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Journal articleSaito M, Yang P, Huang X, et al., 2020, , Geophysical Research Letters, Vol: 47, Pages: 1-10, ISSN: 0094-8276
Two upcoming missions are scheduled to provide novel spaceborne observations of upwelling far鈥恑nfrared spectra. In this study, the accuracy of ice cloud property retrievals using spaceborne middle鈥恡o鈥恌ar鈥恑nfrared (MIR鈥怓IR) measurements is examined toward a better understanding of retrieval biases and uncertainties. Theoretical sensitivity studies demonstrate that the MIR鈥怓IR spectra are sensitive to ice cloud properties, thereby providing a robust means for retrieving cloud properties under nighttime conditions. However, the temperature dependence of the ice refractive index and relevant ice particle shape models need to be incorporated into the retrieval procedure to avoid systematic biases in inferring cloud optical thickness and effective particle radius. Furthermore, prior information of subpixel cloud fractions is essential to mitigation of substantial systematic retrieval biases due to inconsistent subpixel cloud fractions.
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Journal articleLavergne A, Sandoval D, Hare VJ, et al., 2020, , Global Change Biology, Vol: 26, Pages: 7158-7172, ISSN: 1354-1013
Atmospheric aridity and drought both influence physiological function in plant leaves, but their relative contributions to changes in the ratio of leaf-internal to ambient partial pressure of CO2 (χ) - an index of adjustments in both stomatal conductance and photosynthetic rate to environmental conditions - are difficult to disentangle. Many stomatal models predicting χ include the influence of only one of these drivers. In particular, the least-cost optimality hypothesis considers the effect of atmospheric demand for water on χ but does not predict how soils with reduced water further influence χ, potentially leading to an overestimation of χ under dry conditions. Here we use a large network of stable carbon isotope measurements in C3 woody plants to examine the acclimated response of χ to soil water stress. We estimate the ratio of cost factors for carboxylation and transpiration (β) expected from the theory to explain the variance in the data, and investigate the responses of β (and thus χ) to soil water content and suction across seed plant groups, leaf phenological types and regions. Overall, β decreases linearly with soil drying, implying that the cost of water transport along the soil-plant-atmosphere continuum increases as water available in the soil decreases. However, despite contrasting hydraulic strategies, the stomatal responses of angiosperms and gymnosperms to soil water tend to converge, consistent with the optimality theory. The prediction of β as a simple, empirical function of soil water significantly improves χ predictions by up to 6.3 ± 2.3% (mean ± sd of adjusted-R2 ) over 1980-2018 and results in a reduction of around 2% of mean χ values across the globe. Our results highlight the importance of soil water status on stomatal functions and plant water-use efficiency, and suggest the implementation of trait-based hydraulic functions into the model to account for soil water stre
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Conference paperSouthwood D, Cao H, Hunt G, et al., 2020, , Europlanet Science Congress 2020, Publisher: American Geophysical Union
Between April and September 2017 in the final stages of the Cassini Saturn Orbiter mission the spacecraft executed 22 orbits passing planetward of the innermost ring, the D-ring. During periapsis passes on all these orbits oscillations were detected in the azimuthal magnetic field components on typical time scales from a few minutes to 10 minutes. We argue that the time-varying signals detected on the spacecraft are also primarily time-varying in the plasma frame. Nonetheless, we show that nearly all signals exhibit a distinct spatial effect, namely a magnetic node near the effective field line equator. The oscillations thus have a standing structure along the background magnetic field and it follows that they are field line resonances associated with Alfvén waves. The form of the signals suggests that the local field line resonances are most likely pumped from global sources. This is the first detection in a giant planet magnetosphere of a phenomenon known to be important at Earth.
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Journal articleGaland M, Feldman PD, Bockelee-Morvan D, et al., 2020, , Nature Astronomy, Vol: 4, Pages: 1084-1091, ISSN: 2397-3366
Having a nucleus darker than charcoal, comets are usually detected from Earth through the emissions from their coma. The coma is an envelope of gas that forms through the sublimation of ices from the nucleus as the comet gets closer to the Sun. In the far-ultraviolet portion of the spectrum, observations of comae have revealed the presence of atomic hydrogen and oxygen emissions. When observed over large spatial scales as seen from Earth, such emissions are dominated by resonance fluorescence pumped by solar radiation. Here, we analyse atomic emissions acquired close to the cometary nucleus by the Rosetta spacecraft and reveal their auroral nature. To identify their origin, we undertake a quantitative multi-instrument analysis of these emissions by combining coincident neutral gas, electron and far-ultraviolet observations. We establish that the atomic emissions detected from Rosetta around comet 67P/Churyumov-Gerasimenko at large heliocentric distances result from the dissociative excitation of cometary molecules by accelerated solar-wind electrons (and not by electrons produced from photo-ionization of cometary molecules). Like the discrete aurorae at Earth and Mars, this cometary aurora is driven by the interaction of the solar wind with the local environment. We also highlight how the oxygen line O I at wavelength 1,356 Å could be used as a tracer of solar-wind electron variability.
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Journal articleGreaves J, Richards A, Bains W, et al., 2020, , Nature Astronomy, Vol: 5, Pages: 655-664, ISSN: 2397-3366
Measurements of trace gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbour, Venus, has cloud decks that are temperate but hyperacidic. Here we report the apparent presence of phosphine (PH3) gas in Venus’s atmosphere, where any phosphorus should be in oxidized forms. Single-line millimetre-waveband spectral detections (quality up to ~15σ) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH3 at ~20 ppb abundance is inferred. The presence of PH3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life. Other PH3 spectral features should be sought, while in situ cloud and surface sampling could examine sources of this gas.
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Journal articleZhang YC, Dai L, Rong ZJ, et al., 2020, , Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-11, ISSN: 2169-9380
In this study, we reported the large鈥恆mplitude and fast鈥恉amped flapping of the plasma sheet, which co鈥恛ccurred with magnetic reconnection. Data from the Double Star TC鈥1 and Cluster satellites were used to analyze the features of the plasma sheet flapping 1.4 RE earthward of an ongoing magnetic reconnection event. The flapping was rapidly damped, and its amplitude decreased from the magnetohydrodynamics scale to the subion scale in 5 min. The variation in the flapping period (from 224 to 20 s) indicated that the source of the flapping had highly dynamic temporal characteristics. The plasma sheet flapping propagated duskward through a kink鈥恖ike wave with a velocity of 100 km/s, which was in agreement with the group velocity of the ballooning perturbation. A correlation analysis between the magnetic reconnection and plasma sheet flapping indicated that the magnetic reconnection likely facilitated the occurrence of ballooning instability by altering the state of plasma in the downstream plasma sheet. In this regard, the reconnection鈥恑nduced ballooning instability could be a potential mechanism to generate the flapping motion of the plasma sheet.
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Journal articleWilliams RG, Ceppi P, Katavouta A, 2020, , Environmental Research Letters, Vol: 15, ISSN: 1748-9326
The surface warming response to carbon emissions is diagnosed using a suite of Earth system models, 9 CMIP6 and 7 CMIP5, following an annual 1\% rise in atmospheric CO$_2$ over 140 years. This surface warming response defines a climate metric, the Transient Climate Response to cumulative carbon Emissions (TCRE), which is important in estimating how much carbon may be emitted to avoid dangerous climate. The processes controlling these intermodel differences in the TCRE are revealed by defining the TCRE in terms of a product of three dependences: the surface warming dependence on radiative forcing (including the effects of physical climate feedbacks and planetary heat uptake), the radiative forcing dependence on changes in atmospheric carbon and the airborne fraction. Intermodel differences in the TCRE are mainly controlled by the thermal response involving the surface warming dependence on radiative forcing, which arise through large differences in physical climate feedbacks that are only partly compensated by smaller differences in ocean heat uptake. The other contributions to the TCRE from the radiative forcing and carbon responses are of comparable importance to the contribution from the thermal response on timescales of 50 years and longer for our subset of CMIP5 models and 100 years and longer for our subset of CMIP6 models. Hence, providing tighter constraints on how much carbon may be emitted based on the TCRE requires providing tighter bounds for estimates of the physical climate feedbacks, particularly from clouds, as well as to a lesser extent for the other contributions from the rate of ocean heat uptake, and the terrestrial and ocean cycling of carbon.
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Journal articleBrown Z, Koskinen T, Muller-Wodarg I, et al., 2020, , Nature Astronomy, Vol: 4, Pages: 872-879, ISSN: 2397-3366
Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag1,2. A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyse these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturn’s thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy3, we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70° to 30° latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag.
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Journal articleMozer FS, Agapitov OV, Bale SD, et al., 2020, , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380
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- Citations: 26
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Journal articleLotekar A, Vasko IY, Mozer FS, et al., 2020, , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380
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- Citations: 44
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Journal articleZhu X, He J, Verscharen D, et al., 2020, , ASTROPHYSICAL JOURNAL LETTERS, Vol: 901, ISSN: 2041-8205
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- Citations: 25
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Journal articleGood SW, Kilpua EKJ, Ala-Lahti M, et al., 2020, , ASTROPHYSICAL JOURNAL LETTERS, Vol: 900, ISSN: 2041-8205
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- Citations: 17
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Journal articleBradley TJ, Cowley SWH, Bunce EJ, et al., 2020, , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380
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- Citations: 16
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Journal articleShatwell P, Czaja A, Ferreira D, 2020, , Geophysical Research Letters, Vol: 47, Pages: 1-9, ISSN: 0094-8276
To study the role of the Atlantic meridional overturning circulation (AMOC) in transient climate change, we perform an abrupt CO2鈥恉oubling experiment using a coupled atmosphere鈥恛cean鈥恑ce model with a simple geometry that separates the ocean into small and large basins. The small basin exhibits an overturning circulation akin to the AMOC. Over the simulated 200 years of change, it stores heat at a faster rate than the large basin by 0.6 ± 0.2 W m−2. We argue that this is due to the small basin MOC. However, we find that as the MOC weakens significantly, it has little impact on the small basin's heat storage rate. We suggest this is due to the effects of both compensating warming patterns and interbasin heat transports. Thus, although the presence of a MOC is important for enhanced heat storage, MOC weakening is surprisingly unimportant.
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Journal articleBantges R, 2020,
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Journal articleWalker AP, De Kauwe MG, Bastos A, et al., 2020, , New Phytologist, ISSN: 0028-646X
Atmospheric carbon dioxide concentration ([CO2 ]) is increasing, which increases leaf-scale photosynthesis and intrinsic water-use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO2 ] increase and thus climate change. However, ecosystem CO2 -responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO2 ]-driven terrestrial carbon sink can appear contradictory. Here we synthesise theory and broad, multi-disciplinary evidence for the effects of increasing [CO2 ] (iCO2) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre-industry. Established theory, supported by experiments, indicates that iCO2 is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO2-responses are high in comparison with experiments and theory. Plant mortality and soil carbon iCO2-responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO2, albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change.
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Journal articleFranci L, Stawarz JE, Papini E, et al., 2020, , The Astrophysical Journal, Vol: 898, ISSN: 0004-637X
Magnetospheric Multiscale (MMS) observations of plasma turbulence generated by a Kelvin–Helmholtz (KH) event at the Earth's magnetopause are compared with a high-resolution two-dimensional (2D) hybrid direct numerical simulation of decaying plasma turbulence driven by large-scale balanced Alfvénic fluctuations. The simulation, set up with four observation-driven physical parameters (ion and electron betas, turbulence strength, and injection scale), exhibits a quantitative agreement on the spectral, intermittency, and cascade-rate properties with in situ observations, despite the different driving mechanisms. Such agreement demonstrates a certain universality of the turbulent cascade from magnetohydrodynamic to sub-ion scales, whose properties are mainly determined by the selected parameters, also indicating that the KH instability-driven turbulence has a quasi-2D nature. The fact that our results are compatible with the validity of the Taylor hypothesis, in the whole range of scales investigated numerically, suggests that the fluctuations at sub-ion scales might have predominantly low frequencies. This would be consistent with a kinetic Alfvén wave-like nature and/or with the presence of quasi-static structures. Finally, the third-order structure function analysis indicates that the cascade rate of the turbulence generated by a KH event at the magnetopause is an order of magnitude larger than in the ambient magnetosheath.
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Journal articleMadanian H, Burch JL, Eriksson AI, et al., 2020, , Planetary and Space Science, Vol: 187, ISSN: 0032-0633
The Rosetta spacecraft detected transient and sporadic diamagnetic regions around comet 67P/Churyumov-Gerasimenko. In this paper we present a statistical analysis of bulk and suprathermal electron dynamics, as well as a case study of suprathermal electron pitch angle distributions (PADs) near a diamagnetic region. Bulk electron densities are correlated with the local neutral density and we find a distinct enhancement in electron densities measured over the southern latitudes of the comet. Flux of suprathermal electrons with energies between tens of eV to a couple of hundred eV decreases each time the spacecraft enters a diamagnetic region. We propose a mechanism in which this reduction can be explained by solar wind electrons that are tied to the magnetic field and after having been transported adiabatically in a decaying magnetic field environment, have limited access to the diamagnetic regions. Our analysis shows that suprathermal electron PADs evolve from an almost isotropic outside the diamagnetic cavity to a field-aligned distribution near the boundary. Electron transport becomes chaotic and non-adiabatic when electron gyroradius becomes comparable to the size of the magnetic field line curvature, which determines the upper energy limit of the flux variation. This study is based on Rosetta observations at around 200 鈥媖m cometocentric distance when the comet was at 1.24 AU from the Sun and during the southern summer cometary season.
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Journal articleFarrell WM, MacDowall RJ, Gruesbeck JR, et al., 2020, , ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 249, ISSN: 0067-0049
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- Citations: 44
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Journal articleHofstadter MD, Fletcher LN, Simon AA, et al., 2020, , Space Science Reviews, Vol: 216, Pages: 1-17, ISSN: 0038-6308
Other papers in this special issue have discussed the diversity of planetary atmospheres and some of the key science questions for giant planet atmospheres to be addressed in the future. There are crucial measurements that can only be made by orbiters of giant planets and probes dropped into their atmospheres. To help the community be more effective developers of missions and users of data products, we summarize how NASA and ESA categorize their planetary space missions, and the restrictions and requirements placed on each category. We then discuss the atmospheric goals to be addressed by currently approved giant-planet missions as well as missions likely to be considered in the next few years, such as a joint NASA/ESA Ice Giant orbiter with atmospheric probe. Our focus is on interplanetary spacecraft, but we acknowledge the crucial role to be played by ground-based and near-Earth telescopes, as well as theoretical and laboratory work.
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Journal articleErgun RE, Ahmadi N, Kromyda L, et al., 2020, , ASTROPHYSICAL JOURNAL, Vol: 898, ISSN: 0004-637X
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- Citations: 38
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Journal articleVasko IY, Kuzichev I, Artemyev A, et al., 2020, , PHYSICS OF PLASMAS, Vol: 27, ISSN: 1070-664X
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- Citations: 25
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Journal articleSimon Wedlund C, Behar E, Nilsson H, et al., 2020, , Astronomy and Astrophysics: a European journal, Vol: 640, Pages: C3-C3, ISSN: 0004-6361
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Journal articleErgun RE, Ahmadi N, Kromyda L, et al., 2020, , ASTROPHYSICAL JOURNAL, Vol: 898, ISSN: 0004-637X
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- Citations: 53
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Journal articleBowen TA, Bale SD, Bonnell JW, et al., 2020, , ASTROPHYSICAL JOURNAL, Vol: 899, ISSN: 0004-637X
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- Citations: 35
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Journal articleWilder FD, Schwartz SJ, Ergun RE, et al., 2020, , GEOPHYSICAL RESEARCH LETTERS, Vol: 47, ISSN: 0094-8276
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- Citations: 5
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Journal articleFujita R, Morimoto S, Maksyutov S, et al., 2020, , JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, Vol: 125, ISSN: 2169-897X
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- Citations: 32
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Journal articleGibbins G, Haigh JD, 2020,
Entropy production rates of the climate
, Journal of the Atmospheric Sciences, ISSN: 0022-4928There is ongoing interest in the global entropy production rate as a climate diagnostic and predictor, but progress has been limited by ambiguities in its definition; different conceptual boundaries of the climate system give rise to different internal production rates. Three viable options are described, estimated and investigated here, two of which -- the material and the total radiative (here `planetary') entropy production rates -- are well-established and a third which has only recently been considered but appears very promising. This new option is labelled the `transfer' entropy production rate and includes all irreversible processes that transfer heat within the climate, radiative and material, but not those involved in the exchange of radiation with space. Estimates in three model climates put the material rate in the range 27-48 mW/m^2K, the transfer rate 67-76mW/m^2K, and the planetary rate 1279-1312 mW/m^2K. The climate-relevance of each rate is probed by calculating their responses to climate changes in a simple radiative-convective model. An increased greenhouse effect causes a significant increase in the material and transfer entropy production rates but has no direct impact on the planetary rate. When the same surface temperature increase is forced by changing the albedo instead, the material and transfer entropy production rates increase less dramatically and the planetary rate also registers an increase. This is pertinent to solar radiation management as it demonstrates the difficulty of reversing greenhouse gas-mediated climate changes by albedo alterations. It is argued that the transfer perspective has particular significance in the climate system and warrants increased prominence.
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Journal articleAkhavanTafti M, Palmroth M, Slavin JA, et al., 2020, , Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-22, ISSN: 2169-9380
The Vlasiator hybrid鈥怴lasov code was developed to investigate global magnetospheric dynamics at ion鈥恔inetic scales. Here, we focus on the role of magnetic reconnection in the formation and evolution of the magnetic islands at the low鈥恖atitude magnetopause, under southward interplanetary magnetic field (IMF) conditions. The simulation results indicate that: 1) the magnetic reconnection ion kinetics, including the Earthward鈥恜ointing Larmor electric field on the magnetospheric鈥恠ide of an X鈥恜oint and anisotropic ion distributions, are well鈥恈aptured by Vlasiator, thus enabling the study of reconnection鈥恉riven magnetic island evolution processes, 2) magnetic islands evolve due to continuous reconnection at adjacent X鈥恜oints, ‘coalescence’ which refers to the merging of neighboring islands to create a larger island, ‘erosion’ during which an island loses magnetic flux due to reconnection, and ‘division’ which involves the splitting of an island into smaller islands, and 3) continuous reconnection at adjacent X鈥恜oints is the dominant source of magnetic flux and plasma to the outer layers of magnetic islands resulting in cross鈥恠ectional growth rates up to +0.3 RE2/min. The simulation results are compared to the Magnetospheric Multiscale (MMS) measurements of a chain of ion鈥恠cale flux transfer events (FTEs) sandwiched between two dominant X鈥恖ines. The MMS measurements similarly reveal: 1) anisotropic ion populations, and 2) normalized reconnection rate ~0.18, in agreement with theory and the Vlasiator predictions. Based on the simulation results and the MMS measurements, it is estimated that the observed ion鈥恠cale FTEs may grow Earth鈥恠ized within ~10 minutes, which is comparable to the average transport time for FTEs formed in the subsolar region to the high鈥恖atitude magnetopause. Future simulations shall revisit reconnection鈥恉riven island evolution processes with improved spatial resolutions.
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Journal articleEggington JWB, Eastwood JP, Mejnertsen L, et al., 2020, , Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-17, ISSN: 2169-9380
The Earth’s dipole tilt angle changes both diurnally and seasonally and introduces numerous variabilities in the coupled magnetosphere鈥恑onosphere system. By altering the location and intensity of magnetic reconnection, the dipole tilt influences convection on a global scale. However, due to the nonlinear nature of the system, various other effects like dipole rotation, varying IMF orientation and non鈥恥niform ionospheric conductance can smear tilt effects arising purely from changes in coupling with the solar wind. To elucidate the underlying tilt angle鈥恉ependence, we perform MHD simulations of the steady鈥恠tate magnetosphere鈥恑onosphere system under purely southward IMF conditions for tilt angles from 0°鈥90°. We identify the location of the magnetic separator in each case, and find that an increasing tilt angle shifts the 3鈥怐 X鈥恖ine southward on the magnetopause due to changes in magnetic shear angle. The separator is highly unsteady above 50° tilt angle, characteristic of regular FTE generation on the magnetopause. The reconnection rate drops as the tilt angle becomes large, but remains continuous across the dayside such that the magnetosphere is open even for 90°. These trends map down to the ionosphere, with the polar cap contracting as the tilt angle increases, and region鈥怚 field鈥恆ligned current (FAC) migrating to higher latitudes with changing morphology. The tilt introduces a north鈥恠outh asymmetry in magnetospheric convection, thus driving more FAC in the northern (sunward鈥恌acing) hemisphere for large tilt angles than in the south independent of conductance. These results highlight the strong sensitivity to onset time in the potential impact of a severe space weather event.
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