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Conference paperSergis N, Bunce EJ, Carbary JF, et al., 2018,
The Ring Current of Saturn
, AGU Chapman Conference on Currents in Geospace and Beyond, Publisher: AMER GEOPHYSICAL UNION, Pages: 139-154, ISSN: 0065-8448- Author Web Link
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- Citations: 5
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Conference paperFranci L, Hellinger P, Guarrasi M, et al., 2018,
Three-dimensional simulations of solar wind turbulence with the hybrid code CAMELIA
, 12th International Conference on Numerical Modeling of Space Plasma Flows (ASTRONUM), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588- Author Web Link
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- Citations: 23
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Journal articleDavies E, Masters A, Dougherty M, et al., 2017,
Swept Forward Magnetic Field Variability in High-Latitude Regions of Saturn's Magnetosphere
, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 12328-12337, ISSN: 2169-9380Swept forward field is the term given to configurations of magnetic field wherein the field lines deviate from the meridional planes of a planet in the direction of its rotation. Evidence is presented for swept-forward field configurations on Cassini orbits around Saturn from the first half of 2008. These orbits were selected on the basis of high inclination, spatial proximity, and temporal proximity, allowing for the observation of swept-forward field and resolution of dynamic effects using data from the Cassini magnetometer. Nine orbits are surveyed; all show evidence of swept-forward field, with typical sweep angle found to be 23°. Evidence is found for transient events that lead to temporary dramatic increases in sweep-forward angle. The Michigan Solar Wind Model is employed to investigate temporal correlation between the arrivals of solar wind shocks at Saturn with these transient events, with two shown to include instances corresponding with solar wind shock arrivals. Measurements of equatorial electron number density from anode 5 of the Cassini Plasma Spectrometer instrument are investigated for evidence of magnetospheric compression, corresponding with predicted shock arrivals. Potential mechanisms for the transfer of momentum from the solar wind to the magnetosphere are discussed.
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Journal articleMejnertsen L, Eastwood J, Hietala H, et al., 2017,
Global MHD simulations of the Earth's bow shock shape and motion under variable solar wind conditions
, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 259-271, ISSN: 2169-9380Empirical models of the Earth's bow shock are often used to place in situ measurements in context and to understand the global behavior of the foreshock/bow shock system. They are derived statistically from spacecraft bow shock crossings and typically treat the shock surface as a conic section parameterized according to a uniform solar wind ram pressure, although more complex models exist. Here a global magnetohydrodynamic simulation is used to analyze the variability of the Earth's bow shock under real solar wind conditions. The shape and location of the bow shock is found as a function of time, and this is used to calculate the shock velocity over the shock surface. The results are compared to existing empirical models. Good agreement is found in the variability of the subsolar shock location. However, empirical models fail to reproduce the two-dimensional shape of the shock in the simulation. This is because significant solar wind variability occurs on timescales less than the transit time of a single solar wind phase front over the curved shock surface. Empirical models must therefore be used with care when interpreting spacecraft data, especially when observations are made far from the Sun-Earth line. Further analysis reveals a bias to higher shock speeds when measured by virtual spacecraft. This is attributed to the fact that the spacecraft only observes the shock when it is in motion. This must be accounted for when studying bow shock motion and variability with spacecraft data.
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Journal articleMackie A, Palmer PI, Brindley H, 2017,
Characterizing energy budget variability at a Sahelian site: a test of NWP model behaviour
, Atmospheric Chemistry and Physics, Vol: 17, Pages: 15095-15119, ISSN: 1680-7316We use observations of surface and top-of-theatmosphere(TOA) broadband radiation fluxes determinedfrom the Atmospheric Radiation Measurement programmemobile facility, the Geostationary Earth Radiation Budget(GERB) and Spinning Enhanced Visible and Infrared Imager(SEVIRI) instruments and a range of meteorologicalvariables at a site in the Sahel to test the ability of theECMWF Integrated Forecasting System cycle 43r1 to describeenergy budget variability. The model has daily averagebiases of −12 and 18 W m−2for outgoing longwaveand reflected shortwave TOA radiation fluxes, respectively.At the surface, the daily average bias is 12(13) W m−2for the longwave downwelling (upwelling) radiation fluxand −21(−13) W m−2for the shortwave downwelling (upwelling)radiation flux. Using multivariate linear models ofobservation–model differences, we attribute radiation fluxdiscrepancies to physical processes, and link surface andTOA fluxes. We find that model biases in surface radiationfluxes are mainly due to a low bias in ice water path (IWP),poor description of surface albedo and model–observationdifferences in surface temperature. We also attribute observeddiscrepancies in the radiation fluxes, particularly duringthe dry season, to the misrepresentation of aerosol fieldsin the model from use of a climatology instead of a dynamicapproach. At the TOA, the low IWP impacts the amount ofreflected shortwave radiation while biases in outgoing longwaveradiation are additionally coupled to discrepancies inthe surface upwelling longwave flux and atmospheric humidity
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Journal articleCeppi P, Gregory JM, 2017,
Relationship of tropospheric stability to climate sensitivity and Earth's observed radiation budget
, Proceedings of the National Academy of Sciences of the United States of America, Vol: 114, Pages: 13126-13131, ISSN: 0027-8424Climate feedbacks generally become smaller in magnitude over time under CO2 forcing in coupled climate models, leading to an increase in the effective climate sensitivity, the estimated global-mean surface warming in steady state for doubled CO2. Here, we show that the evolution of climate feedbacks in models is consistent with the effect of a change in tropospheric stability, as has recently been hypothesized, and the latter is itself driven by the evolution of the pattern of sea-surface temperature response. The change in climate feedback is mainly associated with a decrease in marine tropical low cloud (a more positive shortwave cloud feedback) and with a less negative lapse-rate feedback, as expected from a decrease in stability. Smaller changes in surface albedo and humidity feedbacks also contribute to the overall change in feedback, but are unexplained by stability. The spatial pattern of feedback changes closely matches the pattern of stability changes, with the largest increase in feedback occurring in the tropical East Pacific. Relationships qualitatively similar to those in the models among sea-surface temperature pattern, stability, and radiative budget are also found in observations on interannual time scales. Our results suggest that constraining the future evolution of sea-surface temperature patterns and tropospheric stability will be necessary for constraining climate sensitivity.
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Journal articleChadney JM, Koskinen TT, Galand M, et al., 2017,
Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b
, Astronomy and Astrophysics, Vol: 608, ISSN: 0004-6361Stellar flares are a frequent occurrence on young low-mass stars around whichmany detected exoplanets orbit. Flares are energetic, impulsive events, andtheir impact on exoplanetary atmospheres needs to be taken into account wheninterpreting transit observations. We have developed a model to describe theupper atmosphere of Extrasolar Giant Planets (EGPs) orbiting flaring stars. Themodel simulates thermal escape from the upper atmospheres of close-in EGPs.Ionisation by solar radiation and electron impact is included and photochemicaland diffusive transport processes are simulated. This model is used to studythe effect of stellar flares from the solar-like G star HD209458 and the youngK star HD189733 on their respective planets. A hypothetical HD209458b-likeplanet orbiting the active M star AU Mic is also simulated. We find that theneutral upper atmosphere of EGPs is not significantly affected by typicalflares. Therefore, stellar flares alone would not cause large enough changes inplanetary mass loss to explain the variations in HD189733b transit depth seenin previous studies, although we show that it may be possible that an extremestellar proton event could result in the required mass loss. Our simulations dohowever reveal an enhancement in electron number density in the ionosphere ofthese planets, the peak of which is located in the layer where stellar X-raysare absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flarelevels and enhanced electron densities last from about 3 to 10 hours after theonset of the flare. The strength of the flare and the width of its spectralenergy distribution affect the range of altitudes that see enhancements inionisation. A large broadband continuum component in the XUV portion of theflaring spectrum in very young flare stars, such as AU Mic, results in a broadrange of altitudes affected in planets orbiting this star.
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Journal articleGraven H, Allison CE, Etheridge DM, et al., 2017,
Compiled records of carbon isotopes in atmospheric CO2 for historical simulations in CMIP6
, Geoscientific Model Development, Vol: 10, Pages: 4405-4417, ISSN: 1991-959XThe isotopic composition of carbon (Δ14C and δ13C) in atmospheric CO2 and in oceanic and terrestrial carbon reservoirs is influenced by anthropogenic emissions and by natural carbon exchanges, which can respond to and drive changes in climate. Simulations of 14C and 13C in the ocean and terrestrial components of Earth system models (ESMs) present opportunities for model evaluation and for investigation of carbon cycling, including anthropogenic CO2 emissions and uptake. The use of carbon isotopes in novel evaluation of the ESMs' component ocean and terrestrial biosphere models and in new analyses of historical changes may improve predictions of future changes in the carbon cycle and climate system. We compile existing data to produce records of Δ14C and δ13C in atmospheric CO2 for the historical period 1850–2015. The primary motivation for this compilation is to provide the atmospheric boundary condition for historical simulations in the Coupled Model Intercomparison Project 6 (CMIP6) for models simulating carbon isotopes in the ocean or terrestrial biosphere. The data may also be useful for other carbon cycle modelling activities.
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Journal articleTan X, Bao M, Hartmann DL, et al., 2017,
The role of synoptic waves in the formation and maintenance of the Western Hemisphere circulation pattern
, Journal of Climate, Vol: 30, Pages: 10259-10274, ISSN: 0894-8755Previous studies have demonstrated that the NAO, the leading mode of atmospheric low-frequency variability over the North Atlantic, could be linked to northeast Pacific climate variability via the downstream propagation of synoptic waves. In those studies, the NAO and the northeast Pacific climate variability are considered as two separate modes that explain the variance over the North Atlantic sector and the east Pacific–North American sector, respectively. A newly identified low-frequency atmospheric regime—the Western Hemisphere (WH) circulation pattern—provides a unique example of a mode of variability that accounts for variance over the whole North Atlantic–North American–North Pacific sector. The role of synoptic waves in the formation and maintenance of the WH pattern is investigated using the ECMWF reanalysis datasets. Persistent WH events are characterized by the propagation of quasi-stationary Rossby waves across the North Pacific–North American–North Atlantic regions and by associated storm-track anomalies. The eddy-induced low-frequency height anomalies maintain the anomalous low-frequency ridge over the Gulf of Alaska, which induces more equatorward propagation of synoptic waves on its downstream side. The eddy forcing favors the strengthening of the midlatitude jet and the deepening of the mid-to-high-latitude trough over the North Atlantic, whereas the deepening of the trough over eastern North America mostly arises from the quasi-stationary waves propagating from the North Pacific. A case study for the 2013/14 winter is examined to illustrate the downstream development of synoptic waves. The roles of synoptic waves in the formation and maintenance of the WH pattern and in linking the northeast Pacific ridge anomaly with the NAO are discussed.
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Journal articleHewitt HT, Bell MJ, Chassignet EP, et al., 2017,
Will high-resolution global ocean models benefit coupled predictions on short-range to climate timescales?
, OCEAN MODELLING, Vol: 120, Pages: 120-136, ISSN: 1463-5003 -
Journal articleRhodin AP, Belmonte MT, Engstrom L, et al., 2017,
Lifetime measurements and oscillator strengths in singly ionized scandium and the solar abundance of scandium
, Monthly Notices of the Royal Astronomical Society, Vol: 472, Pages: 3337-3353, ISSN: 0035-8711The lifetimes of 17 even-parity levels (3d5s, 3d4d, 3d6s and 4p2) in the region57 743–77 837 cm−1 of singly ionized scandium (Sc II) were measured by two-step timeresolvedlaser induced fluorescence spectroscopy. Oscillator strengths of 57 lines from thesehighly excited upper levels were derived using a hollow cathode discharge lamp and a Fouriertransform spectrometer. In addition, Hartree–Fock calculations where both the main relativisticand core-polarization effects were taken into account were carried out for both low- andhigh-excitation levels. There is a good agreement for most of the lines between our calculatedbranching fractions and the measurements of Lawler & Dakin in the region 9000–45 000 cm−1for low excitation levels and with our measurements for high excitation levels in the region23 500–63 100 cm−1. This, in turn, allowed us to combine the calculated branching fractionswith the available experimental lifetimes to determine semi-empirical oscillator strengths fora set of 380 E1 transitions in Sc II. These oscillator strengths include the weak lines that wereused previously to derive the solar abundance of scandium. The solar abundance of scandiumis now estimated to log = 3.04 ± 0.13 using these semi-empirical oscillator strengths toshift the values determined by Scott et al. The new estimated abundance value is in agreementwith the meteoritic value (logmet = 3.05 ± 0.02) of Lodders, Palme & Gail.
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Journal articleLee D, Min S-K, Jin J, et al., 2017,
Thermodynamic and dynamic contributions to future changes in summer precipitation over Northeast Asia and Korea: a multi-RCM study
, CLIMATE DYNAMICS, Vol: 49, Pages: 4121-4139, ISSN: 0930-7575 -
Journal articlePalmroth M, Hoilijoki S, Juusola L, et al., 2017,
Tail reconnection in the global magnetospheric context: Vlasiator first results
, Annales Geophysicae, Vol: 35, Pages: 1269-1274, ISSN: 0992-7689The key dynamics of the magnetotail have beenresearched for decades and have been associated with eitherthree-dimensional (3-D) plasma instabilities and/or magneticreconnection. We apply a global hybrid-Vlasov code, Vlasiator, to simulate reconnection self-consistently in the ion kinetic scales in the noon–midnight meridional plane, including both dayside and nightside reconnection regions withinthe same simulation box. Our simulation represents a numerical experiment, which turns off the 3-D instabilities butmodels ion-scale reconnection physically accurately in 2-D.We demonstrate that many known tail dynamics are presentin the simulation without a full description of 3-D instabilities or without the detailed description of the electrons. Whilemultiple reconnection sites can coexist in the plasma sheet,one reconnection point can start a global reconfiguration process, in which magnetic field lines become detached and aplasmoid is released. As the simulation run features temporally steady solar wind input, this global reconfiguration isnot associated with sudden changes in the solar wind. Further, we show that lobe density variations originating fromdayside reconnection may play an important role in stabilising tail reconnection.
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Journal articleBellisario C, Brindley H, Murray J, et al., 2017,
Retrievals of the Far Infrared surface emissivity over the Greenland Plateau using the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS).
, Journal of Geophysical Research, Vol: 122, Pages: 12152-12166, ISSN: 0148-0227The Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) measured near surface upwelling and downwelling radiances within the far infrared (FIR) over Greenland during two flights in March 2015. Here we exploit observations from one of these flights to provide in-situ estimates of FIR surface emissivity, encompassing the range 80-535 cm-1. The flight campaign and instrumental set-up is described as well as the retrieval method, including the quality control performed on the observations. The combination of measurement and atmospheric profile uncertainties means that the retrieved surface emissivity has the smallest estimated error over the range 360-535 cm-1, (18.7-27.8 μm), lying between 0.89 and 1 with an associated error which is of the order ± 0.06. Between 80 and 360 cm-1, the increasing opacity of the atmosphere, coupled with the uncertainty in the atmospheric state, means that the associated errors are larger and the emissivity values cannot be said to be distinct from 1. These FIR surface emissivity values are, to the best of our knowledge, the first ever from aircraft-based measurements. We have compared them to a recently developed theoretical database designed to predict the infrared surface emissivity of frozen surfaces. When considering the FIR alone, we are able to match the retrievals within uncertainties. However, when we include contemporaneous retrievals from the mid infrared (MIR), no single theoretical representation is able to capture the FIR and MIR behaviour simultaneously. Our results point towards the need for model improvement and further testing, ideally including in-situ characterisation of the underlying surface conditions.
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Journal articleYao ZH, Radioti A, Rae IJ, et al., 2017,
Mechanisms of Saturn's Near-Noon Transient Aurora: In Situ Evidence From Cassini Measurements
, GEOPHYSICAL RESEARCH LETTERS, Vol: 44, Pages: 11217-11228, ISSN: 0094-8276Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in the magnetosphere while remote sensing of the aurora. We report a dynamic auroral event identified with the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Saturn on 13 July 2008 with coordinated measurements of the magnetic field and plasma in the magnetosphere. The auroral intensification was transient, only lasting for ∼30 min. The magnetic field and plasma are perturbed during the auroral intensification period. We suggest that this intensification was caused by wave mode conversion generated field-aligned currents, and we propose two potential mechanisms for the generation of this plasma wave and the transient auroral intensification. A survey of the Cassini UVIS database reveals that this type of transient auroral intensification is very common (10/11 time sequences, and ∼10% of the total images).
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Journal articleFranci L, Cerri SS, Califano F, et al., 2017,
Magnetic Reconnection as a Driver for a Sub-ion-scale Cascade in Plasma Turbulence
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 850, ISSN: 2041-8205A new path for the generation of a sub-ion-scale cascade in collisionless space and astrophysical plasma turbulence, triggered by magnetic reconnection, is uncovered by means of high-resolution two-dimensional hybrid-kinetic simulations employing two complementary approaches, Lagrangian and Eulerian, and different driving mechanisms. The simulation results provide clear numerical evidence that the development of power-law energy spectra below the so-called ion break occurs as soon as the first magnetic reconnection events take place, regardless of the actual state of the turbulent cascade at MHD scales. In both simulations, the reconnection-mediated small-scale energy spectrum of parallel magnetic fluctuations exhibits a very stable spectral slope of $\sim -2.8$, whether or not a large-scale turbulent cascade has already fully developed. Once a quasi-stationary turbulent state is achieved, the spectrum of the total magnetic fluctuations settles toward a spectral index of $-5/3$ in the MHD range and of $\sim -3$ at sub-ion scales.
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Journal articleChen P, Wang T, Dong M, et al., 2017,
Characterization of major natural and anthropogenic source profiles for size-fractionated PM in Yangtze River Delta
, SCIENCE OF THE TOTAL ENVIRONMENT, Vol: 598, Pages: 135-145, ISSN: 0048-9697- Author Web Link
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- Citations: 41
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Journal articleShi J, Zhang Z, Torkar K, et al., 2017,
Distribution of Field-Aligned Electron Events in the High-Altitude Polar Region: Cluster Observations
, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 11245-11255, ISSN: 2169-9380Field-aligned electrons (FAEs) are important for the energy transport in the solar wind-magnetosphere-ionosphere coupling. However, the distribution of FAEs and the concerning physical mechanism in different altitudes of the polar region are still unclear. In this paper, data from the Cluster spacecraft were used to study the characteristics of FAEs in high-altitude polar region. We selected FAE events with a flux higher than 3 × 10 8 (cm 2 s) -1 for our analysis. Their distribution was double peaked around the auroral oval. The main peak occurred around the cusp region (magnetic local time (MLT) 0700-1500) which leaned to the dawnside. The other peak appeared in the evening sector with MLT 2100-2300 just before midnight. The durations of the FAE events covered a wide range from 4 to 475 s, with most of the FAE events lasting less than 40 s. The possible physical mechanisms are discussed, namely, that the downward FAEs may consist of decelerated solar wind and reflected up flowing ionospheric electrons in the potential drops, whereas the upward ones may be mirrored solar wind electrons and accelerated ionospheric up flowing electrons.
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Journal articleMasters A, 2017,
Model-based assessments of magnetic reconnection and Kelvin-Helmholtz instability at Jupiter’s magnetopause
, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 11154-11174, ISSN: 2169-9380The interaction between the solar wind and Jupiter's magnetic field confines the planetary field to the largest magnetosphere in the Solar System. However, the full picture of when and where key processes operate at the magnetopause boundary of the system remains unclear. This is essential for testing understanding with observations and for determining the relative importance of different drivers of Jovian magnetospheric dynamics. Here we present a global analytical model of Jovian magnetopause conditions under steady state, which forms the basis of boundary process assessments. Sites of magnetic reconnection at Jupiter's magnetopause are expected to be in regions of sufficiently high magnetic shear across the boundary, controlled by the orientation of the interplanetary magnetic field. Reconnection rates are also most sensitive to changes in the highly variable IMF, followed by changes in the solar wind plasma mass density. The largest plasma flow shear across the boundary is in the equatorial dawn region, producing a region that is typically unstable to growth of the Kelvin-Helmholtz (K-H) instability. Compared to magnetopause reconnection site locations, this K-H-unstable region at dawn is less sensitive to changing conditions. Motion of K-H boundary perturbations typically includes dawn-to-dusk motion across the subsolar region. Model-predicted reconnection voltages are typically hundreds of kV but rely on steady solar wind conditions on a time scale that is longer than typical at Jupiter's orbit. How the reconnection voltage compares to the voltage applied due to the “viscous-like” interaction involving K-H instability remains unclear.
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Journal articleKajdic P, Hietala H, Blanco-Cano X, 2017,
Different Types of Ion Populations Upstream of the 2013 October 8 Interplanetary Shock
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 849, ISSN: 2041-8205 -
Journal articleKasper JC, Klein KG, Weber T, et al., 2017,
A Zone of Preferential Ion Heating Extends Tens of Solar Radii from the Sun
, ASTROPHYSICAL JOURNAL, Vol: 849, ISSN: 0004-637X- Author Web Link
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- Citations: 36
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Journal articleStjern CW, Samset BH, Myhre G, et al., 2017,
Rapid Adjustments Cause Weak Surface Temperature Response to Increased Black Carbon Concentrations
, Journal of Geophysical Research: Atmospheres, Vol: 122, Pages: 462-481, ISSN: 2169-897XWe investigate the climate response to increased concentrations of black carbon (BC), as part of the Precipitation Driver Response Model Intercomparison Project (PDRMIP). A tenfold increase in BC is simulated by nine global coupled-climate models, producing a model median effective radiative forcing of 0.82 (ranging from 0.41 to 2.91) W m−2, and a warming of 0.67 (0.16 to 1.66) K globally and 1.24 (0.26 to 4.31) K in the Arctic. A strong positive instantaneous radiative forcing (median of 2.10 W m−2 based on five of the models) is countered by negative rapid adjustments (−0.64 W m−2 for the same five models), which dampen the total surface temperature signal. Unlike other drivers of climate change, the response of temperature and cloud profiles to the BC forcing is dominated by rapid adjustments. Low-level cloud amounts increase for all models, while higher-level clouds are diminished. The rapid temperature response is particularly strong above 400 hPa, where increased atmospheric stabilization and reduced cloud cover contrast the response pattern of the other drivers. In conclusion, we find that this substantial increase in BC concentrations does have considerable impacts on important aspects of the climate system. However, some of these effects tend to offset one another, leaving a relatively small median global warming of 0.47 K per W m−2—about 20% lower than the response to a doubling of CO2. Translating the tenfold increase in BC to the present-day impact of anthropogenic BC (given the emissions used in this work) would leave a warming of merely 0.07 K.
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Journal articleOmidi N, Sulaiman AH, Kurth W, et al., 2017,
A Single Deformed Bow Shock for Titan-Saturn System
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 11058-11075, ISSN: 2169-9380During periods of high solar wind pressure, Saturn’s bow shock is pushed inside Titan’s orbitexposing the moon and its ionosphere to the solar wind. The Cassini spacecraft’s T96 encounter with Titanoccurred during such a period and showed evidence for shocks associated with Saturn and Titan. It alsorevealed the presence of two foreshocks: one prior to the closest approach (foreshock 1) and one after(foreshock 2). Using electromagnetic hybrid (kinetic ions and fluid electrons) simulations and Cassiniobservations,we showthat the origin of foreshock 1 is tied to the formation of a single deformed bow shock forthe Titan-Saturn system. We also report the observations of a structure in foreshock 1 with properties consistentwith those of spontaneous hot flow anomalies formed in the simulations and previously observed at Earth,Venus, and Mars. The results of hybrid simulations also show the generation of oblique fast magnetosonicwaves upstream of the outbound Titan bow shock in agreement with the observations of large-amplitudemagnetosonic pulsations in foreshock 2. We also discuss the implications of a single deformed bow shock fornew particle acceleration mechanisms and also Saturn’s magnetopause and magnetosphere.
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Journal articleHajra R, Henri P, Vallières X, et al., 2017,
Impact of a cometary outburst on its ionosphere: Rosetta Plasma Consortium observations of the outburst exhibited by comet 67P/Churyumov-Gerasimenko on 19 February 2016
, Astronomy and Astrophysics, Vol: 607, Pages: 1-10, ISSN: 0004-6361We present a detailed study of the cometary ionospheric response to a cometary brightness outburst using in situ measurements for the first time. The comet 67P/Churyumov-Gerasimenko (67P) at a heliocentric distance of 2.4 AU from the Sun, exhibited an outburst at ∼1000 UT on 19 February 2016, characterized by an increase in the coma surface brightness of two orders of magnitude. The Rosetta spacecraft monitored the plasma environment of 67P from a distance of 30 km, orbiting with a relative speed of ∼0.2 m s-1. The onset of the outburst was preceded by pre-outburst decreases in neutral gas density at Rosetta, in local plasma density, and in negative spacecraft potential at ∼0950 UT. In response to the outburst, the neutral density increased by a factor of ∼1.8 and the local plasma density increased by a factor of ∼3, driving the spacecraft potential more negative. The energetic electrons (tens of eV) exhibited decreases in the flux of factors of ∼2 to 9, depending on the energy of the electrons. The local magnetic field exhibited a slight increase in amplitude (~5 nT) and an abrupt rotation (∼36.4°) in response to the outburst. A weakening of 10-100 mHz magnetic field fluctuations was also noted during the outburst, suggesting alteration of the origin of the wave activity by the outburst. The plasma and magnetic field effects lasted for about 4 h, from ∼1000 UT to 1400 UT. The plasma densities are compared with an ionospheric model. This shows that while photoionization is the main source of electrons, electron-impact ionization and a reduction in the ion outflow velocity need to be accounted for in order to explain the plasma density enhancement near the outburst peak.
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Journal articleGombosi TI, Baker DN, Balogh A, et al., 2017,
Anthropogenic Space Weather
, SPACE SCIENCE REVIEWS, Vol: 212, Pages: 985-1039, ISSN: 0038-6308- Author Web Link
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- Citations: 24
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Journal articleKilpua EKJ, Balogh A, von Steiger R, et al., 2017,
Geoeffective Properties of Solar Transients and Stream Interaction Regions
, SPACE SCIENCE REVIEWS, Vol: 212, Pages: 1271-1314, ISSN: 0038-6308- Author Web Link
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- Citations: 118
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Journal articleKoskinen HEJ, Baker DN, Balogh A, et al., 2017,
Achievements and Challenges in the Science of Space Weather
, SPACE SCIENCE REVIEWS, Vol: 212, Pages: 1137-1157, ISSN: 0038-6308- Author Web Link
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- Citations: 43
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Journal articleBelmonte MT, Pickering JC, Ruffoni MP, et al., 2017,
Fe I Oscillator Strengths for Transitions from High-lying Odd-parity Levels
, Astrophysical Journal, Vol: 848, ISSN: 0004-637XWe report new experimental Fe I oscillator strengths obtained by combining measurements of branching fractionsmeasured with a Fourier Transform spectrometer and time-resolved, laser-induced fluorescence lifetimes. Thisstudy covers the spectral region ranging from 213 to 1033 nm. A total of 120 experimental log( ) gf -values comingfrom 15 odd-parity energy levels are provided, 22 of which have not been reported previously and 63 of whichhave values with lower uncertainty than the existing data. The radiative lifetimes for 60 upper energy levels arepresented, 39 of which have no previous measurements.
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Journal articleShawki D, Field RD, Tippett MK, et al., 2017,
Long-Lead Prediction of the 2015 Fire and Haze Episode in Indonesia
, Geophysical Research Letters, Vol: 44, Pages: 9996-10005, ISSN: 0094-8276We conducted a case study of National Centers for Environmental Prediction Climate Forecast System version 2 seasonal model forecast performance over Indonesia in predicting the dry conditions in 2015 that led to severe fire, in comparison to the non-El Niño dry season conditions of 2016. Forecasts of the Drought Code (DC) component of Indonesia's Fire Danger Rating System were examined across the entire equatorial Asia region and for the primary burning regions within it. Our results show that early warning lead times of high observed DC in September and October 2015 varied considerably for different regions. High DC over Southern Kalimantan and Southern New Guinea were predicted with 180 day lead times, whereas Southern Sumatra had lead times of up to only 60 days, which we attribute to the absence in the forecasts of an eastward decrease in Indian Ocean sea surface temperatures. This case study provides the starting point for longer-term evaluation of seasonal fire danger rating forecasts over Indonesia.
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Journal articleBall WT, Alsing J, Mortlock DJ, et al., 2017,
Reconciling differences in stratospheric ozone composites
, Atmospheric Chemistry and Physics, Vol: 17, Pages: 12269-12302, ISSN: 1680-7316Observations of stratospheric ozone from multipleinstruments now span three decades; combining these intocomposite datasets allows long-term ozone trends to be estimated.Recently, several ozone composites have been published,but trends disagree by latitude and altitude, even betweencomposites built upon the same instrument data. Weconfirm that the main causes of differences in decadal trendestimates lie in (i) steps in the composite time series when theinstrument source data changes and (ii) artificial sub-decadaltrends in the underlying instrument data. These artefacts introducefeatures that can alias with regressors in multiple linearregression (MLR) analysis; both can lead to inaccuratetrend estimates. Here, we aim to remove these artefacts usingBayesian methods to infer the underlying ozone time seriesfrom a set of composites by building a joint-likelihoodfunction using a Gaussian-mixture density to model outliersintroduced by data artefacts, together with a data-driven prioron ozone variability that incorporates knowledge of problemsduring instrument operation. We apply this Bayesianself-calibration approach to stratospheric ozone in 10◦ bandsfrom 60◦ S to 60◦ N and from 46 to 1 hPa (∼ 21–48 km) for1985–2012. There are two main outcomes: (i) we independentlyidentify and confirm many of the data problems previouslyidentified, but which remain unaccounted for in existingcomposites; (ii) we construct an ozone composite, withuncertainties, that is free from most of these problems – wecall this the BAyeSian Integrated and Consolidated (BASIC)composite. To analyse the new BASIC composite, we usedynamical linear modelling (DLM), which provides a morerobust estimate of long-term changes through Bayesian inferencethan MLR. BASIC and DLM, together, provide astep forward in improving estimates of decadal trends. Ourresults indicate a significant recovery of ozone since 1998 inthe upper stratosphere, of both northern and southern midlatitudes,in all f
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