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Journal articleThomas C, Voulgarakis A, Lim G, et al., 2021,
An unsupervised learning approach to identifying blocking events: the case of European summer
, Weather and Climate Dynamics, Vol: 2, Pages: 581-608, ISSN: 2698-4016Atmospheric blocking events are mid-latitude weather patterns, which obstruct the usual path of the polar jet streams. They are often associated with heat waves in summer and cold snaps in winter. Despite being central features of mid-latitude synoptic-scale weather, there is no well-defined historical dataset of blocking events. Various blocking indices (BIs) have thus been suggested for automatically identifying blocking events in observational and in climate model data. However, BIs show significant regional and seasonal differences so that several indices are typically applied in combination to ensure scientific robustness. Here, we introduce a new BI using self-organizing maps (SOMs), an unsupervised machine learning approach, and compare its detection skill to some of the most widely applied BIs. To enable this intercomparison, we first create a new ground truth time series classification of European blocking based on expert judgement. We then demonstrate that our method (SOM-BI) has several key advantages over previous BIs because it exploits all of the spatial information provided in the input data and reduces the dependence on arbitrary thresholds. Using ERA5 reanalysis data (1979–2019), we find that the SOM-BI identifies blocking events with a higher precision and recall than other BIs. In particular, SOM-BI already performs well using only around 20 years of training data so that observational records are long enough to train our new method. We present case studies of the 2003 and 2019 European heat waves and highlight that well-defined groups of SOM nodes can be an effective tool to diagnose such weather events, although the domain-based approach can still lead to errors in the identification of certain events in a fashion similar to the other BIs. We further test the red blocking detection skill of SOM-BI depending on the meteorological variable used to study blocking, including geopotential height, sea level pressure and four variables related to
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Journal articleZazzeri G, Xu X, Graven H, 2021,
Efficient sampling of atmospheric methane for radiocarbon analysis and quantification of fossil methane.
, Environmental Science and Technology (Washington), Vol: 55, Pages: 8535-8541, ISSN: 0013-936XRadiocarbon (14C) measurements offer a unique investigative tool to study methane emissions by identifying fossil-fuel methane in air. Fossil-fuel methane is devoid of 14C and, when emitted to the atmosphere, causes a strong decrease in the ratio of radiocarbon to total carbon in methane (Δ14CH4). By observing the changes in Δ14CH4, the fossil fraction of methane emissions can be quantified. Presently, there are very few published Δ14CH4 measurements, mainly because it is challenging to collect and process the large volumes of air needed for radiocarbon measurements. We present a new sampling system that collects enough methane carbon for high precision Δ14CH4 measurements without having to transport large volumes of air. The system catalytically combusts CH4 into CO2 and adsorbs the combustion-derived CO2 onto a molecular sieve trap, after first removing CO2, CO, and H2O. Tests using reference air show a Δ14CH4 measurement repeatability of 5.4‰, similar or better than the precision in the most recent reported measurements. We use the system to produce the first Δ14CH4 measurements in central London and show that day-to-day differences in Δ14CH4 in these samples can be attributed to fossil methane input. The new system could be deployed in a range of settings to investigate CH4 sources.
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Journal articleChen Y-J, Hwang Y-T, Ceppi P, 2021,
The impacts of cloud-radiative changes on poleward atmospheric and oceanic energy transport in a warmer climate
, Journal of Climate, Vol: 34, Pages: 7857-7874, ISSN: 0894-8755Based on theory and climate model experiments, previous studies suggest most of the uncertainties in projected future changes in meridional energy transport and zonal mean surface temperature can be attributed to cloud feedback. To investigate how radiative and dynamical adjustments modify the influence of cloud-radiative changes on energy transport, this study applies a cloud-locking technique in a fully-coupled climate model, CESM. Under global warming, the impacts of cloud-radiative changes on the meridional energy transport are asymmetric in the two hemispheres. In the Northern Hemisphere, the cloud-radiative changes have little impact on energy transport, because 89% of the cloud-induced heating is balanced locally by increasing outgoing longwave radiation. In the Southern Hemisphere, on the other hand, cloud-induced dynamical changes in the atmosphere and the ocean cause enhanced poleward energy transport, accounting for most of the increase in energy transport under warming. Our experiments highlight that the local longwave radiation adjustment induced by temperature variation can partially offset the impacts of cloud-radiative changes on energy transport, making the estimated impacts smaller than those obtained from directly integrating cloud-radiative changes in previous studies. It is also demonstrated that the cloud-radiative impacts on temperature and energy transport can be significantly modulated by the oceanic circulation, suggesting the necessity of considering atmospheric-oceanic coupling when estimating the impacts of cloud-radiative changes on the climate system.
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Journal articleJoyce CJ, McComas DJ, Schwadron NA, et al., 2021,
Energetic particle evolution during coronal mass ejection passage from 0.3 to 1 AU
, ASTRONOMY & ASTROPHYSICS, Vol: 651, ISSN: 0004-6361- Author Web Link
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- Citations: 11
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Journal articleGreaves JS, Richards AMS, Bains W, et al., 2021,
Phosphine gas in the cloud deck of Venus (vol 5, pg 655, 2021)
, Nature Astronomy, Vol: 5, Pages: 726-728, ISSN: 2397-3366 -
Journal articleDuan D, He J, Bowen TA, et al., 2021,
Anisotropy of solar wind turbulence in the inner heliosphere at kinetic scales: PSP observations
, Letters of the Astrophysical Journal, Vol: 915, Pages: 1-7, ISSN: 2041-8205The anisotropy of solar wind turbulence is a critical issue in understanding the physics of energy transfer between scales and energy conversion between fields and particles in the heliosphere. Using the measurement of Parker Solar Probe (PSP), we present an observation of the anisotropy at kinetic scales in the slow, Alfvénic, solar wind in the inner heliosphere. The magnetic compressibility behaves as expected for kinetic Alfvénic turbulence below the ion scale. A steepened transition range is found between the inertial and kinetic ranges in all directions with respect to the local background magnetic field direction. The anisotropy of k⊥ ≫ k∥ is found evident in both transition and kinetic ranges, with the power anisotropy P⊥/P∥ > 10 in the kinetic range leading over that in the transition range and being stronger than that at 1 au. The spectral index varies from αt∥ = −5.7 ± 1.0 to αt⊥ = −3.7 ± 0.3 in the transition range and αk∥ = −3.12 ± 0.22 to αk⊥ = −2.57 ± 0.09 in the kinetic range. The corresponding wavevector anisotropy has the scaling of ${k}_{\parallel }\sim {k}_{\perp }^{0.71\pm 0.17}$ in the transition range, and changes to ${k}_{\parallel }\sim {k}_{\perp }^{0.38\pm 0.09}$ in the kinetic range, consistent with the kinetic Alfvénic turbulence at sub-ion scales.
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Journal articleMadanian H, Schwartz SJ, Fuselier SA, et al., 2021,
Direct Evidence for Magnetic Reflection of Heavy Ions from High Mach Number Collisionless Shocks
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 915, ISSN: 2041-8205- Author Web Link
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- Citations: 5
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Journal articleChen L, Ma B, Wu D, et al., 2021,
An Interplanetary Type IIIb Radio Burst Observed by Parker Solar Probe and Its Emission Mechanism
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 915, ISSN: 2041-8205- Cite
- Citations: 7
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Journal articleFarrell WM, Rasca AP, MacDowall RJ, et al., 2021,
Switchback Boundary Dissipation and Relative Age
, ASTROPHYSICAL JOURNAL, Vol: 915, ISSN: 0004-637X- Author Web Link
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- Citations: 6
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Journal articleYao Z, Dunn WR, Woodfield EE, et al., 2021,
Revealing the source of Jupiter's x-ray auroral flares
, SCIENCE ADVANCES, Vol: 7, ISSN: 2375-2548- Cite
- Citations: 14
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Journal articleHalekas JS, Bercic L, Whittlesey P, et al., 2021,
The Sunward Electron Deficit: A Telltale Sign of the Sun's Electric Potential
, ASTROPHYSICAL JOURNAL, Vol: 916, ISSN: 0004-637X- Author Web Link
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- Citations: 22
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Journal articleWang R, Vasko IY, Mozer FS, et al., 2021,
Electrostatic Solitary Waves in the Earth's Bow Shock: Nature, Properties, Lifetimes, and Origin
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 126, ISSN: 2169-9380- Author Web Link
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- Citations: 29
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Journal articleGristey JJ, Su W, Loeb NG, et al., 2021,
Shortwave radiance to irradiance conversion for earth radiation budget satellite observations: a review
, Remote Sensing, Vol: 13, ISSN: 2072-4292Observing the Earth radiation budget (ERB) from satellites is crucial for monitoring and understanding Earth’s climate. One of the major challenges for ERB observations, particularly for reflected shortwave radiation, is the conversion of the measured radiance to the more energetically relevant quantity of radiative flux, or irradiance. This conversion depends on the solar-viewing geometry and the scene composition associated with each instantaneous observation. We first outline the theoretical basis for algorithms to convert shortwave radiance to irradiance, most commonly known as empirical angular distribution models (ADMs). We then review the progression from early ERB satellite observations that applied relatively simple ADMs, to current ERB satellite observations that apply highly sophisticated ADMs. A notable development is the dramatic increase in the number of scene types, made possible by both the extended observational record and the enhanced scene information now available from collocated imager information. Compared with their predecessors, current shortwave ADMs result in a more consistent average albedo as a function of viewing zenith angle and lead to more accurate instantaneous and mean regional irradiance estimates. One implication of the increased complexity is that the algorithms may not be directly applicable to observations with insufficient accompanying imager information, or for existing or new satellite instruments where detailed scene information is not available. Recent advances that complement and build on the base of current approaches, including machine learning applications and semi-physical calculations, are highlighted.
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Journal articleKuhn-Régnier A, Voulgarakis A, Nowack P, et al., 2021,
Quantifying the Importance of antecedent fuel-related vegetationproperties for burnt area using random forests
, Biogeosciences, Vol: 8, ISSN: 1726-4170The seasonal and longer-term dynamics of fuel accumulation affect fire seasonality and the occurrence of extreme wildfires. Failure to account for their influence mayhelp to explain why state-of-the-art fire models do not simulate the length and timing of the fire season or interannual variability in burnt area well. We investigated the impact of accounting for different timescales of fuel production and accumulation on burnt area using a suite of random forest regression models that included the immediateimpact of climate, vegetation, and human influences in agiven month and tested the impact of various combinationsof antecedent conditions in four productivity-related vegetation indices and in antecedent moisture conditions. Analyses were conducted for the period from 2010 to 2015 inclusive. Inclusion of antecedent vegetation conditions representing fuel build-up led to an improvement of the global,climatological out-of-sample R2from 0.579 to 0.701, but theinclusion of antecedent vegetation conditions on timescales≥ 1 year had no impact on simulated burnt area. Currentmoisture levels were the dominant influence on fuel drying. Additionally, antecedent moisture levels were importantfor fuel build-up. The models also enabled the visualisationof interactions between variables, such as the importanceof antecedent productivity coupled with instantaneous drying. The length of the period which needs to be consideredvaries across biomes; fuel-limited regions are sensitive to antecedent conditions that determine fuel build-up over longertime periods (∼ 4 months), while moisture-limited regionsare more sensitive to current conditions that regulate fuel drying.
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Journal articleKuhn- Regnier A, Voulgarakis A, Nowack P, et al., 2021,
The importance of antecedent vegetation and drought conditions as global drivers of burnt areas
, Biogeosciences, Vol: 18, Pages: 3861-3879, ISSN: 1726-4170The seasonal and longer-term dynamics of fuel accumulation affect fire seasonality and the occurrence of extreme wildfires. Failure to account for their influence may help to explain why state-of-the-art fire models do not simulate the length and timing of the fire season or interannual variability in burnt area well. We investigated the impact of accounting for different timescales of fuel production and accumulation on burnt area using a suite of random forest regression models that included the immediate impact of climate, vegetation, and human influences in a given month and tested the impact of various combinations of antecedent conditions in four productivity-related vegetation indices and in antecedent moisture conditions. Analyses were conducted for the period from 2010 to 2015 inclusive. Inclusion of antecedent vegetation conditions representing fuel build-up led to an improvement of the global, climatological out-of-sample R2 from 0.579 to 0.701, but the inclusion of antecedent vegetation conditions on timescales ≥ 1 year had no impact on simulated burnt area. Current moisture levels were the dominant influence on fuel drying. Additionally, antecedent moisture levels were important for fuel build-up. The models also enabled the visualisation of interactions between variables, such as the importance of antecedent productivity coupled with instantaneous drying. The length of the period which needs to be considered varies across biomes; fuel-limited regions are sensitive to antecedent conditions that determine fuel build-up over longer time periods (∼ 4 months), while moisture-limited regions are more sensitive to current conditions that regulate fuel drying.
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Journal articleMalaspina DM, Wilson LB, Ergun RE, et al., 2021,
Electron Bernstein waves and narrowband plasma waves near the electron cyclotron frequency in the near-Sun solar wind
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361- Cite
- Citations: 13
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Journal articleStansby D, Bercic L, Matteini L, et al., 2021,
Sensitivity of solar wind mass flux to coronal temperature
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361 -
Journal articleCattell C, Glesener L, Leiran B, et al., 2021,
Periodicities in an active region correlated with Type Ill radio bursts observed by Parker Solar Probe
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361- Cite
- Citations: 9
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Journal articleLaker R, Horbury TS, Bale SD, et al., 2021,
Statistical analysis of orientation, shape, and size of solar wind switchbacks
, Astronomy & Astrophysics, Vol: 650, Pages: 1-7, ISSN: 0004-6361One of the main discoveries from the first two orbits of Parker Solar Probe(PSP) was the presence of magnetic switchbacks, whose deflections dominated themagnetic field measurements. Determining their shape and size could provideevidence of their origin, which is still unclear. Previous work with a singlesolar wind stream has indicated that these are long, thin structures althoughthe direction of their major axis could not be determined. We investigate ifthis long, thin nature extends to other solar wind streams, while determiningthe direction along which the switchbacks within a stream were aligned. We tryto understand how the size and orientation of the switchbacks, along with theflow velocity and spacecraft trajectory, combine to produce the observedstructure durations for past and future orbits. We searched for the alignmentdirection that produced a combination of a spacecraft cutting direction andswitchback duration that was most consistent with long, thin structures. Theexpected form of a long, thin structure was fitted to the results of the bestalignment direction, which determined the width and aspect ratio of theswitchbacks for that stream. The switchbacks had a mean width of $50,000 \,\rm{km}$, with an aspect ratio of the order of $10$. We find that switchbacksare not aligned along the background flow direction, but instead aligned alongthe local Parker spiral, perhaps suggesting that they propagate along themagnetic field. Since the observed switchback duration depends on how thespacecraft cuts through the structure, the duration alone cannot be used todetermine the size or influence of an individual event. For future PSP orbits,a larger spacecraft transverse component combined with more radially alignedswitchbacks will lead to long duration switchbacks becoming less common.
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Journal articleWoodham L, Horbury T, Matteini L, et al., 2021,
Enhanced proton parallel temperature inside patches of switchbacks in the inner heliosphere
, Astronomy and Astrophysics: a European journal, Vol: 650, Pages: 1-7, ISSN: 0004-6361Context. Switchbacks are discrete angular deflections in the solar wind magnetic field that have been observed throughout the helio-sphere. Recent observations by Parker Solar Probe(PSP) have revealed the presence of patches of switchbacks on the scale of hours to days, separated by ‘quieter’ radial fields. Aims. We aim to further diagnose the origin of these patches using measurements of proton temperature anisotropy that can illuminate possible links to formation processes in the solar corona. Methods. We fit 3D bi-Maxwellian functions to the core of proton velocity distributions measured by the SPAN-Ai instrument onboard PSP to obtain the proton parallel, Tp,‖, and perpendicular, Tp,⊥, temperature. Results. We show that the presence of patches is highlighted by a transverse deflection in the flow and magnetic field away from the radial direction. These deflections are correlated with enhancements in Tp,‖, while Tp,⊥remains relatively constant. Patches sometimes exhibit small proton and electron density enhancements. Conclusions. We interpret that patches are not simply a group of switchbacks, but rather switchbacks are embedded within a larger-scale structure identified by enhanced Tp,‖that is distinct from the surrounding solar wind. We suggest that these observations are consistent with formation by reconnection-associated mechanisms in the corona.
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Journal articleDrake JF, Agapitov O, Swisdak M, et al., 2021,
Are switchbacks signatures of magnetic flux ropes generated by interchange reconnection in the corona?
, Astronomy and Astrophysics: a European journal, Vol: 650, Pages: 1-8, ISSN: 0004-6361The structure of magnetic flux ropes injected into the solar wind duringreconnection in the coronal atmosphere is explored with particle-in-cellsimulations and compared with {\it in situ} measurements of magnetic"switchbacks" from the Parker Solar Probe. We suggest that multi-x-linereconnection between open and closed flux in the corona will inject flux ropesinto the solar wind and that these flux ropes can convect outward over longdistances before disintegrating. Simulations that explore the magneticstructure of flux ropes in the solar wind reproduce key features of the"switchback" observations: a rapid rotation of the radial magnetic field intothe transverse direction (a consequence of reconnection with a strong guidefield); and the potential to reverse the radial field component. The potentialimplication of the injection of large numbers of flux ropes in the coronalatmosphere for understanding the generation of the solar wind is discussed.
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Journal articleAllen RC, Ho GC, Jian LK, et al., 2021,
A living catalog of stream interaction regions in the Parker Solar Probe era
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361- Author Web Link
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- Citations: 28
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Journal articleHalekas JS, Whittlesey PL, Larson DE, et al., 2021,
Electron heat flux in the near-Sun environment
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361- Author Web Link
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- Citations: 32
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Journal articleShi C, Velli M, Panasenco O, et al., 2021,
Alfvenic versus non-Alfvenic turbulence in the inner heliosphere as observed by Parker Solar Probe
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361- Cite
- Citations: 42
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Journal articleFroment C, Krasnoselskikh V, de Wit TD, et al., 2021,
Direct evidence for magnetic reconnection at the boundaries of magnetic switchbacks with Parker Solar Probe
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361- Author Web Link
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- Citations: 36
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Journal articleQuaas J, Gryspeerdt E, Vautard R, et al., 2021,
Climate impact of aircraft-induced cirrus assessed from satellite observations before and during COVID-19
, Environmental Research Letters, Vol: 16, Pages: 1-6, ISSN: 1748-9326Aircraft produce condensation trails, which are thought to increase high-level cloudiness under certain conditions. Howeverthe magnitude of such an effect and whether this contributes substantially to the radiative forcing due to the aviation sectorremain uncertain. The very substantial, near-global reduction in air traffic in response to the COVID-19 outbreak offers anunprecedented opportunity to identify the anthropogenic contribution to the observed cirrus coverage and thickness. Here weshow, using an analysis of satellite observations for the period March-May 2020, that in the 20% of the Northern Hemispheremid-latitudes with the largest air traffic reduction, cirrus fraction was reduced by ~9 ± 1.5% on average, and cirrus emissivitywas reduced by ~2 ±5% relative to what they should have been with normal air traffic. The changes are corroborated by aconsistent estimate based on linear trends over the period 2011 – 2019. The change in cirrus translates to a global radiativeforcing of 61 ±39 mWm-2. This estimate is somewhat smaller than previous assessments.
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Journal articleJagarlamudi VK, de Wit TD, Froment C, et al., 2021,
Whistler wave occurrence and the interaction with strahl electrons during the first encounter of Parker Solar Probe
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361- Author Web Link
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- Citations: 30
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Journal articleJoyce CJ, McComas DJ, Schwadron NA, et al., 2021,
Time evolution of stream interaction region energetic particle spectra in the inner heliosphere
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361- Author Web Link
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- Citations: 16
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Journal articleBandyopadhyay R, Matthaeus WH, McComas DJ, et al., 2021,
Energetic particle behavior in near-Sun magnetic field switchbacks from PSP
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361- Cite
- Citations: 16
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Journal articleBadman ST, Bale SD, Rouillard AP, et al., 2021,
Measurement of the open magnetic flux in the inner heliosphere down to 0.13 AU
, ASTRONOMY & ASTROPHYSICS, Vol: 650, ISSN: 0004-6361- Author Web Link
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- Citations: 29
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