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High-resolution meteor exploration with tristatic radar methods
Umeå University, Faculty of Science and Technology, Physics.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Högupplösta meteorstudier med trestatisk radarteknik (Swedish)
Abstract [en]

A meteor observed with the naked eye is colloquially called a shooting star. The streak of light is generated by an extra-terrestrial particle, a meteoroid, entering the Earth’s atmosphere. The term meteor includes both luminosity detectable by optical means and ionization detectable by radar. The radar targets of meteor head echoes have the same motion as the meteoroids on their atmospheric flight and are relatively independent of aspect angle. They appear to be compact regions of plasma created at around 100 km altitude and have no appreciable duration.

This thesis reviews the meteor head echo observations carried out with the tristatic 930 MHz EISCAT UHF radar system during four 24h runs between 2002 and 2005, and a 6h run in 2003 with the monostatic 224 MHz EISCAT VHF radar. It contains the first strong observational evidence of a submillimeter-sized meteoroid breaking apart into two distinct fragments. This discovery promises to be useful in the further understanding of the interaction processes of meteoroids with the Earth’s atmosphere and thus also the properties of interplanetary/interstellar dust.

The tristatic capability of the EISCAT UHF system makes it a unique tool for investigating the physical properties of meteoroids and the meteor head echo scattering process. The thesis presents a method for determining the position of a compact radar target in the common volume of the antenna beams and demonstrates its applicability for meteor studies. The inferred positions of the meteor targets are used to estimate their velocities, decelerations, directions of arrival and radar cross sections (RCS) with unprecedented accuracy. The head echoes are detected at virtually all possible aspect angles all the way out to 130° from the meteoroid trajectory, limited by the antenna pointing directions. The RCS of individual meteors simultaneously observed with the three receivers are equal within the accuracy of the measurements with a very slight trend suggesting that the RCS decreases with increasing aspect angle.

A statistical evaluation of the measurement technique shows that the determined Doppler velocity agrees with the target range rate. This demonstrates that no contribution from slipping plasma is detected and that the Doppler velocities are unbiased within the measurement accuracy. The velocities of the detected meteoroids are in the range of 19-70 km/s, but with very few detections at velocities below 30 km/s. The thesis compares observations with a numerical single-body ablation model, which simulates the physical processes during meteoroid flight through the atmosphere. The estimated meteoroid masses are in the range of 10-9 - 10-5.5 kg.

Abstract [sv]

Meteorer är ljusfenomen på natthimlen som i vardagligt tal kallas fallande stjärnor. Ljusstrimmorna alstras av meteoroider, små partiklar på banor genom solsystemet, som kolliderar med jordens atmosfär. Förutom ljus genererar meteoroider regioner av joniserat plasma, som är detekterbara med radar. Meteoriska huvudekon tycks komma från kompakta radarmål på ungefär 100 km höjd och rör sig genom atmosfären med de infallande meteoroidernas hastighet. Huvudekons signalstyrka förefaller oberoende av vinkeln mellan radarmålens rörelseriktning och riktningen från vilken radiovågorna infaller och sprids.

Avhandlingen sammanfattar huvudekoobservationer från fyra 24-timmarsmätningar mellan 2002 och 2005 med det trestatiska 930 MHz EISCAT UHF-radarsystemet och en 6-timmarsmätning under 2003 med den monostatiska 224 MHz EISCAT VHF-radarn. Avhandlingen innehåller den första observationella bekräftelsen på att en meteoroid av sub-millimeterstorlek faller sönder i två distinkta fragment i atmosfären. Upptäckten är betydelsefull för studier av meteoroiders växelverkansprocesser med atmosfären och interplanetärt/interstellärt stofts materialegenskaper.

EISCAT UHF-systemet består av tre vitt åtskilda mottagarstationer, vilket gör det till ett unikt mätinstrument för studier av meteoroiders egenskaper och hur radiovågor sprids från de radarmål som ger upphov till huvudekon. Avhandlingen presenterar en metod med vilken ett radarmåls position kan bestämmas om det detekteras simultant med de tre mottagarna. Metoden används till att med hög noggrannhet beräkna meteorers radartvärsnitt samt meteoroiders hastighet och atmosfärsinbromsning. De detekterade huvudekona genereras av meteoroider med i princip alla av mätgeometrin tillåtna rörelseriktningar i förhållande till radarstrålen, ända ut till 130° från radiovågornas spridningsriktning. Enskilda meteorers radartvärsnitt är likvärdiga inom mätnoggrannheten i de tre mottagarstationernas dataserier, men en svag trend antyder att radartvärsnittet minskar med ökande vinkel mellan meteoroidernas rörelseriktning och spridningsriktningen.

En statistisk utvärdering av mättekniken visar att den uppmätta dopplerhastigheten stämmer överens med radarmålens flygtidshastighet. Detta innebär att dopplerhastigheterna är väntevärdesriktiga och opåverkade av bidrag från det spår av plasma som meteoroiderna lämnar efter sig. De uppmätta hastigheterna är 19-70 km/s, men bara ett fåtal detekterade meteoroider är långsammare än 30 km/s. Meteoroidmassorna är uppskattade till 10-9 – 10-5.5 kg genom jämförelser av observationerna med simuleringar av meteoroiders färd genom atmosfären i en numerisk ablationsmodell.

Place, publisher, year, edition, pages
Umeå: Fysik , 2008. , 40 p.
Series
IRF Scientific Report, ISSN 0284-1703 ; 293
Keyword [en]
meteors, meteoroids, interplanetary dust, radar, head echoes, ablation, meteoroid fragmentation
Keyword [sv]
radar, huvudekon, ablation, meteorisk fragmentation, meteorer, meteoroider, interplanetärt stoft
National Category
Aerospace Engineering
Identifiers
URN: urn:nbn:se:umu:diva-1602ISBN: 978-91-977255-3-8 (print)OAI: oai:DiVA.org:umu-1602DiVA: diva2:141525
Public defence
2008-04-25, IRF:s aula, Institutet för rymdfysik (IRF), Etian 100, Kiruna, 10:00 (English)
Opponent
Supervisors
Available from: 2008-04-02 Created: 2008-04-02 Last updated: 2009-06-24Bibliographically approved
List of papers
1. Determination of meteoroid physical properties from tristatic radar observations
Open this publication in new window or tab >>Determination of meteoroid physical properties from tristatic radar observations
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2008 (English)In: Annales Geophysicae, ISSN 0992-7689, Vol. 26, 2217-2228 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Interplanetary physics, Interplanetary dust, Instruments and techniques, radio science
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-11237 (URN)
Available from: 2008-12-01 Created: 2008-12-01 Last updated: 2009-06-24Bibliographically approved
2. On the meteoric head echo radar cross section angular dependence
Open this publication in new window or tab >>On the meteoric head echo radar cross section angular dependence
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2008 (English)In: Geophysical Research Letters, ISSN 0094-8276, Vol. 35, no L07101, 5- p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-11233 (URN)doi:10.1029/2008GL033402 (DOI)
Available from: 2008-12-01 Created: 2008-12-01 Last updated: 2009-06-24Bibliographically approved
3. Three-dimensional radar observation of a submillimeter meteoroid fragmentation
Open this publication in new window or tab >>Three-dimensional radar observation of a submillimeter meteoroid fragmentation
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2008 (English)In: Geophysical Research Letters, ISSN 0094-8276, Vol. 35, no L04101, 5- p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-11234 (URN)doi:10.1029/2007GL032733 (DOI)
Available from: 2008-12-01 Created: 2008-12-01 Last updated: 2009-06-24Bibliographically approved
4. Power fluctuations in meteor head echoes observed with the EISCAT VHF radar
Open this publication in new window or tab >>Power fluctuations in meteor head echoes observed with the EISCAT VHF radar
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2004 (English)In: Earth, moon, and planets, ISSN 0167-9295, Vol. 95, no 1-4, 633-638 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
HPLA radar, meteor head echo, meteoroid rotation, plasma effects
Identifiers
urn:nbn:se:umu:diva-3073 (URN)10.1007/s11038-005-3090-0 (DOI)
Available from: 2008-04-02 Created: 2008-04-02 Last updated: 2009-06-24Bibliographically approved
5. Properties of the meteor head echoes observed with the EISCAT radars
Open this publication in new window or tab >>Properties of the meteor head echoes observed with the EISCAT radars
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2005 (English)In: Proceedings of RadioVetenskap och Kommunikation, Linköping 2005, 2005, 197-200 p.Chapter in book (Other academic)
Identifiers
urn:nbn:se:umu:diva-3074 (URN)
Available from: 2008-04-02 Created: 2008-04-02 Last updated: 2009-06-24Bibliographically approved
6. The EISCAT meteor code
Open this publication in new window or tab >>The EISCAT meteor code
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2008 (English)In: Annales Geophysicae, ISSN 0992-7689, Vol. 26, 2303-2309 p.Article in journal (Refereed) Published
Abstract [en]

Abstract. The EISCAT UHF system has the unique capability to determine meteor vector velocities from the head echo Doppler shifts measured at the three sites. Since even meteors spending a very short time in the common volume produce analysable events, the technique lends itself ideally to mapping the orbits of meteors arriving from arbitrary directions over most of the upper hemisphere.

A radar mode optimised for this application was developed in 2001/2002. A specially selected low-sidelobe 32-bit pseudo-random binary sequence is used to binary phase shift key (BPSK) the transmitted carrier. The baud-length is 2.4μs and the receiver bandwidth is 1.6MHz to accommodate both the resulting modulation bandwidth and the target Doppler shift. Sampling is at 0.6μs, corresponding to 90-m range resolution. Target range and Doppler velocity are extracted from the raw data in a multi-step matched-filter procedure. For strong (SNR>5) events the Doppler velocity standard deviation is 100–150 m/s. The effective range resolution is about 30 m, allowing very accurate time-of-flight velocity estimates. On average, Doppler and time-of-flight (TOF) velocities agree to within about one part in 103. Two or more targets simultaneously present in the beam can be resolved down to a range separation <300m as long as their Doppler shifts differ by more than a few km/s.

Keyword
Electromagnetics, Plasmas, Signal processing and adaptive antennas, Ionosphere, Instruments and techniques
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-11235 (URN)
Available from: 2008-12-01 Created: 2008-12-01 Last updated: 2009-06-24Bibliographically approved

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