radiants between radar and optical determinations of 1.5°, with 1/3 of events having radiant agreement to less than one degree. MAARSY tends to record average speeds roughly 0.5km/s and 1.3km higher than optical records, in part due to the higher sensitivity of MAARSY as compared to the optical instruments. More than 98% of all head echoes are not detected with the optical system. Using this non-detection ratio and the known limiting sensitivity of the cameras, we estimate that the limiting meteoroid detection mass of MAARSY is in the 10−9–10−10kg (astronomical limiting meteor magnitudes of +11 to +12) appropriate to speeds from 30 to 60km/s. There is a clear trend of higher peak RCS for brighter meteors between 35 and −30 dBsm. For meteors with similar magnitudes, the MAARSY head echo radar cross-section is larger at higher speeds. Brighter meteors at fixed heights and similar speeds have consistently, on average, larger RCS values, in accordance with established scattering theory. However, our data show RCS ∝ v/2, much weaker than the normally assumed RCS ∝ v3, a consequence of our requiring head echoes to also be detectable optically. Most events show a smooth variation of RCS with height broadly following the light production behavior. A significant minority of meteors show large variations in RCS relative to the optical light curve over common height intervals, reflecting fragmentation or possibly differential ablation. No optically detected meteor occurring in the main radar beam and at times when the radar was collecting head echo data went unrecorded by MAARSY. Thus there does not appear to be any large scale bias in MAARSY head echo detections for the (comparatively) larger optical events in our dataset, even at very low speeds.

Formation of plasma around a small meteoroid: 2. Implications for radar head echo

AbstractThis paper calculates the spatial distribution of the plasma responsible for radar head echoes by applying the kinetic theory developed in the companion paper. This results in a set of analytic expressions for the plasma density as a function of distance from the meteoroid. It shows that at distances less than a collisional mean free path from the meteoroid surface, the plasma density drops in proportion to 1/R where R is the distance from the meteoroid center; and, at distances much longer than the mean‐free‐path behind the meteoroid, the density diminishes at a rate proportional to 1/R2. The results of this paper should be used for modeling and analysis of radar head echoes.

VHF antenna pattern characterization by the observation of meteor head echoes

Abstract. The Middle Atmosphere Alomar Radar System (MAARSY) with its active phased array antenna is designed and used for studies of phenomena in the mesosphere and lower atmosphere. The flexible beam forming and steering combined with a large aperture array allows for observations with a high temporal and angular resolution. For both the analysis of the radar data and the configuration of experiments, the actual radiation pattern needs to be known. For that purpose, various simulations as well as passive and active experiments have been conducted. Here, results of meteor head echo observations are presented, which allow us to derive detailed information of the actual radiation pattern for different beam-pointing positions and the current health status of the entire radar. For MAARSY, the described method offers robust beam pointing and width estimations for a minimum of a few days of observations.

Plasma distributions in meteor head echoes and implications for radar cross section interpretation

The derivation of meteoroid masses from radar measurements requires conversion of the measured radar cross section (RCS) to meteoroid mass. Typically, this conversion passes first through an estimate of the meteor plasma density derived from the RCS. However, the conversion from RCS to meteor plasma density requires assumptions on the radial electron density distribution. We use simultaneous triple-frequency measurements of the RCS for 63 large meteor head echoes to derive estimates of the meteor plasma size and density using five different possible radial electron density distributions. By fitting these distributions to the observed meteor RCS values and estimating the goodness-of-fit, we determine that the best fit to the data is a 1/r2 plasma distribution, i.e. the electron density decays as 1/r2 from the center of the meteor plasma. Next, we use the derived plasma distributions to estimate the electron line density q for each meteor using each of the five distributions. We show that depending on the choice of distribution, the line density can vary by a factor of three or more. We thus argue that a best estimate for the radial plasma distribution in a meteor head echo is necessary in order to have any confidence in derived meteoroid masses.

Should dental assessments routinely be carried out on patients with brain abscesses?

Introduction: A brain abscess is a rare but serious and potentially a life threatening condition. Presenting symptoms range from headaches to hemiparesis.Common sources of microbial infection are from either direct or indirect spread from ear- nose and throat, cardiac or pulmonary origin. Therefore these areas are routinely screened using CT head,chest X-ray and transthoracic echo. A recent study found that odontogenic infections could be accountable for up to 32% of cases.

Neutral density estimation derived from meteoroid measurements using high‐power, large‐aperture radar

AbstractWe present a new method to estimate the neutral density of the lower thermosphere/upper mesosphere given deceleration measurements from meteoroids as they enter Earth's atmosphere. By tracking the plasma (referred to as head echoes) surrounding the ablating meteoroid, we are able to measure the range and velocity of the meteoroid in 3‐D. This is accomplished at Advanced Research Projects Agency Long‐Range Tracking and Instrumentation Radar (ALTAIR) with the use of four additional receiving horns. Combined with the momentum and ablation equations, we can feed large quantities of data into a minimization function which estimates the associated constants related to the ablation process and, more importantly, the density ratios between successive layers of the atmosphere. Furthermore, if we take statistics of the masses and bulk densities of the meteoroids, we can calculate the neutral densities and its associated error by the ratio distribution on the minimum error statistic. A standard deviation of approximately 10% can be achieved, neglecting measurement error from the radar. Errors in velocity and deceleration compound this uncertainty, which in the best case amounts to an additional 4% error. The accuracy can be further improved if we take increasing amounts of measurements, limited only by the quality of the ranging measurements and the probability of knowing the median of the distribution. Data analyzed consist mainly of approximately 500 meteoroids over a span of 20 min on two separate days. The results are compared to the existing atmospheric model NRLMSISE‐00, which predicts lower density ratios and static neutral densities at these altitudes.

Fragmentation of specular overdense meteor trail echoes observed with Gadanki MST radar

The pulse-integrated signal to noise ratio as a function of time known as radar meteor light curve (analogous to optical light curve), is an indicative of ablation processes during meteoroid flight in the atmosphere. In this study, we present and discuss few examples of light curves of long duration specular overdense meteor echoes detected with 53 MHz Gadanki (\(13.5^{\circ}N\), \(79.2^{\circ}E\)) MST radar. These echoes are of several seconds duration, where pulsation in amplitude is about ten cycles within few seconds. This means the fluctuations in amplitude are much slower than typical Fresnel oscillations of underdense as well as the head echo fluctuations. These light curves reveal several features unreported previously in the radar meteor returns that are consistent with meteoroid fragmentation. Some of them provide the strong observational evidence of a sub-millimeter-sized meteoroid, breaking apart into two distinct fragments. The pulsations in light curves are interpreted as being due to interference from two distinct scattering centers. Some other meteor events such as meteoroids undergoing quasi-continuous disintegration are also discussed.

MU radar head echo observations of the 2012 October Draconid outburst

MU radar head echo observations of the 2012 October Draconid outburst

On the size and velocity distribution of cosmic dust particles entering the atmosphere.

The size and velocity distribution of cosmic dust particles entering the Earth's atmosphere is uncertain. Here we show that the relative concentrations of metal atoms in the upper mesosphere, and the surface accretion rate of cosmic spherules, provide sensitive probes of this distribution. Three cosmic dust models are selected as case studies: two are astronomical models, the first constrained by infrared observations of the Zodiacal Dust Cloud and the second by radar observations of meteor head echoes; the third model is based on measurements made with a spaceborne dust detector. For each model, a Monte Carlo sampling method combined with a chemical ablation model is used to predict the ablation rates of Na, K, Fe, Mg, and Ca above 60 km and cosmic spherule production rate. It appears that a significant fraction of the cosmic dust consists of small (<5 µg) and slow (<15 km s−1) particles.

An FDTD model of scattering from meteor head plasma

AbstractWe have developed a three‐dimensional finite difference time domain (FDTD) model of scattering of radar waves from meteor head plasma. The model treats the meteor head plasma as a cold, collisional, and magnetized plasma, and solves Maxwell's equations and the Langevin equation simultaneously and self‐consistently in and around the plasma. We use this model to investigate scattering of radar waves from a meteor head (the “head echo”) under a range of plasma densities, meteor scale sizes, and wave frequencies. In this way we relate the radar cross section (RCS) to these variable parameters. We find that computed RCS disagrees with previous analytical theory at certain meteor sizes and densities, in some cases by over an order of magnitude. We find that the calculated meteor head RCS is monotonically related to the “overdense area” of the meteor, defined as the cross‐section area of the part of the meteor where the plasma frequency exceeds the wave frequency. These results provides a physical measure of the meteor size and density that can be inferred from measured RCS values from ground‐based radars. Meteoroid mass can then be inferred from the meteor plasma distribution using established methods.