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Radio meteors above the Arctic Circle: radiants, orbits and estimated magnitudes
Umeå University, Faculty of Science and Technology, Physics.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Radiometeorer ovan polcirkeln : radianter, banor och uppskattade magnituder (Swedish)
Abstract [en]

This thesis presents results based on data collected with the 930 MHz EISCAT UHF radar system and three SKiYMet specular meteor radars. It describes in detail a method for meteoroid orbit calculation.

The EISCAT UHF system comprises three identical 32 m parabolic antennae: one high-power transmitter/receiver and two remote receivers. Precise meteoroid deceleration and radar cross section are determined from 410 meteor head echoes simultaneously observed with all three receivers between 2002 and 2005, during four 24h runs at the summer/winter solstice and the vernal/autumnal equinox. The observations are used to calculate meteoroid orbits and estimate meteor visual magnitudes. None of the observed meteors appear to be of extrasolar or asteroidal origin; comets, particularly short period (<200 years) ones, may be the dominant source for the particles observed. About 40% of the radiants are associated with the north apex sporadic meteor source and 58% of the orbits are retrograde. The geocentric velocity distribution is bimodal with a prograde population centred around 38 km/s and a retrograde population peaking at 59 km/s. The absolute visual magnitudes of meteors are estimated to be in the range of +9 to +5 using a single-object numerical ablation model. They are thus observable using intensified CCD cameras with telephoto lenses.

The thesis also investigates diurnal meteor rate differences and sporadic meteor radiant distributions at different latitudes using specular meteor trail radar measurements from 68°N, from 55°N and from 8°S. The largest difference in amplitude of the diurnal flux variation is at equatorial latitudes, the lowest variation is found at high latitudes. The largest seasonal variation of the diurnal flux is observed with the high-latitude meteor radar. The investigations show a variation in the sources with both latitude and time of day.

The EISCAT UHF system and the high-latitude meteor radar are located close to the Arctic Circle. Such a geographical position means that zenith points towards the North Ecliptic Pole (NEP) once every day all year round. This particular geometry allows the meteoroid influx from the north ecliptic hemisphere to be compared throughout the year as the ecliptic plane coincides with the local horizon. Considering only the hour when NEP is closest to zenith, the EISCAT UHF head echo rate is about a factor of three higher at summer solstice than during the other seasons, a finding which is consistent with the high-latitude meteor radar measurements.

Abstract [sv]

Avhandlingens resultat är baserade på mätningar med den trestatiska EISCAT UHF-radarn och tre SKiYMet meteorradarsystem. En metod för meteoroidbanberäkning presenteras i detalj.

EISCAT UHF-systemet består av tre identiska, 32 m stora parabolantenner: en högeffektssändare/mottagare och två fjärrstyrda mottagare. Under fyra 24-timmarsmätningar vid vår-/höstdagjämning och sommar-/vintersolstånd mellan 2002 och 2005 detekterades 410 meteoriska huvudekon simultant med alla tre mottagare. Dessa trestatiska meteorers atmosfärsinbromsning och radartvärsnitt har fastställts mycket noggrant och använts till att beräkna meteoroidernas banor samt uppskatta meteorernas luminositeter. Ingen av de observerade meteoroiderna verkar vara av interstellärt eller asteroidursprung. Deras troligaste ursprung är kometer, framför allt kortperiodskometer (<200 år). Ungefär 40% av meteorradianterna kan associeras till norra apex, ett källområde för sporadiska meteorer, och totalt är 58% av partiklarnas banor retrograda. Meteoroidernas geocentriska hastighetsfördelning har två lokala maxima: ett för den prograda populationen vid 38 km/s och ett för den retrograda vid 59 km/s. Genom att anpassa datat till en numerisk ablationsmodell som simulerar meteoroidernas färd genom atmosfären har de detekterade meteorernas absoluta visuella magnituder uppskattats till mellan +9 och +5. Detta innebär att de är observerbara med bildförstärkta, teleskopiska CCD-kameror.

Avhandlingen diskuterar även hur sporadiska meteorers dygns- och säsongsinflöde beror på geografisk latitud och meteorradianternas distribution på himmelssfären. Detta utreds med hjälp av spårekon detekterade under perioden 1999-2004 med tre meteorradarsystem på latituderna 68°N, 55°N och 8°S. Dygnsinflödet varierar mest på låga latituder och minst på höga. Ju högre latitud, desto mer förändras däremot dygnsinflödet över året. Avhandlingen visar att de dominerande källområdena varierar med säsong, över dygnet och med latitud.

Både EISCAT UHF-systemet och meteorradarn på 68°N är belägna nära polcirkeln. Detta innebär att norra ekliptiska polen (NEP) är i zenit en gång per dygn, året om. Vid just denna tidpunkt sammanfaller ekliptikan med den lokala horisonten, vilket möjliggör att det observerade meteorinflödet från norra ekliptiska hemisfären kan jämföras över året. Under timmen då NEP är närmast zenit har EISCAT UHF uppmätt ett ungefär tre gånger högre meteorinflöde vid sommarsolståndet än under de andra säsongerna, vilket överensstämmer med resultaten från meteorradarn på 68°N.

Place, publisher, year, edition, pages
Umeå: Fysik , 2008. , 39 p.
Series
IRF Scientific Report, ISSN 0284-1703 ; 294
Keyword [en]
meteors, meteoroids, dust, meteor radiants, meteoroid orbits, sporadic sources, radar
Keyword [sv]
meteorradianter, meteoroidbanor, sporadiska källområden, radar, meteorer, meteoroider, interplanetärs stoft
National Category
Aerospace Engineering
Identifiers
URN: urn:nbn:se:umu:diva-1603ISBN: 978-91-977255-2-1 (print)OAI: oai:DiVA.org:umu-1603DiVA: diva2:141531
Public defence
2008-05-06, IRF:s aula, Institutet för rymdfysik (IRF), Etian 100, Kiruna, 09:00 (English)
Opponent
Supervisors
Available from: 2008-04-02 Created: 2008-04-02 Last updated: 2010-01-21Bibliographically approved
List of papers
1. Orbit characteristics of the tristatic EISCAT UHF meteors
Open this publication in new window or tab >>Orbit characteristics of the tristatic EISCAT UHF meteors
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2008 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, Vol. 388, no 1, 15-25 p.Article in journal (Refereed) Published
Abstract [en]

The tristatic EISCAT 930-MHz UHF system is used to determine the absolute geocentric velocities of meteors detected with all three receivers simultaneously at 96 km, the height of the common radar volume. The data used in this study were taken between 2002 and 2005, during four 24-h runs at summer/winter solstice and vernal/autumnal equinox to observe the largest seasonal difference. The observed velocities of 410 tristatic meteors are integrated back through the Earth atmosphere to find their atmospheric entry velocities using an ablation model. Orbit calculations are performed by taking zenith attraction, Earth rotation as well as obliquity of the ecliptic into account. The results are presented in the form of different orbital characteristics. None of the observed meteors appears to be of extrasolar or asteroidal origin; comets, particularly short-period (<200 yr) ones, may be the dominant source for the particles observed. About 40 per cent of the radiants can be associated with the north apex sporadic meteor source and 58 per cent of the orbits are retrograde. There is evidence of resonance gaps at semimajor axis values corresponding to commensurabilities with Jupiter, which may be the first convincing evidence of Jupiter's gravitational influence on the population of small sporadic meteoroids surveyed by radar. The geocentric velocity distribution is bimodal with a prograde population centred around 38 km s−1 and a retrograde population peaking at 59 km s−1. The EISCAT radar system is located close to the Arctic Circle, which means that the North Ecliptic Pole (NEP) is near zenith once every 24 h, i.e. during each observational period. In this particular geometry, the local horizon coincides with the ecliptic plane. The meteoroid influx should therefore be directly comparable throughout the year.

Keyword
meteors, meteoroids
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-11238 (URN)doi:10.1111/j.1365-2966.2008.13374.x (DOI)
Available from: 2008-12-01 Created: 2008-12-01 Last updated: 2010-01-21Bibliographically approved
2. Estimated visual magnitudes of the EISCAT UHF meteors
Open this publication in new window or tab >>Estimated visual magnitudes of the EISCAT UHF meteors
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2008 (English)In: Earth, moon, and planets, ISSN 0167-9295, Vol. 102, no 1-4, 373-378 p.Article in journal (Refereed) Published
Abstract [en]

We have investigated the conditions for simultaneous meteor observations with the EISCAT UHF radar system and telescopic optical devices. The observed characteristics of 410 meteors detected by all three UHF receivers are compared with model simulations and their luminosity is calculated as a part of a meteoroid ablation model using a fifth order Runge–Kutta numerical integration technique. The estimated absolute visual magnitudes are in the range of +9 to +5. The meteors should therefore be observable using intensified CCD or EMCCD (Electron Multiplying CCD) cameras with telephoto lenses. A possible setup of a coordinated radar and optical campaign is suggested.

Keyword
Meteor, EISCAT, HPLA, Radar, Magnitude, Optical detection
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-8424 (URN)doi:10.1007/s11038-007-9206-y (DOI)
Available from: 2008-12-03 Created: 2008-12-03 Last updated: 2009-06-24Bibliographically approved
3. Latitudinal variations of diurnal meteor rates
Open this publication in new window or tab >>Latitudinal variations of diurnal meteor rates
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2004 (English)In: Earth, Moon, and Planets, ISSN 0167-9295, Vol. 95, no 1-4, 101-107 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-3078 (URN)
Available from: 2008-04-02 Created: 2008-04-02 Last updated: 2009-06-24Bibliographically approved
4. Radar studies of the sporadic meteoroid complex
Open this publication in new window or tab >>Radar studies of the sporadic meteoroid complex
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2005 (English)In: In Proceedings of RadioVetenskap och Kommunikation, Linköping, 2005, 2005, 191-196 p.Chapter in book (Other academic)
Identifiers
urn:nbn:se:umu:diva-3079 (URN)
Available from: 2008-04-02 Created: 2008-04-02 Last updated: 2009-06-24Bibliographically approved
5. Quantitative Comparison of a New Ab Initio Micrometeor Ablation Model with an Observationally Verifiable Standard Model
Open this publication in new window or tab >>Quantitative Comparison of a New Ab Initio Micrometeor Ablation Model with an Observationally Verifiable Standard Model
2008 (English)In: Earth, moon, and planets, ISSN 0167-9295, Vol. 102, no 1-4, 411-415 p.Article in journal (Refereed) Published
Abstract [en]

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 atmosphere properties 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 end of 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 the features 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 to vacuum technology, 2003) for ablation, (2) uses velocity dependent functions fit from experimental data on heat transfer, luminosity and ionization efficiencies measured by Friichtenicht and Becker (NASA Special Publication 319: 53, 1973) for micron sized particles, (3) assumes a density and temperature dependence of the micrometeoroids and ablation 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.

Keyword
Ablation, Meteor, HPLA, Radar; AO
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-8429 (URN)doi:10.1007/s11038-007-9222-y (DOI)
Available from: 2008-12-03 Created: 2008-12-03 Last updated: 2010-01-21Bibliographically approved

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