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  • Journal article
    Lin Y, Hwang Y, Ceppi P, Gregory Jet al., 2019,

    Uncertainty in the evolution of climate feedback traced to the strength of the Atlantic Meridional Overturning Circulation

    , Geophysical Research Letters, Vol: 46, Pages: 12331-12339, ISSN: 0094-8276

    In most coupled climate models, effective climate sensitivity increases for a few decades following an abrupt CO2 increase. The change in the climate feedback parameter between the first 20 years and the subsequent 130 years is highly model dependent. In this study, we suggest that the intermodel spread of changes in climate feedback can be partially traced to the evolution of the Atlantic Meridional Overturning Circulation. Models with stronger Atlantic Meridional Overturning Circulation recovery tend to project more amplified warming in the Northern Hemisphere a few decades after a quadrupling of CO2. Tropospheric stability then decreases as the Northern Hemisphere gets warmer, which leads to an increase in both the lapse‐rate and shortwave cloud feedbacks. Our results suggest that constraining future ocean circulation changes will be necessary for accurate climate sensitivity projections.
  • Journal article
    Gryspeerdt E, Smith T, O'Keeffe E, Christensen M, Goldsworth Fet al., 2019,

    The impact of ship emission controls recorded by cloud properties

    , Geophysical Research Letters, Vol: 46, Pages: 12547-12555, ISSN: 0094-8276

    The impact of aerosols on cloud properties is one of the leading uncertainties in the human forcing of the climate. Ships are large, isolated sources of aerosol creating linear cloud formations known as shiptracks. These are an ideal opportunity to identify and measure aerosol-cloud interactions. This work uses over 17,000 shiptracks during the implementation of fuel sulphur content regulations to demonstrate the central role of sulphate aerosol in ship exhaust for modifying clouds. By connecting individual shiptracks to transponder data, it is shown that almost half of shiptracks are likely undetected, masking a significant contribution to the climate impact of shipping. A pathway to retrieving ship sulphate emissions is demonstrated, showing how cloud observations could be used to monitor air pollution.
  • Journal article
    Siddle AG, Mueller-Wodarg ICF, Stone SW, Yelle RVet al., 2019,

    Global characteristics of gravity waves in the upper atmosphere of Mars as measured by MAVEN/NGIMS

    , Icarus, Vol: 333, Pages: 12-21, ISSN: 0019-1035

    We present an analysis of gravity waves in Mars' upper atmosphere above 120 km. Using in-situ data from NGIMS onboard MAVEN we have been able to characterise waves from nearly 4000 orbits. We have used temperature and density profiles to extract perturbations and interpret these as vertically propagating gravity waves which we characterise by their amplitude and wavelength. In this region of the atmosphere gravity waves have amplitudes of around 10%. Vertical wavelengths are found to be around 10–30 km. We observe an increase in gravity wave amplitudes with increasing solar zenith angle. Gravity wave amplitudes appear invariant in altitude on the dayside, however increase with altitude on the nightside.
  • Journal article
    Trenchi L, Coxon JC, Fear RC, Eastwood JP, Dunlop MW, Trattner KJ, Gershman DJ, Graham DB, Khotyaintsev Y, Lavraud Bet al., 2019,

    Signatures of magnetic separatrices at the borders of a crater flux transfer event connected to an active X‐line

    , Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 8600-8616, ISSN: 2169-9380

    In this paper, we present Magnetospheric Multiscale (MMS) observations of a flux transfer event (FTE) characterized by a clear signature in the magnetic field magnitude, which shows maximum at the center flanked by two depressions, detected during a period of stable southward interplanetary magnetic field. This class of FTEs are called “crater‐FTEs” and have been suggested to be connected with active reconnection X line. The MMS burst mode data allow the identification of intense fluctuations in the components of the electric field and electron velocity parallel to the magnetic field at the borders of the FTE, which are interpreted as signatures of the magnetic separatrices. In particular, the strong and persistent fluctuations of the parallel electron velocity at the borders of this crater‐FTE reported for the first time in this paper, sustain the field‐aligned current part of the Hall current system along the separatrix layer, and confirm that this FTE is connected with an active reconnection X line. Our observations suggest a stratification of particles inside the reconnection layer, where electrons are flowing toward the X line along the separatrix, are flowing away from the X line along the reconnected field lines adjacent to the separatrices, and more internally ions and electrons are flowing away from the X line with comparable velocities, forming the reconnection jets. This stratification of the reconnection layer forming the FTE, together with the reconnection jet at the trailing edge of the FTE, suggests clearly that this FTE is formed by the single X line generation mechanism.
  • Journal article
    Ceppi P, Shepherd TG, 2019,

    Contributions of climate feedbacks to changes in atmospheric circulation

    , Journal of Climate, Vol: 30, Pages: 9097-9118, ISSN: 0894-8755

    The projected response of the atmospheric circulation to the radiative changes induced by CO2 forcing and climate feedbacks is currently uncertain. In this modeling study, the impact of CO2-induced climate feedbacks on changes in jet latitude and speed is assessed by imposing surface albedo, cloud, and water vapor feedbacks as if they were forcings in two climate models, CAM4 and ECHAM6. The jet response to radiative feedbacks can be broadly interpreted through changes in midlatitude baroclinicity. Clouds enhance baroclinicity, favoring a strengthened, poleward-shifted jet; this is mitigated by surface albedo changes, which have the opposite effect on baroclinicity and the jet, while water vapor has opposing effects on upper- and lower-level baroclinicity with little net impact on the jet. Large differences between the CAM4 and ECHAM6 responses illustrate how model uncertainty in radiative feedbacks causes a large spread in the baroclinicity response to CO2 forcing. Across the CMIP5 models, differences in shortwave feedbacks by clouds and albedo are a dominant contribution to this spread. Forcing CAM4 with shortwave cloud and albedo feedbacks from a representative set of CMIP5 models yields a wide range of jet responses that strongly correlate with the meridional gradient of the anomalous shortwave heating and the associated baroclinicity response. Differences in shortwave feedbacks statistically explain about 50% of the intermodel spread in CMIP5 jet shifts for the set of models used, demonstrating the importance of constraining radiative feedbacks for accurate projections of circulation changes.
  • Journal article
    Hofstadter M, Simon A, Atreya S, Banfield D, Fortney Jonathan, Hayes A, Hedman M, Hospodarsky G, Mandt K, Masters A, Showalter M, Soderlund K, Turrini D, Turtle E, Reh K, Elliot J, Arora N, Petropoulos A, Ice giant mission study teamet al., 2019,

    Uranus and Neptune missions: a study in advance of the next Planetary Science Decadal Survey

    , Planetary and Space Science, Vol: 177, ISSN: 0032-0633

    The ice giant planets, Uranus and Neptune, represent an important and unexplored class of planets. Mostof our detailed information about themcomes from fleeting looks by the Voyager 2 spacecraftin the 1980s.Voyager,and ground-based work since then, found that these planets, their satellites, rings, and magnetospheres, challenge our understanding of the formation and evolution of planetarysystems. We also now knowthat Uranus-Neptune size planetsare common around other stars. These are some of the reasons ice giant exploration was a high priority in NASA’smost recent Planetary Science Decadal Survey. In preparation for the next Decadal Survey,NASA, with ESA participation,conducted a broad study of possible ice giant missions in the 2024 –2037 timeframe. This paper summarizes the key resultsof the study,and addressesquestionsthat have been raised by the science communityand in a recent NASA review. Foremost amongstthese are questions about the science objectives, the science payload, and the importance of an atmospheric probe. Theconclusions ofthe NASA/ESA study remain valid. In particular, it is a high priority to sendan orbiterand atmospheric probeto at least one of the ice giants, with instrumentationto studyall components of an ice giant system.Uranus and Neptune are found to be equally compelling as science targets. The two planets are not equivalent, however, and each systemhas thingsto teach us the other cannot. An additional mission study is needed to refine plans for future exploration of these worlds.
  • Journal article
    Jackman CM, Thomsen MF, Dougherty MK, 2019,

    Survey of Saturn's Magnetopause and Bow Shock Positions Over the Entire Cassini Mission: Boundary Statistical Properties and Exploration of Associated Upstream Conditions

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 8865-8883, ISSN: 2169-9380
  • Journal article
    Provan G, Cowley SWH, Bradley TJ, Bunce EJ, Hunt GJ, Cao H, Dougherty MKet al., 2019,

    Magnetic Field Observations on Cassini's Proximal Periapsis Passes: Planetary Period Oscillations and Mean Residual Fields

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 8814-8864, ISSN: 2169-9380
  • Journal article
    Hodnebrog O, Myhre G, Samset BH, Alterskjaer K, Andrews T, Boucher O, Faluvegi G, Flaeschner D, Forster PM, Kasoar M, Kirkevag A, Lamarque J-F, Olivie D, Richardson TB, Shawki D, Shindell D, Shine KP, Stier P, Takemura T, Voulgarakis A, Watson-Parris Det al., 2019,

    Water vapour adjustments and responses differ between climate drivers

    , Atmospheric Chemistry and Physics, Vol: 19, Pages: 12887-12899, ISSN: 1680-7316

    Water vapour in the atmosphere is the source of a major climate feedback mechanism and potential increases in the availability of water vapour could have important consequences for mean and extreme precipitation. Future precipitation changes further depend on how the hydrological cycle responds to different drivers of climate change, such as greenhouse gases and aerosols. Currently, neither the total anthropogenic influence on the hydrological cycle nor that from individual drivers is constrained sufficiently to make solid projections. We investigate how integrated water vapour (IWV) responds to different drivers of climate change. Results from 11 global climate models have been used, based on simulations where CO2, methane, solar irradiance, black carbon (BC), and sulfate have been perturbed separately. While the global-mean IWV is usually assumed to increase by ∼7 % per kelvin of surface temperature change, we find that the feedback response of IWV differs somewhat between drivers. Fast responses, which include the initial radiative effect and rapid adjustments to an external forcing, amplify these differences. The resulting net changes in IWV range from 6.4±0.9 % K−1 for sulfate to 9.8±2 % K−1 for BC. We further calculate the relationship between global changes in IWV and precipitation, which can be characterized by quantifying changes in atmospheric water vapour lifetime. Global climate models simulate a substantial increase in the lifetime, from 8.2±0.5 to 9.9±0.7 d between 1986–2005 and 2081–2100 under a high-emission scenario, and we discuss to what extent the water vapour lifetime provides additional information compared to analysis of IWV and precipitation separately. We conclude that water vapour lifetime changes are an important indicator of changes in precipitation patterns and that BC is particularly efficient in prolonging the mean time, and therefore like
  • Journal article
    Starkey MJ, Fuselier SA, Desai M, Burch JL, Gomez RG, Mukherjee J, Russell CT, Lai H, Schwartz SJet al., 2019,

    Acceleration of Interstellar Pickup He<SUP>+</SUP> at Earth's Perpendicular Bow Shock

    , GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 10735-10743, ISSN: 0094-8276
  • Journal article
    Nakamura T, Stawarz JE, Hasegawa H, Narita Y, Franci L, Wilder FD, Nakamura R, Nystrom WDet al., 2019,

    Effects of fluctuating magnetic field on the growth of the Kelvin-Helmholtz instability at the Earth's magnetopause
  • Journal article
    Cerri SS, Groselj D, Franci L, 2019,

    Kinetic Plasma Turbulence: Recent Insights and Open Questions From 3D3V Simulations

    , FRONTIERS IN ASTRONOMY AND SPACE SCIENCES, Vol: 6, ISSN: 2296-987X
  • Journal article
    Guo RL, Yao ZH, Sergis N, Wei Y, Xu XJ, Coates AJ, Delamere PA, Roussos E, Arridge CS, Waite JH, Krupp N, Mitche D, Burch J, Dougherty MK, Wan WXet al., 2019,

    Long-standing Small-scale Reconnection Processes at Saturn Revealed by <i>Cassini</i>

    , ASTROPHYSICAL JOURNAL LETTERS, Vol: 884, ISSN: 2041-8205
  • Journal article
    Gregory JM, Andrews T, Ceppi P, Mauritsen T, Webb MJet al., 2019,

    How accurately can the climate sensitivity to CO₂ be estimated from historical climate change?

    , Climate Dynamics, Vol: 54, Pages: 129-157, ISSN: 0930-7575

    The equilibrium climate sensitivity (ECS, in K) to CO2 doubling is a large source of uncertainty in projections of future anthropogenic climate change. Estimates of ECS made from non-equilibrium states or in response to radiative forcings other than 2×CO2 are called “effective climate sensitivity” (EffCS, in K). Taking a “perfect-model” approach, using coupled atmosphere–ocean general circulation model (AOGCM) experiments, we evaluate the accuracy with which CO2 EffCS can be estimated from climate change in the “historical” period (since about 1860). We find that (1) for statistical reasons, unforced variability makes the estimate of historical EffCS both uncertain and biased; it is overestimated by about 10% if the energy balance is applied to the entire historical period, 20% for 30-year periods, and larger factors for interannual variability, (2) systematic uncertainty in historical radiative forcing translates into an uncertainty of ±30to45% (standard deviation) in historical EffCS, (3) the response to the changing relative importance of the forcing agents, principally CO2 and volcanic aerosol, causes historical EffCS to vary over multidecadal timescales by a factor of two. In recent decades it reached its maximum in the AOGCM historical experiment (similar to the multimodel-mean CO2 EffCS of 3.6 K from idealised experiments), but its minimum in the real world (1.6 K for an observational estimate for 1985–2011, similar to the multimodel-mean value for volcanic forcing). The real-world variations mean that historical EffCS underestimates CO2 EffCS by 30% when considering the entire historical period. The difference for recent decades implies that either unforced variability or the response to volcanic forcing causes a much stronger regional pattern of sea surface temperature change in the real world than in AOGCMs. We speculate that this could be explained by a deficiency in simulated coupled atmosphere
  • Journal article
    Hellinger P, Matteini L, Landi S, Franci L, Verdini A, Papini Eet al., 2019,

    Turbulence versus Fire-hose Instabilities: 3D Hybrid Expanding Box Simulations

    , The Astrophysical Journal, Vol: 883, Pages: 178-178
  • Journal article
    Ceppi P, Gregory JM, 2019,

    A refined model for the Earth’s global energy balance

    , Climate Dynamics, Vol: 53, Pages: 4781-4797, ISSN: 0930-7575

    A commonly-used model of the global radiative budget assumes that the radiative response to forcing, R, is proportional to global surface air temperature T, R= λT. Previous studies have highlighted two unresolved issues with this model: first, the feedback parameter λ depends on the forcing agent; second, λ varies with time. Here, we investigate the factors controlling R in two atmosphere–slab ocean climate models subjected to a wide range of abrupt climate forcings. It is found that R scales not only with T, but also with the large-scale tropospheric stability S (defined here as the estimated inversion strength area-averaged over ocean regions equatorward of 50∘). Positive S promotes negative R, mainly through shortwave cloud and lapse-rate changes. A refined model of the global energy balance is proposed that accounts for both temperature and stability effects. This refined model quantitatively explains (1) the dependence of climate feedbacks on forcing agent (or equivalently, differences in forcing efficacy), and (2) the time evolution of feedbacks in coupled climate model experiments. Furthermore, a similar relationship between R and S is found in observations compared with models, lending confidence that the refined energy balance model is applicable to the real world.
  • Journal article
    Wedlund Simon C, Behar E, Nilsson H, Alho M, Kallio E, Gunell H, Bodewits D, Heritier K, Galand M, Beth A, Rubin M, Altwegg K, Volwerk M, Gronoff G, Hoekstra Ret al., 2019,

    Solar wind charge exchange in cometary atmospheres III. Results from the Rosetta mission to comet 67P/Churyumov-Gerasimenko

    , Astronomy and Astrophysics, Vol: 630, ISSN: 0004-6361

    Solar wind charge-changing reactions are of paramount importance to thephysico-chemistry of the atmosphere of a comet. The ESA/Rosetta mission tocomet 67P/Churyumov-Gerasimenko (67P) provides a unique opportunity to studycharge-changing processes in situ. To understand the role of these reactions inthe evolution of the solar wind plasma, and interpret the complex in-situmeasurements made by Rosetta, numerical or analytical models are necessary. Weuse an extended analytical formalism describing solar wind charge-changingprocesses at comets along solar wind streamlines. The model is driven by solarwind ion measurements from the Rosetta Plasma Consortium-Ion CompositionAnalyzer (RPC-ICA) and neutral density observations from the RosettaSpectrometer for Ion and Neutral Analysis-Comet Pressure Sensor (ROSINA-COPS),as well as charge-changing cross sections of hydrogen and helium particles in awater gas. A mission-wide overview of charge-changing efficiencies at comet 67Pis presented. Electron capture cross sections dominate and favor the productionof He and H energetic neutral atoms, with fluxes expected to rival those of H+and He2+ ions. Neutral outgassing rates are retrieved from local RPC-ICA fluxmeasurements, and match ROSINA's estimates very well. From the model, we findthat solar wind charge exchange is unable to fully explain the magnitude of thesharp drop of solar wind ion fluxes observed by Rosetta for heliocentricdistances below 2.5 AU. This is likely because the model does not take intoaccount the relative ion dynamics and, to a lesser extent, ignore the formationof bow shock-like structures upstream of the nucleus. This work also shows thatthe ionization by solar EUV radiation and energetic electrons dominates thesource of cometary ions, although solar wind contributions may be significantduring isolated events.
  • Journal article
    Moore L, Melin H, O'Donoghue J, Stallard TS, Moses J, Galand M, Miller S, Schmidt CAet al., 2019,

    Modelling H-3(+) in planetary atmospheres: effects of vertical gradients on observed quantities

    , Philosophical Transactions of the Royal Society A. Mathematical, Physical and Engineering Sciences, Vol: 377, Pages: 1-19, ISSN: 1364-503X

    Since its detection in the aurorae of Jupiter approximately 30 years ago, the H3+ ion has served as an invaluable probe of giant planet upper atmospheres. However, the vast majority of monitoring of planetary H3+ radiation has followed from observations that rely on deriving parameters from column-integrated paths through the emitting layer. Here, we investigate the effects of density and temperature gradients along such paths on the measured H3+ spectrum and its resulting interpretation. In a non-isothermal atmosphere, H3+ column densities retrieved from such observations are found to represent a lower limit, reduced by 20% or more from the true atmospheric value. Global simulations of Uranus' ionosphere reveal that measured H3+ temperature variations are often attributable to well-understood solar zenith angle effects rather than indications of real atmospheric variability. Finally, based on these insights, a preliminary method of deriving vertical temperature structure is demonstrated at Jupiter using model reproductions of electron density and H3+ measurements. The sheer diversity and uncertainty of conditions in planetary atmospheres prohibits this work from providing blanket quantitative correction factors; nonetheless, we illustrate a few simple ways in which the already formidable utility of H3+ observations in understanding planetary atmospheres can be enhanced.This article is part of a discussion meeting issue ‘Advances in hydrogen molecular ions: H3+, H5+ and beyond’.
  • Journal article
    Wedlund CS, Bodewits D, Alho M, Hoekstra R, Behar E, Gronoff G, Gunell H, Nilsson H, Kallio E, Beth Aet al., 2019,

    Solar wind charge exchange in cometary atmospheres I. Charge-changing and ionization cross sections for He and H particles in H2O

    , Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361
  • Journal article
    Wedlund CS, Behar E, Kallio E, Nilsson H, Alho M, Gunell H, Bodewits D, Beth A, Gronoft G, Hoekstra Ret al., 2019,

    Solar wind charge exchange in cometary atmospheres II. Analytical model

    , Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361
  • Journal article
    Luspay-Kuti A, Altwegg K, Berthelier JJ, Beth A, Dhooghe F, Fiethe B, Fuselier SA, Gombosi T, Hansen KC, Hassig M, Livadiotis G, Mall U, Mandt KE, Mousis O, Petrinec SM, Rubin M, Trattner KJ, Tzou C-Y, Wurz Pet al., 2019,

    Comparison of neutral outgassing of comet 67P/Churyumov-Gerasimenko inbound and outbound beyond 3 AU from ROSINA/DFMS

    , ASTRONOMY & ASTROPHYSICS, Vol: 630, ISSN: 1432-0746
  • Journal article
    Mandt KE, Eriksson A, Beth A, Galand M, Vigren Eet al., 2019,

    Influence of collisions on ion dynamics in the inner comae of four comets

    , ASTRONOMY & ASTROPHYSICS, Vol: 630, Pages: 1-8, ISSN: 1432-0746

    Context. Collisions between cometary neutrals in the inner coma of a comet and cometary ions that have been picked up into the solar wind flow and return to the coma lead to the formation of a broad inner boundary known as a collisionopause. This boundary is produced by a combination of charge transfer and chemical reactions, both of which are important at the location of the collisionopause boundary. Four spacecraft measured ion densities and velocities in the inner region of comets, exploring the part of the coma where an ion-neutral collisionopause boundary is expected to form.Aims. The aims are to determine the dominant physics behind the formation of the ion-neutral collisionopause and to evaluate where this boundary has been observed by spacecraft.Methods. We evaluated observations from three spacecraft at four different comets to determine if a collisionopause boundary was observed based on the reported ion velocities. We compared the measured location of the ion-neutral collisionopause with measurements of the collision cross sections to evaluate whether chemistry or charge exchange are more important at the location where the collisionopause is observed.Results. Based on measurements of the cross sections for charge transfer and for chemical reactions, the boundary observed by Rosetta appears to be the location where chemistry becomes the more probable result of a collision between H2O and H2O+ than charge exchange. Comparisons with ion observations made by Deep Space 1 at 19P/Borrelly and Giotto at 1P/Halley and 26P/Grigg-Skjellerup show that similar boundaries were observed at 19P/Borrelly and 1P/Halley. The ion composition measurements made by Giotto at Halley confirm that chemistry becomes more important inside of this boundary and that electron-ion dissociative recombination is a driver for the reported ion pileup boundary.
  • Journal article
    Hoang M, Garnier P, Gourlaouen H, Lasue J, Reme H, Altwegg K, Balsiger H, Beth A, Calmonte U, Fiethe B, Galli A, Gasc S, Jaeckel A, Korth A, Le Roy L, Mall U, Rubin M, Semon T, Tzou C-Y, Waite JH, Wurz Pet al., 2019,

    Two years with comet 67P/Churyumov-Gerasimenko: H2O, CO2, and CO as seen by the ROSINA/RTOF instrument of Rosetta

    , Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361
  • Journal article
    Myllys M, Henri P, Galand M, Heritier KL, Gilet N, Goldstein R, Eriksson A, Johansson F, Deca Jet al., 2019,

    Plasma properties of suprathermal electrons near comet 67P/Churyumov-Gerasimenko with Rosetta

    , Astronomy and Astrophysics: a European journal, Vol: 630, Pages: 1-14, ISSN: 0004-6361

    Context. The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko from 2014 to September 2016. The mission provided in situ observations of the cometary plasma during different phases of the cometary activity, which enabled us to better understand its evolution as a function of heliocentric distance.Aims. In this study, different electron populations, called warm and hot, observed by the Ion and Electron Sensor (IES) of the Rosetta Plasma Consortium (RPC) are investigated near the comet during the escorting phase of the Rosetta mission.Methods. The estimates for the suprathermal electron densities and temperatures were extracted using IES electron data by fitting a double-kappa function to the measured velocity distributions. The fitting results were validated using observations from other RPC instruments. We give upgraded estimates for the warm and hot population densities compared to values previously shown in literature.Results. The fitted density and temperature estimates for both electron populations seen by IES are expressed as a function of heliocentric distance to study their evolution with the cometary activity. In addition, we studied the dependence between the electron properties and cometocentric distance.Conclusions. We observed that when the neutral outgassing rate of the nucleus is high (i.e., near perihelion) the suprathermal electrons are well characterized by a double-kappa distribution. In addition, warm and hot populations show a significant dependence with the heliocentric distance. The populations become clearly denser near perihelion while their temperatures are observed to remain almost constant. Moreover, the warm electron population density is shown to be strongly dependent on the radial distance from the comet. Finally, based on our results we reject the hypothesis that hot electron population seen by IES consists of solely suprathermal (halo) solar wind electrons, while we suggest that the hot electron population mainly consists of
  • Journal article
    Pancost R, Rogelj J, Scoones I, Haigh JD, Whitmarsh L, Fankhauser S, Williams Jet al., 2019,

    The pathway toward a net-zero-emissions future

    , One Earth, Vol: 1, Pages: 18-20, ISSN: 2590-3322

    The UK government recently committed to achieving net-zero carbon emissions by 2050. We asked a selection of UK-based experts to reflect upon this commitment, the challenges ahead, and the actions required to make it a reality.
  • Journal article
    Beth A, Galand M, Heritier K, 2019,

    Comparative study of photo-produced ionosphere in the close environment of comets

    , Astronomy & Astrophysics, Vol: 630, ISSN: 0004-6361

    Context. The Giotto and Rosetta missions gave us the unique opportunity of probing the close environment of cometary ionospheres of 1P/Halley (1P) and 67P/Churyumov-Gerasimenko (67P). The plasma conditions encountered at these two comets were very different from each other, which mainly stem from the different heliocentric distances, which drive photoionization rates, and from the outgassing activities, which drive the neutral densities.Aims. We asses the relative contribution of different plasma processes that are ongoing in the inner coma: photoionization, transport, photoabsorption, and electron–ion dissociative recombination. The main goal is to identify which processes are at play to then quantitatively assess the ionospheric density.Methods. We provide a set of analytical formulas to describe the ionospheric number density profile for cometary environments that take into account some of these processes. We discuss the validity of each model in the context of the Rosetta and Giotto missions.Results. We show that transport is the dominant loss process at large cometocentric distances and low outgassing rates. Chemical plasma loss through e−-ion dissociative recombination matters around 67P near perihelion and at 1P during the Giotto flyby: its effects increase as the heliocentric distance decreases, that is, at higher outgassing activity and higher photoionization frequency. Photoabsorption is of importance for outgassing rates higher than 1028 s−1 and only close to the cometary nucleus, well below the location of both spacecraft. Finally, regardless of the processes we considered, the ion number density profile always follows a 1∕r law at large cometocentric distances.
  • Journal article
    Carnielli G, Galand M, Leblanc F, Leclercq L, Modolo R, Beth A, Huybrighs HLF, Jia Xet al., 2019,

    First 3D test particle model of Ganymede's ionosphere

    , Icarus, Vol: 330, Pages: 42-59, ISSN: 0019-1035

    We present the first three-dimensional multi-species ionospheric model for Ganymede, based on a test particle Monte Carlo approach. Inputs include the electromagnetic field configuration around the moon from the magnetospheric models developed by Leclercq et al. (2016) and by Jia et al. (2009), and the number density, bulk velocity and temperature distributions of the neutral exosphere simulated by Leblanc et al. (2017). According to our simulations, O2+ is the most abundant ion species, followed by O+, H2+ and H2O+. For O+ and O2+, the majority of ions produced impact the moon's surface, while for the other species the majority escapes Ganymede's magnetosphere. For all ion species, the escape occurs either in the direction of corotation of the Jovian plasma or through the Alfvén wings.To validate our model, the output of our simulations, performed under the Galileo G2 flyby conditions, are compared to the observations. These include the electron density derived by the plasma wave instrument (PWS), the ion energy spectrogram measured by the plasma analyzer (PLS) and the associated plasma moments (Frank et al., 1997a).On the one hand, the electron density found by our model is consistently underestimated throughout the flyby, being at least one order of magnitude lower compared to observations. We argue that the prime reason for this discrepancy comes from the exospheric density, which may be underestimated. On the other hand, we find a remarkably good agreement between the modeled ion energy spectrogram and that recorded by PLS, providing a validation of the test particle model. Finally, we compare the modeled plasma moments along the G2 flyby with those analyzed by Frank et al. (1997a). The data seems to be more consistent with an ionosphere dominated by O2+ instead of H+ or O+, as suggested previously in the literature. This supports our finding that O2+ is the dominant ion species close to the surface.
  • Journal article
    Kuzichev I, Vasko IY, Soto-Chavez AR, Tong Y, Artemyev A, Bale SD, Spitkovsky Aet al., 2019,

    Nonlinear Evolution of the Whistler Heat Flux Instability

    , ASTROPHYSICAL JOURNAL, Vol: 882, ISSN: 0004-637X
  • Journal article
    Krupar V, Magdalenic J, Eastwood JP, Gopalswamy N, Kruparova O, Szabo A, Nemec Fet al., 2019,

    Statistical survey of coronal mass ejections and interplanetary type II bursts

    , The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 882, Pages: 1-5, ISSN: 0004-637X

    Coronal mass ejections (CMEs) are responsible for most severe space weather events, such as solar energetic particle events and geomagnetic storms at Earth. Type II radio bursts are slow drifting emissions produced by beams of suprathermal electrons accelerated at CME-driven shock waves propagating through the corona and interplanetary medium. Here, we report a statistical study of 153 interplanetary type II radio bursts observed by the two STEREO spacecraft between 2008 March and 2014 August. The shock associated radio emission was compared with CME parameters included in the Heliospheric Cataloguing, Analysis and Techniques Service catalog. We found that faster CMEs are statistically more likely to be associated with the interplanetary type II radio bursts. We correlate frequency drifts of interplanetary type II bursts with white-light observations to localize radio sources with respect to CMEs. Our results suggest that interplanetary type II bursts are more likely to have a source region situated closer to CME flanks than CME leading edge regions.
  • Journal article
    Perrone D, Stansby D, Horbury TS, Matteini Let al., 2019,

    Thermodynamics of pure fast solar wind: radial evolution of the temperature-speed relationship in the inner heliosphere

    , Monthly Notices of the Royal Astronomical Society, Vol: 488, Pages: 2380-2386, ISSN: 0035-8711

    A strong correlation between speed and proton temperature has been observed, across many years, on hourly averaged measurements in the solar wind. Here, we show that this relationship is also observed at a smaller scale on intervals of a few days, within a single stream. Following the radial evolution of a well-defined stream of coronal-hole plasma, we show that the temperature–speed (T–V) relationship evolves with distance, implying that the T–V relationship at 1 au cannot be used as a proxy for that near the Sun. We suggest that this behaviour could be a combination of the anticorrelation between speed and flux-tube expansion factor near the Sun and the effect of a continuous heating experienced by the plasma during the expansion. We also show that the cooling index for the radial evolution of the temperature is a function of the speed. In particular, T⊥ in faster wind, although higher close to the Sun, decreases more quickly with respect to slower wind, suggesting that it has less time to interact with the mechanism(s) able to heat the plasma. Finally, we predict the expected T–V relationship in fast streams closer to the Sun with respect to the Helios observations, which Parker Solar Probe will explore in the near future.

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