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  • Journal article
    Chen CHK, Matteini L, Burgess D, Horbury TSet al., 2015,

    Erratum: Magnetic field rotations in the solar wind at kinetic scales (Monthly Notices of the Royal Astronomical Society: Letters DOI: 10.1093/mnrasl/slv107)

    , Monthly Notices of the Royal Astronomical Society: Letters, Vol: 455, ISSN: 1745-3925

    This is an erratum to the paper entitled 'Magnetic field rotations in the solar wind at kinetic scales", published in MNRAS, 2015, 453, L64. Fig. 1 appeared without the key. The correct image is given below.
  • Journal article
    Graven HD, 2015,

    Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century

    , Proceedings of the National Academy of Sciences of the United States of America, Vol: 112, Pages: 9542-9545, ISSN: 1091-6490

    Radiocarbon analyses are commonly used in a broad range of fields, including earth science, archaeology, forgery detection, isotope forensics, and physiology. Many applications are sensitive to the radiocarbon ((14)C) content of atmospheric CO2, which has varied since 1890 as a result of nuclear weapons testing, fossil fuel emissions, and CO2 cycling between atmospheric, oceanic, and terrestrial carbon reservoirs. Over this century, the ratio (14)C/C in atmospheric CO2 (Δ(14)CO2) will be determined by the amount of fossil fuel combustion, which decreases Δ(14)CO2 because fossil fuels have lost all (14)C from radioactive decay. Simulations of Δ(14)CO2 using the emission scenarios from the Intergovernmental Panel on Climate Change Fifth Assessment Report, the Representative Concentration Pathways, indicate that ambitious emission reductions could sustain Δ(14)CO2 near the preindustrial level of 0‰ through 2100, whereas "business-as-usual" emissions will reduce Δ(14)CO2 to -250‰, equivalent to the depletion expected from over 2,000 y of radioactive decay. Given current emissions trends, fossil fuel emission-driven artificial "aging" of the atmosphere is likely to occur much faster and with a larger magnitude than previously expected. This finding has strong and as yet unrecognized implications for many applications of radiocarbon in various fields, and it implies that radiocarbon dating may no longer provide definitive ages for samples up to 2,000 y old.
  • Journal article
    Weiss Z, Steers EBM, Pickering JC, 2015,

    Transition rates and transition rate diagrams in atomic emission spectroscopy: A review

    , SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY, Vol: 110, Pages: 79-90, ISSN: 0584-8547
  • Journal article
    Auster H-U, Apathy I, Berghofer G, Fornacon K-H, Remizov A, Carr C, Güttler C, Haerendel G, Heinisch P, Hercik D, Hilchenbach M, Kührt E, Magnes W, Motschmann U, Richter I, Russell CT, Przyklenk A, Schwingenschuh K, Sierks H, Glassmeier K-Het al., 2015,

    The nonmagnetic nucleus of comet 67P/Churyumov-Gerasimenko

    , Science, Vol: 349, ISSN: 0036-8075

    <jats:p> Knowledge of the magnetization of planetary bodies constrains their origin and evolution, as well as the conditions in the solar nebular at that time. On the basis of magnetic field measurements during the descent and subsequent multiple touchdown of the Rosetta lander Philae on the comet 67P/Churyumov-Gerasimenko (67P), we show that no global magnetic field was detected within the limitations of analysis. The Rosetta Magnetometer and Plasma Monitor (ROMAP) suite of sensors measured an upper magnetic field magnitude of less than 2 nanotesla at the cometary surface at multiple locations, with the upper specific magnetic moment being &lt;3.1 × 10 <jats:sup>−5</jats:sup> ampere–square meters per kilogram for meter-size homogeneous magnetized boulders. The maximum dipole moment of 67P is 1.6 × 10 <jats:sup>8</jats:sup> ampere–square meters. We conclude that on the meter scale, magnetic alignment in the preplanetary nebula is of minor importance. </jats:p>
  • Journal article
    Ryder CL, McQuaid JB, Flamant C, Rosenberg PD, Washington R, Brindley HE, Highwood EJ, Marsham JH, Parker DJ, Todd MC, Banks JR, Brooke JK, Engelstaedter S, Estelles V, Formenti P, Garcia-Carreras L, Kocha C, Marenco F, Sodemann H, Allen CJT, Bourdon A, Bart M, Cavazos-Guerra C, Chevaillier S, Crosier J, Darbyshire E, Dean AR, Dorsey JR, Kent J, O'Sullivan D, Schepanski K, Szpek K, Trembath J, Woolley Aet al., 2015,

    Advances in understanding mineral dust and boundary layer processes over the Sahara from Fennec aircraft observations

    , Atmospheric Chemistry and Physics, Vol: 15, Pages: 8479-8520, ISSN: 1680-7324

    The Fennec climate programme aims to improve understanding of the Saharan climate system through a synergy of observations and modelling. We present a description of the Fennec airborne observations during 2011 and 2012 over the remote Sahara (Mauritania and Mali) and the advances in the understanding of mineral dust and boundary layer processes they have provided. Aircraft instrumentation aboard the UK FAAM BAe146 and French SAFIRE (Service des Avions Français Instrumentés pour la Recherche en Environnement) Falcon 20 is described, with specific focus on instrumentation specially developed for and relevant to Saharan meteorology and dust. Flight locations, aims and associated meteorology are described. Examples and applications of aircraft measurements from the Fennec flights are presented, highlighting new scientific results delivered using a synergy of different instruments and aircraft. These include (1) the first airborne measurement of dust particles sizes of up to 300 microns and associated dust fluxes in the Saharan atmospheric boundary layer (SABL), (2) dust uplift from the breakdown of the nocturnal low-level jet before becoming visible in SEVIRI (Spinning Enhanced Visible Infra-Red Imager) satellite imagery, (3) vertical profiles of the unique vertical structure of turbulent fluxes in the SABL, (4) in situ observations of processes in SABL clouds showing dust acting as cloud condensation nuclei (CCN) and ice nuclei (IN) at −15 °C, (5) dual-aircraft observations of the SABL dynamics, thermodynamics and composition in the Saharan heat low region (SHL), (6) airborne observations of a dust storm associated with a cold pool (haboob) issued from deep convection over the Atlas Mountains, (7) the first airborne chemical composition measurements of dust in the SHL region with differing composition, sources (determined using Lagrangian backward trajectory calculations) and absorption properties between 2011 and 2012, (8) coincident ozone and
  • Journal article
    Yates JN, Southwood DJ, Dougherty MK, 2015,

    Reply to the comment by Cowley et al. on “Magneticphase structure of Saturn’s 10.7h oscillations”

    , Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 5691-5693, ISSN: 2169-9402
  • Journal article
    Voulgarakis A, Marlier ME, Faluvegi G, Shindell DT, Tsigaridis K, Mangeon Set al., 2015,

    Interannual variability of tropospheric trace gases and aerosols: The role of biomass burning emissions

    , Journal of Geophysical Research: Atmospheres, Vol: 120, Pages: 7157-7173, ISSN: 2169-897X

    Fires are responsible for a range of gaseous and aerosol emissions. However, their influence onthe interannual variability of atmospheric trace gases and aerosols has not been systematically investigatedfrom a global perspective. We examine biomass burning emissions as a driver of interannual variability oflarge-scale abundances of short-lived constituents such as carbon monoxide (CO), hydroxyl radicals (OH),ozone, and aerosols using the Goddard Institute for Space Studies ModelE composition-climate model and arange of observations, with an emphasis on satellite information. Our model captures the observed variabilityof the constituents examined in most cases, but with substantial underestimates in boreal regions. Thestrongest interannual variability on a global scale is found for carbon monoxide (~10% for its global annualburden), while the lowest is found for tropospheric ozone (~1% for its global annual burden). Regionally,aerosol optical depth shows the largest variability which exceeds 50%. Areas of strong variability of bothaerosols and CO include the tropical land regions (especially Equatorial Asia and South America) and northernhigh latitudes, while even regions in the northern midlatitudes experience substantial interannual variability ofaerosols. Ozone variability peaks over equatorial Asia in boreal autumn, partly due to varying biomass burningemissions, and over the western and central Pacific in the rest of the year, mainly due to meteorologicalfluctuations. We find that biomass burning emissions are almost entirely responsible for global CO interannualvariability, and similarly important for OH variability. The same is true for global and regional aerosol variability,especially when not taking into account dust and sea-salt particles. We show that important implications canarise from such interannual influences for regional climate and air quality
  • Journal article
    Marsham JH, Parker DJ, Todd MC, Banks JR, Brindley HE, Garcia-Carreras L, Roberts AJ, Ryder CLet al., 2015,

    The contrasting roles of water and dust in controlling daily variations in radiative heating of the summertime Saharan Heat Low

    , Atmospheric Chemistry and Physics Discussions, Vol: 15, Pages: 19447-19476, ISSN: 1680-7367

    The summertime Sahara Heat Low (SHL) is a key component of the West African Monsoon (WAM) system. Considerable uncertainty remains over the relative roles of water vapour and dust aerosols in controlling the radiation budget over the Sahara and therefore our ability to explain variability and trends in the SHL, and in turn, the WAM. Here, new observations from the Fennec field campaign during June 2011 and June 2012, together with satellite retrievals from GERB, are used to quantify how total column water vapour (TCWV) and dust aerosols (from aerosol optical depth, AOD) control day-to-day variations in energy balance in both observations and ECWMF reanalyses (ERA-I). The data show that the earth-atmosphere system is radiatively heated in June 2011 and 2012. It is TCWV that largely determines variations in daily mean TOA net flux and the net heating of the earth-atmosphere system. In contrast, dust provides the primary control on surface heating, but the decreased surface heating from dust is largely compensated by increased atmospheric heating, and so dust control on net TOA radiation is weak. Dust and TCWV are both important for direct atmospheric heating. ERA-I captures the control of TOA net flux by TCWV, with a positive correlation (r = 0.6) between observed and modelled TOA net radiation, despite the use of a monthly dust climatology in ERA-I that cannot capture the daily variations in dustiness. Variations in surface net radiation, and so the vertical profile of radiative heating, are not captured in ERA-I, since it does not capture variations in dust. Results show that ventilation of the SHL by cool moist air leads to a radiative warming, stabilising the SHL with respect to such perturbations. It is known that models struggle to capture the advective moistening of the SHL, especially that associated with mesoscale convective systems. Our results show that the typical model errors in Saharan water vapour will lead to substantial errors in the modelled TOA ener
  • Journal article
    Balikhin MA, Shprits YY, Walker SN, Chen L, Cornilleau-Wehrlin N, Dandouras I, Santolik O, Carr C, Yearby KH, Weiss Bet al., 2015,

    Observations of discrete harmonics emerging from equatorial noise

    , Nature Communications, Vol: 6, ISSN: 2041-1723

    A number of modes of oscillations of particles and fields can exist in space plasmas. Since the early 1970s, space missions have observed noise-like plasma waves near the geomagnetic equator known as 'equatorial noise'. Several theories were suggested, but clear observational evidence supported by realistic modelling has not been provided. Here we report on observations by the Cluster mission that clearly show the highly structured and periodic pattern of these waves. Very narrow-banded emissions at frequencies corresponding to exact multiples of the proton gyrofrequency (frequency of gyration around the field line) from the 17th up to the 30th harmonic are observed, indicating that these waves are generated by the proton distributions. Simultaneously with these coherent periodic structures in waves, the Cluster spacecraft observes 'ring' distributions of protons in velocity space that provide the free energy for the waves. Calculated wave growth based on ion distributions shows a very similar pattern to the observations.
  • Journal article
    Gryspeerdt E, Stier P, White BA, Kipling Zet al., 2015,

    Wet scavenging limits the detection of aerosol effects on precipitation

    , Atmospheric Chemistry and Physics, Vol: 15, Pages: 7557-7570, ISSN: 1680-7324
  • Journal article
    Good SW, Forsyth RJ, Raines JM, Gershman DJ, Slavin JA, Zurbuchen THet al., 2015,

    Radial Evolution of a Magnetic Cloud: MESSENGER, STEREO, and Venus Express Observations

    , Astrophysical Journal, Vol: 807, Pages: 177-189, ISSN: 1538-4357

    The Solar Orbiter and Solar Probe Plus missions will provide observations of magnetic clouds closer to the Sunthan ever before, and it will be good preparation for these missions to make full use of the most recent in situ datasets from the inner heliosphere—namely, those provided by MErcury Surface, Space ENvironment, GEochemistry,and Ranging (MESSENGER) and Venus Express—for magnetic cloud studies. We present observations of thesame magnetic cloud made by MESSENGER at Mercury and later by Solar TErrestrial RElations Observatory-B(STEREO-B), while the spacecraft were radially aligned in 2011 November. Few such radial observations ofmagnetic clouds have been previously reported. Estimates of the solar wind speed at MESSENGER are alsopresented, calculated through the application of a previously established technique. The cloudʼs flux rope has beenanalyzed using force-free fitting; the rope diameter increased from 0.18 to 0.41 AU (corresponding to an rH0.94dependence on heliocentric distance, rH), and the axial magnetic field strength dropped from 46.0 to 8.7 nT (an -rH1.84 dependence) between the spacecraft, clear indications of an expanding structure. The axial magnetic flux was∼0.50 nT AU2 at both spacecraft, suggesting that the rope underwent no significant erosion through magneticreconnection between MESSENGER and STEREO-B. Further, we estimate the change in the cloudʼs angular widthby assuming helicity conservation. It has also been found that the rope axis rotated by 30° between the spacecraftto lie close to the solar equatorial plane at STEREO-B. Such a rotation, if it is a common feature of coronal massejection propagation, would have important implications for space weather forecasting.
  • Journal article
    Campagnola S, Boutonnet A, Martens W, Masters Aet al., 2015,

    Mission design for the exploration of Neptune and Triton

    , IEEE Aerospace and Electronic Systems Magazine, Vol: 30, Pages: 6-17, ISSN: 0885-8985
  • Journal article
    Messori G, Czaja A, 2015,

    On local and zonal pulses of atmospheric heat transport in reanalysis data

    , QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Vol: 141, Pages: 2376-2389, ISSN: 0035-9009
  • Journal article
    Sagnieres LBM, Galand MIF, Cui J, Lavvas P, Vigren E, Vuitton V, Yelle R, Wellbrock A, Coates Aet al., 2015,

    Influence of local ionization on ionospheric densities in Titan’s upper atmosphere

    , Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 5899-5921, ISSN: 2169-9402

    Titan has the most chemically complex ionosphere of the Solar System. The main sources of ions on the dayside are ionization by EUV solar radiation and on the nightside include ionization by precipitated electrons from Saturn's magnetosphere and transport of ions from the dayside, but many questions remain open. How well do models predict local ionization rates? How strongly do the ionization processes drive the ionospheric densities locally? To address these questions, we have carried out an analysis of ion densities from the Ion and Neutral Mass Spectrometer (INMS) from 16 close flybys of Titan's upper atmosphere. Using a simple chemical model applied to the INMS dataset, we have calculated the ion production rates and local ionization frequencies associated with primary ions inline image and inline image. We find that on the dayside the solar energy deposition model overestimates the INMS-derived inline image production rates by a factor of 2. On the nightside, however, the model driven by suprathermal electron intensities from the Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) sometimes agrees, other times underestimates the INMS-derived inline image production rates by a factor of up to 2-3. We find that below 1200 km, all ion number densities correlate with the local ionization frequency, although the correlation is significantly stronger for short-lived ions than long-lived ions. Furthermore, we find that for a given N2 local ionization frequency inline image has higher densities on the day-side than on the nightside. We explain that this is due to inline image being more efficiently ionized by solar photons than by magnetospheric electrons for a given amount of N2 ionization.
  • Journal article
    Lavvas P, Yelle RV, Heays AN, Campbell L, Brunger MJ, Galand M, Vuitton Vet al., 2015,

    N-2 state population in Titan's atmosphere

    , Icarus, Vol: 260, Pages: 29-59, ISSN: 1090-2643

    We present a detailed model for the vibrational population of all non pre-dissociating excited electronic states of N2, as well as for the ground and ionic states, in Titan’s atmosphere. Our model includes the detailed energy deposition calculations presented in the past (Lavvas, P. et al. [2011]. Icarus 213(1), 233–251) as well as the more recent developments in the high resolution N2 photo-absorption cross sections that allow us to calculate photo-excitation rates for different vibrational levels of singlet nitrogen states, and provide information for their pre-dissociation yields. In addition, we consider the effect of collisions and chemical reactions in the population of the different states. Our results demonstrate that above 600 km altitude, collisional processes are efficient only for a small sub-set of the excited states limited to the A and W(ν = 0) triplet states, and to a smaller degree to the a′ singlet state. In addition, we find that a significant population of vibrationally excited ground state N2 survives in Titan’s upper atmosphere. Our calculations demonstrate that this hot N2 population can improve the agreement between models and observations for the emission of the View the MathML source state that is significantly affected by resonant scattering. Moreover we discuss the potential implications of the vibrationally excited population on the ionospheric densities.
  • Book
    Haigh JD, Cargill P, 2015,

    The Sun's Influence on Climate

    , ISBN: 9780691153841

    &quot;--Peter Pilewskie, University of Colorado Boulder &quot;This succinct volume will be invaluable to scientists and general readers who want to learn more about the Sun and its effects on our climate system.
  • Journal article
    Horaites K, Boldyrev S, Krasheninnikov SI, Salem C, Bale SD, Pulupa Met al., 2015,

    Self-Similar Theory of Thermal Conduction and Application to the Solar Wind

    , PHYSICAL REVIEW LETTERS, Vol: 114, ISSN: 0031-9007
  • Journal article
    Lucas DD, Kwok CY, Cameron-Smith P, Graven H, Bergmann D, Guilderson TP, Weiss R, Keeling Ret al., 2015,

    Designing optimal greenhouse gas observing networks that consider performance and cost

    , Geoscientific Instrumentation, Methods and Data Systems, Vol: 4, Pages: 121-137, ISSN: 2193-0864

    Emission rates of greenhouse gases (GHGs) enteringinto the atmosphere can be inferred using mathematicalinverse approaches that combine observations from a networkof stations with forward atmospheric transport models.Some locations for collecting observations are better thanothers for constraining GHG emissions through the inversion,but the best locations for the inversion may be inaccessibleor limited by economic and other non-scientific factors.We present a method to design an optimal GHG observingnetwork in the presence of multiple objectives that may bein conflict with each other. As a demonstration, we use ourmethod to design a prototype network of six stations to monitorsummertime emissions in California of the potent GHG1,1,1,2-tetrafluoroethane (CH2FCF3, HFC-134a). We use amultiobjective genetic algorithm to evolve network configurationsthat seek to jointly maximize the scientific accuracyof the inferred HFC-134a emissions and minimize the associatedcosts of making the measurements. The genetic algorithmeffectively determines a set of “optimal” observingnetworks for HFC-134a that satisfy both objectives (i.e., thePareto frontier). The Pareto frontier is convex, and clearlyshows the tradeoffs between performance and cost, and thediminishing returns in trading one for the other. Without dif-ficulty, our method can be extended to design optimal networksto monitor two or more GHGs with different emissionspatterns, or to incorporate other objectives and constraintsthat are important in the practical design of atmosphericmonitoring networks.
  • Journal article
    Lucas DD, Kwok CY, Cameron-Smith P, Graven H, Bergmann D, Guilderson TP, Weiss R, Keeling Ret al., 2015,

    Designing optimal greenhouse gas observing networks that consider performance and cost

    , GEOSCIENTIFIC INSTRUMENTATION METHODS AND DATA SYSTEMS, Vol: 4, Pages: 705-749, ISSN: 2193-0856
  • Journal article
    Horbury TS, Archer MO, Brown P, Eastwood JP, Oddy TM, Whiteside BJ, Sample JGet al., 2015,

    The MAGIC of CINEMA: First in-flight science results from a miniaturised anisotropic magnetoresistive magnetometer

    , Annales Geophysicae, Vol: 33, Pages: 725-735, ISSN: 1432-0576

    We present the first in-flight results from a novel miniaturised anisotropic magnetoresistive space magnetometer, MAGIC (MAGnetometer from Imperial College), aboard the first CINEMA (CubeSat for Ions, Neutrals, Electrons and MAgnetic fields) spacecraft in low Earth orbit. An attitude-independent calibration technique is detailed using the International Geomagnetic Reference Field (IGRF), which is temperature dependent in the case of the outboard sensor. We show that the sensors accurately measure the expected absolute field to within 2% in attitude mode and 1% in science mode. Using a simple method we are able to estimate the spacecraft's attitude using the magnetometer only, thus characterising CINEMA's spin, precession and nutation. Finally, we show that the outboard sensor is capable of detecting transient physical signals with amplitudes of ~ 20–60 nT. These include field-aligned currents at the auroral oval, qualitatively similar to previous observations, which agree in location with measurements from the DMSP (Defense Meteorological Satellite Program) and POES (Polar-orbiting Operational Environmental Satellites) spacecraft. Thus, we demonstrate and discuss the potential science capabilities of the MAGIC instrument onboard a CubeSat platform.
  • Journal article
    Nilsson H, Wieser GS, Behar E, Wedlund CS, Kallio E, Gunell H, Edberg NJT, Eriksson AI, Yamauchi M, Koenders C, Wieser M, Lundin R, Barabash S, Mandt K, Burch JL, Goldstein R, Mokashi P, Carr CM, Cupido E, Fox PT, Szego K, Nemeth Z, Fedorov A, Sauvaud JA, Koskinen H, Richter I, Lebreton JP, Henri P, Volwerk M, Vallat C, Geiger Bet al., 2015,

    Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko - Observations between 3.6 and 2.0 AU

    , Astronomy & Astrophysics, Vol: 583, ISSN: 0004-6361

    Context. The Rosetta spacecraft is escorting comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 AU, wherethe comet activity was low, until perihelion at 1.24 AU. Initially, the solar wind permeates the thin comet atmosphere formed fromsublimation.Aims. Using the Rosetta Plasma Consortium Ion Composition Analyzer (RPC-ICA), we study the gradual evolution of the comet ionenvironment, from the first detectable traces of water ions to the stage where cometary water ions accelerated to about 1 keV energyare abundant. We compare ion fluxes of solar wind and cometary origin.Methods. RPC-ICA is an ion mass spectrometer measuring ions of solar wind and cometary origins in the 10 eV–40 keV energyrange.Results. We show how the flux of accelerated water ions with energies above 120 eV increases between 3.6 and 2.0 AU. The 24 haverage increases by 4 orders of magnitude, mainly because high-flux periods become more common. The water ion energy spectraalso become broader with time. This may indicate a larger and more uniform source region. At 2.0 AU the accelerated water ion fluxis frequently of the same order as the solar wind proton flux. Water ions of 120 eV–few keV energy may thus constitute a significantpart of the ions sputtering the nucleus surface. The ion density and mass in the comet vicinity is dominated by ions of cometary origin.The solar wind is deflected and the energy spectra broadened compared to an undisturbed solar wind.Conclusions. The flux of accelerated water ions moving from the upstream direction back toward the nucleus is a strongly nonlinearfunction of the heliocentric distance.
  • Journal article
    Carr CM, Edberg NJT, Eriksson AI, Odelstad E, Henri P, Lebreton J-P, Gasc S, Rubin M, Andre M, Gill R, Johansson EPG, Johansson F, Vigren E, Wahlund JE, Cupido E, Glassmeier K-H, Goldstein R, Koenders C, Mandt K, Nemeth Z, Nilsson H, Richter I, Stenberg Wieser G, Szego K, Volwerk Met al., 2015,

    Spatial distribution of low-energy plasma around 2 comet 67P/CG from Rosetta measurements

    , Geophysical Research Letters, Vol: 42, Pages: 4263-4269, ISSN: 1944-8007

    We use measurements from the Rosetta plasma consortium Langmuir probe and mutual impedance probe to study the spatial distribution of low-energy plasma in the near-nucleus coma of comet 67P/Churyumov-Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e., the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be ∼1–2·10−6, at a cometocentric distance of 10 km and at 3.1 AU from the Sun. A clear 6.2 h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisionless plasma within 260 km from the nucleus falls off with radial distance as ∼1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low-energy plasma around the comet.
  • Journal article
    Ceppi P, Hartmann DL, 2015,

    Connections between clouds, radiation, and midlatitude dynamics: a review

    , Current Climate Change Reports, Vol: 1, Pages: 94-102, ISSN: 2198-6061

    We review the effects of dynamical variability on clouds and radiation in observations and models and discuss their implications for cloud feedbacks. Jet shifts produce robust meridional dipoles in upper-level clouds and longwave cloud-radiative effect (CRE), but low-level clouds, which do not simply shift with the jet, dominate the shortwave CRE. Because the effect of jet variability on CRE is relatively small, future poleward jet shifts with global warming are only a second-order contribution to the total CRE changes around the midlatitudes, suggesting a dominant role for thermodynamic effects. This implies that constraining the dynamical response is unlikely to reduce the uncertainty in extratropical cloud feedback. However, we argue that uncertainty in the cloud-radiative response does affect the atmospheric circulation response to global warming, by modulating patterns of diabatic forcing. How cloud feedbacks can affect the dynamical response to global warming is an important topic of future research.
  • Journal article
    Mangeon T, Field R, Fromm M, McHugh C, Voulgarakis Aet al., 2015,

    Satellite versus ground-based estimates of burned area: a comparison between MODIS based burned area and fire agency reports over North America in 2007

    , The Anthropocene Review, Vol: 3, Pages: 76-92, ISSN: 2053-0196

    North American wildfire management teams routinely assess burned area on site during firefighting campaigns; meanwhile, satellite observations provide systematic and global burned-area data. Here we compare satellite and ground-based daily burned area for wildfire events for selected large fires across North America in 2007 on daily timescales. In a sample of 26 fires across North America, we found the Global Fire Emissions Database Version 4 (GFED4) estimated about 80% of the burned area logged in ground-based Incident Status Summary (ICS-209) over 8-day analysis windows. Linear regression analysis found a slope between GFED and ICS-209 of 0.67 (with R = 0.96). The agreement between these data sets was found to degrade at short timescales (from R = 0.81 for 4-day to R = 0.55 for 2-day). Furthermore, during large burning days (> 3000 ha) GFED4 typically estimates half of the burned area logged in the ICS-209 estimates.
  • Journal article
    Cargill PJ, Warren HP, Bradshaw SJ, 2015,

    Modelling nanoflares in active regions and implications for coronal heating mechanisms

    , PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 373, ISSN: 1364-503X
  • Journal article
    Voulgarakis A, Field RD, 2015,

    Fire influences on atmospheric composition, air quality, and climate

    , Current Pollution Reports, Vol: 1, Pages: 70-81, ISSN: 2198-6592

    Fires impact atmospheric composition through their emissions, which range from long-lived gases to short-lived gases and aerosols. Effects are typically larger in the tropics and boreal regions but can also be substantial in highly populated areas in the northern mid-latitudes. In all regions, fire can impact air quality and health. Similarly, its effect on large-scale atmospheric processes, including regional and global atmospheric chemistry and climate forcing, can be substantial, but this remains largely unexplored. The impacts are primarily realised in the boundary layer and lower free troposphere but can also be noticeable in upper troposphere/lower stratosphere (UT/LS) region, for the most intense fires. In this review, we summarise the recent literature on findings related to fire impact on atmospheric composition, air quality and climate. We explore both observational and modelling approaches and present information on key regions and on the globe as a whole. We also discuss the current and future directions in this area of research, focusing on the major advances in emission estimates, the emerging efforts to include fire as a component in Earth system modelling and the use of modelling to assess health impacts of fire emissions.
  • Journal article
    Dunlop MW, Yang J-Y, Yang Y-Y, Xiong C, Lühr H, Bogdanova YV, Shen C, Olsen N, Zhang Q-H, Cao J-B, Fu H-S, Liu W-L, Carr CM, Ritter P, Masson A, Haagmans Ret al., 2015,

    Simultaneous field-aligned currents at Swarm and Cluster satellites

    , Geophysical Research Letters, Vol: 42, Pages: 3683-3691, ISSN: 1944-8007

    We show for the first time, with direct, multispacecraft calculations of electric current density, and other methods, matched signatures of field-aligned currents (FACs) sampled simultaneously near the ionosphere at low (~500 km altitude) orbit and in the magnetosphere at medium (~2.5 RE altitude) orbits using a particular Swarm and Cluster conjunction. The Cluster signatures are interpreted and ordered through joint mapping of the ground/magnetospheric footprints and estimation of the auroral zone boundaries (taken as indication of the boundaries of Region 1 and Region 2 currents). We find clear evidence of both small-scale and large-scale FACs and clear matching of the behavior and structure of the large-scale currents at both Cluster and Swarm. The methodology is made possible through the joint operations of Cluster and Swarm, which contain, in the first several months of Swarm operations, a number of close three-spacecraft configurations.
  • Journal article
    Archer M, Plaschke F, 2015,

    What frequencies of standing surface waves can the subsolar magnetopause support?

    , Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 3632-3646, ISSN: 2169-9380

    It is has been proposed that the subsolar magnetopause may support its own eigenmode, consisting of propagating surface waves which reflect at the northern/southern ionospheres forming a standing wave. While the eigenfrequencies of these so‐called Kruskal‐Schwarzschild (KS) modes have been estimated under typical conditions, the potential distribution of frequencies over the full range of solar wind conditions is not known. Using models of the magnetosphere and magnetosheath applied to an entire solar cycle's worth of solar wind data, we perform time‐of‐flight calculations yielding a database of KS mode frequencies. Under nonstorm times or northward interplanetary magnetic field (IMF), the most likely fundamental frequency is calculated to be urn:x-wiley:jgra:media:jgra51793:jgra51793-math-0001 mHz, consistent with previous estimates and indirect observational evidence for such standing surface waves of the subsolar magnetopause. However, the distributions exhibit significant spread (of order ±0.3 mHz) demonstrating that KS mode frequencies, especially higher harmonics, should vary considerably depending on the solar wind conditions. The implications of such large spread on observational statistics are discussed. The subsolar magnetopause eigenfrequencies are found to be most dependent on the solar wind speed, southward component of the IMF, and the Dst index, with the latter two being due to the erosion of the magnetosphere by reconnection and the former an effect of the expression for the surface wave phase speed. Finally, the possible occurrence of KS modes is shown to be controlled by the dipole tilt angle.
  • Journal article
    Carr CM, Erikksson S, Lapenta G, Newman DL, Phan TD, Gosling JT, Lavraud B, Khotyaintsev YV, Markidis S, Goldman MVet al., 2015,

    On Multiple Reconnection X-lines and Tripolar Perturbations of Strong Guide Magnetic Fields

    , The Astrophysical Journal, Vol: 805, ISSN: 0004-637X

    We report new multi-spacecraft Cluster observations of tripolar guide magnetic field perturbations at a solar windreconnection exhaust in the presence of a guide field BM which is almost four times as strong as the reversing fieldBL. The novel tripolar field consists of two narrow regions of depressed BM, with an observed 7%–14% ΔBMmagnitude relative to the external field, which are found adjacent to a wide region of enhanced BM within theexhaust. A stronger reversing field is associated with each BM depression. A kinetic reconnection simulation forrealistic solar wind conditions and the observed strong guide field reveals that tripolar magnetic fields preferentiallyform across current sheets in the presence of multiple X-lines as magnetic islands approach one another and mergeinto fewer and larger islands. The simulated ΔBM/ΔXN over the normal width ΔXN between a BM minimum andthe edge of the external region agree with the normalized values observed by Cluster. We propose that a tripolarguide field perturbation may be used to identify candidate regions containing multiple X-lines and interactingmagnetic islands at individual solar wind current sheets with a strong guide field.
  • Journal article
    Kilpua EKJ, Hietala H, Turner DL, Koskinen HEJ, Pulkkinen TI, Rodriguez JV, Reeves GD, Claudepierre SG, Spence HEet al., 2015,

    Unraveling the drivers of the storm time radiation belt response

    , GEOPHYSICAL RESEARCH LETTERS, Vol: 42, Pages: 3076-3084, ISSN: 0094-8276

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