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  • Journal article
    Ceppi P, Zappa G, Shepherd TG, Gregory JMet al., 2018,

    Fast and slow components of the extratropical atmospheric circulation response to CO2 forcing

    , Journal of Climate, Vol: 31, Pages: 1091-1105, ISSN: 0894-8755

    Poleward shifts of the extratropical atmospheric circulation are a common response to CO2 forcing in global climate models (GCMs), but little is known about the time dependence of this response. Here it is shown that in coupled climate models, the long-term evolution of sea surface temperatures (SSTs) induces two distinct time scales of circulation response to steplike CO2 forcing. In most GCMs from phase 5 of the Coupled Model Intercomparison Project as well as in the multimodel mean, all of the poleward shift of the midlatitude jets and Hadley cell edge occurs in a fast response within 5–10 years of the forcing, during which less than half of the expected equilibrium warming is realized. Compared with this fast response, the slow response over subsequent decades to centuries features stronger polar amplification (especially in the Antarctic), enhanced warming in the Southern Ocean, an El Niño–like pattern of tropical Pacific warming, and weaker land–sea contrast. Atmosphere-only GCM experiments demonstrate that the SST evolution drives the difference between the fast and slow circulation responses, although the direct radiative effect of CO2 also contributes to the fast response. It is further shown that the fast and slow responses determine the long-term evolution of the circulation response to warming in the representative concentration pathway 4.5 (RCP4.5) scenario. The results imply that shifts in midlatitude circulation generally scale with the radiative forcing, rather than with global-mean temperature change. A corollary is that time slices taken from a transient simulation at a given level of warming will considerably overestimate the extratropical circulation response in a stabilized climate.
  • Journal article
    Nakamura R, Varsani A, Genestreti KJ, Le Contel O, Nakamura T, Baumjohann W, Nagai T, Artemyev A, Birn J, Sergeev VA, Apatenkov S, Ergun RE, Fuselier SA, Gershman DJ, Giles BJ, Khotyaintsev YV, Lindqvist P-A, Magnes W, Mauk B, Petrukovich A, Russell CT, Stawarz J, Strangeway RJ, Anderson B, Burch JL, Bromund KR, Cohen I, Fischer D, Jaynes A, Kepko L, Le G, Plaschke F, Reeves G, Singer HJ, Slavin JA, Torbert RB, Turner DLet al., 2018,

    Multiscale Currents Observed by MMS in the Flow Braking Region

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 1260-1278, ISSN: 2169-9380
  • Journal article
    Tang T, Shindell D, Samset BH, Boucher O, Forster PM, Hodnebrog Ø, Myhre G, Sillmann J, Voulgarakis A, Andrews T, Faluvegi G, Fläschner D, Iversen T, Kasoar M, Kharin V, Kirkevåg A, Lamarque J-F, Olivié D, Richardson T, Stjern CW, Takemura Tet al., 2018,

    Mediterranean Precipitation Response to Greenhouse Gases andAerosols

    <jats:p>Abstract. Atmospheric aerosols and greenhouse gases affect cloud properties, radiative balance and thus, the hydrological cycle. Observations show that precipitation has decreased in the Mediterranean since the 20th century, and many studies have investigated possible mechanisms. So far, however, the effects of aerosol forcing on Mediterranean precipitation remain largely unknown. Here we compare Mediterranean precipitation responses to individual forcing agents in a set of state-of-the-art global climate models (GCMs). Our analyses show that both greenhouse gases and aerosols can cause drying in the Mediterranean, and that precipitation is more sensitive to black carbon (BC) forcing than to well-mixed greenhouse gases (WMGHGs) or sulfate aerosol. In addition to local heating, BC appears to reduce precipitation by causing an enhanced positive North Atlantic Oscillation (NAO)/Arctic Oscillation (AO)-like sea level pressure (SLP) pattern, characterized by higher SLP at mid-latitudes and lower SLP at high-latitudes. WMGHGs cause a similar SLP change, and both are associated with a northward diversion of the jet stream and storm tracks, reducing precipitation in the Mediterranean while increasing precipitation in Northern Europe. Though the applied forcings were much larger, if forcings are scaled to those of the historical period of 1901–2010, roughly one-third (31 ± 17 %) of the precipitation decrease would be attributable to global BC forcing with the remainder largely attributable to WMGHGs whereas global scattering sulfate aerosols have negligible impacts. The results from this study suggest that future BC emissions may significantly affect regional water resources, agricultural practices, ecosystems, and the economy in the Mediterranean region. </jats:p>
  • Journal article
    Franci L, Landi S, Verdini A, Matteini L, Hellinger Pet al., 2018,

    Solar Wind Turbulent Cascade from MHD to Sub-ion Scales: Large-size 3D Hybrid Particle-in-cell Simulations

    , ASTROPHYSICAL JOURNAL, Vol: 853, ISSN: 0004-637X
  • Journal article
    Yang L, Wang L, Li G, Wimmer-Schweingruber RF, He J, Tu C, Tian H, Bale SDet al., 2018,

    The Strongest Acceleration of &gt;40 keV Electrons by ICME-driven Shocks at 1 au

    , ASTROPHYSICAL JOURNAL, Vol: 853, ISSN: 0004-637X
  • Journal article
    Broll JM, Fuselier SA, Trattner KJ, Schwartz SJ, Burch JL, Giles BL, Anderson BJet al., 2018,

    MMS Observation of Shock-Reflected He<SUP>++</SUP> at Earth's Quasi-Perpendicular Bow Shock

    , GEOPHYSICAL RESEARCH LETTERS, Vol: 45, Pages: 49-55, ISSN: 0094-8276
  • Journal article
    Bruneau N, Toumi R, Wang S, 2018,

    Impact of wave whitecapping on land falling tropical cyclones

    , Scientific Reports, Vol: 8, ISSN: 2045-2322

    Predicting tropical cyclone structure and evolution remains challenging. Particularly, the surface wave interactions with thecontinental shelf and their impact on tropical cyclones have received very little attention. Through a series of state-of-the-arthigh-resolution, fully-coupled ocean-wave and atmosphere-ocean-wave experiments, we show here, for the first time, thatin presence of continental shelf waves can cause substantial cooling of the sea surface. Through whitecapping there is atransfer of momentum from the surface which drives deeper vertical mixing. It is the waves and not just the wind which becomethe major driver of stratified coastal ocean ahead-of-cyclone cooling. In the fully-coupled atmosphere-ocean-wave model anegative feedback is found. The maximum wind speed is weaker and the damaging footprint area of hurricane-force winds isreduced by up to 50% due to the strong wave induced ocean cooling ahead. Including wave-ocean coupling is important toimprove land falling tropical cyclone intensity predictions for the highly populated and vulnerable coasts.
  • Conference paper
    Zabori B, Hirn A, Eastwood J, Brown P, Palla C, Oddy T, Nolbert D, Santin G, Nieminen P, Marosy Get al., 2018,

    Space radiation and magnetic field environment specification for the Radcube space weather related CubeSat mission

    , ISSN: 0074-1795

    To study space weather environment in space, as a first step, it is necessary to develop and establish an advanced, real-time monitoring system. Such a monitoring system may be able to provide scientific data on space radiation (electron and proton spectra, flux of heavier ions) and the status of the magnetosphere in order to gain the possibility for a reliable forecast capability. The expansion of the CubeSat/SmallSat industry will make it possible in the near future to launch orbital constellations with relevant, miniaturised instrumentation in order to study the space weather environment in near real-time. Thus the development of RADCUBE, a 3U CubeSat demonstration mission lead by a Hungarian company, called C3S LLC, for space weather monitoring purpose, has begun within the European Space Agency (ESA) CubeSat programme. As part of the development a new, combined, space weather monitoring instrument package (called RadMag) has been initiated at the Centre for Energy Research, Hungarian Academy of Sciences in the framework of ESA General Support Technology Programme (GSTP) in collaboration with Imperial College London and Astronika. The RadMag measurement capabilities were defined by reconstructing the expected space radiation and magnetic field environments for different orbit scenarios. The space radiation environment was analyzed considering the following parameters: flux of Galactic Cosmic Rays, trapped protons and electrons, solar particle events, corresponding Linear Energy Transfer (LET) spectra and Total Ionizing Dose (TID) levels. The expected magnetic field environment was modeled with the IGRF2015 + Tsyganenko-96 model both for quiet and stormy conditions. This paper addresses the results of these radiation and magnetic field environment reconstruction and calculations for the different possible orbital parameters of the RADCUBE mission in order to characterise the expected performance of the RadMag instrument during the RADCUBE mission. An overview of
  • Conference paper
    Murray JE, Pickering JC, Brindley H, Fox S, Ade P, Tucker C, Fox C, Harlow C, Last Aet al., 2018,

    Spectrally resolved radiative observations of the Earth in the Far-Infrared using the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS)

    The Far-Infrared contributes up to 50% of the radiative emission from Earth to space, however, FIR observations are lacking. Satellite instruments are proposed or set for deployment, we discuss the practicalities of supporting these missions with TAFTS.
  • Conference paper
    Murray JE, Brindley HE, Fox S, Wellpott A, Pickering JC, Fox C, Harlow C, Last Aet al., 2018,

    Far-Infrared emissivity of ice and snow: Resolving the paucity of observational data

    Far-Infrared up-welling radiance measurements over Greenland have yielded the first estimates of the emissivity of ice and snow in this spectral region. I describe the complexity of undertaking such work and what the future holds.
  • Journal article
    Samset BH, Myhre G, Forster PM, Hodnebrog O, Andrews T, Boucher O, Faluvegi G, Flaeschner D, Kasoar M, Kharin V, Kirkevag A, Lamarque J-F, Olivie D, Richardson TB, Shindell D, Takemura T, Voulgarakis Aet al., 2018,

    Weak hydrological sensitivity to temperature change over land, independent of climate forcing

    , npj Climate and Atmospheric Science, Vol: 1, ISSN: 2397-3722

    We present the global and regional hydrological sensitivity (HS) to surface temperature changes, for perturbations to CO2, CH4, sulfate and black carbon concentrations, and solar irradiance. Based on results from ten climate models, we show how modeled global mean precipitation increases by 2–3% per kelvin of global mean surface warming, independent of driver, when the effects of rapid adjustments are removed. Previously reported differences in response between drivers are therefore mainly ascribable to rapid atmospheric adjustment processes. All models show a sharp contrast in behavior over land and over ocean, with a strong surface temperature-driven (slow) ocean HS of 3–5%/K, while the slow land HS is only 0–2%/K. Separating the response into convective and large-scale cloud processes, we find larger inter-model differences, in particular over land regions. Large-scale precipitation changes are most relevant at high latitudes, while the equatorial HS is dominated by convective precipitation changes. Black carbon stands out as the driver with the largest inter-model slow HS variability, and also the strongest contrast between a weak land and strong sea response. We identify a particular need for model investigations and observational constraints on convective precipitation in the Arctic, and large-scale precipitation around the Equator.
  • Journal article
    Taylor SA, Coates AJ, Jones GH, Wellbrock A, Fazakerley AN, Desai RT, Caro-Carretero R, Michiko MW, Schippers P, Waite JHet al., 2018,

    Modeling, Analysis, and Interpretation of Photoelectron Energy Spectra at Enceladus Observed by Cassini

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 287-296, ISSN: 2169-9380
  • Conference paper
    Krupp N, Roussos E, Mitchell DG, Kollmann P, Dougherty MKet al., 2018,

    Charged particle measurements in the Saturnian magnetosphere during the Cassini era 2004-2017

    , Symposium Celebrating Prof. Wing-Huen Ip's 70th Birthday - Serendipities in the Solar System and Beyond, Publisher: ASTRONOMICAL SOC PACIFIC, Pages: 163-175, ISSN: 1050-3390
  • Conference paper
    Sergis N, Bunce EJ, Carbary JF, Cowley SWH, Jia X, Hamilton DC, Krimigis SM, Mitchell DG, Dougherty MKet 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
  • Conference paper
    Franci L, Hellinger P, Guarrasi M, Chen CHK, Papini E, Verdini A, Matteini L, Landi Set 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
  • Journal article
    Mejnertsen L, Eastwood J, Hietala H, Chittenden Jet 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: 2169-9380

    Empirical 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.
  • Journal article
    Davies E, Masters A, Dougherty M, Hansen K, Coates A, Hunt Get 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: 2169-9380

    Swept 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.
  • Journal article
    Mackie 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: 1680-7316

    We 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
  • Journal article
    Ceppi 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: 0027-8424

    Climate 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.
  • Journal article
    Chadney JM, Koskinen TT, Galand M, Unruh YC, Sanz-Forcada Jet al., 2017,

    Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b

    , Astronomy and Astrophysics, Vol: 608, ISSN: 0004-6361

    Stellar 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.
  • Journal article
    Graven H, Allison CE, Etheridge DM, Hammer S, Keeling RF, Levin I, Meijer HAJ, Rubino M, Tans PP, Trudinger CM, Vaughn BH, White JWCet al., 2017,

    Compiled records of carbon isotopes in atmospheric CO2 for historical simulations in CMIP6

    , Geoscientific Model Development, Vol: 10, Pages: 4405-4417, ISSN: 1991-959X

    The 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.
  • Journal article
    Tan X, Bao M, Hartmann DL, Ceppi Pet 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: 0894-8755

    Previous 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.
  • Journal article
    Rhodin AP, Belmonte MT, Engstrom L, Lundberg H, Nilsson H, Hartman H, Pickering JC, Clear C, Quinet P, Fivet V, Palmeri Pet 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: 0035-8711

    The 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.
  • Journal article
    Hewitt HT, Bell MJ, Chassignet EP, Czaja A, Ferreira D, Griffies SM, Hyder P, McClean JL, New AL, Roberts MJet 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 article
    Palmroth M, Hoilijoki S, Juusola L, Pulkkinen TI, Hietala H, Pfau-Kempf Y, Ganse U, von Alfthan S, Vainio R, Hesse Met al., 2017,

    Tail reconnection in the global magnetospheric context: Vlasiator first results

    , Annales Geophysicae, Vol: 35, Pages: 1269-1274, ISSN: 0992-7689

    The 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.
  • Journal article
    Bellisario C, Brindley H, Murray J, Last A, Pickering J, Chawn Harlow R, Fox S, Fox C, Newman S, Smith M, Anderson D, Huang X, Chen Xet 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: 0148-0227

    The 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.
  • Journal article
    Yao ZH, Radioti A, Rae IJ, Liu J, Grodent D, Ray LC, Badman SV, Coates AJ, Gerard J-C, Waite JH, Yates JN, Shi QQ, Wei Y, Bonfond B, Dougherty MK, Roussos E, Sergis N, Palmaerts Bet 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: 0094-8276

    Although 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).
  • Journal article
    Franci L, Cerri SS, Califano F, Landi S, Papini E, Verdini A, Matteini L, Jenko F, Hellinger Pet al., 2017,

    Magnetic Reconnection as a Driver for a Sub-ion-scale Cascade in Plasma Turbulence

    , ASTROPHYSICAL JOURNAL LETTERS, Vol: 850, ISSN: 2041-8205

    A 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.
  • Journal article
    Chen P, Wang T, Dong M, Kasoar M, Han Y, Xie M, Li S, Zhuang B, Li M, Huang Tet 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
  • Journal article
    Shi J, Zhang Z, Torkar K, Cheng Z, Farzakeley A, Dunlop M, Carr Cet 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: 2169-9380

    Field-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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