13 research outputs found
Radar efficiency and the calculation of decade-long PMSE backscatter cross-section for the Resolute Bay VHF radar
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
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, βip 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
The Southern Argentina Agile Meteor Radar Orbital System (SAAMER-OS): An initial sporadic meteoroid orbital survey in the southern sky
We present an initial survey in the southern sky of the sporadic meteoroid orbital environment obtained with the Southern Argentina Agile MEteor Radar (SAAMER) Orbital System (OS), in which over three-quarters of a million orbits of dust particles were determined from 2012 January through 2015 April. SAAMER-OS is located at the southernmost tip of Argentina and is currently the only operational radar with orbit determination capability providing continuous observations of the southern hemisphere. Distributions of the observed meteoroid speed, radiant, and heliocentric orbital parameters are presented, as well as those corrected by the observational biases associated with the SAAMER-OS operating parameters. The results are compared with those reported by three previous surveys performed with the Harvard Radio Meteor Project, the Advanced Meteor Orbit Radar, and the Canadian Meteor Orbit Radar, and they are in agreement with these previous studies. Weighted distributions for meteoroids above the thresholds for meteor trail electron line density, meteoroid mass, and meteoroid kinetic energy are also considered. Finally, the minimum line density and kinetic energy weighting factors are found to be very suitable for meteroid applications. The outcomes of this work show that, given SAAMER's location, the system is ideal for providing crucial data to continuously study the South Toroidal and South Apex sporadic meteoroid apparent sources.Facultad de Ciencias Astronómicas y Geofísica
Modeling the Altitude Distribution of Meteor Head Echoes Observed with HPLA Radars-Implications on the Radar Detectability of Meteoroid Populations
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
Similarities and differences in polar mesosphere summer echoes observed in the Arctic and Antarctica
The Southern Argentina Agile Meteor Radar Orbital System (SAAMER-OS): An initial sporadic meteoroid orbital survey in the southern sky
We present an initial survey in the southern sky of the sporadic meteoroid orbital environment obtained with the Southern Argentina Agile MEteor Radar (SAAMER) Orbital System (OS), in which over three-quarters of a million orbits of dust particles were determined from 2012 January through 2015 April. SAAMER-OS is located at the southernmost tip of Argentina and is currently the only operational radar with orbit determination capability providing continuous observations of the southern hemisphere. Distributions of the observed meteoroid speed, radiant, and heliocentric orbital parameters are presented, as well as those corrected by the observational biases associated with the SAAMER-OS operating parameters. The results are compared with those reported by three previous surveys performed with the Harvard Radio Meteor Project, the Advanced Meteor Orbit Radar, and the Canadian Meteor Orbit Radar, and they are in agreement with these previous studies. Weighted distributions for meteoroids above the thresholds for meteor trail electron line density, meteoroid mass, and meteoroid kinetic energy are also considered. Finally, the minimum line density and kinetic energy weighting factors are found to be very suitable for meteroid applications. The outcomes of this work show that, given SAAMER's location, the system is ideal for providing crucial data to continuously study the South Toroidal and South Apex sporadic meteoroid apparent sources.Facultad de Ciencias Astronómicas y Geofísica
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
Radar efficiency and the calculation of decade-long PMSE backscatter cross-section for the Resolute Bay VHF radar
The Resolute Bay VHF radar, located in Nunavut, Canada (75.0° N,
95.0° W) and operating at 51.5 MHz, has been used to investigate
Polar Mesosphere Summer Echoes (PMSE) since 1997. PMSE are a unique
form of strong coherent radar echoes, and their understanding has
been a challenge to the scientific community since their discovery more
than three decades ago. While other high latitude radars have recorded strong
levels of PMSE activities, the Resolute Bay radar has observed
relatively lower levels of PMSE strengths. In order to derive
absolute measurements of PMSE strength at this site, a technique is
developed to determine the radar efficiency using cosmic (sky) noise
variations along with the help of a calibrated noise source. VHF
radars are only rarely calibrated, but determination of efficiency
is even less common. Here we emphasize the importance of efficiency
for determination of cross-section measurements. The significant
advantage of this method is that it can be directly applied to any
MST radar system anywhere in the world as long as the sky noise
variations are known. The radar efficiencies for two on-site radars
at Resolute Bay are determined. PMSE backscatter cross-section is
estimated, and decade-long PMSE strength variations at this location
are investigated. It was noticed that the median of the backscatter
cross-section distribution remains relatively unchanged, but over
the years a great level of variability occurs in the high power tail
of the distribution
Radar Detectability Studies of Slow and Small Zodiacal Dust Cloud Particles. II. A Study of Three Radars with Different Sensitivity
The sensitivity of radar systems to detect different velocity populations of the incoming micrometeoroid flux is often the first argument considered to explain disagreements between models of the Near-Earth dust environment and observations. Recently, this was argued by Nesvorný et al. to support the main conclusions of a Zodiacal Dust Cloud (ZDC) model which predicts a flux of meteoric material into the Earth's upper atmosphere mostly composed of small and very slow particles. In this paper, we expand on a new methodology developed by Janches et al. to test the ability of powerful radars to detect the meteoroid populations in question. In our previous work, we focused on Arecibo 430 MHz observations since it is the most sensitive radar that has been used for this type of observation to date. In this paper, we apply our methodology to two other systems, the 440 MHz Poker Flat Incoherent Scatter Radar and the 46.5 Middle and Upper Atmosphere radar. We show that even with the less sensitive radars, the current ZDC model over-predicts radar observations. We discuss our results in light of new measurements by the Planck satellite which suggest that the ZDC particle population may be characterized by smaller sizes than previously believed. We conclude that the solution to finding agreement between the ZDC model and sensitive high power and large aperture meteor observations must be a combination of a re-examination not only of our knowledge of radar detection biases, but also the physical assumptions of the ZDC model itself
Similarities and differences in polar mesosphere summer echoes observed in the Arctic and Antarctica
Polar Mesosphere Summer Echoes (PMSE) have been observed in the high
latitudes of the Northern and Southern Hemisphere for several years using VHF
radars located at Andenes/Norway (69° N, 16° E), Resolute
Bay/Canada (75° N, 95° W), and Davis/Antarctica (69° S,
78° E). The VHF radars at the three sites were calibrated using the
same methods (noise source and delayed transmitting signal) and identical
equipment. Volume reflectivity was derived from the calibrated echo power and
the characteristics of the seasonal variation of PMSE were estimated at the
sites for the years 2004 to 2007. The largest peak volume reflectivity of
about 2×10−9 m−1 was observed at Andenes compared with their
counterparts at Davis (~4×10−11 m−1) and Resolute Bay
(~6×10−12 m−1). The peak of the PMSE height distribution is
85.6 km at Davis which is about 1 km higher than at Andenes. At Resolute Bay
the height distribution peaks at about 85 km but only a few layers were
found below 84 km. The mean PMSE occurrence rate is 83% at Andenes, 38% at
Davis with larger variability and only 18% at Resolute Bay (in late summer).
The duration of the PMSE season varies at Andenes from 104 to 113 days and at
Davis from 88 to 93 days. In general the PMSE seasons starts about 5 days
later at Davis and ends about 10 days earlier compared to Andenes. In all
three seasons the PMSE occurrence suddenly drops to a much lower level at
Davis about 32 days after solstice whereas the PMSE season decays smoothly at
Andenes. The duration of the PMSE season at Andenes and Davis is highly
correlated with the presence of equatorward directed winds, the observed
differences in PMSE occurrence are related to the mesospheric temperatures at
both sites