| 1. |
- Campbell-Brown, Margaret, et al.
(författare)
-
Photometric and ionization masses of meteors with simultaneous EISCAT UHF radar and intensified video observations
- 2012
-
Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 117, s. A09323-
-
Tidskriftsartikel (refereegranskat)abstract
- There are significant uncertainties in the calculation of photometric and ionization masses of meteors, particularly those derived from meteor head echoes observed by high power, large aperture radars. Simultaneous observations of meteors with the EISCAT UHF tristatic system and narrow field two-station intensified video were conducted in October 2007; 11 hours of data produced four useful meteors observed on all three radar receivers and both cameras. The positions and speeds calculated on the two systems generally agree to within the observational uncertainty. The photometric and ionization masses for each meteor were calculated using several values of luminous efficiency and ionization probability from literature, and all of these masses were found to agree to within the estimated error in the methods. More observations are required to select among the various values of ionization coefficient and luminous efficiency.
|
|
| 2. |
- Kero, Johan, et al.
(författare)
-
A meteor head echo analysis algorithm for the lower VHF band
- 2012
-
Ingår i: Annales Geophysicae. - Gottingen : Copernicus Gesellschaft mbH. - 0992-7689 .- 1432-0576. ; 30:4, s. 639-659
-
Tidskriftsartikel (refereegranskat)abstract
- We have developed an automated analysis scheme for meteor head echo observations by the 46.5 MHz Middle and Upper atmosphere (MU) radar near Shigaraki, Japan (34.85A degrees N, 136.10A degrees E). The analysis procedure computes meteoroid range, velocity and deceleration as functions of time with unprecedented accuracy and precision. This is crucial for estimations of meteoroid mass and orbital parameters as well as investigations of the meteoroid-atmosphere interaction processes. In this paper we present this analysis procedure in detail. The algorithms use a combination of single-pulse-Doppler, time-of-flight and pulse-to-pulse phase correlation measurements to determine the radial velocity to within a few tens of metres per second with 3.12 ms time resolution. Equivalently, the precision improvement is at least a factor of 20 compared to previous single-pulse measurements. Such a precision reveals that the deceleration increases significantly during the intense part of a meteoroid's ablation process in the atmosphere. From each received pulse, the target range is determined to within a few tens of meters, or the order of a few hundredths of the 900 m long range gates. This is achieved by transmitting a 13-bit Barker code oversampled by a factor of two at reception and using a novel range interpolation technique. The meteoroid velocity vector is determined from the estimated radial velocity by carefully taking the location of the meteor target and the angle from its trajectory to the radar beam into account. The latter is determined from target range and bore axis offset. We have identified and solved the signal processing issue giving rise to the peculiar signature in signal to noise ratio plots reported by Galindo et al. (2011), and show how to use the range interpolation technique to differentiate the effect of signal processing from physical processes.
|
|
| 3. |
- Kero, Johan, et al.
(författare)
-
Determination of meteoroid physical properties from tristatic radar observations
- 2008
-
Ingår i: Annales Geophysicae. - 0992-7689 .- 1432-0576. ; 26, s. 2217-2228
-
Tidskriftsartikel (refereegranskat)abstract
- In this work we give a review of the meteor head echo observations carried out with the tristatic 930MHz EISCAT UHF radar system during four 24 h runs between 2002 and 2005 and compare these with earlier observations. A total number of 410 tristatic meteors were observed. We describe a method to determine the position of a compact radar target in the common volume monitored by the three receivers and demonstrate its applicability for meteor studies. The inferred positions of the meteor targets have been utilized to estimate their velocities, decelerations and directions of arrival as well as their radar cross sections with unprecedented accuracy. The velocity distribution of the meteoroids is bimodal with peaks at 35–40 km/s and 55–60 km/s, and ranges from 19–70 km/s. The estimated masses are between 10−9–10−5.5 kg. There are very few detections below 30 km/s. The observations are clearly biased to high-velocity meteoroids, but not so biased against slow meteoroids as has been presumed from previous tristatic measurements. Finally, we discuss how the radial deceleration observed with a monostatic radar depends on the meteoroid velocity and the angle between the trajectory and the beam. The finite beamwidth leads to underestimated meteoroid masses if radial velocity and deceleration of meteoroids approaching the radar are used as estimates of the true quantities in a momentum equation of motion.
|
|
| 4. |
- Kero, Johan, et al.
(författare)
-
On the meteoric head echo radar cross section angular dependence
- 2008
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35, s. 5-
-
Tidskriftsartikel (refereegranskat)abstract
- We present radar cross section (RCS) measurements of meteor head echoes observed with the tristatic 930 MHz EISCAT UHF radar system. The three receivers offer a unique possibility to accurately compare the monostatic RCS of a meteor target with two simultaneously probed bistatic RCSs at different aspect angles. Meteoroids from all possible directions entering the common volume monitored by the three receivers are detected, out to an aspect angle of 130° from the meteoroid trajectories. The RCS of individual meteors as observed by the three receivers are equal within the accuracy of the measurements. This is consistent with an essentially isotropic scattering process as has previously been inferred from polarization measurements by S. Close et al. (2002). There is a very weak trend present in our data suggesting that the RCS may decrease at a rate of 0.2 dB per 10° with increasing aspect angle.
|
|
| 5. |
- Kero, Johan, et al.
(författare)
-
Power fluctuations in meteor head echoes observed with the EISCAT VHF radar
- 2004
-
Ingår i: Earth, moon, and planets. - : Springer Science and Business Media LLC. - 0167-9295 .- 1573-0794. ; 95:1-4, s. 633-638
-
Tidskriftsartikel (refereegranskat)abstract
- We present observations and preliminary results from a meteor experiment carried out with the 224 MHz EISCAT VHF radar in Tromsø, Norway, which was run for 6 h on November 26, 2003. The data set contains echoes with peculiar pulsations in received power in the frequency range 20–200 Hz, limited by instrumental parameters. The process causing the echo power pulsations has not yet been identified. Plasma effects are the most likely cause, a possible mechanism is for instance asymmetrical dust grains in rotation causing a modulation of the ionization rate.
|
|
| 6. |
|
|
| 7. |
- Kero, Johan, et al.
(författare)
-
Three-dimensional radar observation of a submillimeter meteoroid fragmentation
- 2008
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35, s. 5-
-
Tidskriftsartikel (refereegranskat)abstract
- We present and discuss two examples of pulsating meteor events observed with the tristatic 930 MHz EISCAT UHF radar system. One of them provides the first strong observational evidence of a submillimeter-sized meteoroid breaking apart into two distinct fragments. The received power fluctuates regularly in the time profiles of all three receivers, but the fluctuation frequencies are different. The pulsations are interpreted as being due to interference from two distinct scattering centers and the three detected pulsation rates are utilized to calculate the differential velocity of the fragments. The result is consistent with interference from two fragments of unequal cross-sectional area over mass ratio, separating from each other due to different deceleration along the trajectory of their parent meteoroid. The other meteor event is an example of a meteoroid undergoing quasi-continuous disintegration. This manifests itself as simultaneous pulsations at all three receivers. Both observations indicate a head echo target upper size limit of the order of half the 32 cm radar wavelength.
|
|
| 8. |
- Meisel, David D., et al.
(författare)
-
Physical Characteristics of Kazan Minor Showers as Determined by Correlations with the Arecibo UHF radar
- 2008
-
Ingår i: Advances in Meteoroid and Meteor Science. - New York, NY : Springer. - 9780387784182
-
Bokkapitel (refereegranskat)abstract
- In the northern hemisphere, the month of February is characterized by a lack of major meteor shower activity yet a number of weak minor showers are present as seen bythe Kazan radar. Using the Feller transformation to obtain the distribution of true meteor velocities from the distribution of radial velocities enables the angle of incidence to be obtained for the single beam AO (Arecibo Observatory) data. Thus the loci of AO radiants become beam-centered circles on the sky and one can, with simple search routines, find where these circles intersect on radiants determined by other means. Including geocentric velocity as an additional search criterion, we have examined a set of February radiants obtained at Kazan for coincidence in position and velocity. Although some may be chance associations, only those events with probabilities of association[0.5 have been kept. Roughly 90 of the Kazan showers have been verified in this way with mass, radius anddensity histograms derived from the AO results. By comparing these histograms with those of the ‘‘background’’ in which the minor showers are found, a qualitative scale of dynamical minor shower age can be formulated. Most of the showers are found outside the usual ‘‘apex’’ sporadic source areas where it is easiest to detect discrete showers with less confusion from the background.
|
|
| 9. |
- Meisel, David D., et al.
(författare)
-
Physical Characteristics of Kazan Minor Showers as Determined by Correlations with the Arecibo UHF Radar
- 2008
-
Ingår i: Earth, moon, and planets. - : Springer Netherlands. - 0167-9295 .- 1573-0794. ; 102:1-4, s. 315-322
-
Tidskriftsartikel (refereegranskat)abstract
- In the northern hemisphere, the month of February is characterized by a lack of major meteor shower activity yet a number of weak minor showers are present as seen by the Kazan radar. Using the Feller transformation to obtain the distribution of true meteor velocities from the distribution of radial velocities enables the angle of incidence to be obtained for the single beam AO (Arecibo Observatory) data. Thus the loci of AO radiants become beam-centered circles on the sky and one can, with simple search routines, find where these circles intersect on radiants determined by other means. Including geocentric velocity as an additional search criterion, we have examined a set of February radiants obtained at Kazan for coincidence in position and velocity. Although some may be chance associations, only those events with probabilities of association[0.5 have been kept. Roughly 90 of the Kazan showers have been verified in this way with mass, radius and density histograms derived from the AO results. By comparing these histograms with those of the ‘‘background’’ in which the minor showers are found, a qualitative scale of dynamical minor shower age can be formulated. Most of the showers are found outside the usual ‘‘apex’’ sporadic source areas where it is easiest to detect discrete showers with less confusion from the background.
|
|
| 10. |
- Meisel, David D., et al.
(författare)
-
Quantitative Comparison of a New Ab Initio Micrometeor Ablation Model with an Observationally Verifiable Standard Model
- 2008
-
Ingår i: Advances in Meteoroid and Meteor Science. - New York, NY : Springer. - 9780387784182 ; , s. 411-415
-
Bokkapitel (refereegranskat)abstract
- The Arecibo UHF radar is able to detect the head-echos of micron-sized meteoroids up to velocities of 75 km/s over a height range of 80–140 km. Because of their small size there are many uncertainties involved in calculating their above atmosphereproperties as needed for orbit determination. An ab initio model of meteor ablation has been devised that should work over the mass range 10-16 kg to 10-7 kg, but the faint endof this range cannot be observed by any other method and so direct verification is not possible. On the other hand, the EISCAT UHF radar system detects micrometeors in the high mass part of this range and its observations can be fit to a ‘‘standard’’ ablation model and calibrated to optical observations (Szasz et al. 2007). In this paper, we present a preliminary comparison of the two models, one observationally confirmable. Among thefeatures of the ab initio model that are different from the ‘‘standard’’ model are: (1) uses the experimentally based low pressure vaporization theory of O’Hanlon (A users’s guide tovacuum technology, 2003) for ablation, (2) uses velocity dependent functions fit from experimental data on heat transfer, luminosity and ionization efficiencies measured byFriichtenicht and Becker (NASA Special Publication 319: 53, 1973) for micron sized particles, (3) assumes a density and temperature dependence of the micrometeoroids andablation product specific heats, (4) assumes a density and size dependent value for the thermal emissivity and (5) uses a unified synthesis of experimental data for the most important meteoroid elements and their oxides through least square fits (as functions of temperature, density, and/or melting point) of the tables of thermodynamic parameters given in Weast (CRC Handbook of Physics and Chemistry, 1984), Gray (American Institute of Physics Handbook, 1972), and Cox (Allen’s Astrophysical Quantities 2000). This utilization of mostly experimentally determined data is the main reason for calling this an ab initio model and is made necessary by the fact that individual average meteoroid mass densities are now derivable from Arecibo observations.
|
|
