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Journal articleParfitt R, Russell JE, Bantges RJ, et al., 2016,
A study of the time evolution of GERB shortwave calibration by comparison with CERES Edition-3A data
, Remote Sensing of Environment, Vol: 186, Pages: 416-427, ISSN: 0034-4257This study examines the evolution of the GERB-2 and GERB-1 Edition 1 shortwave radiance calibration between 2004-2007 and 2007-2012 respectively, through comparison with CERES instrument FM1 Edition 3A SSF instantaneous radiances. Two periods when simultaneous observations from both GERB-2 and GERB-1 were available, January 13th to February 11th 2007 and May 1st to May 10th 2007, are also compared. For these two overlap periods respectively, averaged over all CERES ‘unfiltered-to-filtered radiance ratio’ subsets, the GERB-1/CERES unfiltered radiance ratio is on average found to be 1.6% and 1.9% lower than the associated GERB-2/CERES unfiltered radiance ratio. Over the two longer time series the GERB/CERES unfiltered radiance ratio shows a general decrease with time for both GERB-2 and GERB-1. The rate of decrease varies through time but no significant seasonal dependence is seen. Averaged over all subsets the GERB-2/CERES unfiltered radiance ratio showed a decrease of 1.9% between June 2004 and June 2006. Between June 2007 and June 2012, the corresponding decrease in the GERB-1/CERES unfiltered radiance ratio was 6.5%. The evolution of the GERB/CERES unfiltered radiance ratio for both GERB-2 and GERB-1 shows a strong dependence on the CERES unfiltered-to-filtered radiance ratio, indicating that it is spectrally dependent. Further time-series analysis and theoretical work using simulated spectral radiance curves suggests that for GERB-1 the evolution is consistent with a darkening in the GERB shortwave spectral response function which is most pronounced at the shortest wavelengths. For GERB-2, no single spectral cause can be identified, suggesting that the evolution is likely due to a combination of several different effects.
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Journal articleWatson-Parris D, Schutgens N, Cook N, et al., 2016,
Community Intercomparison Suite (CIS) v1.4.0: a tool for intercomparing models and observations
, Geoscientific Model Development, Vol: 9, Pages: 3093-3110, ISSN: 1991-9603The Community Intercomparison Suite (CIS) is an easy-to-use command-line tool which has been developed to allow the straightforward intercomparison of remote sensing, in situ and model data. While there are a number of tools available for working with climate model data, the large diversity of sources (and formats) of remote sensing and in situ measurements necessitated a novel software solution. Developed by a professional software company, CIS supports a large number of gridded and ungridded data sources "out-of-the-box", including climate model output in NetCDF or the UK Met Office pp file format, CloudSat, CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization), MODIS (MODerate resolution Imaging Spectroradiometer), Cloud and Aerosol CCI (Climate Change Initiative) level 2 satellite data and a number of in situ aircraft and ground station data sets. The open-source architecture also supports user-defined plugins to allow many other sources to be easily added. Many of the key operations required when comparing heterogenous data sets are provided by CIS, including subsetting, aggregating, collocating and plotting the data. Output data are written to CF-compliant NetCDF files to ensure interoperability with other tools and systems. The latest documentation, including a user manual and installation instructions, can be found on our website (http://cistools.net). Here, we describe the need which this tool fulfils, followed by descriptions of its main functionality (as at version 1.4.0) and plugin architecture which make it unique in the field.
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Journal articleWilder FD, Ergun RE, Schwartz SJ, et al., 2016,
Observations of large-amplitude, parallel, electrostatic waves associated with the Kelvin-Helmholtz instability by the magnetospheric multiscale mission
, Geophysical Research Letters, Vol: 43, Pages: 8859-8866, ISSN: 1944-8007On 8 September 2015, the four Magnetospheric Multiscale spacecraft encountered a Kelvin-Helmholtz unstable magnetopause near the dusk flank. The spacecraft observed periodic compressed current sheets, between which the plasma was turbulent. We present observations of large-amplitude (up to 100 mV/m) oscillations in the electric field. Because these oscillations are purely parallel to the background magnetic field, electrostatic, and below the ion plasma frequency, they are likely to be ion acoustic-like waves. These waves are observed in a turbulent plasma where multiple particle populations are intermittently mixed, including cold electrons with energies less than 10 eV. Stability analysis suggests a cold electron component is necessary for wave growth.
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ReportThomas R, Graven H, Hoskins B, et al., 2016,
What is meant by ‘balancing sources and sinks of greenhouse gases’ to limit global temperature rise?
, Grantham Institute Briefing Note, Imperial College London, 3In an effort to limit global temperature rise to well below 2˚C, the COP21 Paris Agreement stipulates that a ‘balance’ between anthropogenic (man-made) sources and sinks of greenhouse gases must be reached by 2050-2100. An overall greenhouse gas ‘balance’ must consider individual gases in terms of how strongly they absorb solar infrared radiation, their concentration in the atmosphere, and their lifetime in the atmosphere.• Long-lived greenhouse gases, including carbon dioxide (CO2), accumulate in the atmosphere and continue to affect the climate for many centuries. To stabilise the concentrations of these long-lived gases, and thereby their effect on the climate, their sources must be progressively reduced towards zero. • For short-lived greenhouse gases that remain in the atmosphere for less than 100 years, including methane, stable or decreasing concentrations could be achieved within decades if emissions were stabilised or decreased. However, these gases currently only contribute about 20% of the total warming from greenhouse gases, so their reduction alone cannot successfully stabilise global temperature.• An overall ‘balance’ of sources and sinks of greenhouse gases could be facilitated by deliberate removal of CO2 from the atmosphere, for example, by combining biomass energy production with carbon capture and storage. Most current greenhouse gas emission scenarios that keep global temperature rise below 2˚C include some deliberate removal of CO2 to compensate for continued emissions of CO2 and other greenhouse gases
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Journal articleTownley-Smith K, Nave G, Pickering JC, et al., 2016,
Hyperfine structure constants for singly ionized manganese (Mn II) using Fourier transform spectroscopy
, Monthly Notices of the Royal Astronomical Society, Vol: 461, Pages: 73-78, ISSN: 0035-8711We expand on the comprehensive study of hyperfine structure (HFS) in Mn II conducted byHolt et al. (1999) by verifying hyperfine magnetic dipole constants (A) for 20 levels previouslymeasured by Holt et al. (1999) and deriving A constants for 47 previously unstudied levels. TheHFS patterns were measured in archival spectra from Fourier transform (FT) spectrometers atImperial College London and the National Institute of Standards and Technology. Analysis ofthe FT spectra was carried out in XGREMLIN. Our A constant for the ground level has a loweruncertainty by a factor of 6 than that of Blackwell-Whitehead et al.
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Journal articleCoustenis A, Atreya S, Castillo J, et al., 2016,
Preface to the special issue of PSS on "Surfaces, atmospheres and magnetospheres of the outer planets and their satellites and ring systems: Part XI"
, PLANETARY AND SPACE SCIENCE, Vol: 130, Pages: 1-2, ISSN: 0032-0633 -
Journal articleGrün E, Agarwal J, Altobelli N, et al., 2016,
The 2016 Feb 19 outburst of comet 67P/CG: an ESA Rosetta multi-instrument study
, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S220-S234, ISSN: 1365-2966On 2016 Feb 19, nine Rosetta instruments serendipitously observed an outburst of gas and dust from the nucleus of comet 67P/Churyumov-Gerasimenko. Among these instruments were cameras and spectrometers ranging from UV over visible to microwave wavelengths, in situ gas, dust and plasma instruments, and one dust collector. At 09:40 a dust cloud developed at the edge of an image in the shadowed region of the nucleus. Over the next two hours the instruments recorded a signature of the outburst that significantly exceeded the background. The enhancement ranged from 50 per cent of the neutral gas density at Rosetta to factors >100 of the brightness of the coma near the nucleus. Dust related phenomena (dust counts or brightness due to illuminated dust) showed the strongest enhancements (factors >10). However, even the electron density at Rosetta increased by a factor 3 and consequently the spacecraft potential changed from ∼−16 V to −20 V during the outburst. A clear sequence of events was observed at the distance of Rosetta (34 km from the nucleus): within 15 min the Star Tracker camera detected fast particles (∼25 m s−1) while 100 μm radius particles were detected by the GIADA dust instrument ∼1 h later at a speed of 6 m s−1. The slowest were individual mm to cm sized grains observed by the OSIRIS cameras. Although the outburst originated just outside the FOV of the instruments, the source region and the magnitude of the outburst could be determined.
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Journal articleFuselier SA, Altwegg K, Balsiger H, et al., 2016,
Ion chemistry in the coma of comet 67P near perihelion
, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S67-S77, ISSN: 1365-2966The coma and the comet–solar wind interaction of comet 67P/Churyumov–Gerasimenko changed dramatically from the initial Rosetta spacecraft encounter in 2014 August through perihelion in 2015 August. Just before equinox (at 1.6 au from the Sun), the solar wind signal disappeared and two regions of different cometary ion characteristics were observed. These ‘outer’ and ‘inner’ regions have cometary ion characteristics similar to outside and inside the ion pileup region observed during the Giotto approach to comet 1P/Halley. Rosetta/Double-Focusing Mass Spectrometer ion mass spectrometer observations are used here to investigate the H3O+/H2O+ ratio in the outer and inner regions at 67P/ Churyumov–Gerasimenko. The H3O+/H2O+ ratio and the H3O+ signal are observed to increase in the transition from the outer to the inner region and the H3O+ signal appears to be weakly correlated with cometary ion energy. These ion composition changes are similar to the ones observed during the 1P/Halley flyby. Modelling is used to determine the importance of neutral composition and transport of neutrals and ions away from the nucleus. This modelling demonstrates that changes in the H3O+/H2O+ ratio appear to be driven largely by transport properties and only weakly by neutral composition in the coma.
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Journal articleJones CD, Arora V, Friedlingstein P, et al., 2016,
The C4MIP experimental protocol for CMIP6
, Geoscientific Model Development Discussions, Vol: 9, Pages: 2853-2880, ISSN: 1991-962XCoordinated experimental design and implemen-tation has become a cornerstone of global climate modelling.Model Intercomparison Projects (MIPs) enable systematicand robust analysis of results across many models, by reduc-ing the influence of ad hoc differences in model set-up or ex-perimental boundary conditions. As it enters its 6th phase,the Coupled Model Intercomparison Project (CMIP6) hasgrown significantly in scope with the design and documenta-tion of individual simulations delegated to individual climatescience communities.The Coupled Climate–Carbon Cycle Model Intercompar-ison Project (C4MIP) takes responsibility for design, docu-mentation, and analysis of carbon cycle feedbacks and in-teractions in climate simulations. These feedbacks are poten-tially large and play a leading-order contribution in determin-ing the atmospheric composition in response to human emis-sions of CO2and in the setting of emissions targets to sta-bilize climate or avoid dangerous climate change. For overa decade, C4MIP has coordinated coupled climate–carboncycle simulations, and in this paper we describe the C4MIPsimulations that will be formally part of CMIP6. While theclimate–carbon cycle community has created this experimen-tal design, the simulations also fit within the wider CMIP ac-tivity, conform to some common standards including docu-mentation and diagnostic requests, and are designed to com-plement the CMIP core experiments known as the Diagnos-tic, Evaluation and Characterization of Klima (DECK).C4MIP has three key strands of scientific motivation andthe requested simulations are designed to satisfy their needs:(1) pre-industrial and historical simulations (formally partof the common set of CMIP6 experiments) to enable modelevaluation, (2) idealized coupled and partially coupled sim-ulations with 1 % per year increases in CO2to enable di-agnosis of feedback strength and its components, (3) futurescenario simulations
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Journal articleJones CD, Arora V, Friedlingstein P, et al., 2016,
C4MIP - The Coupled Climate-Carbon Cycle Model Intercomparison Project: experimental protocol for CMIP6
, Geoscientific Model Development, Vol: 9, Pages: 2853-2880, ISSN: 1991-9603Coordinated experimental design and implementation has become a cornerstone of global climate modelling. Model Intercomparison Projects (MIPs) enable systematic and robust analysis of results across many models, by reducing the influence of ad hoc differences in model set-up or experimental boundary conditions. As it enters its 6th phase, the Coupled Model Intercomparison Project (CMIP6) has grown significantly in scope with the design and documentation of individual simulations delegated to individual climate science communities. The Coupled Climate–Carbon Cycle Model Intercomparison Project (C4MIP) takes responsibility for design, documentation, and analysis of carbon cycle feedbacks and interactions in climate simulations. These feedbacks are potentially large and play a leading-order contribution in determining the atmospheric composition in response to human emissions of CO2 and in the setting of emissions targets to stabilize climate or avoid dangerous climate change. For over a decade, C4MIP has coordinated coupled climate–carbon cycle simulations, and in this paper we describe the C4MIP simulations that will be formally part of CMIP6. While the climate–carbon cycle community has created this experimental design, the simulations also fit within the wider CMIP activity, conform to some common standards including documentation and diagnostic requests, and are designed to complement the CMIP core experiments known as the Diagnostic, Evaluation and Characterization of Klima (DECK). C4MIP has three key strands of scientific motivation and the requested simulations are designed to satisfy their needs: (1) pre-industrial and historical simulations (formally part of the common set of CMIP6 experiments) to enable model evaluation, (2) idealized coupled and partially coupled simulations with 1 % per year increases in CO2 to enable diagnosis of feedback strength and its components, (3) future scenario simulations to project how the Earth system will re
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Journal articleEdberg NJT, Alho M, Andre M, et al., 2016,
CME impact on comet 67P/Churyumov-Gerasimenko
, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S45-S46, ISSN: 1365-2966We present Rosetta observations from comet 67P/Churyumov-Gerasimenko during the impactof a coronal mass ejection (CME). The CME impacted on 5-6 Oct 2015, when Rosetta wasabout 800 km from the comet nucleus, and 1.4 AU from the Sun. Upon impact, the plasmaenvironment is compressed to the level that solar wind ions, not seen a few days earlier whenat 1500 km, now reach Rosetta. In response to the compression, the flux of suprathermalelectrons increases by a factor of 5-10 and the background magnetic field strength increasesby a factor of ∼2.5. The plasma density increases by a factor of 10 and reaches 600 cm−3,due to increased particle impact ionisation, charge exchange and the adiabatic compressionof the plasma environment. We also observe unprecedentedly large magnetic field spikes at800 km, reaching above 200 nT, which are interpreted as magnetic flux ropes. We suggestthat these could possibly be formed by magnetic reconnection processes in the coma as themagnetic field across the CME changes polarity, or as a consequence of strong shears causingKelvin-Helmholtz instabilities in the plasma flow. Due to the limited orbit of Rosetta, we arenot able to observe if a tail disconnection occurs during the CME impact, which could beexpected based on previous remote observations of other CME-comet interactions.
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Journal articleMushtaq S, Steers EBM, Hoffmann V, et al., 2016,
Evidence for charge transfer from hydrogen molecular ions to copper atoms in a neon-hydrogen analytical glow discharge
, Journal of Analytical Atomic Spectrometry, Vol: 31, Pages: 2175-2181, ISSN: 1364-5544Extensive investigations have been carried out in recent years on the effects of the presence of small amounts of molecular gases in analytical glow discharges (GDs) using argon as the plasma gas. Complementary studies using neon have shown that, when hydrogen is added to a neon discharge with a copper sample, the intensities of lines from most of the upper energy levels in the Cu II 3d9(2D)4p sub-configuration decrease. However, the 224.700 nm spectral line and other lines from the same upper level (3Po2) increase in intensity and this effect cannot be produced by the excitation processes normally considered. We show that asymmetric charge transfer (ACT) excitation by hydrogen molecules (H2-ACT) is the most likely explanation, the first time this process has been reported for any element in analytical GDs. We also explain why a similar effect is not observed with added nitrogen, although the ionization energies of hydrogen and nitrogen molecules are very similar.
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Journal articleVigren E, Altwegg K, Edberg NJT, et al., 2016,
MODEL-OBSERVATION COMPARISONS OF ELECTRON NUMBER DENSITIES IN THE COMA OF 67P/CHURYUMOV–GERASIMENKO DURING 2015 JANUARY
, Astronomical Journal, Vol: 152, ISSN: 1538-3881During 2015 January 9–11, at a heliocentric distance of ~2.58–2.57 au, the ESA Rosetta spacecraft resided at a cometocentric distance of ~28 km from the nucleus of comet 67P/Churyumov–Gerasimenko, sweeping the terminator at northern latitudes of 43°N–58°N. Measurements by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Comet Pressure Sensor (ROSINA/COPS) provided neutral number densities. We have computed modeled electron number densities using the neutral number densities as input into a Field Free Chemistry Free model, assuming H2O dominance and ion-electron pair formation by photoionization only. A good agreement (typically within 25%) is found between the modeled electron number densities and those observed from measurements by the Mutual Impedance Probe (RPC/MIP) and the Langmuir Probe (RPC/LAP), both being subsystems of the Rosetta Plasma Consortium. This indicates that ions along the nucleus-spacecraft line were strongly coupled to the neutrals, moving radially outward with about the same speed. Such a statement, we propose, can be further tested by observations of H3O+/H2O+ number density ratios and associated comparisons with model results.
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Journal articleMurray JE, Brindley HE, Bryant RG, et al., 2016,
Enhancing weak transient signals in SEVIRI false color imagery: Application to dust source detection in southern Africa
, Journal of Geophysical Research: Atmospheres, Vol: 121, Pages: 10199-10219, ISSN: 2169-897XA method is described to significantly enhance the signature of dust events using observations from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI). The approach involves the derivation of a composite clear-sky signal for selected channels on an individual time-step and pixel basis. These composite signals are subtracted from each observation in the relevant channels to enhance weak transient signals associated with either (a) low levels of dust emission, or (b) dust emissions with high salt or low quartz content. Different channel combinations, of the differenced data from the steps above, are then rendered in false color imagery for the purpose of improved identification of dust source locations and activity. We have applied this clear-sky difference (CSD) algorithm over three [globally significant] source regions in southern Africa: the Makgadikgadi Basin, Etosha Pan, and the Namibian and western South African coast. Case study analyses indicate three notable advantages associated with the CSD approach over established image rendering methods: (i) an improved ability to detect dust plumes, (ii) the observation of source activation earlier in the diurnal cycle, and (iii) an improved ability to resolve and pinpoint dust plume source locations.
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Journal articleMangeon T, Voulgarakis A, Gilham R, et al., 2016,
INFERNO: a fire and emissions scheme for the UK Met Office's Unified Model
, Geoscientific Model Development, Vol: 9, Pages: 2685-2700, ISSN: 1991-9603Warm and dry climatological conditions favour the occurrence of forest fires. These fires then become a significant emission source to the atmosphere. Despite this global importance, fires are a local phenomenon and are difficult to represent in a large-scale Earth System Model (ESM). To address this, the INteractive Fire and Emission algoRithm for Natural envirOnments (INFERNO) was developed. INFERNO follows a reduced complexity approach and is intended for decadal to centennial scale climate simulations and assessment models for policy making. Fuel flammability is simulated using temperature, relative humidity, fuel density as well as precipitation and soil moisture. Combining flammability with ignitions and vegetation, burnt area is diagnosed. Emissions of carbon and key species are estimated using the carbon scheme in the JULES land surface model. JULES also possesses fire index diagnostics which we document and compare with our fire scheme. Two meteorology datasets and three ignition modes are used to validate the model. INFERNO is shown to effectively diagnose global fire occurrence (R = 0.66) and emissions (R = 0.59) through an approach appropriate to the complexity of an ESM, although regional biases remain.
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Journal articleMangeon S, Voulgarakis A, Gilham R, et al., 2016,
INFERNO: a fire and emissions scheme for the UK Met Office's Unified Model
, Geoscientific Model Development, Vol: 9, Pages: 2685-2700<jats:p>Abstract. Warm and dry climatological conditions favour the occurrence of forest fires. These fires then become a significant emission source to the atmosphere. Despite this global importance, fires are a local phenomenon and are difficult to represent in large-scale Earth system models (ESMs). To address this, the INteractive Fire and Emission algoRithm for Natural envirOnments (INFERNO) was developed. INFERNO follows a reduced complexity approach and is intended for decadal- to centennial-scale climate simulations and assessment models for policy making. Fuel flammability is simulated using temperature, relative humidity (RH) and fuel load as well as precipitation and soil moisture. Combining flammability with ignitions and vegetation, the burnt area is diagnosed. Emissions of carbon and key species are estimated using the carbon scheme in the Joint UK Land Environment Simulator (JULES) land surface model. JULES also possesses fire index diagnostics, which we document and compare with our fire scheme. We found INFERNO captured global burnt area variability better than individual indices, and these performed best for their native regions. Two meteorology data sets and three ignition modes are used to validate the model. INFERNO is shown to effectively diagnose global fire occurrence (R = 0.66) and emissions (R = 0.59) through an approach appropriate to the complexity of an ESM, although regional biases remain. </jats:p>
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Journal articleBroiles TW, Livadiotis G, Burch JL, et al., 2016,
Characterizing cometary electrons with kappa distributions
, Journal of Geophysical Research. Solid Earth, Vol: 121, Pages: 7407-7422, ISSN: 2169-9356The Rosetta spacecraft has escorted comet 67P/Churyumov-Gerasimenko since 6 August 2014and has offered an unprecedented opportunity to study plasma physics in the coma. We have usedthis opportunity to make thefirst characterization of cometary electrons with kappa distributions. Twothree-dimensional kappa functions werefit to the observations, which we interpret as two populations ofdense and warm (density=10cm3, temperature=2×105K, invariant kappa index=10>1000), andrarefied and hot (density=0.005cm3, temperature=5×105K, invariant kappa index=1–10) electrons. Wefit the observations on 30 October 2014 when Rosetta was 20km from 67P, and 3AU from the Sun. Werepeated the analysis on 15 August 2015 when Rosetta was 300km from the comet and 1.3AU from the Sun.Comparing the measurements on both days gives thefirst comparison of the cometary electron environmentbetween a nearly inactive comet far from the Sun and an active comet near perihelion. Wefind that the warmpopulation density increased by a factor of 3, while the temperature cooled by a factor of 2, and the invariantkappa index was unaffected. Wefind that the hot population density increased by a factor of 10, whilethe temperature and invariant kappa index were unchanged. We conclude that the hot population islikely the solar wind halo electrons in the coma. The warm population is likely of cometary origin, but itsmechanism for production is not known.
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Journal articlePhan TD, Shay MA, Haggerty CC, et al., 2016,
Ion Larmor radius effects near a reconnection X line at the magnetopause: THEMIS observations and simulation comparison
, Geophysical Research Letters, Vol: 43, Pages: 8844-8852, ISSN: 0094-8276We report a Time History of Events and Macroscale Interactions during Substorms (THEMIS-D) spacecraft crossing of a magnetopause reconnection exhaust ~9 ion skin depths (di) downstream of an X line. The crossing was characterized by ion jetting at speeds substantially below the predicted reconnection outflow speed. In the magnetospheric inflow region THEMIS detected (a) penetration of magnetosheath ions and the resulting flows perpendicular to the reconnection plane, (b) ion outflow extending into the magnetosphere, and (c) enhanced electron parallel temperature. Comparison with a simulation suggests that these signatures are associated with the gyration of magnetosheath ions onto magnetospheric field lines due to the shift of the flow stagnation point toward the low-density magnetosphere. Our observations indicate that these effects, ~2–3 di in width, extend at least 9 di downstream of the X line. The detection of these signatures could indicate large-scale proximity of the X line but do not imply that the spacecraft was upstream of the electron diffusion region.
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Journal articleMejnertsen L, Eastwood JP, Chittenden J, et al., 2016,
Global MHD Simulations of Neptune's Magnetosphere
, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 7497-7513, ISSN: 2169-9380A global magnetohydrodynamic (MHD) simulation has been performed in order to investigate the outer boundaries of Neptune's magnetosphere at the time of Voyager 2's flyby in 1989 and to better understand the dynamics of magnetospheres formed by highly inclined planetary dipoles. Using the MHD code Gorgon, we have implemented a precessing dipole to mimic Neptune's tilted magnetic field and rotation axes. By using the solar wind parameters measured by Voyager 2, the simulation is verified by finding good agreement with Voyager 2 magnetometer observations. Overall, there is a large-scale reconfiguration of magnetic topology and plasma distribution. During the “pole-on” magnetospheric configuration, there only exists one tail current sheet, contained between a rarefied lobe region which extends outward from the dayside cusp, and a lobe region attached to the nightside cusp. It is found that the tail current always closes to the magnetopause current system, rather than closing in on itself, as suggested by other models. The bow shock position and shape is found to be dependent on Neptune's daily rotation, with maximum standoff being during the pole-on case. Reconnection is found on the magnetopause but is highly modulated by the interplanetary magnetic field (IMF) and time of day, turning “off” and “on” when the magnetic shear between the IMF and planetary fields is large enough. The simulation shows that the most likely location for reconnection to occur during Voyager 2's flyby was far from the spacecraft trajectory, which may explain the relative lack of associated signatures in the observations.
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Journal articleHunt GJ, Cowley SWH, Provan G, et al., 2016,
Field-aligned currents in Saturn's magnetosphere: local time dependence of southern summer currents in the dawn sector between midnight and noon
, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 7785-7804, ISSN: 2169-9402We examine and compare the magnetic field perturbations associated with field-aligned ionosphere-magnetosphere coupling currents at Saturn, observed by the Cassini spacecraft during two sequences of highly inclined orbits in 2006/2007 and 2008 under late southern summer conditions. These sequences explore the southern currents in the dawn-noon and midnight sectors, respectively, thus allowing investigation of possible origins of the local time (LT) asymmetry in auroral Saturn kilometric radiation (SKR) emissions, which peak in power at ~8 h LT in the dawn-noon sector. We first show that the dawn-noon field data generally have the same four-sheet current structure as found previously in the midnight data and that both are similarly modulated by “planetary period oscillation” (PPO) currents. We then separate the averaged PPO-independent (e.g., subcorotation) and PPO-related currents for both LT sectors by using the current system symmetry properties. Surprisingly, we find that the PPO-independent currents are essentially identical within uncertainties in the dawn-dusk and midnight sectors, thus providing no explanation for the LT dependence of the SKR emissions. The main PPO-related currents are, however, found to be slightly stronger and narrower in latitudinal width at dawn-noon than at midnight, leading to estimated precipitating electron powers, and hence emissions, that are on average a factor of ~1.3 larger at dawn-noon than at midnight, inadequate to account for the observed LT asymmetry in SKR power by a factor of ~2.7. Some other factors must also be involved, such as a LT asymmetry in the hot magnetospheric auroral source electron population.
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Journal articleKasoar M, Voulgarakis A, Lamarque J-F, et al., 2016,
Regional and global temperature response to anthropogenic SO2 emissions from China in three climate models
, Atmospheric Chemistry and Physics, Vol: 16, Pages: 9785-9804, ISSN: 1680-7324We use the HadGEM3-GA4, CESM1, and GISS ModelE2 climate models to investigate the global and regional aerosol burden, radiative flux, and surface temperature responses to removing anthropogenic sulfur dioxide (SO2) emissions from China. We find that the models differ by up to a factor of 6 in the simulated change in aerosol optical depth (AOD) and shortwave radiative flux over China that results from reduced sulfate aerosol, leading to a large range of magnitudes in the regional and global temperature responses. Two of the three models simulate a near-ubiquitous hemispheric warming due to the regional SO2 removal, with similarities in the local and remote pattern of response, but overall with a substantially different magnitude. The third model simulates almost no significant temperature response. We attribute the discrepancies in the response to a combination of substantial differences in the chemical conversion of SO2 to sulfate, translation of sulfate mass into AOD, cloud radiative interactions, and differences in the radiative forcing efficiency of sulfate aerosol in the models. The model with the strongest response (HadGEM3-GA4) compares best with observations of AOD regionally, however the other two models compare similarly (albeit poorly) and still disagree substantially in their simulated climate response, indicating that total AOD observations are far from sufficient to determine which model response is more plausible. Our results highlight that there remains a large uncertainty in the representation of both aerosol chemistry as well as direct and indirect aerosol radiative effects in current climate models, and reinforces that caution must be applied when interpreting the results of modelling studies of aerosol influences on climate. Model studies that implicate aerosols in climate responses should ideally explore a range of radiative forcing strengths representative of this uncertainty, in addition to thoroughly evaluating the models used against observat
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Journal articleGeen R, Czaja A, Haigh JD, 2016,
The effects of increasing humidity on heat transport by extratropical waves
, Geophysical Research Letters, Vol: 43, Pages: 8314-8321, ISSN: 1944-8007This study emphasizes the separate contributions of the warm and cold sectors of extratropical cyclones to poleward heat transport. Aquaplanet simulations are performed with an intermediate complexity climate model in which the response of the atmosphere to a range of values of saturation vapor pressure is assessed. These simulations reveal stronger poleward transport of latent heat in the warm sector as saturation vapor pressure is increased and an unexpected increase in poleward sensible heat transport in the cold sector. The latter results nearly equally from changes in the background stability of the atmosphere at low levels and changes in the temporal correlation between wind and temperature fields throughout the troposphere. Increased stability at low level reduces the likelihood that movement of cooler air over warmer water results in an absolutely unstable temperature profile, leading to less asymmetric damping of temperature and meridional velocity anomalies in cold and warm sectors.
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Journal articleVigren E, Galand M, Wellbrock A, et al., 2016,
Suprathermal electrons in Titan's sunlit ionosphere: model-observation comparisons
, Astrophysical Journal, Vol: 826, ISSN: 1538-4357 -
Journal articleStiller-Reeve MA, Heuze C, Ball WT, et al., 2016,
Improving together: better science writing through peer learning
, Hydrology and Earth System Sciences, Vol: 20, Pages: 2965-2973, ISSN: 1027-5606Science, in our case the climate and geosciences, is increasingly interdisciplinary. Scientists must therefore communicate across disciplinary boundaries. For this communication to be successful, scientists must write clearly and concisely, yet the historically poor standard of scientific writing does not seem to be improving. Scientific writing must improve, and the key to long-term improvement lies with the early-career scientist (ECS). Many interventions exist for an ECS to improve their writing, like style guides and courses. However, momentum is often difficult to maintain after these interventions are completed. Continuity is key to improving writing.This paper introduces the ClimateSnack project, which aims to motivate ECSs to develop and continue to improve their writing and communication skills. The project adopts a peer-learning framework where ECSs voluntarily form writing groups at different institutes around the world. The group members learn, discuss, and improve their writing skills together. Several ClimateSnack writing groups have formed. This paper examines why some of the groups have flourished and others have dissolved. We identify the challenges involved in making a writing group successful and effective, notably the leadership of self-organized groups, and both individual and institutional time management. Within some of the groups, peer learning clearly offers a powerful tool to improve writing as well as bringing other benefits, including improved general communication skills and increased confidence.
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Journal articleMandt KE, Eriksson A, Edberg NJT, et al., 2016,
RPC observation of the development and evolution of plasma interaction boundaries at 67P/ChuryumovGerasimenko
, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S9-S22, ISSN: 1365-2966One of the primary objectives of the Rosetta Plasma Consortium, a suite of five plasma instruments on-board the Rosetta spacecraft, is to observe the formation and evolution of plasma interaction regions at the comet 67P/Churyumov-Gerasimenko (67P/CG). Observations made between 2015 April and 2016 February show that solar wind–cometary plasma interaction boundaries and regions formed around 2015 mid-April and lasted through early 2016 January. At least two regions were observed, separated by an ion-neutral collisionopause boundary. The inner region was located on the nucleus side of the boundary and was characterized by low-energy water-group ions, reduced magnetic field pileup and enhanced electron densities. The outer region was located outside of the boundary and was characterized by reduced electron densities, water-group ions that are accelerated to energies above 100 eV and enhanced magnetic field pileup compared to the inner region. The boundary discussed here is outside of the diamagnetic cavity and shows characteristics similar to observations made on-board the Giotto spacecraft in the ion pileup region at 1P/Halley. We find that the boundary is likely to be related to ion-neutral collisions and that its location is influenced by variability in the neutral density and the solar wind dynamic pressure.
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Journal articleMasters A, Sulaiman AH, Sergis N, et al., 2016,
Suprathermal electrons at Saturn’s bow shock
, Astrophysical Journal, Vol: 826, ISSN: 1538-4357The leading explanation for the origin of galactic cosmic rays is particle acceleration at the shocks surrounding young supernovaRemnants (SNRs), although crucial aspects of the acceleration process are unclear. The similar collisionless plasma shocks frequently encountered by spacecraft in the solar wind are generally far weaker (lower Mach number) than these SNR shocks.However, the Cassini spacecraft has shown that the shock standingin the solar wind sunward of Saturn (Saturn’s bow shock) can occasionally reach this high-Mach number astrophysical regime.In this regime Cassini has provided the first in situ evidence for electron acceleration under quasi-parallel upstream magnetic conditions. Here we present the full picture of suprathermal electrons at Saturn’s bow shock revealed by Cassini. The downstream thermal electron distribution is resolved in all data taken by the low-energy electron detector (CAPS-ELS, <28 keV)during shock crossings, but the higher energy channels were at(or close to) background. The high-energy electron detector (MIMI-LEMMS, >18 keV) measured a suprathermal electron signatureat 31 of 508 crossings, where typically only the lowest energy channels (<100 keV) were above background. We show that these results are consistent with theory in which the “injection” of thermal electrons into an acceleration process involves interaction with whistler waves at the shock front, and becomes possible for all upstream magnetic field orientations at high Mach numbers like those of the strong shocks around young SNRs. A future dedicated study will analyse the rare crossings with evidence for relativisticelectrons (up to ~1 MeV).
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Journal articleRichter I, Auster H-U, Berghofer G, et al., 2016,
Two-point observations of low-frequency waves at 67P/Churyumov-Gerasimenko during descent of PHILAE: Comparison of RPCMAG and ROMAP
, Annales Geophysicae, Vol: 34, Pages: 609-622, ISSN: 1432-0576The European Space Agency’s spacecraft ROSETTA has reached its final destination,comet 67P/Churyumov-Gerasimenko. Whilst orbiting in the close vicinity of the nucleus theROSETTA magnetometers detected a new type of low-frequency waves possibly generated by across-field current instability due to freshly ionized cometary water group particles. During separation, descent and landing of the lander PHILAE on comet 67P/Churyumov-Gerasimenko, weused the unique opportunity to perform combined measurements with the magnetometers onboardROSETTA (RPCMAG) and its lander PHILAE (ROMAP). New details about the spatial distributionof wave properties along the connection line of the ROSETTA orbiter and the lander PHILAEare revealed. An estimation of the observed amplitude, phase and wavelength distribution will be presented as well as the measured dispersion relation, characterizing the new type of low-frequencywaves. The propagation direction and polarization features will be discussed using the results of aminimum variance analysis. Thoughts about the size of the wave source will complete our study
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Journal articleBeth AA, Garnier, Toublanc, et al., 2016,
Theory for planetary exospheres: III. Radiation pressure effect on the Circular Restricted Three Body Problem and its implication on planetary atmospheres
, Icarus, Vol: 280, Pages: 415-423, ISSN: 1090-2643The planetary exospheres are poorly known in their outer parts, since the neutral densities are low compared with the instruments detection capabilities. The exospheric models are thus often the main source of information at such high altitudes. We present a new way to take into account analytically the additional effect of the stellar radiation pressure on planetary exospheres. In a series of papers, we present with a Hamiltonian approach the effect of the radiation pressure on dynamical trajectories, density profiles and escaping thermal flux. Our work is a generalization of the study by Bishop and Chamberlain (1989). In this third paper, we investigate the effect of the stellar radiation pressure on the Circular Restricted Three Body Problem (CR3BP), called also the photogravitational CR3BP, and its implication on the escape and the stability of planetary exospheres, especially for hot Jupiters. In particular, we describe the transformation of the equipotentials and the location of the Lagrange points, and we provide a modified equation for the Hill sphere radius that includes the influence of the radiation pressure. Finally, an application to the hot Jupiter HD 209458b reveals the existence of a blow-off escape regime induced by the stellar radiation pressure.
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Journal articleChen CHK, Matteini L, Schekochihin AA, et al., 2016,
Multi-species measurements of the firehose and mirror instability thresholds in the solar wind
, Astrophysical Journal Letters, Vol: 825, ISSN: 2041-8213The firehose and mirror instabilities are thought to arise in a variety of space and astrophysical plasmas, constraining the pressure anisotropies and drifts between particle species. The plasma stability depends on all species simultaneously, meaning that a combined analysis is required. Here, we present the first such analysis in the solar wind, using the long-wavelength stability parameters to combine the anisotropies and drifts of all major species (core and beam protons, alphas, and electrons). At the threshold, the firehose parameter was found to be dominated by protons (67%), but also to have significant contributions from electrons (18%) and alphas (15%). Drifts were also found to be important, contributing 57% in the presence of a proton beam. A similar situation was found for the mirror, with contributions of 61%, 28%, and 11% for protons, electrons, and alphas, respectively. The parallel electric field contribution, however, was found to be small at 9%. Overall, the long-wavelength thresholds constrain the data well (${\text{}}\lt 1 \% $ unstable), and the implications of this are discussed.
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Journal articleEriksson S, Wilder FD, Ergun RE, et al., 2016,
Magnetospheric Multiscale Observations of the Electron Diffusion Region of Large Guide Field Magnetic Reconnection
, Physical Review Letters, Vol: 117, ISSN: 1079-7114We report observations from the Magnetospheric Multiscale (MMS) satellites of a large guide field magnetic reconnection event. The observations suggest that two of the four MMS spacecraft sampled the electron diffusion region, whereas the other two spacecraft detected the exhaust jet from the event. The guide magnetic field amplitude is approximately 4 times that of the reconnecting field. The event is accompanied by a significant parallel electric field (E∥) that is larger than predicted by simulations. The high-speed (∼300 km/s) crossing of the electron diffusion region limited the data set to one complete electron distribution inside of the electron diffusion region, which shows significant parallel heating. The data suggest that E∥ is balanced by a combination of electron inertia and a parallel gradient of the gyrotropic electron pressure.
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