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- Harju, Jorma, et al.
(author)
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Radio Interferometric Observation of an Asteroid Occultation
- 2018
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In: Astronomical Journal. - : University of Chicago Press. - 0004-6256 .- 1538-3881. ; 156:4, s. 1-10
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Journal article (peer-reviewed)abstract
- The occultation of the radio galaxy 0141+268 by the asteroid(372)Palma on 2017 May 15 was observed using sixantennas of the Very Long Baseline Array(VLBA). The shadow of Palma crossed the VLBA station at Brewster,Washington. Owing to the wavelength used, and the size and the distance of the asteroid, a diffraction pattern in theFraunhofer regime was observed. The measurement retrieves both the amplitude and the phase of the diffractedelectromagnetic wave. This is thefirst astronomical measurement of the phase shift caused by diffraction. Themaximum phase shift is sensitive to the effective diameter of the asteroid. The bright spot at the shadow’s center,the so called Arago–Poisson spot, is clearly detected in the amplitude time-series, and its strength is a goodindicator of the closest angular distance between the center of the asteroid and the radio source. A sample ofrandom shapes constructed using a Markov chain Monte Carlo algorithm suggests that the silhouette of Palmadeviates from a perfect circle by 26±13%. The best-fitting random shapes resemble each other, and we suggesttheir average approximates the shape of the silhouette at the time of the occultation. The effective diameterobtained for Palma, 192.1±4.8 km, is in excellent agreement with recent estimates from thermal modeling ofmid-infrared photometry. Finally, our computations show that because of the high positional accuracy, a singleradio interferometric occultation measurement can reduce the long-term ephemeris uncertainty by an order ofmagnitude.
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| 2. |
- Korhonen, Kimmo, et al.
(author)
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Ice-nucleating ability of particulate emissions from solid-biomass-fired cookstoves : An experimental study
- 2020
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In: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 20:8, s. 4951-4968
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Journal article (peer-reviewed)abstract
- This research was part of the Salutary Umeå Study of Aerosols in Biomass Cookstove Emissions (SUSTAINE) laboratory experiment campaign. We studied ice-nucleating abilities of particulate emissions from solid-fuel-burning cookstoves, using a portable ice nuclei counter, Spectrometer Ice Nuclei (SPIN). These emissions were generated from two traditional cookstove types commonly used for household cooking in sub-Saharan Africa and two advanced gasifier stoves under research to promote sustainable development alternatives. The solid fuels studied included biomass from two different African tree species, Swedish softwood and agricultural residue products relevant to the region. Measurements were performed with a modified version of the standard water boiling test on polydisperse samples from flue gas during burning and size-selected accumulation mode soot particles from a 15 m3 aerosol-storage chamber. The studied soot particle sizes in nanometers were 250, 260, 300, 350, 400, 450 and 500. From this chamber, the particles were introduced to water-supersaturated freezing conditions (-32 to-43 °C) in the SPIN. Accumulation mode soot particles generally produced an ice-activated fraction of 10-3 in temperatures 1-1.5 °C higher than that required for homogeneous freezing at fixed RHw D 115 %. In five special experiments, the combustion performance of one cookstove was intentionally modified. Two of these exhibited a significant increase in the icenucleating ability of the particles, resulting in a 10-3 ice activation at temperatures up to 5.9 °C higher than homogeneous freezing and the observed increased ice-nucleating ability. We investigated six different physico-chemical properties of the emission particles but found no clear correlation between them and increasing ice-nucleating ability. We conclude that the freshly emitted combustion aerosols form ice via immersion and condensation freezing at temperatures only moderately above homogeneous freezing conditions.
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