Physical Dispersions of Meteor Showers Through High Precision Optical Observations

Meteoroids ejected from comets form meteoroid streams which disperse over time due to gravitational perturbations and non-gravitational forces. When stream meteoroids collide with the Earth\u27s atmosphere, they are visible as meteors emanating from a common point-like area (radiant) in the sky. Measuring the size of meteor shower radiant areas can provide insight into stream formation and age. The tight radiant dispersion of young streams are difficult to determine due to measurement error, but if successfully measured, the dispersion could be used to constrain meteoroid ejection velocities from their parent comets. The estimated ejection velocity is an uncertain, model-dependent value with significant influence on the prediction accuracy of meteor shower models which are operationally used by space agencies to mitigate the meteoroid impact risk. The first part of this work consists of a theoretical investigation of achievable meteor radiant and velocity measurement accuracy using optical observation systems. From dynamical meteoroid stream modelling it has been estimated that a minimum radiant measurement accuracy of 0.1° is needed to begin to resolve the radiant structure of young meteor showers. Using a novel meteor trajectory simulator, it was found that this accuracy can be achieved using narrow field of view optical systems and a newly developed method of meteor trajectory estimation. The measurement accuracy of pre-atmosphere meteoroid velocities remains model-dependent because meteoroids may decelerate up to 750 m/s prior to becoming visible. The second part of the work was observational and done using the Canadian Automated Meteor Observatory (CAMO). Four Electron Multiplying CCD cameras were used to observe the 2018 outburst of the Draconid meteor shower which had a radiant dispersion of 0.25°, consistent with simulations and previous high-precision measurements. A mass index of s = 1.74 ± 0.18 during the peak was estimated using a novel method. The CAMO mirror tracking system was used to observe the 2019 Orionids. For the first time, the Orionid radiant structure was accurately measured, showing indications of two stream branches. As part of the meteoroid modelling work to improve radiant and orbit measurements the compressive strengths of meteoroids were estimated through direct observations of fragmentation. The measured values were a good match to in-situ Rosetta measurements from comet 67P

First holistic modelling of meteoroid ablation and fragmentation: A case study of the Orionids recorded by the Canadian Automated Meteor Observatory

18 mm-sized Orionid meteoroids were captured in 2019 and 2020 by the Canadian Automated Observatory's mirror tracking system. Meteor position measurements were made to an accuracy of $\sim1$ m and the meteors were tracked to a limiting magnitude of about $+7.5$ at the faintest point. The trajectory estimation shows the intrinsic physical dispersion of the Orionid radiant is $0.400^{\circ} \pm 0.062^{\circ}$. An erosion-based entry model was fit to the observations to reproduce ablation and fragmentation for each meteor, simultaneously reproducing the light curve, the dynamics, and the wake. Wake observations were found to directly inform the grain mass distribution released in the modelled erosion. A new luminous efficiency model was derived from simultaneous radar and optical observations and applied in the modelling to improve its accuracy. The results show that the apparent strength of Orionids varies with radiant location and time of appearance during the period of shower activity. The average differential grain mass distribution index was 2.15, higher than found from in-situ estimates, possibly due to the evolution of the physical properties of meteoroids since ejection. All Orionids showed leading fragment morphology which was best explained by stopping the erosion at the peak of the light curve, leaving a non-fragmenting meteoroid with $\sim10\%$ of the original mass. The inverted Orionid meteoroid average bulk density of $\sim300$ kg m$^{-3}$, corresponding to porosities of $\sim90\%$, is consistent with in-situ measurements of larger dust particles by Vega-2 at 1P/Halley and Rosetta at 67P.Comment: Accepted for publication in Icaru

Frankov znak kao čimbenik rizika cerebrovaskularne bolesti

Frank’s sign is a dermatological marker, which has in many studies correlated with coronary heart disease. The aim of the study carried out in a sample of 60 subjects was to define whether Frank’s sign belongs to the group of risk factors for cerebrovascular disease. The subjects with the presence of Frank’s sign were assigned to group A, and those without Frank’s sign to group B. All study subjects underwent color Doppler examination of carotid arteries and determination of their common carotid artery intimal wall thickness (ACC IMT). The value exceeding 0.9 mm was considered as an ACC IMT increase. Statistical analysis by use of Pearson’s χ2-test yielded a value of 11.279 and p=0.001. Assessment of the Frank’s sign value in predicting increased ACC IMT showed a sensitivity of 73%, specificity of 70%, positive predictive value of 71% and negative predictive value of 72%. The study indicated a statistically significant correlation between Frank’s sign and increased ACC IMT, supporting the hypothesis according to which this marker is an uncontrollable risk factor for cerebrovascular disease.Frankov znak je dermatološki biljeg koji je u brojnim studijama povezan s koronarnom bolešću srca. Cilj istraživanja provedenog na uzorku od 60 ispitanika bio je utvrditi pripada li Frankov znak čimbenicima rizika cerebrovaskularne bolesti. U skupinu A svrstani su ispitanici s prisutnim, a u skupinu B ispitanici s odsutnim Frankovim znakom te je svima napravljen obojeni Doppler karotidnih arterija i određena debljina intimalne stijenke (IMT) na zajedničkoj karotidnoj arteriji (ACC). Kao povećanje ACC IMT uzeta je vrijednost iznad 0,9 mm. Statističkom analizom Pearsonovim χ2-testom dobivena je vrijednost od 11,279 i p=0,001. Procjenom vrijednosti Frankovog znaka u predviđanju povećanja ACC IMT utvrđena je osjetljivost od 73%, specifičnost od 70%, pozitivna prediktivna vrijednost od 71% i negativna prediktivna vrijednost od 72%. Istraživanje je pokazalo statistički značajnu povezanost Frankovog znaka i povećanja ACC IMT, što podupire hipotezu prema kojoj je ovaj znak čimbenik rizika cerebrovaskularne bolesti na koji se ne može utjecati

Precision measurements of radar transverse scattering speeds from meteor phase characteristics

We describe an improved technique for using the backscattered phase from meteor radar echo measurements just prior to the specular point ($t_{0}$) to calculate meteor speeds and their uncertainty. Our method, which builds on earlier work of Cervera et al (1997), scans possible speeds in the Fresnel distance - time domain with a dynamic, sliding window and derives a best-speed estimate from the resultant speed distribution. We test the performance of our method, called pre-$t_{0}$ speeds by sliding-slopes technique (PSSST), on transverse scattered meteor echoes observed by the Middle Atmosphere Alomar Radar System (MAARSY) and the Canadian Meteor Orbit Radar (CMOR), and compare the results to time-of-flight and Fresnel transform speed estimates. Our novel technique is shown to produce good results when compared to both model and speed measurements using other techniques. We show that our speed precision is $\pm$5$\%$ at speeds less than 40 km/s and we find that more than 90$\%$ of all CMOR multi-station echoes have PSSST solutions. For CMOR data, PSSST is robust against the selection of critical phase value and poor phase unwrapping. Pick errors of up to $\pm$6 pulses for meteor speeds less than about 50 km/s produce errors of less than $\pm$5$\%$ of the meteoroid speed. In addition, the width of the PSSST speed Kernel density estimate (KDE) is used as a natural measure of uncertainty that captures both noise and $t_0$ pick uncertainties.Comment: Accepted for publication to Radio Science on 2020-06-2